My LPT DAC story

Hackaday featured today LPT DAC circuit. Back in the olden days, around 25 years ago, sound cards were rare on PCs. One hack those days was the original Covox Speech Thing, that connected a simple DAC to to PC parallel port. The Covox Speech Thing (also known as Covox plug) was an external audio device attached to the computer to output digital sound. It allowed to play back sounds in considerably better quality than with PC speaker. A simple resistor ladder was sufficient to convert digital data on parallel port. When a suitable software sent the data to parallel port data pins at the sample rate, the audio was available at the adapter audio output. One nice feature of this circuit was that it could co-exist with parallel port printer on the same port (circuit did not have any effect on printer operation and when audio was played back printer did not react to it, you just can’t do both things at the same time).

The circuit was marketed around 1986 by Covox, Inc of Eugene, Oregon, for about 70 USD with the accompanying software (that included speech synthesis). This hardware was eventually supported by the old DOS games from Sierra and Id. I came by one device and it’s software one day around 1988 or early 1989. I did some thinking and analyzing, and came out with my own circuit simple design that worked same way as the original hardware (this is re-drawn version from my document):

Those circuits are old designs from time when I was not so good in electronics than I am now. All of the designs are not very good and I would do those circuits today differenly. For example the first DAC circuit is not very accururate and R2R construction would give much better performance.

The circuit was built inside simple parallel port connector adapter box:

I posted the circuit to one BBS some day 1989, and the idea started to spread quickly. Immediately someone posted this circuit to some Finnish national BBS network. The plans spread well, file was very much loaded. It went viral without me originally start to spread it widely. Very many people started building this circuit, and started spreading the software that uses it (pirated version of original Covox software was found in many places as well applications made by other people). I don’t think that company imported the original Covox product to Finland like that…

My circuit was not optimal in performance, but it was easy to build and “good enough”. Later plans for better quality R2R circuit (same as shown in Wikipedia) started to spread around:


The plug quickly came quite popular in the demoscene (somebody later even designed stereo version). The popularity faded over years when PC sound cards become more and more popular, and popularity of DOS faded (getting the adapter to work with Windows was a bit tricky).


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