Fmtchunk

Fmtchunk is a chunk used in RIFF-based audio file formats, most notably the WAV format. It contains the essential format information that describes how the audio data in the file should be interpreted. The chunk has an ID of "fmt " (the four ASCII characters, including a trailing space) and a size field that indicates the length of the format data that follows.

In WAV files, the format data is defined by a structure that includes several fields. The format tag (wFormatTag) identifies the encoding of the audio data, with common values such as 1 for PCM and 3 for IEEE floating point. Other fields specify the number of channels (nChannels), the sample rate (nSamplesPerSec), the average bytes per second (nAvgBytesPerSec), the block alignment in bytes (nBlockAlign), and the number of bits per sample (wBitsPerSample). An additional field, cbSize, indicates the size of any extra format data that follows the standard fields. For PCM data, Subchunk1Size is typically 16 and cbSize is 0. For non-PCM formats, or for WAV formats that use extensible structures, there can be extra format data, such as the WAVEFORMATEXTENSIBLE layout, which includes a channel mask and a subformat GUID.

The information in the fmt chunk is used by decoders to allocate buffers, interpret sample data, and convert it to playable form. The fmt chunk is usually followed by the data chunk, which contains the actual audio samples, and together they define how the file’s audio content is stored.