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From: TLC (all posts)
Date: 2000-01-19
Subject: Re: SID with Plus4

Hi!

> Try to implement the pseudo SID card into memory $D400
> (like on a real C64)! There're not so much stuffs what support
> the real SID card, but many games (and others) use some
> C64 music conversion routine (if I'm right?), and in many
> programs there're the original C64 music replay routine,
> and it's writing into $D400, so if you turn off the TED sound
> during this, you'll hear the real SID music!

Right!

Actually, this idea was just invented by BSZ/NST, when he designed his SID-card prototype for the _real Plus/4 (it was a huge PCB with an on-board 985Khz clock generator, a 6581 SID and lots of TTL chips).

The reason that this hack works: most of post-1989 programs use real c64
SID musics that run in the background, and a 'music converter routine'
that simply picks up the values from $d400+x and plays the melodies +
instruments either via the TED squarewaves or digital samples+mixing.

Then the possible way to go: implement a SID library into the emulator,
and map it to $fd40 and $fe80 (you can map either one SID to both addresses, or different SIDs to these addresses and mix them to the right and left channels, since Solder reserved the second SID area for a stereo SID expansion ;-) ). Both of these should run at the Plus/4
single clock frequency (at least supposed to, but could be changed to
985Khz by the user's interaction).

However, if you map a SID to $d400 then you should do it by the users ask in the emulator setup, since this is just supposed but not guaranteed to work. Sometimes the music converter routine also writes to the 'pseudo' SID regs (like mine do) and this can f*** up the instruments if you map a real SID to this location.

Also note that this SID should rather 'run at 985Khz' like real C64 SIDs do (there is the _original C64 player routine in the memory playing on the _original frq table).

Well. One more thing to mention. All Plus/4 SID cards utilize 8580 SIDs
(as found in C64-II). There was only one attempt to use 6581, BSZ's card. (AFAIK Solder also planned to support 6581 in his card, but I don't know if he actually did it. The 6581 needs higher operation voltage, higher current and a lot of discrete components to be of different type in order to work properly).

As speaking of the SID emulation itselves, AFAIK there are at least two popular SID emulation libraries (SIDplay by Michael Schwendt and ReSID by Dag Lem). I can't give URL or something like that but I can either ask them on the HVSC mailing list or you can look for the libraries by HTTP search (or simply subscribe to the HVSC mailing list -
HTTP://www.onelist.com ).

L.