Common Chunk

The Common Chunk describes fundamental parameters of the sampled sound.

#define CommonID 'COMM' /* ckID for Common Chunk */

typedef struct {

ID ckID;

long ckSize;

short numChannels;

unsigned long numSampleFrames;

short sampleSize;

extended sampleRate;

ckID is always 'COMM'. ckSize is the size of the data portion of the chunk, in bytes. It does not include the 8 bytes used by ckID and ckSize. For the Common Chunk, ckSize is always 18.

numChannels contains the number of audio channels for the sound. A value of 1 means monophonic sound, 2 means stereo, and 4 means four channel sound, etc. Any number of audio channels may be represented.

The actual sound samples are stored in another chunk, the Sound Data Chunk, which will be described shortly. For multichannel sounds, single sample points from each channel are interleaved. A set of interleaved sample points is called a sample frame. This is illustrated below for the stereo case.

For monophonic sound, a sample frame is a single sample point.

For multichannel sounds, the following conventions should be observed:

numSampleFrames contains the number of sample frames in the Sound Data Chunk. Note that numSampleFrames is the number of sample frames, not the number of bytes nor the number of sample points in the Sound Data Chunk. The total number of sample points in the file is numSampleFrames times numChannels.

sampleSize is the number of bits in each sample point. It can be any number from 1 to 32. The format of a sample point will be described in the next section, the Sound Data Chunk.

sampleRate is the sample rate at which the sound is to be played back, in sample frames per second.

One and only one Common Chunk is required in every FORM AIFF.

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