The Covox Speech Thing is an external audio device attached to the computer to output digital sound. It was composed of a primitive 8-bit DAC using a resistor ladder and an analogue signal output, and plugged into the printer port of the PC. The Speech Thing was introduced on December 18, 1987 by Covox, Inc of Eugene, Oregon, for about US$70, 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 hardware system with a similar design to the Covox Speech Thing, 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. It used external power and could be turned on/off by software. Contrary to the Speech Thing the output rate is determined by the hardware 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.
Features
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. However the sound quality can increased by software which has the dithering, thus the aliasing noise is minimized and the dynamics increased. Advanced versions of Covox-like devices featured:
Printer port forwarding connectors — allowed to plug printer into the Covox and use both without reconnections, although not at the same time. Whenever something is printed, loud noise is created.
Numerous sound-enhancing features, like filters or equalizers;
Stereo capability, either by using two parallel ports or one port with switching using strobe and line feed signals.
Compatibility
Games
The Covox plug couldn't directly substitute any of the popular cards of that age, 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.
Some Games work with a software emulator called Virtual Soundblaster, such as Wolfenstein 3D, Wolfenstein 3D has also native Support for the Disney Soundsource.
s existed that allowed Covox to act as if there was another soundboard installed:
Virtual SoundBlaster - could emulate Sound Blaster on Covox
Covoxer - could emulate Tandy 1000/2000 music synthesizer
TEMU - could emulate Tandy 1000/2000 music synthesizer and DSS
In reverse, the DOSBox and Fake86 emulators allow to emulate presence of Covox on a machine without such physical device connected. 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.
Operating Systems
Several operating systems have a driver for Covox available for install:
