24-510: 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 (equivalent to $ 188 in 2023) and priced US$ 79.95 as of 1989. People soon started to build their own (DIY) variants, since its communication protocol and DAC
48-445: A digital-to-analog converter ( DAC , D/A , D2A , or D-to-A ) is a system that converts a digital signal into an analog signal . An analog-to-digital converter (ADC) performs the reverse function. There are several DAC architectures ; the suitability of a DAC for a particular application is determined by figures of merit including: resolution , maximum sampling frequency and others. Digital-to-analog conversion can degrade
72-441: A whiffletree mechanism to produce finer steps. The IBM Selectric typewriter uses such a system. DACs are widely used in modern communication systems enabling the generation of digitally-defined transmission signals. High-speed DACs are used for mobile communications and ultra-high-speed DACs are employed in optical communications systems. The most common types of electronic DACs are: The most important characteristics of
96-402: A DAC are: Other measurements, such as phase distortion and jitter , can also be very important for some applications, some of which (e.g. wireless data transmission, composite video) may even rely on accurate production of phase-adjusted signals. Non-linear PCM encodings (A-law / μ-law, ADPCM, NICAM) attempt to improve their effective dynamic ranges by using logarithmic step sizes between
120-540: A continually varying physical signal . Provided that a signal's bandwidth meets the requirements of the Nyquist–;Shannon sampling theorem (i.e., a baseband signal with bandwidth less than the Nyquist frequency ) and was sampled with infinite resolution, the original signal can theoretically be reconstructed from the sampled data. However, an ADC's filtering can't entirely eliminate all frequencies above
144-677: A linear contrast ratio (difference between darkest and brightest output levels) of 1000:1 or greater, equivalent to 10 bits of audio precision even though it may only accept signals with 8-bit precision and use an LCD panel that only represents 6 or 7 bits per channel. Video signals from a digital source, such as a computer, must be converted to analog form if they are to be displayed on an analog monitor. As of 2007, analog inputs were more commonly used than digital, but this changed as flat-panel displays with DVI and/or HDMI connections became more widespread. A video DAC is, however, incorporated in any digital video player with analog outputs. The DAC
168-468: A packaged IC) would typically be extremely high-speed low-resolution power-hungry types, as used in military radar systems. Very high-speed test equipment, especially sampling oscilloscopes , may also use discrete DACs. A DAC converts an abstract finite-precision number (usually a fixed-point binary number ) into a physical quantity (e.g., a voltage or a pressure ). In particular, DACs are often used to convert finite-precision time series data to
192-420: A signal, so a DAC should be specified that has insignificant errors in terms of the application. DACs are commonly used in music players to convert digital data streams into analog audio signals . They are also used in televisions and mobile phones to convert digital video data into analog video signals . These two applications use DACs at opposite ends of the frequency/resolution trade-off. The audio DAC
216-524: Is a low-frequency, high-resolution type while the video DAC is a high-frequency low- to medium-resolution type. Due to the complexity and the need for precisely matched components , all but the most specialized DACs are implemented as integrated circuits (ICs). These typically take the form of metal–oxide–semiconductor (MOS) mixed-signal integrated circuit chips that integrate both analog and digital circuits . Discrete DACs (circuits constructed from multiple discrete electronic components instead of
240-424: 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
264-403: Is then reconstructed into analog using a DAC on the receiving party's end. Video sampling tends to work on a completely different scale altogether thanks to the highly nonlinear response both of cathode ray tubes (for which the vast majority of digital video foundation work was targeted) and the human eye, using a "gamma curve" to provide an appearance of evenly distributed brightness steps across
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#1732780194245288-513: Is usually integrated with some memory ( RAM ), which contains conversion tables for gamma correction , contrast and brightness, to make a device called a RAMDAC . A device that is distantly related to the DAC is the digitally controlled potentiometer , used to control an analog signal digitally. A one-bit mechanical actuator assumes two positions: one when on, another when off. The motion of several one-bit actuators can be combined and weighted with
312-612: 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
336-447: The demoscene included Covox support, for example: Emulators exist that allow a physical Covox to appear as if it is another soundcard: The DOSBox and Fake86 emulators can emulate a virtual Covox (as Disney Sound Source) on machines without a physical Covox. Several operating systems have an installable driver for Covox: Also as described in a 1991 COVOX Company Profile: Digital-to-analog converter In electronics ,
360-408: The digital revolution . To illustrate, consider a typical long-distance telephone call. The caller's voice is converted into an analog electrical signal by a microphone , then the analog signal is converted to a digital stream by an ADC. The digital stream is then divided into network packets where it may be sent along with other digital data , not necessarily audio. The packets are then received at
384-554: The Nyquist frequency, which will alias into the baseband frequency range. And the ADC's digital sampling process introduces some quantization error (rounding error), which manifests as low-level noise. These errors can be kept within the requirements of the targeted application (e.g. under the limited dynamic range of human hearing for audio applications). DACs and ADCs are part of an enabling technology that has contributed greatly to
408-806: The destination, but each packet may take a completely different route and may not even arrive at the destination in the correct time order. The digital voice data is then extracted from the packets and assembled into a digital data stream. A DAC converts this back into an analog electrical signal, which drives an audio amplifier , which in turn drives a speaker , which finally produces sound. Most modern audio signals are stored in digital form (for example MP3s and CDs ), and in order to be heard through speakers, they must be converted into an analog signal. DACs are therefore found in CD players , digital music players , and PC sound cards . Specialist standalone DACs can also be found in high-end hi-fi systems. These normally take
432-492: The digital output of a compatible CD player or dedicated transport (which is basically a CD player with no internal DAC) and convert the signal into an analog line-level output that can then be fed into an amplifier to drive speakers. Similar digital-to-analog converters can be found in digital speakers such as USB speakers, and in sound cards . In voice over IP applications, the source must first be digitized for transmission, so it undergoes conversion via an ADC and
456-442: The display's full dynamic range - hence the need to use RAMDACs in computer video applications with deep enough color resolution to make engineering a hardcoded value into the DAC for each output level of each channel impractical (e.g. an Atari ST or Sega Genesis would require 24 such values; a 24-bit video card would need 768...). Given this inherent distortion, it is not unusual for a television or video projector to truthfully claim
480-478: 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
504-426: 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: Popular DOS-based trackers used in
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#1732780194245528-573: 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. 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
552-643: 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). The Covox plug couldn't directly substitute any of
576-457: 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 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,
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