Covox Speech Thing Explained

Covox Speech Thing
Invent-Name:Covox, Inc.
Conn1:Parallel printer port
Class-Name:Use
Class1:audio digital-to-analog converter attached to computer parallel port with pass-through
Manuf1:Covox, Inc. (original Covox Speech Thing)
Manuf2:Disney (Disney Sound Source variant)
Manuf3:Faster Than Light (FTL Sound Adapter variant)
Manuf4:SiliconSoft (SoundJr variant)
Manuf5:Do it yourself variants

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[1] by Covox, Inc. of Eugene, Oregon, for about US$70[2] and priced US$79.95 as of 1989.[3] 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)[4] 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., 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; for some time, a self-built variant was an inexpensive way to give old computers sound capabilities.[5]

Features

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  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.[6]

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

Compatibility

Games

The Covox plug couldn't directly substitute any of the popular cards of that age (AdLib, Sound Blaster, Gravis UltraSound, etc.), but several games / platforms supported it directly. It is also usually used in tandem with an AdLib sound card as said card officially was a music card and while it could be put into a mode to handle sampled audio, it could not play sampled audio and music at the same time. Notable entries include:

Music trackers

Popular DOS-based trackers used in the demoscene included Covox support, for example:

Emulating other soundcards

Emulators exist that allow a physical Covox to appear as if it is another soundcard:

Emulating Covox

The DOSBox and Fake86 emulators can emulate a virtual Covox (as Disney Sound Source) on machines without a physical Covox.[15]

Operating systems

Several operating systems have an installable driver for Covox:

Later variants

Other Covox products

See main article: Covox.

Also as described in a 1991 COVOX Company Profile:[31]

External links

Notes and References

  1. Web site: Speech Thing trademark . United States Patent and Trademark Office (USPTO).
  2. Web site: Stefan . Göhler . Phonomenal! Covox Speech Thing / Disney Sound Source (1986 / 199x) . 2023-01-23 . Crossfire Designs.
  3. 1989 . Social Science Microcomputer Review . Duke University Press . 7 . 97 . 0885-0011.
  4. US4812847A. Parallel port pass-through digital to analog converter. 1989-03-14. Stewart. Stewart. John L.. Bradley C..
  5. Book: Brychkov, Eugeny . Adding a multimedia capability: a Covox device . GR8BIT . October 19, 2012. https://web.archive.org/web/20140201185718/http://kb.gr8bit.ru/GR8BIT-KB0010-Adding-multimedia-capability-covox-device-23102013.pdf . February 1, 2014 .
  6. Web site: Scali . 2017-11-28 . The Covox years . live . https://web.archive.org/web/20230220063533/https://scalibq.wordpress.com/2017/11/28/the-covox-years/ . 2023-02-20 . 2023-06-25 . Scali's OpenBlog . en.
  7. Book: Pilgrim, Aubrey . Build Your Own Multimedia PC . McGraw-Hill . 1996 . 978-0-07-912226-1 . 282 . registration .
  8. Web site: Some notes on programming for the Disney Sound Source . Mark . Phillips . https://web.archive.org/web/20070101124905/http://thorkildsen.no/faqsys/docs/progss.txt . 2007-01-01.
  9. Web site: The Oldskool PC Carnival Sideshow . 1999 . 2017-08-06.
  10. Web site: The Sound Source Programmer's Guide . live . https://web.archive.org/web/20230625193328/http://www.vgmpf.com/Wiki/images/8/8f/The_Sound_Source_-_Programmer%27s_Guide.pdf . 2023-06-25 . 2023-06-25 . Video Game Music Preservation Foundation.
  11. Web site: Disney Sound Source Pinout \ VOGONS . live . https://web.archive.org/web/20230502181836/http://www.vogons.org/viewtopic.php?t=48604 . 2023-05-02 . 2023-06-26 . www.vogons.org.
  12. US4384170A. Method and apparatus for speech synthesizing. 1983-05-17. Mozer. Stauduhar. Forrest S.. Richard P..
  13. Book: Programmer's Guide to the Disney Sound Source . English.
  14. Web site: Reversing the Disney Sound Source . 2015-02-01 . VOGONS . 2017-08-06.
  15. Web site: 2015 . Sound – Disney Sound Source . 2017-08-06 . DOSBox Wiki.
  16. Web site: Beck . Michael . 2005-01-23 . Alternate Sound Driver for Linux 2.x Version 1.3 (pcsndrv-1.3.readme) . dead . https://web.archive.org/web/20050123212438/http://fresh.t-systems-sfr.com/linux/src/pcsndrv-1.3.readme . 2005-01-23 .
  17. Web site: MENG . YEO . January 2, 2017 . A Science Project: Bringing the Covox Speech Thing to 2017 . YKM'S CORNER ON THE WEB.
  18. Web site: Meng . Yeo . August 10, 2021 . Covox-music-player . GitHub.
  19. Web site: outb(2) - Linux manual page . 2023-11-23 . man7.org.
  20. Book: André LaMothe . Black Art of 3D Game Programming . 1995-09-01.
  21. Web site: Simple LPTSND – Covox Speech Thing . Retroianer.
  22. Web site: CVX4 Vogons Tuning Edition. Serdashop.
  23. Web site: Reversing the Disney Sound Source . VOGONS.
  24. Web site: SoundJr Digital Audio Modules by SiliconSoft.Com . https://web.archive.org/web/20230305200927/http://www.siliconsoft.com/sndjr1.htm . 2023-03-05 . 2023-06-26 . www.siliconsoft.com.
  25. Web site: Freisen . Benedikt . Parallel port audio and joystick adapter . GitHub.
  26. Web site: Dual ISA DAC r0 – Covox Speech Thing on ISA Bus . Retroianer.
  27. https://www.analog.com/media/en/technical-documentation/data-sheets/AD7524.pdf
  28. Web site: Bergheim . Kim . 1984-09-13 . Micros prick up their ears . InfoWorld: The Newsweekly for Microcomputer Users, volume 6, issue 33 . 41.
  29. Web site: COVOX Voice Master Junior User Manual . December 1988 .
  30. Web site: 1988-06-01 . 8-Bit Product Reviews: Covox Voice Master Jr., SSI's Shilob, Panasonic KX-PL09LI Printer . live . https://web.archive.org/web/20220810042022/https://www.atarimagazines.com/v7n2/8BitProductReviews.html . 2022-08-10 . 2023-06-25 . atarimagazines.com . Antic (magazine) vol 77, no 2.
  31. Web site: 1991-09-01 . COVOX: Company Profile (from Covox BBS) . 2023-06-25 . groups.google.com.