Stereo SID for Commodore 64 and 128

The Commodore 64 came equipped with a powerful sound chip called the SID, acronym for sound interface device. In its later years, one of the ways Commodore enthusiasts extended the usefulness of their machines was by adding a second sound chip. Let’s talk about this capability and how many people took advantage of it.

Expanding the Commodore 64 with a second SID chip for stereo

stereo SID for Commodore 64 and 128
The SID was the best sounding chip of its generation, and adding a second one was a niche upgrade for Commodore 64s and 128s that developed a cult following.

When Commodore designed the Commodore 64, they left two areas in the memory space reserved for I/O chips unoccupied to facilitate future expansion. The RAM expansion units used one of those areas to map in a memory management unit to add 128k, 256K, or 512k to the machine. There were some other niche expansions that used those addresses, including a speech synthesizer, an early Yamaha OPL sound chip, and another board that just slapped a pair of 6522 VIA chips in those two spaces, useful for those who wanted to use their Commodore for robotics or home automation projects.

In the late 1980s, hobbyist named Mark Dickinson had the idea to use one of those spaces to add a second SID chip. His modification was a bit hacky, but it worked. He piggybacked a second chip on top of the existing chip, bent out a few of the lines that needed to go elsewhere, and then added the electronic components or jumper wires needed to wire it into the right memory space and route the sound output.

And then he modified a popular music player to use it, and convinced some musicians to make songs to use it. The mod quickly developed a cult following.

With the right combination of care, skill, and luck, the modification worked. If you were lacking in one or more of those areas, you might damage one or both of those chips. A good friend wrecked two chips attempting this modification, although I know of someone local to me who successfully performed the modification at least a couple of times.

A game changer?

I had someone ask me recently if this gave a significant advantage over other computers, such as Atari. It wasn’t a disadvantage, but by the time this modification existed, the Commodore 64 was already the most common home computer on the market. It was a $30 project to make you less jealous of an Amiga or ST.

And if you had an Atari 8-bit computer, a similar modification exists to add a second POKEY chip to an Atari. The stereo SID came first, but Atari enthusiasts were doing something similar within a few months. And today, there is just as much Atari software using stereo POKEY as there is Commodore software using stereo SID.

How common was it?

I also think we have a tendency today to overestimate how many people performed this modification in the 1980s and early 1990s. Initially, the documentation was distributed online. It originated on Quantumlink and found its way onto private bulletin board systems.

But how many people in 1987 were comfortable opening up a $150 computer, prying out the SID chip, and then soldering another chip onto it? I would estimate I knew 200 Commodore owners at one point, and I knew no more than four who tried it.

That leads me to the SID Symphony cartridge, a plug in cartridge for those who wanted the second SID chip but didn’t want to risk damaging their computer to do it.

The SID Symphony cartridge

A company called Dr Evil marketed a plug-in cartridge that made adding a second SID much easier. All you had to do was plug in a cartridge and run an audio cable to your monitor or speaker. The problem with that was by the late 1980s, the cartridge port was frequently already occupied, either by a fast load cartridge or a RAM expansion unit. You could buy an extender that allowed you to plug in more than one cartridge, or you could upgrade from a fast load cartridge to JiffyDOS to free up your cartridge port.

Again, it’s easy to overestimate how many people bought these. But we don’t have to estimate because we have data. Unlike many vintage computer products, we have a very good idea how many were produced. One of the creators of the product, Kent Sullivan, has written an extensive blog post on its history. And near the end he states that his company produced slightly over 1,000 cartridges, and he has a royalty statement from Creative Micro Devices that proves CMD produced about 500 after they took over production from Dr Evil. His exact wording was at least 1,478 cartridges were produced.

But don’t read too much into that. A million units is more than 1,478, but so is 1,479. Given the sizes of the companies involved, this was a niche product that sold around 1,500 units. If we generously estimate that another 1,500 people successfully soldered another SID into their 64 or 128, we’re still talking an upgrade that well under one percent of the user base used. We’re talking something even more niche than the 1581 disk drive here.

The stereo SID today

Today, you can sidestep the issue by buying a PCB that facilitates adding a second SID chip in a socket internally, leaving you to use your cartridge port for whatever else you want. There are countless YouTube videos about the project.

And it’s a modification worth doing. There were some impressive songs written for the modification in the 1980s, and a lot of current Commodore software uses a second sound chip if you have one. You can do more with a second SID today than you could in 1988.

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One thought on “Stereo SID for Commodore 64 and 128

  • March 8, 2023 at 12:13 pm

    I don’t have a second SID on a real C64 but I think I would like to try that modification, specifically, the one on the PCB that can be inserted in the SID socket. It sort of makes me think. Could there be an extension to the Commodore 64 platform? There is a modern VIC-II replacement with added features, modern RAM that COULD be tweaked to also act as an REU (I’m speculating) and some kind of “SuperCPU”. Also, an agreed upon way to utilize the additional signals in the joystick ports for more buttons could make for more interesting game mechanics too.

    There is the MEGA 65 which does seem to be the evolution, in a way, of the C64 but I don’t want to leave development of the C64 behind. I just happen to think that there has to be a way extend the platform through the 21st century without losing its roots.

    Just a blathering…

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