Sound Data Chunk

The Sound Data Chunk contains the actual sample frames.

#define SoundDataID 'SSND' /* ckID for Sound Data Chunk */

typedef struct {

ID ckID;

long ckSize;

unsigned long offset;

unsigned long blockSize;

unsigned char soundData[];

ckID is always 'SSND'. 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.

offset determines where the first sample frame in the soundData starts. offset is in bytes. Most applications won't use offset and should set it to zero. Use for a non-zero offset is explained in the Block-Aligning Sound Data section below.

blockSize is used in conjunction with offset for block-aligning sound data. It contains the size in bytes of the blocks that sound data is aligned to. As with offset, most applications won't use blockSize and should set it to zero. More information on blockSize is in the Block-Aligning Sound Data section below.

soundData contains the sample frames that make up the sound. The number of sample frames in the soundData is determined by the numSampleFrames parameter in the Common Chunk.

Sample Points

Each sample point in a sample frame is a linear, 2's complement value. The sample points are from 1 to 32 bits wide, as determined by the sampleSize parameter in the Common Chunk. Sample points are stored in an integral number of contiguous bytes. One to 8 bit wide sample points are stored in one byte, 9 to 16 bit wide sample points are stored in two bytes, 17 to 24 bit wide sample points are stored in 3 bytes, and 25 to 32 bit wide samples are stored in 4 bytes. When the width of a sample point is less than a multiple of 8 bits, the sample point data is left justified, with the remaining bits zeroed. An example case is illustrated below. A 12 bit sample point, binary 101000010111, is stored left justified in two bytes. The remaining bits are set to zero.

Sample Frames

Sample frames are stored contiguously in order of increasing time. The sample points within a sample frame are packed together, there are no unused bytes between them. Likewise, the sample frames are packed together with no pad bytes.

Block-Aligning Sound Data

There may be some applications that, to insure real time recording and playback of audio, wish to align sampled sound data with fixed-size blocks. This can be accomplished with the offset and blockSize parameters, as shown below.

In the above figure, the first sample frame starts at the beginning of block N. This is accomplished by skipping the first offset bytes of the soundData. Note too that the soundData array can extend beyond valid sample frames, allowing the soundData array to end on a block boundary.

blockSize specifies the size in bytes of the block that is to be aligned to. A blockSize of zero indicates that the sound data does not need to be block-aligned. Applications that don't care about block alignment should set blockSize and offset to zero when writing Audio IFF files. Applications that write block-aligned sound data should set blockSize to the appropriate block size. Applications that modify an existing Audio IFF file should try to preserve alignment of the sound data, although this is not required. If an application doesn't preserve alignment, it should set blockSize and offset to zero. If an application needs to realign sound data to a different sized block, it should update blockSize and offset accordingly.

The Sound Data Chunk is required unless the numSampleFrames field in the Common Chunk is zero. A maximum of one Sound Data Chunk can appear in a FORM AIFF.

[Index | Previous Paragraph | Next Paragraph ]