ADPCM stands for Adaptive Differential Pulse Code Modulation. ADPCM is a lossy compression mechanism. There are various flavors of ADPCM. This particular algorithm was suggested by the International Multimedia Association (IMA). IMA ADPCM compresses data recorded at various sampling rates. Sound is encoded as a succession of 4-bit or 3-bit data packets. Each data packet represents the difference between the current sampled signal value and the previous value. The compression ratio obtained is relatively modest. As an example: 16-bit data samples encoded as 4-bit differences result in 4:1 compression format.
IMA ADPCM is similar to Intel's DVI audio format.
IMA ADPCM is directly supported on most Windows implementations as a native format. Although the quality of IMA ADPCM voice files is not great, the files are portable. There is a real advantage in having compact files that can be played on most Windows PCs.
Windows supports IMA ADPCM ".wav" files in 4-bit format only and at sample rates between 8 and 41KHz.
Vox Studio supports IMA ADPCM ".wav" files in 4-bit or 3-bit format at sample rates from 6KHz to 64KHz.