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Microphone Month 4

AC-1

AC-1 was Dolby’s first digital audio coding scheme. First adopted by systems providers in 1984 when bit rate reduction was in its infancy, AC-1 is a refined form of adaptive delta modulation (ADM), whereby changes in the signal amplitude from moment to moment are transmitted, rather than the absolute values. In addition to coding amplitude changes, a system of dynamic pre- and de-emphasis is used to minimize the audibility of coding noise. It was intended for mass broadcasting applications at a time when the digital signal processing “horsepower” that we are used today just wasn’t available. The solution was to use a simple and therefore cheap decoder driven by a fairly complex encoder. The technology is in use in satellite and cable delivery systems. Encoders and decoders are both sold and licensed.

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