(Short for "digital to analog converter", often abbreviated to DAC) A device that converts a digital signal, represented by a stream of words, into an analog signal. The conversion process is driven by a clock, with successive words being converted at precise intervals. Each word is converted into an analog voltage; the resulting signal consists of a string of stair-steps that approximates the desired signal. Per the Nyquist theory, the stair-step signal actually fully contains the desired signal, but it also contains additional frequencies that are at the clock frequency and harmonics of the clock frequency. Filtering these out with a low pass filter results in the desired signal. (This is easier said than done; the interval between the Nyquist frequency and the clock frequency may be as short as one octave, and a filter with an extremely steep slope is required. The most advanced converters use a variety of techniques such as oversampling to further separate the clock signal frequency from the highest frequency contained within the signal being converted, so that the filtering job becomes easier.)
The D/A converter is an essential component of all samplers, sample playback units, and other types of digital synthesizers and digital effects units. At some point, any digital signal must be converted back to analog before it can be heard by the human ear; although there are amplifiers that can amplify digital signals directly, there is no commercially available loudspeaker or headphone that can accept a digital signal. Note, however, that some digital units have direct digital outputs (such as S/PDIF) that allow a signal in digital form to be sent directly to a digital mixer or recording system, deferring the D/A conversion process until playback of the recorded digital signal.