ADPCM is not a specific sample format but rather a term used to describe a family of compression algorithms which work by predicting the next sample based on previous samples. Usually the first few samples are stored uncompressed to get things going (the actual number depends on how many previous samples the algorithm uses) then the prediction error codes are the data actually stored. ie the next sample is predicted from recent samples and the error code stored in the data stream is used to correct the prediction.
Usually ADPCM formats are arranged in blocks spanning a short period of time to enable a player to jump to an arbitrary point in a long sample file. Since the algorithm requires some raw data to start it must always start from the beginning of such a block.
See also IMA ADPCM for information on PlayIt's support of that format.
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