wFormatTag
Waveform-audio
format type. Format tags are registered with Microsoft Corporation for
many compression algorithms. A complete list of format tags can be
found in the Mmreg.h header file. For one- or two-channel PCM data,
this value should be WAVE_FORMAT_PCM. When this structure is included
in a WAVEFORMATEXTENSIBLE structure, this value must be WAVE_FORMAT_EXTENSIBLE.
nChannels
Number of channels in the waveform-audio data. Monaural data uses one channel and stereo data uses two channels.
nSamplesPerSec
Sample rate, in samples per second (hertz). If wFormatTag is WAVE_FORMAT_PCM, then common values for nSamplesPerSec
are 8.0 kHz, 11.025 kHz, 22.05 kHz, and 44.1 kHz. For non-PCM formats,
this member must be computed according to the manufacturer's
specification of the format tag.
nAvgBytesPerSec
Required average data-transfer rate, in bytes per second, for the format tag. If wFormatTag is WAVE_FORMAT_PCM, nAvgBytesPerSec should be equal to the product of nSamplesPerSec and nBlockAlign. For non-PCM formats, this member must be computed according to the manufacturer's specification of the format tag.
nBlockAlign
Block alignment, in bytes. The block alignment is the minimum atomic unit of data for the wFormatTag format type. If wFormatTag is WAVE_FORMAT_PCM or WAVE_FORMAT_EXTENSIBLE, nBlockAlign must be equal to the product of nChannels and wBitsPerSample
divided by 8 (bits per byte). For non-PCM formats, this member must be
computed according to the manufacturer's specification of the format
tag.
Software must process a multiple of nBlockAlign
bytes of data at a time. Data written to and read from a device must
always start at the beginning of a block. For example, it is illegal to
start playback of PCM data in the middle of a sample (that is, on a
non-block-aligned boundary).
wBitsPerSample
Bits per sample for the wFormatTag format type. If wFormatTag is WAVE_FORMAT_PCM, then wBitsPerSample
should be equal to 8 or 16. For non-PCM formats, this member must be
set according to the manufacturer's specification of the format tag. If
wFormatTag is WAVE_FORMAT_EXTENSIBLE, this value can be any
integer multiple of 8 and represents the container size, not
necessarily the sample size; for example, a 20-bit sample size is in a
24-bit container. Some compression schemes cannot define a value for wBitsPerSample, so this member can be 0.
cbSize
Size, in bytes, of extra format information appended to the end of the WAVEFORMATEX structure. This information can be used by non-PCM formats to store extra attributes for the wFormatTag. If no extra information is required by the wFormatTag,
this member must be set to 0. For WAVE_FORMAT_PCM formats (and only
WAVE_FORMAT_PCM formats), this member is ignored. When this structure
is included in a WAVEFORMATEXTENSIBLE structure, this value must be at least 22.
Remarks
An example of a format that uses extra information is the Microsoft Adaptive Delta Pulse Code Modulation (MS-ADPCM) format. The wFormatTag for MS-ADPCM is WAVE_FORMAT_ADPCM. The cbSize
member will typically be set to 32. The extra information stored for
WAVE_FORMAT_ADPCM is coefficient pairs required for encoding and
decoding the waveform-audio data.
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