DOS Days

ExpertColor MED1931 Sound Card - Part 1

4th January 2024


In early January I picked up this unassuming sound card as I wanted to do a review of the OPTi 82C931 audio chipset. Despite the name, this sound card's manufacturer is a bit of a mystery - with the model number having a 'MED' prefix, I would associate this to mean it's an ExpertColor card, but there's nothing online about a MED1931 card having ever existed - perhaps it was developed solely for the OEM market.

MED1931 OPTI 82C931-based sound card, ca 1996

Many OPTI-based sound cards are very cheap and cheerful, with little more than the OPTi chip itself, a power amplifier chip, and supporting discrete components. This one, however, seems to have been targeted more at the mid-range market as we will see more in today's exploration.

Other almost identical cards are branded Union, or Lung Hwa Electronics SK826. Shuttle also used a similar card, called HOT-235 and HOT-237.

Quick Visual Inspection

Looking around the card, it's a very simple design given that almost everything is handled within the OPTi chip:

  • OPTi 82C931 Integrated Audio Controller
  • ISSI IS92C66 - EEPROM chip
  • C-Media CM2025B - stereo audio amplifier
  • Wavetable daughterboard header (with a few bent pins to fix)
  • IDE CD-ROM header
  • Pads for an OPTi 82C941 and AdMOS QS750
  • Two LS245 bus transceivers for the IDE interface
  • Two white 4-pin headers for CD Audio-in (J3) and MPEG audio-in (J4)
  • One jumper at JP1
  • Faceplate [top-to-bottom]: Speaker-out, Line-out, Line-in, and Mic-in 3.5mm jack sockets, and 15-pin game/MIDI port.

Date-wise, the most recent date stamp on the card's chips is week 23 of 1996, so it was certainly no earlier than June 3rd 1996 when this particular card was manufactured.


The Component Parts

Audio Controller Chip

The OPTi 82C931 was part of OPTi's 2nd-generation of audio chipsets after their 1st-gen 82C924 and 82C925. It was a highly-integrated audio controller, supporting Ad Lib, Sound Blaster Pro and Windows Sound System standards, and was fully pin-compatible with the 82C930. The 931 bettered the 930 with an integrated FM synthesizer, a 7-channel mixer (compared to 5 on the 930), 16-bit fast DMA playback, external modem interface, and added Intel Plug & Play compliance.

The internal FM synthesizer is considered a downgrade, as it uses OPTI's own implementation of OPL3 called OPTiFMTM rather than the 1st-generation OPTi audio chipsets that relied on having an external Yamaha OPL3 chip on the card.

The family of OPTi audio chipsets was developed in 1993 by OPTi's Multimedia division, MediaCHIPS, and was branded "MAD16" (MediaChips 16-Bit Audio Controller). In its first iteration the MAD chip took on the functions of AT bus interface, Sound Blaster Pro-compatible digital audio processor, MIDI interface, Windows Sound System interface, FM synthesizer interface, CODEC/mixer interface, game port interface and interfaces to Sony, Mitsumi and Pansonic CD-ROM drives.

The full list of variants is listed here in chronological order:

MAD Chip Introduced ISA PnP OPL3 WSS MPU-401 Mixer Channels 3D
82C928 (MAD16) 1993 No External External No 5 No
82C929/A (MAD16 Pro) 1994 No External External Yes 5 No
82C930 1995 No External External Yes 5 No
82C924 1995 Yes External External Yes 5 No
82C925 1995 Yes Internal OPTiFMTM External Yes 5 No
82C931 1996 Yes Internal OPTiFMTM Internal Yes 7 No
82C933 1996 Yes Internal OPTiFMTM Internal Yes 7 Yes
The 82C928/929/930/924 were often coupled with the Yamaha OPL3 and Crystal CS4231 or Analog Devices AD1845 / AD1846 / AD1848. Cards based on the 82C924 and 82C931 sometimes came with an onboard wavetable in the form of the OPTi 82C941.



The socketed 8-pin DIP here is an ISSI IS92C66, though other cards might have the equivalent Atmel 93C66. It's a 64 KB EEPROM chip used to store the card's Plug & Play configuration settings (resource data, serial identifier, etc) between system restarts. This is for Windows only, and allows each sound card's manufacturer to customise how the card appears to Windows.

For DOS, the SNDINIT utility pulls the stored configuration from the .CFG file in the same directory when it is run with the '/b' command-line argument. When SNDINIT is run without any command-line args, the program allows you to set up the sound configuration, including port addresses, IRQ lines and DMA channels. It then writes to this .CFG file when you save the settings.

Audio Amplifier

This 2-channel audio amplifier produces up to 2.3W per channel in stereo mode, or 4.7W mono to the Speaker-out jack.

The CM2025B here is a different-branded (Kyocera?) variant of the popular TEA2025B audio amplifier from ST Microelectronics. Some cards have a Samsung (SEC) SA2206N at this location.

The original TEA2025B variant can be found on many ISA sound cards including the Aztech Sound Galaxy BX/NX/NX Pro 16/Basic 16/Pro 16, Creative Music System (CMS), Sound Blaster 1.0/1.5/2.0, Sound Blaster Pro, Sound Blaster 16, AWE32, and various OPTi-based cards with 82C925/82C928/82C930/82C931 chipsets.

Pads for OPTi 82C941 and QS750

The card also has provision for the OPTi 82C941, which is a rebranded AdMOS QDSP QS700 wavetable audio processor and an appropriate ROM chip (QS750 or equivalent) to hold the instrument samples.

The QS700 was released in 1996 and was a cheap wavetable audio processor with options for 512 KB, 1 MB or 2 MB ROM capacities. I put it in the worst-quality category on my Wavetable Audio page along with 14 others, out of 43 chips in total, so it's probably better to just use the wavetable header and fit a better-quality daugherboard to this card if you want decent General MIDI capability.

I guess this MED1931 card came in two distinct flavours: one with the onboard wavetable and this one without. Based on the missing components on my card, getting onboard wavetable audio onto this card would require:

  • OPTi 82C941 or AdMOS QS700 chip (100-pin PQFP)
  • AdMOS QS750 1 MB ROM chip (100-pin PQFP)
  • Three crystal oscillators at X1, X2, and X3 - on some cards X3 is already populated with a 14.318 MHz xtal, even though the wavetable components are missing.
  • 74LS04 or equiv. inverter at U3
  • LM311 or equiv. voltage comparator at U5
  • 13 capacitors
  • 8 resistors

Scouring the internet, I was unable to find any images of this board design with the wavetable components present.

In Part 2, we'll get it installed, drivers running, and test out the sound in some games.