Covox Speech Thing

Lore & Background

The Covox Speech Thing was introduced on December 18, 1987 by Covox, Inc. of Eugene, Oregon, priced at about US$70 and later US$79.95 as of 1989. Its design uses a resistor ladder (100 kΩ and 200 kΩ according to the patent) with deliberately high resistances to prevent excessive loading of the parallel port signals, allowing a printer attached to the output connector to operate normally. The device does not use sequential logic or a clock signal, so theoretically it can operate at any sampling rate; in practice, parallel port speed limits constrained performance—an average 1980s 80286 system could handle about 12 kHz, while a faster 33 MHz 486SX introduced in 1991 could reach 44 kHz. A 15 kΩ load resistor in parallel with a 5 nF capacitor after the R-2R ladder creates a passive RC low-pass filter starting around 3 kHz, limiting analog bandwidth. The CPU had to be interrupted at the sampling rate for every sample, as there was no data buffering or direct memory access. Sound quality could be improved by software dithering, used in players such as Inertia Player and FastTracker 2. The novelty of its patent (filed 1987, granted 1989) was not the resistor ladder itself but the ease of plugging into the parallel port without blocking other devices. People soon built DIY variants because the protocol and DAC were simple, requiring only a few cheap parts. The sound quality of the Covox plug was far superior to the PC speaker.

Reader's Guide

The Covox Speech Thing's significance lies in its role as an affordable audio solution for early PCs before dedicated sound cards became common. It was not compatible with popular cards like AdLib, Sound Blaster, or Gravis UltraSound, but several games and platforms supported it natively, often used alongside an AdLib card because the AdLib could not play sampled audio and music simultaneously. Notable games included 688 Attack Sub, Star Control II, Pinball Fantasies, many Sierra Entertainment titles, and Build Engine games such as Duke Nukem 3D. It was widely supported in DOS-based music trackers used in the demoscene, including Modplay, Inertia Player, Scream Tracker, FastTracker, and Impulse Tracker. Emulators such as Virtual SoundBlaster and Covoxer allowed a physical Covox to emulate other sound cards, while DOSBox and Fake86 could emulate a Covox on machines without one. Operating system drivers existed for Windows 3.1x, Windows 95, Windows 98, Linux, MenuetOS, and CSI-DOS. Later variants included André LaMothe's DigiBlaster design, Simple LPTsnd, CVX-4, and a 2015 reverse-engineered Disney Sound Source remake. The Covox Voice Master and Voice Master Junior were earlier products for the Commodore 64 and Atari, respectively.

Did You Know?

• Its patent novelty was the parallel port pass-through design, not the resistor ladder DAC itself.
• The device has no clock signal or data buffering; the CPU must interrupt for every sample.
• The Disney Sound Source, a superficially similar device, uses a FIFO buffer and a single chip DAC, unlike the Covox's passive R-2R ladder.