The fir filter allows users to graphically place
echoes of different delays and volumes.  At small
delays, this will affect the tone of the sound.
The fir filter can also be used to place echoes
with delays equal to a fraction of a beat.

The largest component in the fir filter is the 
display area.  Grid units are marked off on the
display area for guidance.  The horizontal axis
represents time, and vertical bars are drawn 
every n samples.  The vertical axis represents 
volume.  Clicking with the left button will add
an echo.  An echo can be dragged with the left 
button, and removed with the right.  To the right
of the display area is a scrollbar which controls 
the zoom level in the time domain.

The 'grid unit' adjustment controls the time 
represented by each vertical bar.  This number
is in samples, where 44100 samples equal one 
second.  The time between bars can also be set
as a number of beats.  When the 'beats' 
scrollbar is set to non-zero value, the grid 
unit value is remembered, and this value is 
restored when the beat_factor is set to zero.

The 'normalize' toggle button causes the total 
volume of all echoes to be held constant.  The
maximum value is given by the 'scale' adjustment.
If the volume is not normalized, then the 'scale'
value is applied to each echo's volume.