Atari XL

Sound and Music
89
P and CHAN in line 12060, then calling the routine
normally called when the OPTION key is pressed.
When running, the program constantly cycles
through the two routines FUNKEY and
KEYBOARD which read the two keyboards, until a
keypress is detected. The appropriate subroutine is
called and SOUND parameters for the selected
channel changed by ei ther the DECREASE or
INCREASE subroutines.
After each keypress is processed, the PLAY routine
at 8000 is called to update the noise, and the
program awaits the next keypress.
PLAYING TUNES
To play tunes, we must (feed' a SOUND command
with a series of frequency parameters in turn, and
one way of doing this is shown in this example:
10 SOUND 0,INT(100*RND(I»,10,8:
GOTO 10
As mentioned earlier, all four channels are
independant - allowing us to play four different
tones at once on each channel.
5 REM CACOPHONY!
10 SOUND 0,INT(10*RND(1»,10,8
20 SOUND I,INT(50*RND(1»,10,8
30 SOUND 2,INT(lOO*RND{1»,10,8
40 SOUND 3,INT{250*RND{l»,10,a
50 GOTO 10
Nei ther of these examples could be described as
music, for that we need a more controlled way of
calculating frequencies. The table in Appendix 10
lists the frequency values corresponding to musical
notes (if you are unfamiliar with musical notation,
refer to Appendix 12). Using this table we can build