Westwood IMA ADPCM
- Company: Westwood Studios
Many games published by Westwood Studios use VQA files to transport movie animations and AUD files to transport audio clips. Such titles include the Command & Conquer and Lands of Lore series. Westwood Studios multimedia files store audio using the standard IMA ADPCM algorithm.
VQA is a tagged format with different chunks marked by fourccs. A 'SND2' chunk contains IMA ADPCM nibbles. There is no chunk preamble that specifies initial predictor and index. The predictor and index variables are both initialized to 0 when file playback is started and maintained across chunks. This makes random seeking through Westwood Studios multimedia files quite difficult.
If the audio is mono data, the low nibble is decoded first (bits 3-0) then the high nibble:
byte0 byte1 byte2 byte3 ... n1n0 n3n2 n5n4 n7n6 ...
If the audio is stereo data, left and right bytes are interleaved. Each byte represents 2 samples for either the left of channel:
byte0 byte1 byte2 byte3 ... L1L0 R1R0 L3L2 R3R2 ...