Adaptive Differential Pulse Code Modulation (ADPCM), also called Delta Pulse Code Modulation, is a compression coding for a signal, which is based on differential values similar to Differential Pulse Code Modulation (DPCM), and in which the scaling of the quantization stages is additionally adjusted (adapted) depending on the signal curve. ADPCM was developed in 1973 at Bell Laboratories by P. Cummiskey, N. S. Jayant and James L. Flanagan.
In the field of audio signals, ADPCM is used within the framework of various ITU-T standards such as G.726 . The output data rate can be dynamically adjusted between 16 kbit/s and 64 kbit/s in these applications. Another application example is Digital Enhanced Cordless Telecommunications (DECT) as used in cordless telephones.
ADPCM is a pulse code modulation with prediction function. When the signal is processed, an attempt is made to predict the further course of the signal within the next section. For the quantization of the signal in the next time step only the difference between predicted and real signal is used. Due to this difference formation, fewer bits can be used to describe the signal.
With this method, both the prediction function and the quantization level are "adapted" anew with each work step. This control loop provides a better signal prediction than DPCM.
ADPCM is also used for emulating computer and arcade games. See also M.A.M.E. and Irem M-62.
Well-known implementations are the IMA ADPCM or IMADPCM, Microsoft ADPCM or MSADPCM, Microsoft IMA ADPCM and Apple QuickTime IMA ADPCM.
- G.721 (withdrawn) 32 kbit/s adaptive differential pulse code modulation (ADPCM)
- The content of this Recommendation is now covered by Recommendation ITU-T G.726.
- G.726 : 40, 32, 24, 16 kbps adaptive differential pulse code modulation (ADPCM)
- G.727: 5-, 4-, 3- and 2-bit/sample embedded adaptive differential pulse code modulation (ADPCM)
- G.722 : Subband ADPCM for the frequency range from 50 Hz to 7000 Hz with a sampling rate of 16 kHz and quantized with 14 bit