Sample data

The data chunk holds the audio. It has no header of its own: how to read it comes entirely from the format chunk.

Frames and interleaving

A frame is one sample for every channel at the same moment. Frames follow one another in time order, and inside a frame the channels are interleaved: left then right for stereo, or the order of the channel mask for more channels.

Three numbers follow from the format chunk:

bytes per frame is nBlockAlign, which is nChannels × wBitsPerSample / 8 for PCM and float
the number of frames is the data chunk’s size divided by nBlockAlign
the length in seconds is the number of frames divided by nSamplesPerSec

Sample c of frame f starts at the data start plus f × nBlockAlign plus c × wBitsPerSample / 8.

Sample encodings

The format code and bits per sample together decide how each sample is stored. All multi-byte values are little-endian.

Bits	Format code	Stored as	Silence	Range
8	1	unsigned integer	128	0 to 255
16	1	signed integer	0	−32,768 to 32,767
24	1 or `0xFFFE`	signed integer, 3 bytes	0	−8,388,608 to 8,388,607
32	1 or `0xFFFE`	signed integer	0	−2,147,483,648 to 2,147,483,647
32	3 or `0xFFFE`	IEEE 754 single	0.0	−1.0 to 1.0, louder values allowed
64	3 or `0xFFFE`	IEEE 754 double	0.0	−1.0 to 1.0, louder values allowed
8	6 or 7	A-law or µ-law code	code for 0	a 13 or 14-bit value, compressed

8-bit PCM is the one unsigned case: subtract 128 to get a signed value. Every larger integer size is signed two’s complement.

Bit depths in between

A sample always fills whole bytes. A 20-bit recording is stored in 24-bit slots, with wBitsPerSample set to 24. An extensible format chunk then sets wValidBitsPerSample to 20. The real bits sit at the top of each slot and the unused low bits are zero, so a reader that ignores the valid bit count still plays the audio at the right level.

A worked example

The four frames at the end of the complete file example: 16-bit stereo, so 4 bytes per frame.

0000002C  00 00 00 00 FF 7F 01 80 FF FF 01 00 00 01 00 FF

Four stereo frames, with each left sample marked.

Frame	Left	Right
0	`00 00` = 0	`00 00` = 0
1	`FF 7F` = 32,767, the loudest positive value	`01 80` = −32,767
2	`FF FF` = −1	`01 00` = 1
3	`00 01` = 256	`00 FF` = −256

Compressed formats

For ADPCM and other codecs, the data chunk is a run of blocks of nBlockAlign bytes, each decoding to a fixed number of frames given in the codec’s extra format data. The number of frames cannot be worked out from the size, so these files carry a fact chunk with the frame count.

Padding

When the data chunk’s size is odd, as with 8-bit mono or 24-bit mono audio with an odd number of frames, one zero pad byte follows the samples. It is not part of the audio and is not counted in the chunk size.