AN3225 datasheet(4/13 Pages) STMICROELECTRONICS

4 / 13 page

ADPCM codec

AN3225

4/13

Doc ID 17566 Rev 3

1.3

IMA ADPCM decompression

The IMA ADPCM decompression algorithm compresses samples which are stored as 4-bit

signed two’s complement values. The result is decompressed samples in linear 16-bit two’s

complement format.

The input sample (Sq) is dequantized using an inverse quantizer with an adapted step (q) to

obtain a difference (Sd). The step size is adapted by using a lookup table approximating to

an exponential function. The vector pointing to the lookup table is adjusted according to the

magnitude of Sq. To reduce quantization errors, one half of the step size is always added or

subtracted to Sd. The resulting decompressed linear sample is the sum of Sd and the

predicted sample (Sp). As Sp is a simple zero order hold function, Sd is simply added to the

previous output value. The decoder preconditions are that Sp is cleared and that the

quantization step is set to the smallest one.

Figure 1.

ADPCM decoder

AN3225 Datasheet(PDF) 4 Page - STMicroelectronics