Format chunk

The fmt chunk says how the samples in the data chunk are encoded. It comes in three sizes, each extending the one before, and its first field decides how the rest of the file is read.

Fields

Offsets are from the start of the chunk’s data, after the eight-byte chunk header. In the usual 44-byte layout the data starts at file offset 20.

Offset	Size	Type	Name	Description
0	2	u16	`wFormatTag`	the format code: 1 for PCM, 3 for float, `0xFFFE` for extensible
2	2	u16	`nChannels`	the number of channels
4	4	u32	`nSamplesPerSec`	sample frames per second, such as 44,100 or 48,000
8	4	u32	`nAvgBytesPerSec`	bytes per second: `nSamplesPerSec` × `nBlockAlign` for PCM
12	2	u16	`nBlockAlign`	bytes per frame: `nChannels` × bytes per sample for PCM, or the codec’s block size
14	2	u16	`wBitsPerSample`	bits per sample: 8, 16, 24, 32 or 64 for PCM and float
16	2	u16	`cbSize`	the number of extra bytes after this field: 0, 22 for extensible, or a codec’s own data
18	2	u16	`wValidBitsPerSample`	extensible only: how many bits of each sample are used, such as 20 in a 24-bit container
20	4	u32	`dwChannelMask`	extensible only: which speaker each channel feeds, see channel mask
24	16	GUID	`SubFormat`	extensible only: the real format code, inside a GUID, see SubFormat

Three sizes

The chunk size tells which version a file uses.

Size	Structure	Used for
16	`PCMWAVEFORMAT`	plain PCM: the six fields up to `wBitsPerSample`
18 or more	`WAVEFORMATEX`	adds `cbSize`; non-PCM formats should use it, and codecs put their own data after it
40	`WAVEFORMATEXTENSIBLE`	`wFormatTag` is `0xFFFE` and `cbSize` is 22; needed for more than two channels, more than 16 bits, or a channel mask

Readers should accept all three, and should read cbSize only when the chunk is long enough to hold it.

Format codes

Microsoft registered hundreds of codes. These are the ones commonly met in WAV files.

Code	Name	Audio
0x0001 (1)	`WAVE_FORMAT_PCM`	integer PCM
0x0002 (2)	`WAVE_FORMAT_ADPCM`	Microsoft ADPCM
0x0003 (3)	`WAVE_FORMAT_IEEE_FLOAT`	32 or 64-bit floating point
0x0006 (6)	`WAVE_FORMAT_ALAW`	G.711 A-law, 8-bit codes
0x0007 (7)	`WAVE_FORMAT_MULAW`	G.711 µ-law, 8-bit codes
0x0011 (17)	`WAVE_FORMAT_DVI_ADPCM`	IMA ADPCM, 4 bits per sample
0x0031 (49)	`WAVE_FORMAT_GSM610`	GSM 6.10 speech
0x0050 (80)	`WAVE_FORMAT_MPEG`	MPEG audio layer 1 or 2
0x0055 (85)	`WAVE_FORMAT_MPEGLAYER3`	MP3
0xFFFE (65534)	`WAVE_FORMAT_EXTENSIBLE`	see `SubFormat`

Channel mask

In an extensible format chunk, each set bit of dwChannelMask names a speaker. The channels in each frame appear in the order of the set bits, lowest first. A mask of 0 means the channels have no particular speaker.

Bit	Speaker	Bit	Speaker
`0x1`	front left	`0x200`	side left
`0x2`	front right	`0x400`	side right
`0x4`	front centre	`0x800`	top centre
`0x8`	low frequency (LFE)	`0x1000`	top front left
`0x10`	back left	`0x2000`	top front centre
`0x20`	back right	`0x4000`	top front right
`0x40`	front left of centre	`0x8000`	top back left
`0x80`	front right of centre	`0x10000`	top back centre
`0x100`	back centre	`0x20000`	top back right

Common masks are 0x4 for mono, 0x3 for stereo, 0x3F for 5.1 (front left, front right, centre, LFE, back left, back right) and 0x63F for 7.1 with side speakers.

SubFormat

An extensible chunk carries its real format as a GUID. For the classic formats, the first two bytes are the old format code and the rest is fixed: 0000xxxx-0000-0010-8000-00AA00389B71. A GUID is stored with its first three groups little-endian, so on disk PCM is 01 00 00 00 00 00 10 00 80 00 00 AA 00 38 9B 71 and float starts 03 00 instead.

An example

The format chunk of a 24-bit, 48 kHz, 5.1 surround file.

00000000  66 6D 74 20 28 00 00 00 FE FF 06 00 80 BB 00 00  fmt (...........
00000010  00 2F 0D 00 12 00 18 00 16 00 18 00 3F 00 00 00  ./..........?...
00000020  01 00 00 00 00 00 10 00 80 00 00 AA 00 38 9B 71  .............8.q

An extensible format chunk, with offsets from the start of the chunk.

Field	Bytes	Value
`ckSize`	`28 00 00 00`	40
`wFormatTag`	`FE FF`	`0xFFFE`, extensible
`nChannels`	`06 00`	6
`nSamplesPerSec`	`80 BB 00 00`	48,000
`nAvgBytesPerSec`	`00 2F 0D 00`	864,000
`nBlockAlign`	`12 00`	18 bytes per frame: 6 channels × 3 bytes
`wBitsPerSample`	`18 00`	24
`cbSize`	`16 00`	22
`wValidBitsPerSample`	`18 00`	24
`dwChannelMask`	`3F 00 00 00`	5.1
`SubFormat`	16 bytes	PCM