Commodore VIC-20 User Manual page 251

Chapter 7 Sound
237
never exceed 15 (the highest allowable number in the volume register).
Next, POKE the loop value into the volume register by adding the
following line:
45 POKE 36878, 15-T
This causes the volume to decrease with each pass through the loop.
You can also use the loop variable to decrease the length of the delays
between pulses as the volume decreases. This produces a sound resembling a
dropping ball. Try changing line 40 to
49 FOR F = 9 TO 400 - T*26: NEXT
The number 26 was chosen because 400 divided by 15 is approximately 26.
This divides the steps of the delay into 15 even increments. Subtracting the
loop value from 400 reduces the delay in the increments each time through
the loop.
Try changing the tones used in the first ping-pong program to produce
the sound of a clock ticking. This can be accomplished by using two different
tone values on alternate loops.
AnACK/SUSTAIN/DECAY
When you playa note on a piano, the sound begins loudly and slowly
fades until it finally fades away completely. The start of the sound is called
the attack. The portion of the sound in which the volume is maintained is
called sustain, and the last part of the sound, in which it fades, is called
decay.
Sustain
..,
s
-E
>
Time