5i' Supplement No. 19 INFORMATION ON SOME LOUDNESS LOSS RELATED RATINGS (Melbourne, 1988) (quoted in Recommendations P.79 and G.III) Introduction It is important to determine the electroacoustic performance of telephone sets in terms of a standard which is universally recognized. Recommendation P.79 gives the algorithm as agreed by the CCITT for calculation of loudness ratings (LRs) of telephone sets. In order to avoid confusion, this algorithm should not be changed during the 1989-1992 Study Period. However, it is also clear from several independent investigations that Recommendation P.79 represents only with limited accuracy the speech and hearing characteristics of "ordinary people". This Sup- plement, gives the reader of the P-Series Recommendations a possi- bility to study the back-ground to the problems and provides infor- mation on some other loudness rating systems which have been used. In particular, SS 1,7 and 8 give examples of algorithms found useful by some Administrations for their own national planning. To avoid confusion when dealing with loudness ratings, the reader should also consider the information given in the Prelim- inary Notes | to this Volume. The results of (CCITT) LRs and R25Es calculated by the Chinese algorithm described in S 3 have been found to be in good agreement with the subjectively determined values obtained by the CCITT Laboratory in the past. This algorithm will be used by the CCITT Laboratory for the objective determination of R25Es for Administra- tions and other organizations. 1 The IEEE algorithm for calculating "objective loudness rat- ings" (Contribution from BNR, Canada) Abstract An algorithm for calculating loudness ratings is described. The agorithm is based on objective measurements and computations performed in such a manner that the numerical results obtained reflect the subjective attribute of loudness , but it employs cer- tain simplifying assumptions, to combine simplicity and reasonably close agreement between objectively determined responses and sub- jective responses. 1.1 Introduction The algorithm described below is based on a method [1] which has been in widespread use in North America for several years. The method has proved very adequate for use both in the planning of telephone networks and the characterization of individual com- ponents. The method described may be used for determining the loudness rating of partial or complete connections. For complete connec- tions, comprising overall or sidetone transmission paths, the pro- cedure involves measurement of acoustic input and output pressures. For partial telephone connections comprising transmitting, receiv- ing or electrical connection paths, the procedures involve measure- ment of acoustic pressure and electrical voltages. A particular advantage of this method for planning purposes, is that the sum of the loudness losses determined for individual parts of a connection closely approximates the loudness of the overall connection. 1.2 Definitions 1.2.1 loudness rating The amount of frequency-independent gain that must be inserted into a system under test so that speech sounds from the system under test and a reference system are equal in loudness (see S 1.3.2). 1.2.2 reference system A system that provides 0 dB acoustic gain between a mouth reference point at 25 mm in front of a talker's lips and an ear reference point at the entrance to the ear canal of a listener, when the listener is using an earphone. This system is assigned a loudness rating of 0 dB. The frequency characteristic of the system must be flat over the range 300-33000 Hz and show infinite attenuation outside of this range. 1.2.3 objective loudness rating (OLR) The rating of a connection or its components when measured according to the methodology described within S 1. 1.2.4 overall objective loudness rating (OOLR) OOLR = -20 log10fISfIM _______ (1-1) where SM is the sound pressure at the mouth reference point (in pascals) SE is the pressure at the ear reference point (in pascals). 1.2.5 transmitting objective loudness rating (TOLR) TOLR = -20 log10fISfIM _______ (1-2) where SM is the sound pressure at the mouth reference point (in pascals) VT is the output voltage of the transmitting component (in millivolts). 1.2.6 receiving objective loudness rating (ROLR) ROLR = -20 log10(12VfIW ________ (1-3) where VW is the open-circuit voltage of the electric source (in millivolts) SE is the sound pressure at the ear reference point (in pasclas). 1.2.7 electrical objective loudness rating (EOLR) For an electrical network, EOLR = -20 log10(12VfIW ________ (1-4) where VW is the open-circuit voltage of the electric source (in millivolts) VT is the output voltage of the network (in millivolts). 1.2.8 Loudness equation Loudness voltages (in millivolts) and pressures (in pascals) are determined in accordance with Equation (1-5). where xj is the signal level of SE, SM, VW | or VT | (in dBPa or dBmV) at frequency fj X the value SE, SM, VW | or VT | in accordance with the value used for Xj fj are specific frequencies of the N | frequencies selected for analysis. Loudness voltages and pressures are expressed in decibel-like form using Equation (1-6). S ` E, S ` M, V ` W | r V ` T= 20 log1\d0X (1-6) 1.3 Practical considerations 1.3.1 Voltage and pressure levels Voltage and pressure levels (VJ, VW, SM | and SE) as used in the definitions above may be measured using exactly the same pro- cedure as used in measuring the corresponding levels (i.e. VJ, EJ, PM, PE) in Recommendation P.64. 1.3.2 Analysis bandwith The loudness equation given above in Equation 1-5 is broadly applicable to any arbitrary bandwidth. However, for most transmis- sion planning purposes the bandwidth generally selected is 300-3300 Hz. This is because the use of partial connection ratings as engineering tools implicitly requires that for any given connec- tion, the sum of the partial ratings (for example, transmitting plus receiving) should approximately equal the overall rating. Thus the bandwidth used to obtain these ratings should approximate the bandwidth of the most restrictive element(s) in order to avoid cumulating bandwidth penalties when summing partial ratings. The specific limits of 300 Hz and 3300 Hz were selected largely on the basis of bandwidth capabilities of broad-band carrier systems with a 4 kHz channel spacing. In some cases, for example evaluation of a telephone sidetone path, a wider analysis band (e.g. 100-5000 Hz) may permit better estimation of the loudness loss. The method described above may still be used in such cases. It should be noted that if an actual reference system is con- structed for subjective comparison purposes, the system response at 300 and 3300 Hz shall be down 3 _ 1 dB relative to the midband response. The gain of the system shall be adjusted to compensate for the finite slope of the filter skirts (i.e. in comparison to the infinite slope inherent in the definition of S 1.2.2) and devi- ation from flatness of the pass-band. The amount of this adjustment can be determined by first calculating the OLR (S 1.2.3) over a frequency range that includes at least the -50 dB points of the real response, and next calculating the OLR of the ideal response over the same frequency range. The difference between the OLRs is the required gain adjustment. 1.3.3 Number of frequency points As a practical matter, measurement frequencies from which a loudness computation is made may be evenly spaced on either a linear frequency scale (1) or logarithmic frequency scale (2). For (1), no fewer than 31 frequencies should be used. For (2), no fewer than 12 frequencies should be used, but there is no signifi- cant improvement in accuracy if more than 20 frequencies are used. 1.3.4 Conversion factors between IEEE and Rec. P.79 loud- ness ratings The following empirical conversion factors have been found useful among North American Administrations for converting between loudness ratings derived according to the IEEE method described above and loudness ratings derived according to Rec. P.79, for 500-type (or equivalent) telephones using the G-handset. Send : SLR (P.79) = TOLR (IEEE) + 56 dB Receive : RLR (P.79) = ROLR (IEEE) - 50 dB Overall : OLR (P.79) = OOLR (IEEE) + 6 dB Sidetone : STMR (P.79) = SOLR + 8 dB For send, receive and overall, these relationships give agree- ment between the different ratings with a tolerance of about _ | dB; for sidetone the tolerance is about _ | dB. 1.4 Conclusions An alternative algorithm for calculating loudness ratings has been described. This algorithm has been in widespread use in North America for several years and has been found very satisfactory both for transmission planning purposes and characterization of indivi- dual network components. One of the main advantages is its relative simplicity. 2 Algorithms for calculation of loudness ratings (Contribu- tion from the Australian Administration) 2.1 Introduction There is growing evidence (see S 4) that the algorithm defined in Recommendation P.79 for the calculation of loudness ratings (LRs) is non-optimum, giving undue weight to the lower frequencies. This prompted a study within Telecom Australia to seek a better algorithm. The approach involved determining the loudness rating of many telephone paths and then optimizing the parameters in the algorithm for best agreement between subjective and computed values. An insert earphone type headset and a (pseudo) loudspeaking telephone were also included in the programme of work. In view of the physical differences from handset telephones, particularly on receiving, it was expected that different algorithms would be required. 2.2 Basic algorithm A method for the computation of loudness ratings (LRs) is derived in Recommendation P.79 and results in a formula of the form: LR = -10/m log ~" 10 (Si - Wo\di)m /10 where: m is the loudness growth coefficient Si is the overall acoustic-acoustic sensitivity in dB of the unknown telephone path (completed by the IRS, if necessary) Wo\di is the (negative) weighting function of frequency, in dB i is the 1/3 octave (strictly 1/10 decade) frequency step number. In the derivation, Si | efers to real mouth and real ear sen- sitivities, but if the correction factors for using artificial equivalents are included in the definition of Wo\di, then Sican be re-defined to be the measured sensitivity with artificial mouth and ear. Wo\dialso includes other components such as the spectral den- sity of human speech, the frequency sensitivity of the human ear, and normalization so that computed loudness rating of the IRS + IRS connection is 0 dB. 2.3 Determination of parameters The weighting function in Rec. P.79 was derived by determining each of the above components and then combining them. In the present work, the weighting function was derived directly. This direct approach leads naturally to consideration of non-handset telephones, such as headsets which may have insert type receivers and handsfree loudspeaking telephones. In the latter case the weighting function must also take into account the diffraction of sound around the human head, the effect of listening with two ears instead of one, and the use of an open rather than occluded ear. The method involved the insertion of a series of five low-pass and five high-pass filters into various telephone connections, measuring the LR of each subjectively, and then optimizing the parameters to give best agreement (in a least-squares error sense) with the computed values. The overall acoustic-acoustic sensitivi- ties of each connection were first measured using an artificial mouth (B&K type 4219) and an artificial ear (IEC type 318 by B&K) for handsets, and IEC type 711 (B&K type 4157) for the insert receiver. 2.4 Telephone paths The telephone paths involved several different telephone types which are in use in Australia, and are listed in the first column of Tables 2-1 to 2-5. If necessary, the connection was completed using the appropriate IRS end. Since the 802 type was fitted with a carbon transmitter, the send and receive sensitivities were meas- ured using a speech weighted random noise signal. The pseudo loudspeaking telephone (LST) paths were similarly measured to reduce the effect of standing waves in the test room. All other telephones were measured using sine waves. The equalized IRS con- nections were obtained by first equalizing to give a reasonably flat overall sensitivity (measured objectively) and then adding further equaliz- ers to give either a falling response or a rising response (about 6 dB/octave in both cases). The Featherset headset has an insert type receiver and a noise cancelling electret microphone which is held near the side of the mouth by a boom. The pseudo loudspeaking telephone for send measurements con- sisted of a 1/2 inch condenser microphone plus measuring amplifier with a sound level meter A - weighting function. The microphone was mounted on a goose-neck extension piece which held the microphone just above the surface of the table. For receive measurements, the equipment consisted of a power amplifier and a small loudspeaker lying on the long side of its enclosure, with the axis horizontal and pointing to the listener. A real loudspeaking telephone was not used to avoid complications associated with voice switching. 2.5 Form of weighting function Various parametric forms of the weighting function were tried, but a parabola gave almost as good a result as more complicated forms, including higher order polynomials. A parabola can be described in terms of the coordinates of its minimum (in this case) and a coefficient controlling its breadth, by a procedure known as "completing the square", viz. Wo\di= A + C (i - B )2 In order to compare the weighting functions derived using dif- ferent values of loudness growth coefficients m , it is more mean- ingful to consider the product Wo. This quantity may be interpreted as being proportional to the negative of the decibel equivalent of the weighting function which multiplies the band loudness (as dis- tinct from band power) in each of the 1/3 octave (1/10 decade) fre- quency bands. The value of i | ranges from 0 to 17 for frequencies from 100 Hz to 5012 Hz. 2.6 Optimum parameters The optimum values of m , Am , B , C | nd Cm | are given in Table 2-1 for the various telephone paths con- sidered. Also included in the table are the subjective-objetive error standard deviations (means = 0 dB) and the computed LR of the IRS + IRS connection (which ideally should be 0 dB). The standard deviations range from 0.1 to 0.4 dB, showing good fit of the model whem optimized for the particular path. Examina- tion of the distribution of the individual errors showed no trends with filter cut-off frequencies. The values of B , Cm | and m | are fairly consistent with different paths, the biggest differences in B | occurring with the different equalizer responses used with the IRS. Note that although the Featherset and loudspeaking telephone have quite different receive characteristics, B , Cm | and m | are within the range of those for conventional handset telephones. Am | is significantly dif- ferent, however, and this is reflected in the error of the computed LR of the IRS + IRS connection. This suggests that a single fre- quency weighting shape may be satisfactory for all telephones, whether handset, headset or handsfree, provided that a constant correction factor is applied in certain cases. Note that the value A | (and hence Am ) for the loudspeaking telephone on receive is now believed to be in error. This is dis- cussed later in S 2.11. H.T. [T1.19] TABLE 2-1 Optimum parameters for each path and error statistics ____________________________________________________________________________________________________ Parameters Errors Path m A Am B C Cm Std. dev. IRS ____________________________________________________________________________________________________ 802 send 0.255 39.67 10.12 9.64 1.225 0.312 0.2 -0.6 802 receive 0.249 42.72 10.63 9.04 0.889 0.221 0.3 - 0.1 Flip-phone send 0.308 34.72 10.69 9.35 0.732 0.226 0.3 -1.6 Flip-phone receive 0.286 40.69 11.63 8.85 0.513 0.147 0.4 - 0.5 807 send 0.315 36.38 11.46 9.66 0.648 0.204 0.2 -0.3 807 receive 0.263 43.37 11.41 8.95 0.533 0.140 0.1 - 0.1 Commander T210 send 0.312 33.84 10.56 9.45 0.934 0.291 0.5 -0.9 Commander T210 receive 0.279 38.28 10.68 8.72 0.704 0.196 0.4 -0.9 Siemens Trans. Cour. send 0.290 35.83 10.39 9.50 1.119 0.325 0.4 -0.2 Siemens Trans. Cour. receive 0.337 35.69 12.03 9.33 0.751 0.253 0.3 - 2.3 Equalized, IRS flat 0.270 42.47 11.47 9.64 0.581 0.157 0.3 -0.1 Equalized, IRS falling 0.299 40.21 12.02 10.31 0.398 0.119 0.2 -0.6 Equalized, IRS rising 0.300 35.07 10.52 6.66 0.496 0.149 0.3 - 0.3 Featherset send 0.285 36.48 10.40 9.55 0.684 0.195 0.3 -3.2 Featherset receive 0.330 42.63 14.07 9.28 0.525 0.173 0.3 - 6.9 Pseudo LST send 0.244 40.29 9.83 8.89 0.776 0.189 0.4 -3.8 Pseudo LST receive 0.232 27.36 6.35 9.40 0.352 0.082 0.3 -23.4 ____________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Table 2-1 [T1.19], p.1 2.7 Global optimization Parameters A , B | and C | are partly dependent on loudness rating specifics, but m | is a pure psycho-acoustic phenomenon. The average value of m | in Table 2-1 is 0.2855 (median = 0.29). The optimization process was therefore repeated with m | held at 0.2855, with results given in Table 2-2. The standard deviations increased only slightly (about 0.1 dB) verifying that a single value of m | is practicable. H.T. [T2.19] TABLE 2-2 Optimum parameters and error statistics with m = 0.2855 ____________________________________________________________________________________________ Parameters Errors Path A Am B C Cm Std. dev. IRS ____________________________________________________________________________________________ 802 send 39.91 10.25 9.64 1.208 0.345 0.3 -0.4 802 receive 37.68 10.76 9.08 0.901 0.257 0.4 - 0.3 Flip-phone send 37.31 10.65 9.29 0.712 0.203 0.3 -1.7 Flip-phone receive 40.70 11.62 8.85 0.513 0.147 0.4 - 0.5 807 send 39.76 11.35 9.57 0.603 0.172 0.3 -0.5 807 receive 40.17 11.47 9.03 0.569 0.162 0.2 - 0.3 Commander T210 send 36.55 10.44 9.38 0.919 0.262 0.5 -1.1 Commander T210 receive 37.46 10.69 8.74 0.711 0.203 0.4 -0.8 Siemens Trans. Cour. send 36.42 10.40 9.49 1.111 0.317 0.4 -0.2 Siemens Trans. Cour. receive 41.05 11.72 9.21 0.691 0.197 0.5 - 1.9 Equalized, IRS flat 40.16 11.47 9.63 0.606 0.173 0.3 -0.1 Equalized, IRS falling 42.03 12.00 10.43 0.373 0.107 0.3 -0.6 Equalized, IRS rising 36.92 10.54 6.56 0.476 0.136 0.3 - 0.3 Featherset send 36.42 10.40 9.55 0.685 0.196 0.3 -3.1 Featherset receive 47.08 13.44 9.06 0.490 0.140 0.4 - 6.4 Pseudo LST send 34.42 9.83 9.02 0.817 0.233 0.5 -3.4 Pseudo LST receive 18.75 5.35 9.28 0.390 0.111 0.4 -23.1 ____________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Table 2-2 [T2.19], p.2 Next, parameters m , B | and C were optimized globally, but individual values of A | were permitted, to investigate the feasi- bility of using the same shape for the weighting function, for all telephone types (handset, headset and handsfree), but permitting a correction constant if necessary. Optimization gave m = 0.2855, B = 9.19 and C = 0.7723, with A | and errors as shown in Table 2-3. The standard deviations have now increased signifi- cantly, the worst being for the IRS with rising frequency response. The errors for this path also show a clear trend with filter cut-off frequency, indicating a lack of fit of the model. Note how- ever that the standard deviations for the headset and handsfree telephone are still comparable with handset telephones in general. The value of A | necessary to give a computed LR of 0 dB for the IRS is 38.45. Table 2-4 gives the errors for a new algorithm (denoted D4 for convenience) based on the above data. The most significant mean errors are -22.4 dB (but see S 2.11) for the loudspeaking telephone receive, 6.9 dB for the headset receive, -3.6 dB for loudspeaking telephone send and -3.0 dB for headset send. There are obvious reasons why the mean errors on receive would not be zero, but the main reason for the errors on send are thought to be due to incorrect pressure distribution as a function of distance of the artificial mouth (B&K type 4219). Another reason might be due to the handset mouth cap affecting the pressure of the feedback micro- phone in the artificial mouth, while no significant effect occurred with the headset and loudspeaking telephone. Errors for handset telephones are smaller but unfortunately not negligible. These are thought to be mainly due to limitations of the artificial mouth and ear, including the effect of earcap leakage which is not modelled at all, and has to be included in the weighting function. H.T. [T3.19] TABLE 2-3 Optimum A and errors for the case of other parameters globally optimized __________________________________________________________________________________________________________________________________ High pass (Hz) Low pass (Hz) Errors Path A 158 225 380 630 1020 630 780 1260 2040 3120 Std. dev. IRS __________________________________________________________________________________________________________________________________ 802 send 38.03 -0.3 -0.6 -0.2 -0.2 -0.6 2.2 0.7 -0.2 -0.4 -0.5 0.9 -0.4 802 receive 38.62 -0.6 0.1 0.3 0.6 0.9 0.0 -0.8 -0.1 -0.5 -0.1 0.5 0.2 Flip-phone send 36.82 0.0 0.0 -0.2 0.0 -0.8 0.3 0.2 0.4 -0.3 0.5 0.4 -1.6 Flip-phone receive 38.95 0.3 0.0 -0.3 0.9 -0.3 -0.3 -0.5 0.5 0.5 -0.7 0.5 0.5 807 send 38.42 -0.4 -0.2 -0.3 -0.5 -1.9 1.2 1.1 0.6 0.5 0.1 0.9 -0.1 807 receive 38.84 -0.2 -0.2 0.1 0.2 0.2 -0.3 0.1 0.5 -0.1 -0.5 0.3 0.4 Commander T210 send 37.27 -0.3 0.2 0.5 0.7 -1.0 0.8 -0.1 -0.3 -0.2 -0.3 0.6 -1.2 Commander T210 receive 37.30 0.0 0.4 0.2 1.3 0.9 -1.0 -1.4 -0.1 -0.3 -0.1 0.8 -1.2 Siemens Trans. Cour. send 38.25 -0.1 -0.3 -0.1 0.4 -0.9 1.5 0.4 -0.6 -0.1 0.0 0.7 -0.2 Siemens Trans. Cour. receive 40.53 1.0 0.0 0.1 -0.2 -0.8 -0.3 0.0 0.1 -0.2 0.2 0.5 2.1 Equalized, IRS flat 38.89 -0.5 0.0 -0.9 -0.7 -1.1 1.0 0.9 1.2 0.5 -0.4 0.8 0.4 Equalized, IRS falling 39.37 0.1 0.1 -0.1 -0.1 -2.2 0.1 1.0 0.8 0.6 -0.3 0.9 0.9 Equalized, IRS rising 37.39 0.8 1.2 2.1 3.8 5.4 -7.1 -4.2 -1.8 0.0 -0.2 3.7 -1.1 Featherset send 35.47 -0.3 -0.3 0.0 -0.4 -1.4 1.6 0.8 0.6 0.4 -1.0 0.9 -3.0 Featherset receive 45.31 0.2 0.0 0.2 0.3 -1.5 -0.1 0.8 0.3 0.2 -0.3 0.6 6.9 Pseudo LST send 34.81 -0.2 -0.3 0.3 0.9 0.9 -0.1 -0.7 0.2 -0.8 0.0 0.6 -3.6 Pseudo LST receive 16.07 -0.4 -0.5 -0.6 0.3 0.0 -0.4 0.9 0.9 0.4 -0.4 0.6 -22.3 | __________________________________________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 2-3 [T3.19], p.3 H.T. [T4.19] TABLE 2-4 Errors for algorithm D4 _____________________________________________________________________________________________________________________________________________ High pass (Hz) Low pass (Hz) Errors Path 158 225 380 630 1020 630 780 1260 2040 3120 Mean Std. dev. _____________________________________________________________________________________________________________________________________________ 802 send -0.7 -1.0 -0.6 -0.7 -1.0 1.8 0.3 -0.7 -0.8 -0.9 -0.4 0.9 802 receive -0.4 0.2 0.5 0.7 1.1 0.2 -0.6 0.1 -0.3 0.1 0.2 0.5 Flip-phone send -1.6 -1.7 -1.9 -1.6 -2.4 -1.4 -1.4 -1.2 -1.9 -1.1 -1.6 0.4 Flip-phone receive 0.8 0.5 0.2 1.4 0.2 0.2 0.0 1.0 1.0 -0.2 0.5 0.5 807 send -0.5 -0.3 -0.4 -0.5 -2.0 1.1 1.0 0.5 0.4 0.1 -0.1 0.9 807 receive 0.2 0.2 0.5 0.6 0.6 0.1 0.5 0.9 0.3 -0.1 0.4 0.3 Commander T210 send -1.4 -0.9 -0.7 -0.5 -2.1 -0.4 -1.3 -1.5 -1.4 -1.4 -1.2 0.5 Commander T210 receive -1.1 -0.8 -1.0 0.2 -0.2 -2.1 -2.5 -1.2 -1.5 -1.2 -1.1 0.8 Siemens Trans. Cour. send -0.3 -0.5 -0.3 0.2 -1.1 1.3 0.2 -0.8 -0.3 -0.2 -0.2 0.7 Siemens Trans. Cour. receive 3.1 2.1 2.1 1.9 1.2 1.8 2.1 2.2 1.9 2.3 2.1 0.5 Equalized, IRS flat 0.0 0.4 -0.4 -0.2 -0.6 1.4 1.4 1.7 1.0 0.0 0.5 0.8 Equalized, IRS falling 1.0 1.0 0.8 0.8 -1.3 1.1 1.9 1.7 1.6 0.6 0.9 0.9 Equalized, IRS rising -0.3 0.1 1.1 2.7 4.3 -8.2 -5.2 -2.8 -1.1 -1.3 -1.1 3.7 Featherset send -3.2 -3.3 -3.0 -3.4 -4.4 -1.4 -2.1 -2.4 -2.6 -4.0 -3.0 0.9 Featherset receive 7.0 6.9 7.0 7.1 5.4 6.7 7.7 7.1 7.1 6.6 6.9 0.6 Pseudo LST send -3.9 -3.9 -3.3 -2.8 -2.7 -3.8 -4.4 -3.4 -4.4 -3.7 -3.6 0.6 Pseudo LST receive -22.8 -22.9 -23.0 -22.1 -22.3 -22.8 -21.5 -21.5 -22.0 -22.8 -22.4 0.6 _____________________________________________________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 2-4 [T4.19], p.4 2.8 Comparison with other algorithms Table 2-5 compares algorithm D4 with other algorithms. D2 is an algorithm based on preliminary work in which m = 0.2976 and the weighting function is defined by A = 40.50, B = 9.867 and C = 0.423. P.79 is the current Recommendation while P.XXE is the draft upon which it is based. Note that draft Rec. P.XXE as pub- lished in [2] is in error. On page 178 it states that the mean LR\dM\dEis -4.72 dB, but in fact it should be -0.1 dB. Thus 4.6 dB should be subtracted if the tabulated data are used. Zw is a com- plicated algorithm based on the work of E. Zwicker and published as ISO Rec. R532B. The paths are as previously discussed except that the first item is the set of 14 sidetone responses reported in an earlier work [3]. The group mean errors for the handset telephones are fairly small for all algorithms, but the group standard deviation for P.79 is about twice that of the others. Note that the complicated Zw method does not seem to offer any significant advantage, and still gives rather large errors for the IRS + IRS + equalizer connec- tions. Naturally D4 gives a reasonably good fit because it was optimized for these conditions. The values of Wo | as a function of frequency for algorithms P.XXE, P.79, D2 and D4 are shown in Figure 2-1. Note that P.XXE and D4 are very similar, and that P.79 shows much smaller (negative) weight at low frequencies. H.T. [T5.19] TABLE 2-5 Comparison of errors for five algorithms _______________________________________________________________________________________________________________________________________ D2 D4 P.79 P.XXE Zw Path Mean Std. dev. Mean Std. dev. Mean Std. dev. Mean Std. dev. Mean Std. dev. _______________________________________________________________________________________________________________________________________ Sidetone [3] -1.2 0.5 -1.2 0.7 1.9 1.4 -0.8 0.6 -0.6 0.7 _______________________________________________________________________________________________________________________________________ 802 send -0.7 0.9 -0.4 0.8 -1.4 3.4 -1.0 1.2 -0.4 1.8 802 receive -0.4 1.5 0.2 0.5 0.1 2.2 -0.1 0.5 0.6 0.9 Flip-phone send -2.2 1.0 -1.6 0.4 -1.9 3.0 -1.9 0.7 -1.1 1.7 Flip-phone receive 0.1 1.6 0.5 0.5 -0.5 2.6 0.2 0.5 1.3 1.3 807 send -0.6 0.5 0.0 0.9 -0.5 3.6 -0.3 1.1 0.7 2.1 807 receive -0.2 1.2 0.4 0.3 -0.3 2.6 0.0 0.3 1.0 1.4 Commander T210 send -1.6 1.4 -1.2 0.5 -2.7 2.5 -1.6 0.8 -0.6 1.7 Commander T210 receive -2.1 1.7 -1.2 0.8 -1.3 1.8 -1.5 0.6 -0.7 1.0 Siemens Trans. Cour. send -0.9 1.0 -0.2 0.6 -0.6 3.1 -0.6 1.0 0.0 2.1 Siemens Trans. Cour. receive 1.1 1.0 2.1 0.4 2.1 2.7 1.8 0.8 2.8 1.3 Equalized, IRS flat 0.0 0.4 0.4 0.8 3.2 3.6 0.5 0.9 1.4 2.1 Equalized, IRS falling 0.1 0.5 0.9 0.8 4.7 3.8 0.9 1.2 2.1 2.2 Equalized, IRS rising -1.1 4.7 -1.1 3.5 0.3 1.5 -1.0 3.4 -0.5 2.5 Featherset send -3.3 0.8 -3.0 0.8 -4.1 3.1 -3.3 1.0 -2.4 2.0 Featherset receive 6.2 1.1 6.9 0.6 5.0 2.5 6.3 0.8 7.2 1.6 Pseudo LST send -4.4 1.6 -3.6 0.6 -4.3 1.8 -4.0 0.5 -3.3 0.9 Pseudo LST receive -23.4 0.7 -22.4 0.5 -22.0 2.5 -22.7 0.6 -21.9 1.0 _______________________________________________________________________________________________________________________________________ Handset -0.61 0.95 -0.09 1.02 -0.09 2.09 -0.35 1.06 0.51 1.19 _______________________________________________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Table 2-5 [T5.19], p. Figure 2-1, p. 2.9 Validation of new algorithm Table 2-6 gives the subjective-objective errors for test results which are not used in deriving D4. Two samples of 802 tele- phone (local designations 82/YZ and 82/IA) were each fitted with one of four 20E non-carbon transmitters (designated 101, 165, 310 and 313) for send measurements. Only one receive measurement was made for each telephone. Three lines were used, viz. zero, 1.6 km and 4.2 km of 0.4 mm cable. One consistent trend is that the errors become more positive with increasing line length, and range from 0.6 dB for D2 through 0.9 dB for D4, P.XXE and Zw, to 1.2 dB for P.79. A possible reason for this trend is the progressive high frequency loss which occurs with line length, and inadequacies in the loudness models to cope with this. This is also consistent with the errors associated with the equalized IRS results in Table 2-5, where the falling response gives the most positive error and the rising response the most negative error of the set of three. H.T. [T6.19] TABLE 2-6 Errors for 802 telephones (20E non-carbon transmitters) plus lines for five algorithms Unable to convert table Table 2-6 [T6.19], p. 2.10 Attempts to reduce errors In order to explore whether another weighting function would simultaneously give small errors for the 802 telephone only, with both filters and lines, the 802 + lines data described above was combined with the 802 + filter data described earlier. A new weighting function was then optimized, with A | constrained to give 0 dB error for the LR of the IRS. It was found however that the optimum parameters were not greatly different from those in D4 and that the range of errors with line length was only reduced by 0.1 dB to 0.8 dB. It was thought possible that forcing a polynomial fit to the weighting function may be partly responsible for this poor agree- ment, so a piecewise linear weighting function was tried, with break frequencies at i = 4, 7, 10 and 13 (f = 250, 500, 1000 and 2000 Hz respectively). It was found that the range of errors with line length was unchanged at 0.8 dB. Thus the weighting function shape does not seem to be at fault. A simplification inherent in all algorithms from P.XXE to D4 is that the weighting function does not cause any frequency band to be masked, whereas it is assumed in the derivation of these models that it is only the band loudness above threshold which contributes to loudness. The basic formula was therefore changed to include a threshold rather than a weighting function. Summation is only over those bands which are above threshold. A disadvantage of this algo- rithm is that it is now not possible to make loudness rating the subject of the formula, and an iterative approach is necessary. A parabolic threshold function was assumed, and it was found that the range of errors with line length was only reduced a further 0.1 dB to 0.7. The marginal improvement does not justify the extra compli- cation of this method. Finally, the effect of frequency masking was included by investigating whether a better way of using Zwicker's loudness algorithm could be found. In addition to the sensitivity of hearing which is inherent in Zwicker's algorithm, a LR algorithm must also include the spectral density and level of the speech signal, the ear cap leakage loss and the junction loss to give the same loud- ness through the IRS + IRS path as the NOSFER system with 25 dB in its junction. These may be combined to form an auxiliary function analogous to an input signal to the telephone path, where the out- put is fed to Zwicker's loudness algorithm. Assuming a parabolic shape to this auxiliary function, it was found that the range of mean errors with line length was 0.8 dB and thus comparable to that of previous algorithms, such as D4. A possible reason why none of the methods was successful in reducing errors to a low and random value (i.e. no trend with line length) may be that the subjects changed their bases of listening to the speech from one filter condition to the next. They may not listen to the signal as a whole, but base their comparison on a smaller band or bands where the main energy lies (formants). The location of the band or bands could vary depending on the cut-off frequencies of the filters. Zwicker based his method on subjective data gathered on non-speech signals, but it is known that people listen to speech in a different way to other sounds, and this may affect the judgement of loudness. Other possible sources of discrepancy are possible, including the effect of changes in the voice-ear team membership during the course of the investigation. 2.11 Postscript on the correction factor for loudspeaking telephone receive The receive correction factor found initially for the loudspeaker and amplifier combination was about -22.4 dB, but in subsequent work a drift in this value was observed. Whether this was due to set-up errors, hardware faults or to changing bases of rating loudness by the voice-ear team has not been resolved. Subse- quent tests repeating those reported above and others have yielded a correction factor of about -14.0 dB, and this is now believed to be more correct. (The D4 loudness algorithm continued to give good consistency in the repeat tests, with a standard deviation of 0.7 dB over the range of filters.) 2.12 Conclusion A revised algorithm has been found which is remarkably similar to the draft Recommendation upon which the present Recommendation P.79 was based. Using either of these methods gives about half the standard deviation of the difference between subjec- tive and objective measurements which would be obtained with Rec. P.79. A general accuracy of about _ | dB can be expected, which is about the order of accuracy of subjective tests, but with better repeatability and lower cost. Although it was expected that a different weighting function would be required for headsets and loudspeaking telephones, in fact it was found that a constant correction for each path type proved to be all that was necessay for practical purposes. In particular, the following corrections should be added to the calculated LRs: Headset Send: -3.0 dB Receive: 6.9 dB ( insert receiver only) Loudspeaking telephone Send: - 3.6 dB Receive: -14.0 dB As far as the revision of Rec. P.79 is concerned, two courses of action seem possible. Preferably, i) pool all the data available worldwide and derive a global average using the principles described above, or alternatively ii) return to the algorithm weights of draft Rec. P.XXE. 3 Uniform algorithms for the calculation of R25 equivalents and loudness ratings (from the Ministry of Post and Telecommunica- tions of the People's Republic of China) 3.1 Introduction The subjective test team of the CCITT Laboratory has been changed since 1985. From the periodic stability check reports of the CCITT Laboratory, it can be ascertained that the recent subjec- tively determined value x2(see Recommendation P.78) is about 18 dB which is close to the value determined at other laboratories, and different from the previously determined value of 12 dB. In addi- tion, the SR25E and RR25E values of telephone systems determined recently by the CCITT Laboratory are several decibels lower than the results previously obtained, and close to those measured by other laboratories. In this connection, it is possible to use a uniform algorithm, similar to the simple algorithm in Recommendation P.79, for the calculation of R25 equivalents and loudness ratings, with values for the slope parameter m and the G -functions different from those given in Recommendation P.79. In order to obtain a suitable algorithm and appropriate parameters, four different algorithms were used in order to calcu- late the values of R25E and LR, and the results were compared. Three of them are similar to that used for the calculation of loud- ness ratings described in Recommendation P.79, except that dif- ferent values of the slope parameter m and the G -functions are used. These values: - are taken from draft Recommendation P.XXE [2]; - correspond to the Chinese test team; - correspond to the old test team of the CCITT Laboratory, but with LEcorrected in the NOSFER receiving system. The fourth algorithm used was the ISO-532B (Zwicker) algo- rithm. 3.2 Comparison of various algorithms The four algorithms used here are labelled as the P.XXE, the Chinese, the P.79 Cor. and the ISO-532B algorithms. 3.2.1 SFC of the reference system 3.2.1.1 The sensitivity/frequency characteristic (SFC) data of the sending system and the receiving system (without leakage) of the NOSFER are taken from Recommendation P.42 (Red Book). The cou- pling loss at the receiving part of the NOSFER is included in the receiving SFC in the calculation. Several years ago the Chinese Administration pointed out that the SFC data of the NOSFER receiving system measured by the IEC 318 artificial ear with the flat plate differed considerably from those measured with the operator's ear, and measured the values of LEcorresponding to the earphone type DR-701 used by the Chinese test crew in the receiving system of the NOSFER. This point of view has been verified by many Administrations and has been generally accepted by CCITT Study Group XII. The values of LEused here are those corresponding to the CCITT Labora- tory test team, as given by the French Administration (Contribution COM XII-111, 1985-1988) (see Table 3-1). 3.2.1.2 The SFC data of IRS are taken from Recommendation P.48 and the SFC values of the receiving system are corrected using the LEgiven in Recommendation P.79. 3.2.2 Slope parameter m and G-functions Methods for estimating m and G are described in Contribution COM XII-3 and COM XII-10 (1981-1984). 3.2.2.1 P.79 Cor. algorithm (m = 0.175) The values of the slope parameter m and the G -functions in Recommendation P.79 are derived from the results of the filter loudness loss test of the old CCITT Laboratory test team; the leak- age between the ear of the operator and the earphone of NOSFER is not included. The values of the G -functions given in Recommendation P.79 must therefore be corrected. The results of the G -functions with correction of LEare listed in Table 3-2. H.T. [T7.19] TABLE 3-1 Acoustic coupling loss LvE used in calculation _________________________________ Frequency L NOSFER L P.79 _________________________________ 100 0.9 20.0 125 0.2 16.5 160 -0.6- 12.5 200 -1.6- 8.4 250 -2.9- 4.9 315 -4.2- 1.0 400 -5.3- -0.7- 500 -5.4- -2.2- 630 -4.9- -2.6- 800 -4.6- -3.2- 1000 -4.5- -2.3- 1250 -3.9- -1.2- 1600 -4.6- -0.1- 2000 -3.3- 3.6 2500 -3.2- 7.4 3150 -3.3- 6.7 4000 -3.7- 8.8 5000 -2.9- 10.0 6300 -0.8- 12.5 8000 -0.8- 15.0 _________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 3-1 [T7.19], p.8 H.T. [T8.19] TABLE 3-2 10 logv1v0 G of various algorithms ____________________________________________ Frequency P.79 Cor. P.XXE Chinese ____________________________________________ 100 -31.86 -35.90 -30.67 125 -28.58 -34.11 -30.63 160 -27.14 -32.94 -30.68 200 -28.13 -31.50 -30.81 250 -28.48 -30.96 -31.02 315 -31.22 -31.21 -31.35 400 -30.10 -31.15 -31.79 500 -33.02 -30.97 -32.33 630 -33.46 -32.13 -33.00 800 -34.34 -33.05 -33.83 1000 -35.51 -34.50 -34.74 1250 -37.97 -35.91 -35.78 1600 -38.60 -37.14 -37.10 2000 -41.22 -38.50 -38.46 2500 -41.66 -39.66 -39.96 3150 -45.77 -41.11 -41.70 4000 -43.54 -43.45 -43.68 5000 -47.03 -45.37 -45.71 6300 -48.03 -48.01 8000 -46.32 -50.60 ____________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 3-2 [T8.19], p.9 3.2.2.2 P.XXE algorithm (m = 0.225) The values for the slope parameter m and the G -functions are taken from Table 1, page 185 of COM XII-1 [2]. Also see Table 3-2 of this Supplement. 3.2.2.3 Chinese algorithm (m = 0.2) Results of smoothed G -functions are used [see Contribution COM XII-233 (1981-1984)]. Values are also given in Table 3-2. The coupling loss of the NOSFER earphone is not included in the estimation of the G -functions but this has little effect on the smoothed result of the G -functions. 3.2.3 W-weights for the calculation of R25E Methods for deriving W -weights are described in Contributions COM XII-3 and COM XII-10 (1981-1984). 3.2.3.1 P.79 Cor. algorithm Weights are derived from the SFC data of NOSFER described in S 3.2.1.1 and the data for m and G -functions given in S 3.2.2.1. 3.2.3.2 P.XXE algorithm W -weights are derived from the SFC data of NOSFER described in S 3.2.1.1 and the data for m and G -functions given in S 3.2.2.2. In the absence of a complete set of data for the G -functions at high and low frequencies, a number of arbitrary values have had to be chosen in this contribution. 3.2.3.3 Chinese algorithm W -weights are derived from the SFC data of NOSFER described in S 3.2.1.1 and the data for m and the G -functions given in S 3.2.2.3. The derived W -weights of the three algorithms discussed above for the calculation of R25E are listed in Table 3-3. 3.2.4 W-weights for the calculation of LR The methods for the derivation of W -weights for the three algorithms are similar to those described in S 3.2.3, except that the SFC data of IRS (with the LEof P.79) are used instead of the SFC data of NOSFER. The derived W -weights of the three algorithms discussed above for the calculation of LR are listed in Table 3-4. 3.2.5 Source of data for the SFC of telephone systems and the subjectively determined values of R25E and LR In making comparisons between the subjectively determined results and the calculated results, use can only be made of the data relating to telephone sets with subjectively determined values established by the new CCITT test team and the corresponding SFC values. 3.2.5.1 For SR25E There are only six sets of sending SFC data provided by three linear telephone sets under 0/L line conditions (i.e. with or without lines). These data are taken from CCITT Laboratory Technical Report 808 (Temporary Document 84, Working Party XII/1, April 1987); the other set of subjectively determined values is taken from CCITT Laboratory Technical Report 797 (Temporary Docu- ment 78, Working Party XII/1, April 1987). 3.2.5.2 For RR25E The subjectively determined values of RR25E of some telephone systems are taken from CCITT Laboratory Technical Report 797 and the corresponding SFC data were given by the Head of the CCITT Laboratory in October 1986. H.T. [T9.19] TABLE 3-3 W-weights for the calculation of R25E ___________________________________________ P.79 Cor. P.XXE Chinese ___________________________________________ | | | | | | | | | | Frequency W W W W W W ______________________________________________________________________ 100 109.6 116.6 106.9 113.9 92.2 99.2 125 82.4 89.8 92.1 99.5 84.5 91.9 160 67.2 75.2 81.2 89.2 78.5 86.5 200 66.6 76.3 69.5 79.2 73.4 83.1 250 60.6 72.7 60.4 72.5 67.2 79.3 315 67.6 82.2 54.3 68.9 61.0 75.6 400 53.8 70.2 47.9 64.3 56.6 73.0 500 63.5 80.8 41.3 58.6 52.9 70.2 630 60.5 75.5 42.4 57.4 51.5 66.5 800 60.8 74.3 42.9 56.4 51.6 65.1 1000 62.6 75.1 45.5 58.0 51.9 64.4 1250 70.5 81.9 47.2 58.6 51.9 63.3 1600 67.3 79.9 47.1 59.7 52.3 64.9 2000 78.4 90.1 50.3 62.0 55.8 67.5 2500 74.8 86.6 50.6 62.4 57.9 69.7 3150 93.2 102.1 53.5 62.4 62.3 71.2 4000 76.7 84.6 61.3 69.2 69.2 77.1 5000 88.8 103.3 63.2 77.7 72.2 86.7 6300 84.9 110.0 92.2 117.3 74.9 100.0 8000 80.4 99.1 | | | | | | | | | | | | | | | | | | | | | | 102.7 | | | | | | | | | | | | | | | | | | | | | | 121.4 | | | | | | | | | | | | | | | | | | | | | | 93.8 | | | | | | | | | | | | | | | | | | | | | | 112.5 ______________________________________________________________________ m = 0.175 m = 0.225 m = 0.2 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 3-3 [T9.19], p.10 3.2.5.3 For SLR and RLR The subjectively determined values are taken from CCITT Laboratory Technical Report 771 (Temporary Document 42, Working Party XII/1, May 1986) and the corresponding SFC data [the sending data measured at LRGP (loudness rating guard-ring position)] were also provided by the CCITT Laboratory. 3.2.6 Method of calculation 3.2.6.1 For the P.79 Cor., P.XXE and the Chinese algorithms, the equations used for the calculation of SR25E, RR25E, SLR and RLR are as follows: H.T. [T10.19] TABLE 3-4 W-weights for the calculation of LR ___________________________________________ P.79 Cor. P.XXE Chinese ___________________________________________ | | | | | | | | | | ___________________________________________________________ Frequency W W W W W W ___________________________________________________________ 100 149.3 147.6 150.0 150.0 135.7 134.0 125 111.4 112.2 150.0 150.0 117.2 118.0 160 85.3 87.6 150.0 150.0 100.3 102.5 200 74.4 82.5 82.5 90.6 85.0 93.1 250 61.5 73.6 66.7 78.8 71.9 84.0 315 62.2 79.2 54.0 71.0 59.3 76.3 400 46.0 65.0 45.1 64.1 52.4 71.4 500 54.6 75.0 37.4 57.8 47.6 68.0 630 49.4 70.0 36.1 56.7 44.2 64.8 800 48.2 68.6 35.5 55.9 42.6 63.0 1000 50.6 69.2 38.8 57.4 43.6 62.2 1250 59.0 75.0 41.0 57.0 44.2 60.2 1600 57.3 71.0 42.3 56.0 45.8 59.5 2000 71.5 80.7 48.6 57.8 52.5 61.7 2500 71.8 75.7 52.7 56.6 58.8 62.7 3150 88.5 92.6 54.0 58.1 61.5 65.6 4000 116.7 113.5 106.5 103.3 112.7 109.5 5000 155.9 143.2 150.0 150.0 143.0 130.3 6300 170.0 163.6 150.0 150.0 163.8 157.4 8000 180.0 165.0 150.0 150.0 196.7 181.7 ___________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | _____________________________________ m = 0.175 m = 0.225 m = 0.2 _____________________________________ | | | | | | | | Tableau 3-4 [T10.19], p.11 It should be noted that: - different values of m and W -weights are used for the three different algorithms discussed above; - the values of Wsfor the calculation of SR25E are different from the values used for the calculation of SLR within the same algorithm. The same rule applies to WRfor the calculation of RR25E and RLR. - SU\dM\dJare the sending SFC values of telephone systems measured at RESP (reference equivalent speaking position) and the RGP for the calculation of SR25E and SLR respectively; and - the LEvalues listed in Recommendation P.79 are used to correct the telephone receiving systems for the calculation of RR25E and RLR. 3.2.6.2 Method using the ISO-532B algorithm a) Input signal: the long-term speech spectrum given in Recommendation P.50 and 1/3 octave data are used. b) Reference system: NOSFER sending + 25 dB + NOSFER receiving for R25E, and IRS sending + 18 dB + IRS receving for LR. c) Tested system: varies depending on the items. For example, for SR25E, sending tested + variable attenuator (X ) + NOSFER receiving. Proceed as follows: 1) Calculate the loudness of the reference system using the ISO-532B algorithm on the basis of the output levels in 1/3 octave bands of the reference system. 2) Calculate the loudness of the tested system using the ISO-532B algorithm on the basis of the output levels in 1/3 octave bands of the tested system, change the attenuation value X of the variable attenuator until the calculated loudness is the same as that in the reference system. 3) Then: R 25 E = 25 - X LR = 18 - X In calculating R25E and LR with the ISO-532B algorithm, the SFC data of the reference systems and the telephone systems are the same as those used in the other algorithms discussed above. 3.2.7 Calculated results The subjectively determined values of SR25E, RR25E, SLR and RLR, the result calculated by using various algorithms, and the differences between the subjectively determined values and the cal- culated results are given in Tables 3-5 a) to 3-5 d). For the sake of comparison, the mean results calculated by the four algorithms are summarized in Table 3-6. 3.3 Discusion Before analyzing the calculated results, it is necessary to bear in mind the effect of the diffraction by the human head and the reverberation of the test room on the sending SFC and NOSFER. As a result of this effect, the difference in the sending SFC between the mouth reference point of the NOSFER system and a point 140 mm in front of the operator's lips is less than 13.46 dB under ideal conditions, i.e. with the virtual sound source 6 mm behind the lips being taken to be the actual human sound source and assum- ing the sound to be transmitted in a free field. In the Chinese subjective test room, this difference has been mesured with an average correction of 1 to 1.5 dB for each frequency (see contribu- tion COM XII-209 (1985-1988)). This effect has not been included in any of the four algorithms discussed above. 3.3.1 The calculated results of SR25E and RR25E using the P.79 Cor. algorithm are about 1.5 to 2 dB higher than the subjectively determined values. This is understandable because the values of slope parameter m and the G -functions were estimated on the basis of the filter test results of the old CCITT test team. 3.3.2 Both the P.XXE and the Chinese algorithms can be used as the uniform algorithm for the calculation of R25E and LR. The SR25E calculated by the P.XXE algorithm is about 1 dB lower than the sub- jective result, but after correction for diffraction by the human head and the reverberation of the test chamber as discussed in S 3.3, there may be fairly good agreement between the subjective and objective values of SR25. In view of the fact that some values, at high and low frequencies, of the G -functions and W -weights used in the P.XXE algorithm are chosen arbitrarily and that a correction has to be made to the sending SFC of NOSFER, the Chinese algorithm may be better than the P.XXE algorithm in use. 3.3.3 The results calculated using ISO-532B agree with the corresponding subjective test results. It has been noticed, how- ever, that the standard deviation for the mean values of the differences of SR25, SLR and RLR is larger than that for the other algorithms. Furthermore, the ISO-532B algorithm is much more com- plicated than the other algorithms. This algorithm would not there- fore be the best choice. 3.3.4 The difference of the results of SLR and of RLR calculated with the P.79 Cor. algorithm and with the original P.79 algorithm, respectively, is generally less than 0.1 dB. 3.3.5 It is not advisable to use the P.79 Cor. algorithm to calculate R25E values because of the considerable difference between the calculated values and the subjectively determined values. The difference between the subjectively determined values of SR25E for a telephone set obtained by the old and by the new test team is about 4 to 6 dB, respectively, while the difference calcu- lated by the P.79 Cor. algorithm and by the Chinese algorithm is about 2 dB. The value of R25E calculated by the P.79 Cor. algorithm does not agree with the subjectively determined value of the old test team either. 3.4 Conclusion A simpler algorithm such as the Chinese algorithm can be used as the standard algorithm for the calculation of R25E and LR. There is good agreement between the calculated results and the results subjectively determined by the new test team of the CCITT Labora- tory. The statement appearing in some Recommendations to the effect that a simple algorithm cannot be used for the comparison of the loudness of wideband systems should be revised. 4 Loudness rating coefficients derived from subjective meas- urements on high-pass (HP) and low-pass (LP) filtered speech (Con- tribution from ELLEMTEL, Sweden) 4.1 Introduction The exact shape of the frequency-weighting of the loudness rating (LR) algorithm is not very critical when computing LR values for routine planning evaluations. However, reasonable realistic values of the coefficients are needed for a more detailed analysis of, for instance, attenuation distortion and bandwidth restriction effects. Loudness rating parameters may be derived from known statis- tics of the "average" speech power spectrum and the "average" hear- ing threshold frequency response curves. An alternative direct way is to make use of subjective listen- ing tests of the influence of variable low-pass and high-pass filters in a NOSFER type circuit. Such measurements have been made many times in the past. This section uses four sets of data of which three originate from STL [4] and one from the People's Repub- lic of China [5]. As is well known, subjective evaluation of loudness has its difficulties. A prime requirement is that the test team must represent "ordinary people" with regard to speech and hearing. Also, the team must be instructed to judge specifically "loudness impression" and not "quality impression" of bandwidth limitation. The CCITT test team seems not to have fulfilled these criterions when performing the measurements for the P.79 algorithm. 4.2 Derivation of loudness rating coefficients For complex noise spectra of time-constant nature the masking effects between frequency bands have to be considered, i.e. the Zwicker algorithm should be used for evaluating loudness. However, it is rather doubtful whether this complex method is really neces- sary (or even correct) for speech signals. Instead, the simpler conventional " physiological loudness impression " model for interpreting the subjective results will be tried. The expression for the loudness loss A caused by a filter introduced in the electric part of the transmission path of speech sounds from mouth to ear will be given. To facilitate the mathemat- ical treatment, the usual series summation over the third-octave bands is replaced by a continuous integration over a logarithmic frequency scale. H.T. [1T11.19] _________________________________________________________________________________________ TABLE 3-5 { a) Comparison of subjective and calculated results using the P.79 Cor. algorithm } Unable to convert table _________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | Tableau 3-5a) [1T11.19], p.12 a l'italienne H.T. [2T11.19] _____________________________________________________________________________________ { TABLE 3-5 (continued) } { b) Comparison of subjective and calculated results using the P.XXE algorithm } Unable to convert table _____________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 3-5b) [2T11.19], p.13 a l'italienne H.T. [3T11.19] _______________________________________________________________________________________ { TABLE 3-5 (continued) } { c) Comparison of subjective and calculated results using the Chinese algorithm } Unable to convert table _______________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 3-5c) [3T11.19], p.14 a l'italienne H.T. [4T11.19] ___________________________________________________________________________ TABLE 3-5 (end) { d) Comparison of subjective and calculated results using the ISO-532B algorithm } Unable to convert table ___________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | Tableau 3-5d) [4T11.19], p.15 a l'italienne H.T. [T12.19] TABLE 3-6 Summarized results showing the mean differences and standard deviations between the subjective and calculated R25Es and LRs using various algorithms ______________________________________________________________________________________________________ | | | | SR25E RR25E SLR RLR | | Mean | ua) Std. dev. | ua) Mean Std. dev. Mean Std. dev. Mean Std. dev. ______________________________________________________________________________________________________ Chinese -0.57 -0.22 0.68 0.48 +0.67 0.54 +0.12 0.83 +0.63 1.21 ______________________________________________________________________________________________________ P.XXE -1.41 -1.06 0.64 0.45 -0.33 0.60 +0.12 0.82 +0.80 1.27 ______________________________________________________________________________________________________ P.79 Cor +1.56 +1.91 0.87 0.57 +1.83 0.79 -0.39 0.85 +0.01 1.11 ______________________________________________________________________________________________________ ISO-532B -1.15 -0.06 0.92 1.05 +0.23 0.52 +0.10 0.93 +0.67 1.42 ______________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | a) Two values in each block correspond to subjective test results quoted from different Technical Reports of the CCITT Laboratory. Tableau 3-6 [T12.19], p.16 Thus, the loudness loss becomes: Formula [F3.19], p. where m is the loudness growth factor X log1\d0 { | fIF /F0 } F0= 1 kHz K (X ) is the loudness weighting factors 4.2 L (X ) is the attenuation of the filter For K (X ) it is stipulated that Formula [F4.19], p. Otherwise, K (X ) remains to be determined as well as the value of m . [In Equation (4-1) however, the exact value of m has only a second-order effect as has been discussed in other contribu- tions.] For a high-pass filter with negligible loss in the pass band and sharp cutoff at F = Fc(X = Xc) we get: Similarly, for a low-pass filter Using Equation (4-3) we get For a chosen value of m , we may now plot as a function of Xc S -shaped curves are obtained as in Figure 4-1 a). If the two curves more or less coincide as in Figure 4-1 b) the "best" value of m has been found. Then a mathematical expression for a curve Y0which fits the coincidence curve reasonably well is sought. The derivative of Y0thus gives K (X ). Figure 4-1, p. Of course S -shaped curves can be described by an infinite number of mathematical functions. However, the normal error integral turns out to be a suitable choice. Plotting Y (A ) on a "normal distribution diagram" paper gives, in essence, straight lines. Figures 4-2, 4-3 and 4-4 present the results from data given in [4]. (The measurements were made at STL in January 1986, May 1975 and February 1975.) It is interesting to note how well the points cluster around straight lines, especially in Figure 4-2. The corresponding K (X )-curves are plotted in Figure 4-5 together with a curve derived from [5] as presented in [6]. 4.3 Discussion and conclusions It is remarkable that the weighting curves depicted in Fig- ure 4-5 coincide so closely although they were made by very dif- ferent test teams. Curve 4 in Figure 4-5 has been used as a kind of reference in the further development of the "simplified" algorithm P.79A. The STL HP-LP measurements seem to confirm that this "weighting refer- ence" is quite suitable. Thus, the P.79A algorithm will give a rea- sonable estimation of attenuation distortion and bandwidth limita- tion effects. Another conclusion is that the loudness loss caused by attenuation distortion and bandwidth limitation can be explained by the simple loudness rating model without resorting to the Zwicker algorithm Curve 4 in Figure 4-5 was used to compute the corresponding 20-weights for the 1/3-octave frequencies in the series summation for the 0.1-8 kHz band, see [6]. These are shown in Figure 4-6 together with the equivalent Ki-values for P.79. As can be seen, the P.79 curve has some absurd peaks and gives more emphasis to lower frequencies and less to higher frequencies. Thus, P.79 can be expected to underestimate the effect of how attenuation slope as a function of frequency influences the loudness loss of a connec- tion. This seems to be verified experimentally, as reported in [7]. FIGURE 4-2, p.18 FIGURE 4-3, p.19 FIGURE 4-4, p.20 FIGURE 4-5, p.21 FIGURE 4-6, p.22 5 Loudness ratings and bandwidth in transmission planning (Contri- bution from ELLEMTEL, Sweden) It is shown below that loudness ratings can be specified as "basic" parameters in the "common" band 0.3-3.4 kHz complemented with an E -factor for the band edges down to 0.2 kHz and up to 4 kHz. The E -factor can be determined numerically from attenuation values or by some simple network rules. The advantage of the method is a simplification for the transmission planner. 5.1 Introduction Many Administrations seek to maintain good transmission pro- perties in a telephone channel with a band of 200 to 4000 Hz, at least in the subscriber network. Under those circumstances it may seem natural to compute loudness ratings (LRs) using parameters specified for this band [8]. Because in this case the loss distor- tion is limited within the band, the additivity properties of the LRs will be satisfactory, i.e.: OLR = SLR + JLR + RLR . However, a connection may often contain links with an appreci- able band edge attenuation distortion, virtually limiting the band to 300-3400 Hz. (This will be true for many interntional calls.) Such hard band-limiting corresponds to an increase of several deci- bels in LR. If LRs are computed for the band 0.1-8 kHz or even 0.2-4 kHz they can no longer be added without noticeable errors [9] which may cause confusion in the transmission planning. In principle there are several ways of resolving the dilemma. The first is simply to ignore the improvement of a few dBs which some "wideband" local connections may possess. Thus, the American IEEE practice for objective loudness ratings is to use the band 0.3-3-4 kHz when computing (or measuring) the LRs. The second method is to apply bandwidth correction factors to the LRs. One may compare with the CCITT concept of "corrected reference equivalents" which is tailored to the subscriber's actual loudness impression. A 200-4000 Hz circuit will have a lower CRE value than a 300-3400 circuit having the same midband loss. The effect of band-limiting is taken care of by correcting the wideband values by adding the so-called D -factors according to certain rules. [The CCITT D -factors may not be quite correct, however, as they were derived from measurements using SRAEN filters. These are not truly representative of modern transmission circuits [10].] Considering modern trends of trying to improve the telephone channel's low-frequency response, it seems appropriate for the LR calculations to use a "wideband" method with corrections. Such a methodology will be described below and this can be applied to transmission planning. The LRs are basically calculated for the narrow "common band" 0.3-3.4 kHz. These LRs can be added without loss of accuracy. A correction, the E -factor, is computed for the transmission at the band edges. The E -factor is subtracted from the "common band" OLR to obtain the "wideband" OLR (W ). 5.2 The E-factor as a band edge correction of LR In general, a loudness rating can be thought of as a "frequency-weighted average" of an electro-acoustical attenuation. According to Recommendation P.79 the electro-acoustical pro- perties should be evaluated in the band 0.1-8 kHz. For practical reasons the computations are often limited to the band 0.2-4 kHz. (The Wi-weights are then diminished by 0.3 dB). However, only in the band 0.3-3.4 is one assured of a real transfer of signals under all cir- cumstances. At the band edges, 0.2 to 0.3 and 3.4 to 4 kHz, the attenuation of a specific link in a connection may be so high as virtually to stop transmission. This could result in a reduction of several decibels in a subjectively measured loudness impression of a voice signal. To handle this properly it is convenient to characterize the electro-acoustical attenuations separately for the "common band" 0.3-3.4 kHz and for the band edges. In the common band each link is characterized by the weighted average of the electroacoustical loss, i.e. SLR, RLR or JLR, and the LRs can be added. For example, for the circuit as shown in Fig- ure 5-1, consisting of two telephone sets interconnected via a number of transmssion links, the following relation should hold at any interfacte P between the links: (Any mismatch attenuation effects at the interfaces can be treated as special forms of JLRs). Figure 5-1, p. At the band edges the connection is characterized by its abil- ity to transmit voice signals, i.e. the E -factor. Zero band edge losses means E = 2.5 dB. (Details will be given later). For a complete connection as shown in Figure 5-1, the overall loudness rating is: In the common band 0.3-3.4 kHz OLR = SLR + RLR (5.2) In the full band 0.2-4 kHz OLR (W ) = OLR - E (5-3) In the following, the general mathematical expressions for the LRs and the E -factor are given. It is shown how to apply them to telephone sets and various transmission links. The E -factor may be designated the "loudness improvement". 5.3 General mathematical expressions In the common band 0.3-3.4 kHz the general LR algorithm can be written as: LR = L0+ L (5-4) (the summation being made for fi= 0.315 . | | 3.15, the 1/3-octave ISO frequencies) when Li are the values of electroacoustic loss for the LR in question L0, Ki, m are constants to be specified below. Note - Equations 5-4 and 5-5 are mathematically equivalent to the "Wi-algorithm" as explained in [11] but are more convenient to use in the following. When the spread between minimum and maximum values of the Li's is moderate, the following expression can be used for L . The full band 0.2-4 kHz overall loudness rating was given by Equation 5-3, i.e.: OLR (W ) = OLR - E The expression for the E -factor is: E = C1x 10 -0.1m (L 01 - L ) + C2x 10 -0.1m (L 02 - L ) + C3x 10 -0.1m (L 03 - L ) (5-7) where L0\d1, L0\d2, L0\d3 are the band edge losses at 0,2, 0,25 et 4 kHz respectively. C1, C2 and C3 are constants to be specified below (for the derivation, see Annex A). The constants of Equations 5-4 and 5-5 may in principle be derived from any defined LR algorithms. However, Recommendation P.79 is less suitable for use in the context of transmission planning because of a lack of accuracy as discussed in [11] and [7]. The very simple algorithm, designated by "C " in [11], seems to be just as accurate as any other investigated so far and is therefore chosen here. (It also has the advantage of closely resembling the IEEE objective loudness rating.) The constants used in Equations (5-4), (5-5), (5-6) and (5-7) are given in Tables 5-1 and 5-2. H.T. [T13.19] TABLE 5-1 Constants used in equations (5-4), (5-5) and (5-6) ______________________________ { K = 0.05 for f = 0.315 and 3.15 kHz } { K = 0.1 for f = 0.4, 0.5 . | | 2, 2.5 kHz } m = 0.2 ______________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | _________________________________ LR SLR RLR OLR JLR _________________________________ L 0 -3 12 9 0 _________________________________ | | | | | | | | | | | | | | | | | | | | | | | | Table 5-1 [T13.19], p.24 H.T. [T14.19] TABLE 5-2 Constants used in equation (5-7) __________________________________ C 1 = 0.5 C 2 = 1 C 3 = 1 __________________________________ | | | | | | | | Table 5-2 [T14.19], p.25 A perfectly flat mouth-to-ear acoustic frequency response in the band 0.2-4 kHz will thus result in E = 0.5 + 1 + 1 = 2.5 dB, i.e. OLR (W ) is 2.5 dB lower than OLR for a "flat" channel limited to 0.3-3.4 kHz. In the following the E -factor is computed for a number of cases including "broadband" and "narrowband" telephone sets in com- bination with different types of transmission links. It turns out that some rather simple rules can be set up for the approximate determination of the E -factor. 5.4 Telephone sets Suppose the transmission channel between the sending and receiving telephone sets is flat within the band 0.2-4 kHz. Then the E -factor, the loudness improvement, characterizes the bandwidth performance of the sets. Let this be designated ET. Table 5-3 gives some examples. It is worth noticing that the spread in E around the average value 1.3 is quite moderate. H.T. [T15.19] TABLE 5-3 Examples of E-factors for some telephone sets _______________________________________ Type of set E _______________________________________ { 1) Old-type carbon microphone } 1.9 { 2) Old-type carbon microphone } 1.5 { 3) Old-type carbon microphone } 1.1 4) W.E. type 500 1.3 5) Electret microphone 1.8 { 6) Digital set; new specification } 0.8 { 7) Average of 90 types of sets } 1.3 _______________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Table 5-3 [T15.19], p. 5.5 Transmission links To characterize the loudness improvement performance of the transmission links as such, it is convenient to compute the E -fac- tor under the assumption that the telephone sets have a flat fre- quency response in the band 0.2-4 kHz. Let this transmission chan- nel E -factor be designated EC. In S 5.6 the resulting E -factor will be given for various typical combinations of ETand EC. When connecting several transmission links in tandem, mismatch may occur. These effects can be diminished, however, by using com- plex nominal impedances in the subscriber networks, as many Administrations already do. In general, mismatch losses can be considered by computing their JLRs . The "common band" performance of the links are characterized by JLR = L according to Equations (5-4), (5-5) and Table 5-1. As large attenuation distortions within this band are not allowed, the very simple Equation (5-6) can be used for computing L . (It is interesting to note that this corresponds in effect to averaging the loss over a log (f )-scale, a method which has been verified empirically a long time ago). When several links are connected in tandem, ECcan of course be computed from the total band edge losses. However, some simple approximate rules can be used for the combination of individual EC-factors for JRL . 5.5.1 Subscriber cables Surprisingly, the typical loss curve of a non-loaded sub- scriber cable produces the same loudness improvement as a full-bandwidth channel, i.e. EC = 2.5 dB. This is due to the fact that at the lower band edge the loss is lower than the average L which compensates for the higher loss at the upper band edge. When a subscriber cable is connected in tandem with a narrow band device, it turns out that the EC-factor for that device applies. 5.5.2 Band-limiting equipment Band-limiting in a telephone connection can be caused by heavily loaded subscriber cables, FDM and PCM equipment. Figure 5-2 shows some idealized attenuation curves for which the EC-factors have been computed. Figure 5-2, p. For the attenuation curves in Figure 5-2 the following EC-values are obtained: EC Heavily loaded subscriber cable 1.2 dB 1 FDM link 1.4 dB 1 PCM link 1.9 dB When several PCM and FDM links are connected in tandem the EC-values according to Table 5-4 are obtained. 5.6 Complete connections The loudness improvement, the E -factor, has been computed for a number of combinations of telephone sets and transmission links. For each telephone characteristic the "total" E -factor has been plotted against the "channel" EC-factor. The results are presented in Figures 5-3 and 5-4. H.T. [T16.19] TABLE 5-4 EvC for PCM and FDM links in tandem ________________________________________________________ No. of PCM links 0 1 2 3 4 5 No. of FDM links ________________________________________________________ 0 2.5 1.9 1.6 1.4 1.3 1.2 1 1.4 1.3 1.2 1.1 1.0 1.0 2 1.1 1.0 1.0 0.9 0.9 0.8 3 0.9 0.9 0.8 0.8 0.7 0.7 4 0.8 0.7 0.7 0.7 0.6 0.6 5 0.7 0.6 0.6 0.6 0.5 0.5 ________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 5-4 [T16.19], p.28 Figure 5-3 shows the E -factor for the "average" analog and the "digital" telephone set. (The "average" was taken as the mean of 90 different types of commercial sets. The "digital" corresponds to the new CCITT specification for digital sets). Figure 5-4 illustrates the spread in the E -factor for a number of widely varying analog telephone characteristics. (It is worth noticing that the spread, after all, is fairly moderate). Figure 5-3, p.29 FIGURE 5-4, p.30 Considering the general requirements of transmission planning, there is hardly a need to specify the loudness improvement, the E-factor, more accurately than within steps of 0.5 dB. Therefore, instead of calculating the E-value in each application, one can follow the rules given in Tables 5-5 and 5-6 for analog and digi- tal sets respectively. H.T. [T17.19] TABLE 5-5 E-factor, analog sets ____________________________________________________ E Links in tandem ____________________________________________________ 1.5 { Subscriber cable, non-loaded } 1.0 { 1 x PCM, . | | , 3 x PCM 1 x FDM } 0.5 { Subscriber cable, heavy coding } 4 x PCM 2 x FDM 0.0 { 5 x PCM + 5 x FDM } ____________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note - Non-loaded subscriber cable sections do not affect the E -factor. Tableau 5-5 [T17.19], p.31 H.T. [T18.19] TABLE 5-6 E-factor, digital sets _____________________________________________________________ E Links in tandem _____________________________________________________________ 1.0 All digital connection 0.5 { 1 D/A-A/D to 6 D/A-A/D connections } 0.0 7 D/A-A/D connections _____________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | Tableau 5-6 [T18.19], p.32 5.7 Conclusions The transmission planner can obtain loudness ratings by quite simple numerical methods: computing and adding individual LRs for the "common band" 0.3-3.4 kHz and correcting for the band edge transmission by subtracting the E -factor. The E -factor can be determined by some uncomplicated rules. The results can be expected to be more accurate than calcula- tions based on Recommendation P.79. MONTAGE: S 6 SUR LE RESTE DE CETTE PAGE