.WAV File Format – Format chunk

Fields

Offsets are from the start of the chunk’s data, after the eight-byte chunk header; straight after a 36-byte ds64 chunk the data starts at file offset 56. All numbers are little-endian.

The explorer checks the block align and byte rate of PCM, float and G.711 files against the other fields, and reports a channel count or sample rate of 0.

Sizes

The chunk size tells which form a file uses.

Microsoft’s 1994 revision of WAV says every format other than PCM must use the extended chunk, with cbSize present even when it is 0, and must have a fact chunk. Readers should accept every size, read cbSize only when the chunk is long enough to hold it, and not trust a cbSize larger than the chunk; the explorer reports one.

Format codes

Microsoft registered hundreds of codes, listed in its mmreg.h header and, as of 1998, in RFC 2361. These are the ones commonly met in WAV files.

The values of the Ogg Vorbis modes 1 and 3 plus are confirmed in Microsoft’s headers as published in the winapi bindings; the others follow in sequence.

WAVE_FORMAT_EXTENSIBLE

With the format code 0xFFFE, cbSize is 22 and three fields follow. The real format is then the SubFormat GUID. The explorer reads them in a BW64 file as in any WAV file.

McGill’s notes on WAV give the rules: wBitsPerSample must be the container size, 8 × nBlockAlign / nChannels, and a multiple of 8; wValidBitsPerSample is for information, from 1 to the container size. The extensible form is meant for PCM above 16 bits, for more than two channels, for a container larger than the bit depth, and whenever the speakers need naming.

Channel mask

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. The explorer reports a mask whose number of set bits differs from the channel count.

In BW64 the mask matters less. Wikipedia’s account says BW64 no longer relies on these 18 positions, because the ADM metadata in the axml chunk describes the loudspeaker positions, and the chna chunk says which track is which; this could not be checked against the Recommendation.

The short names are libsndfile’s. 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 field is the old format code and the rest is fixed: 0000xxxx-0000-0010-8000-00AA00389B71. Microsoft names them KSDATAFORMAT_SUBTYPE_PCM (1), _IEEE_FLOAT (3), _ALAW (6), _MULAW (7) and _ADPCM (2).

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. Ambisonic B-format, as libsndfile writes it, uses 00000001-0721-11D3-8644-C8C1CA000000 for PCM and 00000003-0721-11D3-8644-C8C1CA000000 for float.

Microsoft ADPCM

Format code 2. cbSize is 32 with the usual seven coefficient pairs.

The standard pairs are (256, 0), (512, −256), (0, 0), (192, 64), (240, 0), (460, −208) and (392, −232). How a block uses them is on Sample data.

IMA ADPCM

Format code 0x11. cbSize is 2, and the one field is wSamplesPerBlock at offset 18: the frames each block decodes to, which must be 2 × (nBlockAlign − 4 × nChannels) / nChannels + 1. libsndfile refuses a file where it is not. A 256-byte mono block gives 505 frames, a 512-byte mono block 1,017 and a 1,024-byte stereo block 1,017. wBitsPerSample is 4. The blocks are described on Sample data.

MPEG

For MP3, format code 0x55, Microsoft’s MPEGLAYER3WAVEFORMAT has cbSize 12 or more:

For MPEG-1 layers 1 and 2, format code 0x50, MPEG1WAVEFORMAT has cbSize 22; wBitsPerSample is 0:

An example

The format chunk of a real RF64 file, a SciPy test file, which a BW64 file of the same audio would carry unchanged: extensible, 1 channel, 32-bit PCM, 44,100 Hz.

00000000  66 6D 74 20 28 00 00 00 FE FF 01 00 44 AC 00 00  fmt (.......D...
00000010  10 B1 02 00 04 00 20 00 16 00 20 00 00 00 00 00  ...... ... .....
00000020  01 00 00 00 00 00 10 00 80 00 00 AA 00 38 9B 71  .............8.q