Actually, the SID chip was only found in the following CBM models:
- C-65 (had two for stereo sound)
- the PET-II series, consisting of the B128, the B500 series, the C-600 series, the C-700/P500 series.
The SID chip is basically a three voice synthesizer on a chip. It's a weird mixture of digital and analogue circuitry which account for it's distinctive sound.
There were two models of the SID both with several revisions, out of which the 6581R4 is probably the finest sounding there is. (this is of course only my personal opinion.)
The 12V 6581 was the first, and still the one which many consider the real SID. In the turn of the 90s, while the C64 went through a radical redesign (to the worse IMHO) CBM came out with the 9V 8580, that had most of the "bugs" fixed that were present in the 6581. That is the features that differed from the original spec. This might have been a godsend, if the nature of the chip didn't depend on these bugs!
Somewhere in between the 6581 and the 8580 was a chip called the 6582. Yes, it does exist. It's a 9V 6581 that you can drop into your C-128DCR or C-64II. These are pretty scarce and I've never seen one live myself.
The most important bug was, that when you changed the volume, the SID chip emitted a click. Some ingenious programmer found out, that you could emit different frequency clicks by different volume increments/decrements. You can guess what this meant? You could play back PCM samples on the SID, albeit very noisily, by making it click very fast.
The other bugs were voltage leaks and waveform bugs, but all this radically affected the smooth mellow sound of the 6581, transforming it to the more nasal, sharp tones that come out of the 8580.
If that wasn't enough, the filters were changed too, being 470pF capacitors on the 6581 and 22 000pF on the 8580. This also took something out of the magnificent analogue filters present in the chip.
The developer of the SID chip, Bob Yannes later went on to create Ensoniq, a well known synthesizer manufacturer.
All in all, the SID chip is a fine example of what results when an engineer is given literally free hands to design his chip. At the time, the SID was one of the best synthesizers made, and one could argue it still is.
You should check out the High Voltage SID collection at http://www.hvsc.c64.org/
Little updates, added some punctuation, added the 6582 and updated the machine list