WAVEFORMATEXTENSIBLE

The WAVEFORMATEXTENSIBLE structure defines the format of waveform-audio data for formats having more than two channels or higher sample resolutions than allowed by WAVEFORMATEX. It can also be used to define any format that can be defined by WAVEFORMATEX.

Syntax

typedef struct {
  WAVEFORMATEX  Format;
  union {
    WORD  wValidBitsPerSample;
    WORD  wSamplesPerBlock;
    WORD  wReserved;
  } Samples;
  DWORD   dwChannelMask; 
  GUID    SubFormat;
} WAVEFORMATEXTENSIBLE, *PWAVEFORMATEXTENSIBLE;

Members

Format

WAVEFORMATEX structure that specifies the basic format. The wFormatTag member must be WAVE_FORMAT_EXTENSIBLE. The cbSize member must be at least 22.

wValidBitsPerSample

Number of bits of precision in the signal. Usually equal to WAVEFORMATEX.wBitsPerSample. However, wBitsPerSample is the container size and must be a multiple of 8, whereas wValidBitsPerSample can be any value not exceeding the container size. For example, if the format uses 20-bit samples, wBitsPerSample must be at least 24, but wValidBitsPerSample is 20.

wSamplesPerBlock

Number of samples contained in one compressed block of audio data. This value is used in buffer estimation. This value is used with compressed formats that have a fixed number of samples within each block. This value can be set to 0 if a variable number of samples is contained in each block of compressed audio data. In this case, buffer estimation and position information needs to be obtained in other ways.

wReserved

Reserved for internal use by operating system. Set to 0.

dwChannelMask

Bitmask specifying the assignment of channels in the stream to speaker positions.

SubFormat

Subformat of the data, such as KSDATAFORMAT_SUBTYPE_PCM. The subformat information is similar to that provided by the tag in the WAVEFORMATEX structure's wFormatTag member.

Remarks

WAVEFORMATEXTENSIBLE can describe any format that can be described by WAVEFORMATEX, but provides additional support for more than two channels, for greater precision in the number of bits per sample, and for new compression schemes.

WAVEFORMATEXTENSIBLE can safely be cast to WAVEFORMATEX, because it simply configures the extra bytes specified by WAVEFORMATEX.cbSize.

The dwChannelMask member specifies which channels are present in the multichannel stream. The least significant bit corresponds with the front left speaker, the next least significant bit corresponds to the front right speaker, and so on. The bits, in order of significance, are defined as follows.

The channels specified in dwChannelMask must be present in the prescribed order (from least significant bit up). For example, if only SPEAKER_FRONT_LEFT and SPEAKER_FRONT_RIGHT are specified, then the samples for the front left speaker must come first in the interleaved stream. The number of bits set in dwChannelMask should be the same as the number of channels specified in WAVEFORMATEX.nChannels.

For backward compatibility, any wave format that can be specified by a stand-alone WAVEFORMATEX structure can also be defined by a WAVEFORMATEXTENSIBLE structure. Thus, every wave-format tag in mmreg.h has a corresponding SubFormat GUID. The following table shows some typical wave-format tags and their corresponding SubFormat GUIDs. These GUIDs are defined in Ksmedia.h.

Because WAVEFORMATEXTENSIBLE is an extended version of WAVEFORMATEX, it can describe additional formats that cannot be described by WAVEFORMATEX alone. Vendors are free to define their own SubFormat GUIDs to identify proprietary formats for which no wave-format tags exist.

The following structures, for particular extended formats, are defined as WAVEFORMATEXTENSIBLE.

Requirements

**  Windows NT/2000/XP:** Included in Windows XP and later.
**  Windows 95/98/Me:** Not included.
**  Header:** Declared in Mmreg.h.

See Also

Waveform Audio, Waveform Structures