JOTEGO

Exploring C64 SID Schematics

Added 2019-08-26 15:09:45 +0000 UTC

The Commodore 64 SID chip is something it should already be done perfectly. There are schematics for the two SID chips. What else would you need?

Well, after looking at the schematics I can see some reasons why people hear differences between SID implementations, real chips and even between individual chips. Let me explain.

There is a large digital section. This one can be replicated to 100% accuracy. I am assuming everyone got that right. The problem is the analog section which is comprised of resistors, capacitors and amplifiers.

1. Resistors will vary from wafer to wafer typically by ±20% Inside the same chip, one resistor will be different from any other one, with a variation typically within ±5%
2. Capacitors have variations similar to those of resistors
3. Amplifiers in SID appear in a a couple of forms. Each one will have different non linear effects and again, there will not be two equal amplifiers once you hit the non linear regions

We can overcome items 1 and 2 by adding some tuning capability to the SID code. If done in a smart way, it wouldn't need too many parameters to let the user get the feel of the SID he likes.

As for the amplifiers, I am going to look into ways of characterizing them. Once that is done, I can built in the transfer function so we emulate that response with a digital amplifier. This is what is regularly done with valve guitar amplifiers in modern electronics.

I don't know if a parameter-base solution with lots of OSD options will make people happy. But I don't see much else to do. There are just not two equal SID chips out there.