Dialogic ADPCM

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Dialogic ADPCM or VOX/ is an audio file format, optimized for storing digitized voice data at a low sampling rate. VOX files are most commonly found in telephony applications, as well as an occasional arcade redemption game.[citation needed]

Similar to other ADPCM (Adaptive Differential Pulse Code Modulation) formats, Dialogic ADPCM compresses audio data into a series of 4-bit samples. The original Dialogic ADPCM paper (linked to below) does not specify or mention a recording or playback frequency; it may be at the implementer discretion. However, traditionally, files commonly have a sampling rate of 6000 or 8000 samples per second, but 8000 samples per second (8000 Hz) is more common. Unlike a WAV file, a VOX file does not contain a header to specify the encoding format or the sampling rate, so this information must be known in order to play the file. If not known, it is normally assumed that a VOX file is encoded with Dialogic ADPCM at a sampling rate of 8000 Hz. It is possible that a VOX file may be encoded in a format other than Dialogic ADPCM, but this is not common.

Dialogic ADPCM is an open file format.