read this excerpt :
"ADPCM
Adaptive Differential Pulse Code Modulation ADPCM (G.721) allows for the compression of PCM encoded input whose power varies with time. Feedback of a reconstructed version of the input signal is subtracted from the actual input signal, which is then quantised to give a 4 bit output value. This compression gives a 32 kbit/s output rate. This standard was recently extended in G.726 , which replaces both G.721 and G.723 , to allow conversion between 64 kbit/s PCM and 40, 32, 24, or 16 kbit/s channels. G.727 is an extension of G.726 and issued for embedded ADPCM on 40, 32, 24, or 16 kbit/s channels, with the specific intention of being used in packetised speech systems utilizing the Packetized Voice Protocol (PVP), defined in G.764.
The encoding of higher quality speech (50Hz-7kHz) is covered in G.722 and G.725 , and is achieved by utilizing sub-band ADPCM coding on two frequency sub-bands; the output rate is 64 kbit/s. "
o what you are looking for is the ITU-T standard .