Stereo SIDs on the Commodore 64 (last updated 2019-04-18)

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History

The Commodore 64 was blessed with a great little sound chip called the 6581 SID (Sound Interface Device), designed by Bob Yannes (here's an interview with him). It has three independently controlled voices and is capable of a wide range of sounds, including ring modulation, low frequency oscillation, and high-, low-, and band-pass filters. Later versions of the chip included the 6582 (which ran at 9V rather than 12V), and the 8580, which had improved filters but broke a popular way of playing back digitized samples. See the excellent Wikipedia article for more information. There are also some microphotographs of the SID chip circuits available here, which are beautiful to look at even if you don't understand them.

However, the limited memory available to the machine made it impossible to include sound commands in the BASIC interpreter, and setting the hardware registers manually with POKE commands or machine language was very time-consuming. So many software packages for writing and playing music on the Commodore were created, and as usual, one rose to the top (at least here in America). It was a program written by Craig Chamberlain, released in 1985 as a book and disk combination by the publishers of Compute! magazine and popularly known as "the Sidplayer". It was followed by another version in 1986 that added new commands and was known as "the Enhanced Sidplayer" (see photo). Here is a D64 image of the disk that came with the book. More information on the original Sidplayer can be found on Craig's Sidplayer site.

Raymond Day sent me a video of a talk by Craig at the 1999 Chicago Commodore Expo (caution: the file is over 300M). Here's how the show was described in a post on comp.sys.cbm:

On September 25 the Chicago Commodore Expo will be held at the Day's Inn in Lansing, Illinois, and it looks to be bigger and better than last year's show. I'm sorry that I'm going to miss it. What a line-up of C= luminaries! Maurice Randall (Wheels 64/128, GeoShell), Fender and Judi Tucker (Loadstar), Jim Butterfield (programmer/writer), Raymond Day (hardware/software hacker), Tim Lewis (president of the Lansing Area Commodore Club, Michigan), Craig Chamberlain (Enhanced SID Player), Maurine Gutowski (newsletter editor for the Muskegon Commodore Club), and Steve Judd, Jason and Katherine Compton, Mark Seelye, Robin Harbron, and Darren Foulds (programmers).

The Sidplayer still had a fairly steep learning curve, but one of the reasons it became the de facto standard for writing SIDs (as music files became known here in America) was the fact that huge libraries of them were available on the online services, like GEnie and QuantumLink (also affectionately known as "Q-Link", an early Commodore-only dialup network that went on to become America Online). See the resources section to download these collections.

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The Stereo Mod

the late Jerry Roth (DrJ5)

Although people were having a lot of fun making SIDs and playing them back, many were feeling the limitations of the SID chip. For example, there was only a single master volume control rather than one for each voice, making it difficult to balance softer waveforms. In addition, three voices are really not enough for more sophisticated efforts. Enter Mark Dickenson, who, as he puts it, "noticed the hardware addressing had open areas that were perfect for adding a second SID chip." In early 1987, Mark came up with a way of adding the second chip (here is a copy of his original instructions as uploaded to Q-Link), as well as a player that would read two SID files at a time and play them simultaneously, one through each chip (see below for more information on the player). He suggested the idea of stereo SIDs to a couple of the better-known SID writers on Q-Link, and Jerry Roth (a.k.a. DrJ5), whose rock 'n' roll SIDs are still considered some of the most imaginative and technically advanced, took him up on it. The resulting stereo version of "Johnny B. Goode" (MP3) was demonstrated at a SIDfest in Columbus in June 1987. It caused a sensation, and many people either modified their machines themselves or had someone do it for them (Raymond Day did quite a few). Here's a D64 image of a whole bunch of Dr J's SIDs, both mono and stereo.

This is a schematic of the stereo mod (created on a Commodore 64) that Jerry posted to Q-Link. Note the typo: 7/29/78 should be 7/29/87.

Below are some pictures that Larry Anderson sent me of the "piggyback mod", as it came to be known (because the second SID chip was mounted directly on top of the first one). This version adds two RCA outputs on the left side of the computer case and a switch controlling the I/O address of the second SID chip ($DE00 or $DF00) on the right. Click for larger views.

entire board, showing wires to jacks and switch	closeup of the "piggyback" mod itself

Here are a few more pictures of the piggyback mod; these were posted during a discussion in IRC. You can see that although the basic principles are the same, implementations varied widely. The quest for the perfect stereo mod continues to this day; several people are currently working on alternate designs.
	Dopple's mod	Mech's mod

A slightly less painful option is the SID2SID, which is a circuit board designed for use with the Prophet64 cartridge, now known as MSSIAH. It fits inside the Commodore 64 (or 128), and although it requires just as much work, the instructions and parts lists are very clear and detailed. You must provide the second SID chip, as well as sockets, resistors and so forth.

Stereo SID Cartridges

one of Mark's hand-made cartridges

Although a lot of people were modifying their machines to add a second SID chip, many more were squeamish at the thought of opening the machine and taking a soldering iron to it (I still haven't done it to any of my machines, even after 30 years). Eventually, Mark Dickenson came up with another hardware hack: putting the second chip in a plug-in cartridge that would avoid the need for surgery. He built a run of hand-made cartridges (few of which survive today), and sold them on Q-Link. The cartridge had a defeat switch, an RCA plug for the audio output, and a plug-in clip lead that had to be attached to one of the pins of the SID chip inside the machine (which meant that some disassembly was still required). Here are scans of pages one, two, and three of the instructions that came with.

the SID Symphony cartridge

Kent Sullivan, with his imaginatively-named company Dr. Evil Laboratories, improved on the design, replacing the 6581 chip with a 6582, which required 9V rather than 12V (note that these two chips are not interchangeable!). This made it possible to use a 9V battery for power, which rested inside the cartridge case itself. To replace the battery, you would gently pry apart the two halves of the cartridge casing to expose the power leads (see pictures below). Here is the original Q-Link post from Kent Sullivan announcing the cartridge, and here is a recent a blog post about its history. The picture on the left is of the final version (eventually sold by CMD), which did not require a battery. At last, you could just plug in a cartridge and start playing stereo SIDs. When they appear on eBay (which is rare), it's not unusual to see them go for over $100.

first version of the SID Symphony cartridge, opened up to show its construction (click for larger views)

Pictures of Mangelore's stereo cartridge (click for bigger images)

In the late 2000s, someone finally came out with a new stereo cartridge. Fotios Kotsiopoulos ("Mangelore") came up with a device that lets you map the second SID chip into I/O space at any one of sixteen different addresses from $DE00 to $DFE0 (via jumpers and a DIP switch). It also has a pass-thru port that will accept a Prophet64 cartridge.

The only catch is that you must provide your own SID chip; the board comes in 12-volt (6581) and 9-volt (8580) versions. Also, the board has not been made for quite some time, so if you want one, keep an eye on eBay!

Playing Stereo SIDs

Have a stereo SID setup of some kind but don't know how to try it out? Here's a .D64 of Mark Dickenson's StereoPlayer 10.3, which I believe is the last version released. I don't have the docs for it, but here's a .D64 of StereoPlayer 10.2, which does include lots of docs (as well as the "Star Wars" extended .WDS/.ANI demo).

Since there's some room on the .D64 of version 10.3, I put some of my favorite stereo SIDs on it as well so you can get started right away. Here are some quick configuration tips:

• hit the Commodore key to get to the second menu
• use 'A' to set the second SID's I/O address (usually $DE00)
• hit Space on a mono SID to set which voices play in which channels
• use 'M' to simulate stereo mode for mono SIDs
• use 'E' to control the delay between the two mono channels in stereo mode
• the MIDI options don't work (they were never implemented)

The stereo editor, showing the opening commands of Jerry Roth's
arrangement of "Johnny B. Goode"

The Stereo Editor

Stereo SIDs had become wildly popular, but the process of creating them was still very labor-intensive. Each channel of three voices had to be edited as a separate file, then tested using StereoPlayer. If errors were found, it was back to square one to repeat the cycle. In 1988, Robert Stoerrle created a stereo editor that allowed the editing and playback of all six voices at once. At this point, we had the full stack: hardware support (the second chip), a way to create stereo SIDs (the editor), and a way to play them back (the StereoPlayer). The Commodore stereo revolution was born!

Here is a .D64 of the stereo editor, which can be extracted to a Commodore floppy disk or used in an emulator. There are help files on the disk, and the program has a text file viewer for them.

Text Files from Q-Link

These are some text files downloaded from Q-Link back in the Olden Times.

Format of SID files (Dick Thornton)
Stereo Sid-Player docs for version 7.0 (Mark Dickenson)
Sidplayer News #4 part one (April 1986)
Sidplayer News #4 part two
Sidplayer News #8 (February 1987)
SID class transcript 11/08/????
SID class transcript 11/15/????
SID class transcript 11/22/????
SID patches (ADSR settings) from Half Moon Lake BBS

Resources

• A D64 image of some SIDs by Jerry Roth (DrJ5)

• Compute's Gazette SID Collection

• The High Voltage SID Collection

• sidplayer.org (history of Craig Chamberlain's original Sidplayer)

• e-mail interview with Mark Dickenson (Mark agreed to be interviewed for a presentation on stereo SIDs I gave at CommVEx 2006)

• SID in-depth information site (includes SID chip die pictures)

• This is a large picture of a 6581 R3 die.

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