Covox Speech Thing
The Covox Speech Thing is an external audio device attached to the computer to output digital sound. It was composed of the most primitive 8-bit DAC using a resistor ladder and an analogue signal output, and plugged into the printer port of the PC.
Covox Speech Thing
|Invented by||Covox, Inc|
|Connects to||Printer port|
|Use||audio device attached to the computer|
|Common manufacturers||Covox, Disney, others|
The Speech Thing was introduced on December 18th, 1987 by Covox, Inc of Eugene, Oregon, for about 70 USD (79.95 USD as of 1989), but as its parts were much cheaper than the complete plug, and as its design was fairly simple, people soon started to build their own variants. The plug was used long into the 1990s, as sound cards were still very expensive at that time. The plug was also quite popular in the demoscene.
An inherent problem of the design is that it requires very precise resistors. If normal parts are used, the values get shuffled, especially for quiet sounds, resulting in distortion. Nevertheless, the sound quality of the Covox plug is far superior compared to the PC speaker; even today, a self-built Covox plug is still an inexpensive way to give old computers sound capabilities.
- Covox Speech Thing. The simplest hardware DAC, bundled with speech synthesis software, marketed originally as part of voice synthesis and recognition system.
- Disney Sound Source. Covox-idea based DAC, marketed by Disney Software in early 1990s. It consisted of 3 parts: a FIFO and a DAC on PCB plugged into printer port and separate amplifier / speaker box. Its price was set to only $14 and it was supported by many games (see below). It used external power (9 volt battery) and could be turned on/off by software. Contrary to the Speech Thing the output rate is determined by the hardware (7 kHz) and the design features a 16 byte FIFO allowing for autodetection and flow control of the output. In 2015 the hardware was reverse engineered so compatible circuits can be built from easily available off the shelf components. It is also emulated by the popular DOSBox emulator.
In its simplest form, Covox received 8-bit, mono signal through the parallel port and produced analog output that could be amplified and played back on loudspeakers. Sampling rate was not fixed by hardware means, and theoretically Covox can support any sampling rate. In practice, however, parallel port speed limits make it rather hard to achieve even standard CD-quality 44100 Hz. Another limiting factor compared to real sound cards was the need to use the computationally demanding timer interrupt to play background music, since there was no direct memory access available.
Advanced versions of Covox-like devices featured:
- Printer port forwarding connectors — allowed to plug printer into the Covox and use both (playing audio and printing) without reconnections, although not at the same time. Whenever something is printed, loud noise is created.
- Both DAC and ADC converters;
- ADCs with tiny microphone preamplifiers;
- DACs with amplification;
- Sourcing power from serial port;
- Numerous sound-enhancing (hi-fi) features, like filters or equalizers;
- Stereo capability, either by using two parallel ports or one port with switching using strobe (pin #1) and line feed (pin #14) signals.
The Covox plug couldn't directly substitute any of the popular cards of that age (AdLib, Sound Blaster, Gravis Ultrasound, etc.), but several games / platforms supported it directly. Notable entries include:
- SimCity - The original PC release made a selling point of supporting the Covox Speech Thing.
- Lemmings – had special promotional edition named Covox Lemmings, released with Covox sound cards; the game is identical to original game but contains a "Covox" level and seven more additional levels (only works with Covox Soundcards for ISA Bus).
- Pinball Fantasies.
- Most older Sierra Entertainment games, such as King's Quest and Space Quest series, could output the sound to Covox when selecting Disney Sound Source.
- Some Games work with a software emulator called Virtual Soundblaster, such as Wolfenstein 3D, Wolfenstein 3D has also native Support for the Disney Soundsource.
- Others with the Tandy Emulator (TEMU) or Tandy 3 Voice Sound: Eye of the Beholder, Sid Meier's Civilization
- Duke Nukem 3D, Redneck Rampage, Shadow Warrior supports Disney Sound Source.
Popular DOS-based trackers used on demoscene included Covox support, for example:
- Modplay by Mark J. Cox (also ModEdit, DMP, VP and other Mod Players from early 90's)
- Galaxy Music Player (can play MODs even on a Intel 8088)
- Scream Tracker
- Fast Tracker
- Impulse Tracker
Emulators existed that allowed Covox to act as if there was another soundboard installed:
- Virtual SoundBlaster, that could be used to emulate Sound Blaster on Covox
- Covoxer could emulate Tandy 1000/2000 music synthesizer
- TEMU could emulate Tandy 1000/2000 music synthesizer and DSS
As of 2015 the circuit for the Disney Sound Source has been reverse engineered, so Covox plugs can be used with software requiring such hardware without the need of any additional software emulators using an additional plug that goes between the computer's LPT port and the Covox. 
Several operating systems have a driver for Covox available for install:
within in the 8bit computer scene and other demoscene COVOX has been used by
- "Speech Thing trademark registration".
- Göhler, Stefan (March 12, 2003). "Phonomenal! ... a retrospective view the sound card history".
- Social Science Microcomputer Review. Duke University Press. 7: 97. 1989. ISSN 0885-0011. Missing or empty
- Brychkov, Eugeny (2012-10-19). "Adding a multimedia capability: a Covox device" (PDF). AGE Labs. Retrieved 2014-01-20.
- Pilgrim, Aubrey (1996). Build Your Own Multimedia PC. McGraw-Hill. p. 282. ISBN 978-0-07-912226-1.
- Phillips, Mark. "Some notes on programming for the Disney Sound Source". Archived from the original on 2007-07-07.
- "The Oldskool PC Carnival Sideshow". 1999. Retrieved 2017-08-06.
- "Reversing the Disney Sound Source". VOGONS. 2015-02-01. Retrieved 2017-08-06.
- "Sound - Disney Sound Source". DOSBox Wiki. Retrieved 2017-08-06.
- Linux driver for Covox by Michael Beck
- Powering devices from PC parallel port, an article on LPT-port devices, including Covox.
- List of games that support Disney Sound Source at MobyGames.com.
- List of games that support Covox Speech Thing at MobyGames.com.
- A collection of software for Covox, including Windows driver.
- (in Russian) Covox Voice Master schematics and software.
- (in Russian) Covox Digital-Analog Converter, a comprehensive Covox history overview.
- DSS Reverse Engineering on Vogons DSS Reverse Engineering Thread