Atari XL

98
The Ateri XL Handbook
Each increase in voltage is alternately applied then
switched off 100 times, causing a buzzing sound,
which increases in volume with each increase in
voltage. For each voltage the distance travelled by
the speaker cone increases, displacing an increased
amount of air, and so producing a louder sound.
We can change the frequency of the noise by
inserting a delay loop:
35 FOR D=O TO 30:NEXT D
With this delay, you can hear the individual clicks
as the speaker moves in and out.
Unfortunately the speed limitations imposed by
BASIC mean that this technique cannot be
exploited fully without resorting to machine code.
THE POKEY CONTROL REGISTER
This register provides additional control of the
POKEY chip, and opens up many more sound
generating possibili ties.
The frequency of the square waves generated by
the POKEY chip is governed by another square
wave generated by the computer. The frequency of
this stream of pulses (known as clock pulses) has a
bearing on the range of frequencies you can
generate. You can control the frequency of these
clock pulses and thereby greatly increase therange
of frequencies generated by the POKEY chip. Try
this example:
SOUND 0,255,10,8
will generate the lowest frequency tone possible
with a SOUND command. If you now type: