.WAV File Format – Format chunk

Fields

Offsets are from the start of the chunk’s data, after the eight-byte chunk header. All numbers are big-endian.

The format chunk of the SciPy float file on the RIFX chunks page reads 00 03 (float), 00 02 (2 channels), 00 00 AC 44 (44,100 Hz), 00 05 62 20 (352,800 bytes a second), 00 08, 00 20 (32 bits) and 00 00 (cbSize 0). 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.

The 1994 revision 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; SciPy’s float file follows this. 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.

Format codes

The format codes are Microsoft’s, listed in its mmreg.h header and, as of 1998, in RFC 2361. They mean the same in a RIFX file, stored big-endian. These are the ones commonly met in WAV files; in practice RIFX files hold PCM or float.

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.

McGill’s notes 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 should be used for PCM above 16 bits, for more than two channels, for a container larger than the bit depth, and whenever the speakers need naming. No extensible RIFX file was among the files examined for this site.

Channel mask

Each set bit of dwChannelMask, a big-endian u32, 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.

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. In a RIFX file the 5.1 mask is stored 00 00 00 3F.

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 a 32-bit field, two 16-bit fields and eight single bytes. In a RIFF file the first three fields are little-endian. In a RIFX file libsndfile reads them in the file’s byte order, big-endian, and the explorer does the same, so PCM is stored 00 00 00 01 00 00 00 10 80 00 00 AA 00 38 9B 71. No RIFX file with an extensible format chunk was seen, so this follows libsndfile rather than files. Ambisonic B-format, as libsndfile names 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. Here is the format chunk of a mono 8,000 Hz Microsoft ADPCM sample with 256-byte blocks, written in RIFX byte order for this page; no RIFX ADPCM file was seen.

00000000  66 6D 74 20 00 00 00 32 00 02 00 01 00 00 1F 40  fmt ...2.......@
00000010  00 00 10 00 01 00 00 04 00 20 01 F4 00 07 01 00  ......... ......
00000020  00 00 02 00 FF 00 00 00 00 00 00 C0 00 40 00 F0  .............@..
00000030  00 00 01 CC FF 30 01 88 FF 18                    .....0....

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 (01 F9), 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. Its fields are big-endian in a RIFX file, as the explorer reads them:

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

An example

The format chunk of a 24-bit, 48 kHz, 5.1 surround file, written in RIFX byte order for this page.

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