Dialogic ADPCM or VOX is an

audio file format An audio file format is a file format for storing digital audio data on a computer system. The bit layout of the audio data (excluding metadata) is called the audio coding format and can be uncompressed, or compressed to reduce the file size, o ...

, optimized for storing

digitized DigitizationTech Target. (2011, April). Definition: digitization. ''WhatIs.com''. Retrieved December 15, 2021, from https://whatis.techtarget.com/definition/digitization is the process of converting information into a digital (i.e. computer-r ...

voice data at a low

sampling rate In signal processing, sampling is the reduction of a continuous-time signal to a discrete-time signal. A common example is the conversion of a sound wave to a sequence of "samples". A sample is a value of the signal at a point in time and/or sp ...

. VOX files are most commonly found in

telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

applications, as well as an occasional arcade

redemption game Redemption games are typically arcade games of skill that reward the player proportionally to their score in the game. The reward most often comes in the form of tickets, with more tickets being awarded for higher scores. These tickets can th ...

. It uses a

lossy compression In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...

algorithm, optimized for voice, not high fidelity. 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. 8000 Hz matches the sampling rate used in

G.711 G.711 is a narrowband audio codec originally designed for use in telephony that provides toll-quality audio at 64 kbit/s. G.711 passes audio signals in the range of 300–3400 Hz and samples them at the rate of 8,000 samples per second ...

voice systems such as DS1. 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. It matches ITU-T standard G.721, later superseded by

G.726 G.726 is an ITU-T ADPCM speech codec standard covering the transmission of voice at rates of 16, 24, 32, and 40 kbit/s. It was introduced to supersede both G.721, which covered ADPCM at 32 kbit/s, and G.723, which described ADPCM for ...

. The

algorithm In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...

for Dialogic ADPCM was developed by

Oki Electric , commonly referred to as OKI, OKI Electric or the OKI Group, is a Japanese information and communications technology company, headquartered in Toranomon, Minato-ku, Tokyo and operating in over 120 countries around the world. OKI produced the ...

, which also produced

ICs ICS may refer to: Computing * Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy * Industrial control system, computer systems and networks used to control industrial plants and infrastructu ...

such as the Oki Semiconductor MSM7580 to implement the algorithm in hardware. These ICs were used on popular telephony interface cards manufactured by

Dialogic Corporation Dialogic is an American multinational technology company headquartered in Parsippany, New Jersey, United States, with operations in over 25 countries. Dialogic provides a cloud-optimized communications technology for real-time communications me ...

for use in voicemail and similar systems. As this was the most common use for the file format, it became known as "Dialogic ADPCM." Some early BlackBerry phones that don't support MP3 format (e.g. 7100) used that codec for sound files which had ADP filetype extension. These must be of max filesize of 128 Kbytes or less. ADP filetype extension were being just renamed VOX filetype extension. The AD4 extension is also used for files compressed using "Dialogic ADPCM" with a sample rate of 36000 Hz. This means that .ad4 files can be decoded if imported as VOX ADPCM with a sample rate set to 36 kHz on software that allows such settings, like e.g. Audacity.

References

(Dialogic Corp.) - PDF

(Dialogic ADPCM Algorithm) - PDF
Digital container formats {{software-type-stub