Tuesday, January 7, 2014

SID and DOS - Unlikely but True Bedfellows

The Commodore MOS 6581 Sound Interface Device, the SID, is one of the most famous synthesizer chips of all time.  Its inclusion in the Commodore 64 and 128 computers helped to popularize computer music, mostly in Europe.  Like virtually every other popular synthesizer chip of the time, it made its way to the PC in some form.  Commodore had no problem in selling its chips to outside and technically rival companies. During its market life, the SID appeared in two different but related PC sound cards.

I.  The Entertainer

In 1987, Microprose was looking for a way to enhance the sound of some of its DOS games.  These games had access to superior audio hardware on the Commodore 64, Amiga and Atari ST, but the U.S. market was increasingly turning to the PCs.  Unless a PC owner had a Tandy 1000 with its 3-voice chip, he had to settle for the PC speaker, as there was no popular and cheap sound expansion hardware available at the time.  Since Microprose was still developing games for the Commodore 64, it seemed a natural idea to interface its SID chip with the PC.  They advertised a PC sound board called The Entertainer, which included a 6581 and a joystick port on an ISA card.  The only place this advertisement has ever been known to be seen is in an introduction screen and the readme to Gunship v429.04.  The readme discusses the card with the following text :

Support has been added for the MicroProse Sound Board - "The Entertainer".
Presence of the board is detected by the program and enhanced sound is
automatically generated.  Two additional control keys have been added to
control the volume.  Ctrl-V turns the sound down one notch.  Alt-V turns
it up.  Just pressing "V" alone still toggles the sound on & off.

Microprose, according to the ad, sold the card directly and it was available through "your local retailer". Two high-level Microprose employees have been contacted and have no memory of selling the card at that time, so it probably was not actually sold and if it was sold, it did not sell well.  The Entertainer card, if it was actually released by Microprose, was not supported by Microprose except in Gunship, v429.04 & 429.05 and Sid Meier's Pirates!, v432.1, 432.2 and 432.3.  It may also be supported in 432.4, which was a DOS conversion of the floppy PC booter from a Best of Microprose CD.  It was not known to have been intentionally supported in any other company's games.  The in-game ad and the text quoted above was removed for Gunship v429.05, so it is very possible that this was an idea that Microprose considered but which it did not follow through to market.

II.  The Innovation Sound Standard SSI-2001

Not too long after the Adlib Music Synthesizer Card began to be adopted by Sierra and other companies, and the idea (and more, as we will see) of a SID-on-a-chip card was resurrected by a company called Innovation.  It released a card around April, 1989 called the Innovation Sound Standard SSI-2001, which is a 6581 plus a speed-adjustable gameport.  It cost $129.00, but with discounts and coupons you could have purchased one for $69.00.  It may have only been available directly through Innovation.  Support was poor, the following games are the only ones known to support it :

Airball - SFX only, slow PC req'd
Bad Blood - Music only
Battle Chess II - SFX only
BattleTech: The Crescent Hawks' Revenge
F-19 Stealth Fighter
Falcon A.T.
Joe Montana Football - SFX only
Lord of the Rings Volume 1 - Floppy version only, SFX only
Red Storm Rising
Super Jeopardy
Ultima VI - Music only

I have verified that all the above games work with the Innovation emulation in DOSBox except for Super Jeopardy.  While Mobygames claims that Lexi-Cross and Castles and its expansion Castles : The Northern Campaign support Innovation, I have personally verified that these programs do not have install options for Innovation, despite what the system requirements labels on their boxes may indicate.  Asterix: Operation Getafix is also listed by Mobygames as supporting the Innovation, but it actually supports the InterSound MDO, a Covox Speech Thing-like device released by the French gaming company Coktel Vision.

The best informational article describing the card can be found in Computer Gaming World, Number 63, September 1989.  It mentions that only Origin and possibly Mindscape were planning to support the card.  Interplay and Microprose eventually did with more than one title.  Although Commodore made PC clones, it never put its sound chips in them.  The SSI-2001 is the only non-Commodore product I am aware of that used the SID chip prior to Commodore's failure in 1994.  

Technical details about the Innovation card and its SID implementation are as follows.  The card puts the SID chip and its 29 registers directly on the ISA bus.  It can use ports starting at 280, 2A0, 2C0 or 2E0.  280 is the default, and it takes up 32 consecutive I/O ports (3 presumably mirrored or unused).  The SID is clocked using the 14.318180 MHz clock from the ISA bus.  This clock signal is divided by the counter and flip flops on the board by 16 to produce a base frequency of .89488625 MHz.  By contrast, the base frequency of the SID in an NTSC C64 is 1.02272714 MHz and a PAL C64 is .98524851 MHz.  

The filter capacitors on the Innovation board use a value of 1nF.  This was proper according to the datasheet of the 6581, but in practice in the Commodore 64, different values were used.  By comparison, the 6581 SID in a C64 uses 470pF capacitors and an 8580 in a C64C or C128D use 22nF capacitors (1nF = 1000pF).  If Innovation had used the later 8580 chips in their card, the datasheet for the 8580 specifies 6n8/6.8nF/6800pF capacitors.  The POTX and POTY pins are not connected on the Innovation board, so the two registers which correspond to the functionality provided by these pins are useless.

The gameport is addressable at the standard port 201, supports 4 axes and 4 buttons, can be disabled with a jumper and supports three levels of "sensitivity", also selectable by jumper.  The Innovation card is the first sound card known to incorporate a standard gameport, a feature popularized by the Sound Blaster in 1990 and then ubiquitous on sound cards for the rest of the decade.

By using the joystick enable jumper and the four port range selections, one can install four boards and hence four SID chips in one computer, if you had the slots to spare.  Two boards would provide fully-independent stereo channels, but no software was known to take advantage of this capability.  

I know of exactly two Innovation boards known to be in existence, and each has minor layout differences compared with the other.  Both boards clearly show the use of 6581 chips, not the later 6582/8580 chips.  Here are images of both boards known to be in existence :

Earlier Board :

Later Board :

No one I know has ever seen a "The Entertainer" branded card.  It works just like the Innovation board hard coded at port 280, but it also has autodetection port functionality at port 200.  Gunship and Pirates! will only produce SID sound if they read a value of 45h from this port. The code string BA 00 02 EC in Gunship 429.05 executables EGAME.EXE/CGAME.EXE/TGAME.EXE and TITLE.EXE (the only version that uses unencrypted executables) must be replaced with B8 A5 00 90.  Ditto for the disk images of Pirates! Once the code is bypassed and an Innovation card is in the system or the emulation is enabled in DOSBox, you will hear SID sound.  Unfortunately, DOSBox uses the 1.02MHz frequency and 470pF Caps for the SID, so it will not sound exactly like the real boards would.

III.  The Modern Replica SSI-2001

In 2015, the prayers of many were answered when members of the VOGONS forum produced a replica of the SSI-2001 using a redesigned ISA board built mainly from photos of the card.  This is what my card looks like :

Improvements over the old card include :

Can use 6581 or 8580 SIDs.
Jumper based speed setting for NTSC clock frequency or canonical SID clock frequency
Audio input jack
4-pin audio output header (dual mono on pins 1 & 4, pin 2 is ground)
Plated holes for POTX and POTY inputs
Fully labeled jumper settings

The cards have been made by a VOGONS user named Fagear, who is based in the Russian Federation. His work is top quality and he had the good sense to use mini-jacks instead of the RCA jack on the original.  This meant that you can use any old junky ISA sound card for a bracket instead of having to make a custom bracket.  My card works great, although I had to find my 6581 SID chip from another source.

You may notice, if you order from a card from Fagear, that the markings on the ICs, except for the SID, are not the familiar 74 series designations.  The chips on this card are clones or work-alikes of the 74 series from several manufacturers exported by V/O Elektronorgtechnica.  This company was a part of Elorg, the Soviet state owned entity that controlled imports and exports of computer hardware and where Tetris was born.  However, with one apparent exception, the Russian parts appear to be functionally equivalent and use the same pinouts as the western chips.  The identifying information for every part, western or Russian is silkscreened underneath the part, as can be seen here :

The sole exception is the chip just below the silkscreened SID CLOCK on the replica board, hence the jumper if the Russian or Western part is used.  

You cannot just swap a 6581 for an 8580 in these boards whenever you like.  The 6581 uses +12v, which the ISA bus provides.  The 8580 uses +9v, which is not present on the ISA bus.  In order to use an 8580, you must install a 78L09 voltage converter.  The location is just to the left of the POT X holes.  You would probably need to remove it if you revert to a 6581.

Thanks to VOGONS user form member bristlehog, you can play back .sid files through this card through a DOS utility called SIDPLAY.  However, you will need a fast PC, around at least a 500MHz Pentium III, for proper speed.  This is because .sid files use 6510 CPU instructions and rely on the 6526 CIA or the 6569 VIC-II for their timing, so a fair amount of these chips must be emulated.  That means CPU power.

If you want to order a Replica SSI-2001 and download the player software, you can find it in this thread : http://www.vogons.org/viewtopic.php?f=46&t=27045

With this Board, I was able to verify that Super Jeopardy does output digitized sound.  The output volume is so low as to make it almost useless.  Activision got it right in the BattleTech: The Crescent Hawks' Revenge, the voice in the introduction is nice and clear with an Innovation.


  1. I remember there was also similar cards to provide Tandy 3-voice sound to non-Tandy systems, but as I understand it they are even more rare than the SID->PC cards.

  2. Tandy provided a card with a PSSJ chip on it for generic PCs, but only made it available to developers, which explains its rarity.

  3. Shanghai II: Dragon's Eye supports Innovation Sound Standard SSI-2001

  4. now I wouldn't be surprised if I see some graphic card for a PC that features a VIC-2 :-)

  5. Anyone wants to make an USB version? I'm available :)

  6. It seems to me Harpoon supports only PC speaker.