The Covox Speech Thing is an external digital-to-analog converter (DAC) that plugs into the parallel printer port of a PC. It converts 8-bit digital sound using a simple R-2R resistor ladder into an analog signal output.
The Speech Thing was introduced on December 18, 1987 by Covox, Inc. of Eugene, Oregon, for about US$70 (US$79.95 as of 1989). People soon started to build their own (DIY) variants, since its communication protocol and DAC is simple and only requires soldering a few cheap parts. The novelty of its patent "Parallel port pass-through digital to analog converter" (filed in 1987, granted in 1989) wasn't specifically the use of a resistor ladder as a DAC, but rather the patent's discussion is around its ease of plugging into the parallel port and how its resistor ladder design didn't block other devices from using the parallel port. 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 its quality relies on how precisely-matched the resistors are (see ). If unmatched resistors are used, the resulting voltage levels 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 variant is still an inexpensive way to give old computers sound capabilities.
Features
In its simplest form, the Covox plug received an 8-bit digital byte for each digital audio sample from the parallel port and produced a high impedance mono analog output voltage signal though a mini phone connector. That signal could then be amplified and played back on loudspeakers.
The resistances of the R-2R ladder (100 kΩ and 200 kΩ according to the patent) are deliberately high-enough to prevent excessive loading of the signals, so a printer attached to the output connector will operate normally.
The original Covox plug itself doesn't use sequential logic or a clock signal, so theoretically it can operate with any sampling rate. In practice, however, parallel port speed limits make it rather hard to achieve even standard 44100 Hz (the average 1980s 80286 system could handle sampling rates of 12 kHz, while later the faster 33 MHz 486SX introduced in 1991 could handle 44 kHz).
Its 15 kΩ load resistor in parallel with a 5 nF capacitor after the R2R resistor ladder results in a passive RC low-pass filter starting around 3 kHz, thus limiting the analog bandwidth. Many DIY variants do not use the same ladder topology and component values, resulting in different timbre.
Another limiting factor was that the CPU had to be interrupted at the sampling rate to play background audio (thus incurring the cost of a context switch for every sample, many thousands of times a second), since there was no data buffering or direct memory access available.
The sound quality can be increased by software through dithering, which reduces perceptible aliasing noise and increases dynamic range (used in Inertia Player and FastTracker 2 as an interpolating option).
Commercial products
Covox Speech Thing - a R-2R resistor DAC with parallel port pass through, bundled with speech synthesis software, marketed originally as part of voice synthesis and recognition system.