Path: utzoo!utgpu!watmath!isishq!f171.n221.z1.FIDONET.ORG!izot
From: izot@f171.n221.z1.FIDONET.ORG (Geoffrey Welsh)
Newsgroups: comp.sys.cbm
Subject: How to use CB2 on PET's
Message-ID: <1646.23FB6F19@isishq.FIDONET.ORG>
Date: 15 Feb 89 14:30:25 GMT
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 > From: sjk@utastro.UUCP (Scot Kleinman)
 > Message-ID: <3617@utastro.UUCP>
 
 > A while ago, someone asked how to make music (?) through the CB2 on his
 > 8096.  Attempts at E-Mail failed, so I am posting my reply.
 
 > Ok, what you need are a few pokes.  First you must poke 59467,16 to set it
 > up.
 > Than poke 59466,timbre. This sets your timbre.  Then, to get sounds (you
 > may
 > already hear something after the above poke), poke 59464,note.  poke
 > 59464,0 to
 > stop playing your note, or just poke it to another value.  When done, poke
 > 59467,0.  This allows use of the tape drive again. (Apparently, pokeing it
 > to
 > 16 disallows tape drive use.)  Happy music making!
 
   Let me give a more technical information.
 
   CB2 sound is made by shifting data out of the 6522 VIA's serial shift 
register (the same device that, on the 6526, does C128 burst mode serial 
transfers). POKE 59467,16 sets the shift register to shift out indefinitely 
(and recycle) using timer #2. Clobbering cassette would be an incidental side 
effect of the other bits set by that POKE. Using POKE 59467, (PEEK(59467) AND 
227) OR 16 might sidestep that.
 
   The "timbre" at 59466 is the data being shifted out (the waveform). If it 
contains the binary pattern "10101010", then you're going to get a sqaure 
wave (NOTE: if it contains "11001100", you'll still get a square wave of half 
the frequency). Experiment for yourself to see what bit patters produce what 
tones. Note also that changing the waveform to divide the effective frequency 
by two can be used to extend the frequency range down below that offered by 
the timer).
 
   By now you will have guessed that 59464 is the timer value. Once it's 
POKEd, it is decremented every clock cycle (about 1 MHz) and is re-loaded when 
the count underflows (in "free-running mode"... in "one-shot" mode it stops at 
zero). Each time that timer 2 underflows, the shift register is shifted one 
bit to the left and the bit coming out of the leftmost bit is "copied" to the 
CB2 output.
 
   You now know more than you wanted to about PET sound. Just as a sidebar, 
the IBM PC uses a similar timer-underflow scheme to give its sound... but, in 
stead of shifting data out, it simply toggles the bit when the timer 
underflows (giving the standard PC less sound control than the PET)... the 
PCjr has a 3-voice sound generator similar to the C64's...
 
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