Date:  27-Sep-96 19:38:28  MsgID: 308-10774  ToID: 100112,220
From: 0Andreas Boose >INTERNET:boose@unixserv.rz.fh-hannover.de
Subj:  Re: SID internals
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Date: Sat, 28 Sep 1996 00:33:31 +0200 (DST)
From: Andreas Boose <boose@unixserv.rz.fh-hannover.de>
Subject: Re: SID internals
To: Stephen Judd <sjudd@nwu.edu>
Cc: Marko.Makela@HUT.FI, e9426444@stud1.tuwien.ac.at, alstrup@diku.dk,
        3schwend@informatik.uni-hamburg.dk, 100112.220@compuserve.com
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Hi!

>I believe I now understand what SID does quite well.  One thing I would like
>to know is if this is a clever thing Yannes did.  It seems mighty clever
>to me, but perhaps it is a common technique in the chip designing world?
>(I am referring to the way he implemented ring modulation on SID).

The way he implemented ring modulation is definitely common technique.  
Analogue ring modulation requires 4 diodes and 2 transformers, there you  
need a lot of cleverness to achieve a grade of modulation of 1. (I don't  
know the right translation of "Modulationsgrad".) On the other hand,  
digital ring modulation is so incredible cheap, just swapping the high bit  
of the oscillators and a few XORs and the work is done.

IMHO the only true clever thing Bob did is to decide what functions/ 
features should go into silicon and which not. He didn't put in useless or  
redundant things blocking lots of chip space like Peddle did with the  
decimal mode in the 6502.

>One other thing I would be interested in is any reactions you all might have
>had while reading the interview... any surprises?  Disagreements?  New
>insights?  A new outlook on the world, fire pouring down from the heavans
>as the working of the entire universe suddenly becomes crystal clear?

It was interesting to read, it cleared up some minor things about how the  
SID works. There are just two points in the interview that made me wonder:

#1 He defined the possibility to playback digitized samples as feature and
   not as bug. So it was only CBM being so stupid to think it was a bug
   and removed the volume register change click in later SIDs. So now it's
   official confirmed that they removed a feature and not a bug.

#2 He said that random plus waveform can result in an empty shifter,
   because of the way the outputs are multiplexed. That's very obscure. Of
   course he used wired AND, but he must separate the outputs of each
   waveform generator from each other by MOS FETs anyway. And FETs use to
   work in input->output direction only.

>I found it useful to "trap" the pulse waveform at $FF -- that is,
>change the pulse width until the waveform outputs FF, then set the
>PW to zero so it stays there.  Now this FF may be combined with
>another waveform, e.g. sawtooth.  The result is periodic (with the
>fundamental period), and quite bizarre... sometimes the sawtooth
>starts to break through, but most of the time it sits at zero.  My
>intuition, such as it is, suggests to me that there is some really
>nontrivial effect going on inside the multiplexer, like one where you
>might need to solve the quantum mechanical equations to really understand
>what is going on.

Yes, that's hard to understand, especially why does it sit at zero?

I think I should measure it too, but until today when I acquired a new  
"old" C64, I had no C64/SID combination I could trust...

>What the heck...  Attached is a little program I wrote as an instrument
>editor for a music composer that I am writing.  I find it useful for
>performing simple SID experiments.  Keys 1,2, and 3 select voice 123,
>cursor keys move, +/- keys move values up/down, shift +/- moves high
>byte up/down (i.e. moves by large increments), and space toggles.

Neat.

>I just had an intriguing idea.  The waveform is based on the frequency
>accumulator.  Therefore, it should be possible to create new waveforms
>by rapidly varying the frequency register (not to mention the waveform
>selector).  Has anyone ever tried this?

No. But it will surely sound odd.

MfG Andreas


**CCs to:
  100112,220
  INTERNET:MARKO.MAKELA@HUT.FI
  INTERNET:E9426444@STUD1.TUWIEN.AC.AT
  INTERNET:ALSTRUP@DIKU.DK
  INTERNET:3SCHWEND@INFORMATIK.UNI-HAMBURG.DK


