Covox Speech Thing
The Covox Speech Thing (also known as Covox plug) was 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.
The circuit was marketed around 1986 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.
Commercial products 
- 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. A somewhat popular device, marketed by Disney Software in early 1990s, appeared to be a sophisticated Covox-idea based DAC. It consisted of 2 parts: a DAC plugged into printer port and separate amplifier / speaker box. The low price of $14 got the device some popularity. It used external power (ran on batteries) and involved some circuitry to turn it on / off. Sound quality was also superior due to sound filtering schemes used. Games that run on Disney Sound Source should run without problems on simpler Covoxes; however, not all software made for simple Covoxes will run on Disney Sound Source, as it requires additional command bytes to be sent to additional control port to operate.
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:
- 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.
- Most older Sierra Entertainment games, such as King's Quest and Space Quest series, were available to output the sound to Covox as Disney Sound Source.
- Wolfenstein 3D, Return to Zork, Hare Raising Havoc (as Disney Sound Source).
Popular DOS-based trackers used on demoscene included Covox support, for example:
Several operating systems have a driver for Covox available for install:
- Göhler, Stefan (2003-03-12). "Phonomenal! ... a retrospective view the sound card history".
- Social Science Microcomputer Review (Duke University Press) 7: 97. 1989. ISSN 0885-0011.
- 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 2007-01-01.
- The Oldskool PC Carnival Sideshow
- Disney Sound Source programming info
- Linux driver for Covox by Michael Beck
- DOSBox Wiki: Sound article
- 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.
- (Russian) Covox Voice Master schematics and software.
- (Russian) Covox Digital-Analog Converter, a comprehensive Covox history overview.