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Covox Speech Thing
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<p>The Covox Speech Thing is an external <a href="/facts/Digital-to-analog_converter/5loEvAeb">digital-to-analog converter (DAC)</a> that plugs into the <a href="/facts/Parallel_printer_port/qpkjH8ts">parallel printer port</a> of a <a href="/facts/IBM_PC_compatible/zGUpl7Sh">PC</a>. It converts 8-<a href="/facts/Bit/S972NSaD">bit</a> <a href="/facts/Digital_sound/PgugcLAI">digital sound</a> using a simple <a href="/facts/Resistor_ladder/mSq5z9lI">R-2R resistor ladder</a> into an <a href="/facts/Analog_signal/xDbb4uB5">analog signal</a> output. </p><p>The Speech Thing was introduced on December 18, 1987 by <a href="/facts/Covox/sSWwslff">Covox, Inc.</a> of <a href="/facts/Eugene,_Oregon/HkvYT2qx">Eugene, Oregon</a>, for about US$70 (equivalent to $194 in 2024) and priced US$79.95 as of 1989. People soon started to <a href="/facts/Do_it_yourself/C2Gf8VJW">build their own (DIY)</a> variants, since its <a href="/facts/Communication_protocol/5uady7cB">communication protocol</a> and DAC is simple and only requires <a href="/facts/Soldering/fod6dLHD">soldering</a> 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[<i>clarify</i>], as <a href="/facts/Sound_card/bxlF4YBa">sound cards</a> were still very expensive at that time. The plug was also quite popular in the <a href="/facts/Demoscene/yKunscS7">demoscene</a>. </p><p>An inherent problem of the design is that its quality relies on how precisely matched the resistors are (see <a href="/facts/Resistor_ladder/mSq5z9lI">Resistor ladder § Accuracy of R–2R resistor ladders</a>). If unmatched resistors are used, the resulting voltage levels get shuffled, especially for quiet sounds, resulting in <a href="/facts/Distortion/z8RJcwNy">distortion</a>. Nevertheless, the sound quality of the Covox plug is far superior compared to the <a href="/facts/PC_speaker/27mCwdtG">PC speaker</a>; for some time, a self-built variant was an inexpensive way to give old computers sound capabilities. </p>