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 in to 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 orders of magnitude 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.
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