IMA ADPCM (Adaptive Delta Pulse Code Modulation)

PlayIt supports 2, 3 and 4-bit IMA ADPCM. Only 4-bit is actually standard but the others are simple dervatives that are used - albeit rarely. The decoding of these less non-standard variants are compatible with files generated by Syntrillium's CoolEdit for Windows.

4 bit

This is the standard bit-size which produces good quality 1:4 compression of 16-bit data.

In mono mode there are little endian (mode &11) and big endian (mode &13) variants which determine whether the lower 4 bits precede or follow the upper 4 in the sample stream.

In stereo mode PlayIt ignores the endian mode (assuming little-endian) and the interleaf is not as general as uncompressed formats. Only 1, 2, 4, 8 and 16 frame interleaves are supported.

3 bit

PlayIt only works in little-endian mode for this format and interleaf is presently ignored to implement the weird requirements of the Wave implemention which is interleaved by words instead of frames.

2 bit

This non-standard variant is lowish in quality (but not bad) and is useful if compression ratio is more important. Little endian and big endian modes are both supported in both mono and stereo modes. Stereo interleaving is 4 times the value specified to simplify byte alignment (ie it works out as being a byte interleaf rather than a frame interleaf). eg Wave requires 16-frame interleaving which is specified as 4.

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