View source for Covox Speech Thing

{{Short description|External digital-to-analog audio converter}}
{{Use mdy dates|date=February 2016}}
{{Infobox Computer Hardware Generic
|name        = Covox Speech Thing
|image       = Covox Speech Thing.jpg
|caption     = Covox Speech Thing
|invent-name = [[Covox|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|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
|invent-date = {{Start date and age|1987|10|02}}
|introduced  = {{Start date and age|1987|12|18}}
}}

The '''Covox Speech Thing''' is an external [[Digital-to-analog converter|digital-to-analog converter (DAC)]] that plugs into the [[parallel printer port]] of a [[IBM PC compatible|PC]]. It converts 8-[[bit]] [[digital sound]] using a simple [[Resistor ladder|R-2R resistor ladder]] into an [[analog signal]] output.

The Speech Thing was introduced on December 18, 1987<ref>{{cite web |title=Speech Thing trademark |url=https://tsdr.uspto.gov/#caseNumber=73714260&caseType=SERIAL_NO&searchType=statusSearch |url-status= |archive-url= |archive-date= |access-date= |website=United States Patent and Trademark Office (USPTO)}}</ref> by [[Covox|Covox, Inc.]] of [[Eugene, Oregon]], for about US$70<ref>{{Cite web |last=Stefan |first=Göhler |title=Phonomenal! Covox Speech Thing / Disney Sound Source (1986 / 199x) |url=https://www.crossfire-designs.de/index.php?lang=en&what=articles&name=showarticle.htm&article=soundcards/&page=5 |access-date=2023-01-23 |website=Crossfire Designs}}</ref> ({{Inflation|index=US|value=70|start_year=1987|fmt=eq}}) and priced US$79.95 as of 1989.<ref>{{cite journal |year=1989 |journal=Social Science Microcomputer Review |publisher=Duke University Press |volume=7 |page=97 |issn=0885-0011}}</ref> People soon started to [[Do it yourself|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)<ref>{{Cite patent|number=US4812847A|title=Parallel port pass-through digital to analog converter|gdate=1989-03-14|invent1=Stewart|invent2=Stewart|inventor1-first=John L.|inventor2-first=Bradley C.|url=https://patents.google.com/patent/US4812847/en}}</ref> 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. {{clarify span|date=February 2024|The plug was used long into the 1990s}}, as [[sound card]]s 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 {{Slink|Resistor ladder|Accuracy of R–2R resistor ladders}}). 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.<ref>{{Cite book |last=Brychkov |first=Eugeny |url=http://kb.gr8bit.ru/GR8BIT-KB0010-Adding-multimedia-capability-covox-device-23102013.pdf |title=Adding a multimedia capability: a Covox device |publisher=GR8BIT |date=October 19, 2012|archive-url=https://web.archive.org/web/20140201185718/http://kb.gr8bit.ru/GR8BIT-KB0010-Adding-multimedia-capability-covox-device-23102013.pdf |archive-date=February 1, 2014 }}</ref>

== Features ==
[[Image:Covox-Stecker.svg|thumb|One widely used variant]]

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 [[Phone connector (audio)#Miniature size|mini phone connector]]. That signal could then be amplified and played back on [[loudspeaker]]s.

The resistances of the [[Resistor ladder|R-2R ladder]] (100&nbsp;[[kΩ]] and 200&nbsp;kΩ according to the patent) are deliberately high-enough to prevent excessive [[Electrical load|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 [[44,100 Hz|standard 44100&nbsp;Hz]] (the average 1980s [[Intel 80286|80286]] system could handle sampling rates of 12&nbsp;kHz, while later the faster 33&nbsp;MHz [[i486SX|486SX]] introduced in 1991 could handle 44&nbsp;kHz). 

Its 15&nbsp;kΩ load resistor in parallel with a 5&nbsp;[[Nanofarad|nF]] capacitor after the R2R resistor ladder results in a [[Passivity (engineering)|passive]] [[RC circuit|RC]] [[low-pass filter]] starting around 3&nbsp;[[kHz]], thus limiting the [[Bandwidth (signal processing)|analog bandwidth]]. Many DIY variants do not use the same ladder topology and component values, resulting in different [[timbre]].<ref>{{Cite web |last=Scali |date=2017-11-28 |title=The Covox years |url=https://scalibq.wordpress.com/2017/11/28/the-covox-years/ |url-status=live |archive-url=https://web.archive.org/web/20230220063533/https://scalibq.wordpress.com/2017/11/28/the-covox-years/ |archive-date=2023-02-20 |access-date=2023-06-25 |website=Scali's OpenBlog |language=en}}</ref> 

Another limiting factor was that the CPU had to be [[Interrupt|interrupted]] at the sampling rate to play background audio (thus incurring the cost of a [[Context switch#Interrupt handling|context switch]] for every sample, many thousands of times a second), since there was no [[Data buffer|data buffering]] or [[direct memory access]] available.

The sound quality can be increased by software through [[dither]]ing, which reduces perceptible [[aliasing]] noise and increases [[dynamic range]] (used in Inertia Player and [[FastTracker 2]] as an [[Interpolation|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.<ref>{{cite book |title = Build Your Own Multimedia PC |first = Aubrey |last = Pilgrim |publisher = McGraw-Hill |year = 1996 |isbn = 978-0-07-912226-1 |page = [https://archive.org/details/buildyourownmult0000pilg/page/282 282] |url-access = registration |url = https://archive.org/details/buildyourownmult0000pilg/page/282}}</ref>

* '''Disney Sound Source''' – a different design to the Covox Speech Thing, though in a superficially similar case also with parallel pass through, marketed by [[Disney Software]] in early 1990s.
** Consists of a [[FIFO (computing and electronics)|FIFO buffer]] with a DAC on the board that plugs into the parallel printer port, which transmits analog audio over a [[registered jack]] to a separate amplifier / speaker box.<ref>{{cite web |title = Some notes on programming for the Disney Sound Source |url = http://thorkildsen.no/faqsys/docs/progss.txt |first1 = Mark |last1 = Phillips |archiveurl = https://web.archive.org/web/20070101124905/http://thorkildsen.no/faqsys/docs/progss.txt |archivedate = 2007-01-01}}</ref> Its price was set to only $14<ref>{{cite web |url=http://www.oldskool.org/shrines/carny |title=The Oldskool PC Carnival Sideshow |date=1999 |access-date=2017-08-06}}</ref> and it was supported by many games (see below). It used external power (9 volt battery) and could be turned on/off by software. Contrary to the Covox Speech Thing which had no [[FIFO (computing and electronics)|FIFO buffer]], the Disney Sound Source features a 16-byte [[FIFO (computing and electronics)|FIFO]] allowing for autodetection and flow control, which clocks digital output to the resistive DAC at a fixed [[sample rate]] of 7&nbsp;kHz ±5%.<ref>{{Cite web |title=The Sound Source Programmer's Guide |url=http://www.vgmpf.com/Wiki/images/8/8f/The_Sound_Source_-_Programmer%27s_Guide.pdf |url-status=live |archive-url=https://web.archive.org/web/20230625193328/http://www.vgmpf.com/Wiki/images/8/8f/The_Sound_Source_-_Programmer%27s_Guide.pdf |archive-date=2023-06-25 |access-date=2023-06-25 |website=Video Game Music Preservation Foundation}}</ref>
** The printed circuit board DSS007C shows "Ⓟ (1983) U.S. 4,384,170 and others"<ref>{{Cite web |title=Disney Sound Source Pinout \ VOGONS |url=https://www.vogons.org/viewtopic.php?t=48604 |url-status=live |archive-url=https://web.archive.org/web/20230502181836/http://www.vogons.org/viewtopic.php?t=48604 |archive-date=2023-05-02 |access-date=2023-06-26 |website=www.vogons.org}}</ref> which is an earlier patent by [[Forrest S. Mozer]].<ref>{{Cite patent|number=US4384170A|title=Method and apparatus for speech synthesizing|gdate=1983-05-17|invent1=Mozer|invent2=Stauduhar|inventor1-first=Forrest S.|inventor2-first=Richard P.|url=https://patents.google.com/patent/US4384170/en}}</ref> The board a has one [[integrated circuit]] only (labeled "ICS1453") and the schematic<ref>{{Cite book |url=http://archive.org/details/dss-programmers-guide |title=Programmer's Guide to the Disney Sound Source |language=English}}</ref> shows 8 lines from the parallel port going into one chip with input pins D0-D8 that also has a "DAC output" pin going to an output "sound". So it can be inferred that that single chip implements both the FIFO and DAC internally, which differs dramatically from the passive R-2R design of the original Covox.
** In 2015 the hardware was reverse engineered so compatible circuits can be built from easily available off the shelf components.<ref name="vogons">{{cite web |url=http://www.vogons.org/viewtopic.php?f=46&t=42250 |title=Reversing the Disney Sound Source |date=2015-02-01 |website=VOGONS |access-date=2017-08-06}}</ref> It is also emulated by the popular [[DOSBox]] emulator.

== Compatibility ==
[[File:Custom Covox.jpg|thumb|Home-built variant]]

===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:
* ''[[688 Attack Sub]]'' – title music, sound effects
* ''[[Zorro (1995 video game)|Zorro]]'' – sound samples
* ''[[Battle Bugs]]'' – speech, requires [[expanded memory]]
* ''[[Star Control II]]'' – sounds 
* ''[[Pinball Fantasies]]''
* most older [[Sierra Entertainment]] games, such as ''[[King's Quest]]'' and ''[[Space Quest]]'' series
* [[Build Engine]] games including ''[[Duke Nukem 3D]]'', ''[[Redneck Rampage]]'' and ''[[Shadow Warrior (1997 video game)|Shadow Warrior]]''

===Music trackers===
Popular DOS-based [[Music tracker|trackers]] used in the [[demoscene]] included Covox support, for example:
* Modplay (also [[Modedit|ModEdit]], DMP, VP and other Mod players from early 1990s) (Up to 4 channel COVOX output or custom assembler routine for user device)
* Inertia Player (mono, stereo-on-1 covox, with dithering option).
* Multiplayer by DGS Italian Software Company
* Galaxy Music Player (can play MODs even on an [[Intel 8088]])
* [[Scream Tracker]] version 2.3 direct support. Version 3.21 through virtual-soundblaster driver.
* [[FastTracker]] (mono, stereo, stereo-on-1 mode, with dithering).
* [[Impulse Tracker]]

===Emulating other soundcards===
[[Emulator]]s exist that allow a physical Covox to appear as if it is another soundcard:
* Virtual SoundBlaster – can emulate Sound Blaster on Covox
* Covoxer – can emulate [[Tandy 1000]]/[[Tandy 2000|2000]] music synthesizer
* TEMU – can emulate [[Tandy 1000]]/[[Tandy 2000|2000]] music synthesizer and Disney Sound Source

=== Emulating Covox ===
The [[DOSBox]] and Fake86 emulators can emulate a virtual Covox (as Disney Sound Source) on machines without a physical Covox.<ref>{{cite web |date=2015 |title=Sound – Disney Sound Source |url=http://www.dosbox.com/wiki/Sound#Disney_Sound_Source |access-date=2017-08-06 |website=DOSBox Wiki}}</ref>

===Operating systems===
Several operating systems have an installable [[Driver (software)|driver]] for Covox:
* [[Windows 3.1x]], [[Windows 95]], [[Windows 98]]
* [[Linux]]
** "Alternate Sound Driver for Linux 2.x" (pcsndrv) – supports a "Mono DAC" using one lp-port ([[parallel printer port]]) and a "Stereo DAC" using two lp-ports.<ref>{{Cite web |last=Beck |first=Michael |date=2005-01-23 |title=Alternate Sound Driver for Linux 2.x Version 1.3 (pcsndrv-1.3.readme) |url=http://fresh.t-systems-sfr.com/linux/src/pcsndrv-1.3.readme |url-status=dead |archive-url=https://web.archive.org/web/20050123212438/http://fresh.t-systems-sfr.com/linux/src/pcsndrv-1.3.readme |archive-date=2005-01-23 |access-date= |website=}}</ref>
** covox-music-player – not a [[kernel driver]], but rather a [[userspace]] program that outputs sound on modern Linux distros to the Covox via [[port-mapped I/O]] with the ''outb'' (output byte) [[Opcode|instruction]] [[wrapper function]] (which can be called from user space).<ref>{{Cite web |last=MENG |first=YEO |date=January 2, 2017 |title=A Science Project: Bringing the Covox Speech Thing to 2017 |url=https://yeokhengmeng.com/2017/01/a-science-project-bringing-the-covox-speech-thing-to-2017/ |website=YKM'S CORNER ON THE WEB}}</ref><ref>{{Cite web |last=Meng |first=Yeo |date=August 10, 2021 |title=Covox-music-player |url=https://github.com/yeokm1/covox-music-player |website=[[GitHub]]}}</ref><ref>{{Cite web |title=outb(2) - Linux manual page |url=https://man7.org/linux/man-pages/man2/outb.2.html |access-date=2023-11-23 |website=man7.org}}</ref>
* [[MenuetOS]]
* [[CSI-DOS]]

===Later variants===
* [[André LaMothe|André LaMothe's]] 1995 book "Black Art of 3D Game Programming" section "Building Your Own Digital Sound Device-DIGIBLASTER"<ref>{{Cite book |last=André LaMothe |url=http://archive.org/details/BlackArt3DEBook |title=Black Art of 3D Game Programming |date=1995-09-01}}</ref> – while not saying the word "Covox", it does describe how to build a resistor ladder hardware device with a [[Circuit diagram|schematic]] that is essentially a clone of a Covox in stereo that plugs into two parallel printer ports, and describes [[DOS]] [[C (programming language)|C code]] for driving it.
* Simple LPTsnd<ref>{{Cite web |title=Simple LPTSND – Covox Speech Thing |url=https://www.retrokits.de/index.php/simple-lptsnd/ |website=Retroianer}}</ref> – also compatible with SoundJR and FTL sound adapters in addition to Covox Speech Thing, allowing more games to be played.
* CVX-4<ref>{{Cite web|url=https://www.serdashop.com/CVX4|title=CVX4 Vogons Tuning Edition|website=Serdashop}}</ref> – features [[DIP switch|DIP switches]] to select capacitor for the low-pass filter, to provide extra attenuation, and to bypass [[coupling capacitor]].
* Disney Soundsource 2015 Remake<ref>{{Cite web |title=Reversing the Disney Sound Source |url=https://www.vogons.org/viewtopic.php?f=62&t=42250 |website=VOGONS}}</ref> – [[reverse-engineered]] from an original adapter.
** Allows software requiring the original Disney hardware to work without the need of any additional software emulators.<ref name="vogons" />
* Benedikt's remakes:
** Generic 8 bit audio DAC<ref>{{Citation |last=Freisen |first=Benedikt |title=Generic 8 bit audio DAC |date=2023-06-08 |url=https://github.com/roybaer/generic_8_bit_audio_dac |access-date=2023-06-26}}</ref> – includes parallel-port pass-through for printer, with intention of being more similar to original Covox.
** Parallel Port Headphone DAC<ref>{{Citation |last=Freisen |first=Benedikt |title=Parallel Port Headphone DAC |date=2023-06-08 |url=https://github.com/roybaer/parallel_port_headphone_dac |access-date=2023-06-26}}</ref> – replica of 1990s SiliconSoft SoundJr device<ref>{{Cite web |title=SoundJr Digital Audio Modules by SiliconSoft.Com |url=http://www.siliconsoft.com/sndjr1.htm |archive-url=https://web.archive.org/web/20230305200927/http://www.siliconsoft.com/sndjr1.htm |archive-date=2023-03-05 |access-date=2023-06-26 |website=www.siliconsoft.com}}</ref> that can drive headphones with power from the parallel port.
** Parallel port audio and joystick adapter<ref>{{Cite web |last=Freisen |first=Benedikt |title=Parallel port audio and joystick adapter |url=https://github.com/roybaer/parallel_port_audio_and_joystick_adapter |website=[[GitHub]]}}</ref> – compatible with FTL Sound Adapter.
* ISA LPT DAC r0 – Dual-channel board that plugs into the [[ISA bus]].<ref>{{Cite web |title=Dual ISA DAC r0 – Covox Speech Thing on ISA Bus |url=https://www.retrokits.de/index.php/dual-isa-dac-r0-covox-speech-thing-on-isa-bus/ |website=Retroianer}}</ref>
** Each channel uses an [[Analog Devices]] AD7524<ref>https://www.analog.com/media/en/technical-documentation/data-sheets/AD7524.pdf {{Bare URL PDF|date=August 2024}}</ref> 8-bit [[Flip-flop (electronics)|latched]] DAC [[CMOS]] chip containing a precise integrated thin-film resistor network.

== Other Covox products ==
{{Main|Covox}}
* '''Covox Voice Master''' – Earlier 1984 speech-synthesis board for the [[Commodore 64]] which could playback 64 words stored in memory with [[BASIC]] instructions. Word capacity could be extended with disk or tape.<ref>{{Cite web |last=Bergheim |first=Kim |date=1984-09-13 |title=Micros prick up their ears |url=https://books.google.com/books?id=GS8EAAAAMBAJ&pg=PA41 |website=InfoWorld: The Newsweekly for Microcomputer Users, volume 6, issue 33 |page=41}}</ref>
** '''Covox Voice Master Junior''' – Later cheaper ($39.95) variant that plugs into [[Atari]] joystick port and contains an [[electret microphone]] for recording sounds for playback or recognition and a speaker.<ref>{{cite web | url=https://archive.org/details/VoiceMasterJuniorUserManual/page/n5/mode/2up | title=COVOX Voice Master Junior User Manual | date=December 1988 }}</ref><ref>{{Cite web |date=1988-06-01 |title=8-Bit Product Reviews: Covox Voice Master Jr., SSI's Shilob, Panasonic KX-PL09LI Printer |url=https://www.atarimagazines.com/v7n2/8BitProductReviews.html |url-status=live |archive-url=https://web.archive.org/web/20220810042022/https://www.atarimagazines.com/v7n2/8BitProductReviews.html |archive-date=2022-08-10 |access-date=2023-06-25 |website=atarimagazines.com |series=[[Antic (magazine)]] vol 77, no 2}}</ref>

Also as described in a 1991 COVOX Company Profile:<ref>{{Cite web |date=1991-09-01 |title=COVOX: Company Profile (from Covox BBS) |url=https://groups.google.com/g/comp.sys.ibm.pc.soundcard/c/M2T95v7EgsI/m/ibrbO4-KcGQJ |access-date=2023-06-25 |website=groups.google.com}}</ref>

* '''Covox Sound Master''' – 1984 for [[Apple II]], 1989 for [[MS-DOS]]
* '''Covox Voice Master System II''' – 1990
* '''Covox MIDI Maestro''' – 1990
* '''Covox Sound Master II''' – 1990

== References ==
{{reflist}}

== External links ==
{{Commons category}}
* [https://web.archive.org/web/20191113033645/http://www.epanorama.net/circuits/lptpower.html Powering devices from PC parallel port], an article on LPT-port devices, including Covox.
* [http://www.mobygames.com/attribute/sheet/attributeId,44/ List of games that support Disney Sound Source] at MobyGames.com.
* [http://www.mobygames.com/attribute/sheet/attributeId,43/ List of games that support Covox Speech Thing] at MobyGames.com.
* [https://web.archive.org/web/20190722053927/http://margo.student.utwente.nl/el/pc/ad_da/ A collection of software for Covox], including Windows driver.
* {{in lang|ru}} [https://web.archive.org/web/20191113214717/http://www.415.spb.ru/page.php?60 Covox Voice Master schematics and software].
* {{in lang|ru}} [https://web.archive.org/web/20181103030711/http://www.phantom.sannata.ru/konkurs/netskater002.shtml Covox Digital-Analog Converter], a comprehensive Covox history overview.
* [https://web.archive.org/web/20191111010911/https://www.vogons.org/viewtopic.php?f=62&t=42250 DSS Reverse Engineering on Vogons] DSS Reverse Engineering Thread

{{PC sound standards}}

[[Category:Sound cards]]
[[Category:Computer-related introductions in 1987]]