From kayleigh@access.digex.net Tue Dec 27 12:41:38 1994 From: Steve Gallichio Newsgroups: alt.humor.best-of-usenet Subject: [digex.general] QAM Primer (was Re: Baud, BPS, etc.) Followup-To: alt.humor.best-of-usenet.d Date: 24 Dec 1994 20:22:40 -0500 Organization: best of usenet humor NNTP-Posting-Host: unix1.cc.ysu.edu X-Disclaimer: the "Approved" header verifies header information for article transmission and does not imply approval of content. See .sig below. X-Submissions-To: ahbou-sub@acpub.duke.edu Newsgroups: digex.general From: esmythe@digex.net (Erich Smythe) Subject: QAM Primer (was Re: Baud, BPS, etc.) The most common form of high speed modulation over voice telephone lines is Quadrature Amplitude Modulation (QAM). What is that, you ask? QAM sends information by modulating the phase and amplitude of a carrier wave. That was clear, wasn't it? Well, this is best described using a few simple objects found in nearly every modern home. Allow Dr. Nerd to explain: First take a sheet of paper, lay it lengthwise, and draw a horizontal axis roughly in the middle of the page. That is our time axis. Now draw a sine wave with several cycles above and below the axis proceeding from left to right (or right to left if your native writing system does that). That is our carrier wave. For those that found trigonometry boring, a sine wave is a curvy wave that rises above an axis to a point and then below the axis the same distance below the axis. It is symmetrical above and below the axis. The maximum height of the wave is the Amplitude. The wave completes a Cycle when it goes above and below the axis and returns to the same amplitude value. The Phase of the wave is roughly where you are in the cycle. The number of cycles in a second is called the frequency. Now lets modulate. Take a reasonably clear condom, unlubricated and unused, and without all those funny ribs and tickler thingies. Unroll the condom. Cut the condom lengthwise (opposite the "Bobbit Cut") and lay it flat over your sine wave. WARNING: This action renders the condom useless for its intended purpose! Trace the sinewave on the condom using a felt tip pen. Shift the condom along the horizontal axis. This is called Phase Shift Keying (PSK) since you have shifted the phase. Stretch the condom in the vertical axis. This is called Amplitude Shift Keying (ASK). Do PSK and ASK simultaneously and you have QAM. This is how modems communciate. Now stretch the condom along the horizontal axis. You have enlarged the wavelength (men, stop dreaming) and have in effect shifted the frequency. This is called Frequency Shift Keying (FSK) and has nothing to do with QAM but I just felt like throwing it in. Phase shifts are measured in degress (or radians), so that a shift of half a cycle is 180 degress, 3/4ths of a cycles is 270 degrees, and a full cycle is 360 or zero degrees. You can define a QAM Constellation using a phase-amplitude plot. The phase is plotted like a compass, with a ray pointing from the center of the axes. The amplitude is the length of the ray. A modem constellation is a bunch of phase and amplitude points, and for V.32 it is a sqare centered at the orgin. At every baud the phase and amplitude are measured and matched with a constellation point. This is called a symbol. For V.32 without a trellis (9,600 bps) this is done 2400 times a second: the baud rate. How do we get more bits than bauds? For V.32 there are 16 constellation points that encode 4 bits for each symbol. So there are four bits transmitted with every baud. 4 bits times 2400 cycles/sec is 9600 bits/sec, our data rate. V.32bis at 14,400 actually uses a 128 point constellation with 7 bits per point, but because it uses a special encoding called a trellis encoding that eats a bit in each symbol you get six bits/symbol, or 14,400 bps. Trellis codes give Dr. Nerd a headache, and he will not explain those today. V.34 (28,800) uses a 916 point constellation and a four-dimensional trellis code, giving Dr. Nerd four times the headache so he won't explain that one, either. Simple, wasn't it? -erich -- Moderators accept or reject articles based solely on the criteria posted in the Frequently Asked Questions. Article content is the responsibility of the submittor. Submit articles to ahbou-sub@acpub.duke.edu. To write to the moderators, send mail to ahbou-mod@acpub.duke.edu.