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.
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.