Atari XL

Sound and Music
101
allow you to 'joiri' two channels together, forming
one channel with a 16 bit frequency register.
Bit 3 joins channel 2 to channel 3, and bit 4 joins
channel 0 to channel 1. So if you are prepared to
reduce the number of independent tone channels,
you will be able to generate a much wider range of
freq uencies.
The following program demonstrates the increased
frequency range by joining channels 0 an 1.
10 SOUND 0,0,0,0
20 POKE 53768,64+16
30 POKE 53761,160
40 POKE 53763,175
50 FOR Z=O TO 65535
60 HI = INT(Z/256)
70 LO = Z-(HI*256)
80 POKE 53760,LO
90 POKE 53762,HI
100 NEXT Z
110 SOUND 0,0,0,0
The program will take a long time to run, so you
might like to try adding a STEP to the FOR ...
NEXT loop to speed things up a bit.
The range of frequencies can be altered by setting
other bits in peR which control the clock frequency
applied to the POKEY chip. Try changing line 20
to:
20 POKE 53768,16
This will select the normal clock frequency instead
of 1.79 MHz (1 MHz is one million cycles per
second) making the lowest frequency about 1 Hz (or
one click per second). The lowest frequency can be
reduced still further, by setting bit 1 of peR to