Wave Formats

Audio formats are described by the WAVEFORMATEX structure, but the polymorphic nature of this format encompasses several other structures as well.

WAVEFORMATEX

The 'generic' structure of a WAVEFORMATEX is as follows (18 bytes), and is the modern header structure used to encode audio data. This header can be interpreted as other structures based on the wFormatTag value, and can encode variable-length header extensions.

WORD  wFormatTag;
WORD  nChannels;
DWORD nSamplesPerSec;
DWORD nAvgBytesPerSec;
WORD  nBlockAlign;
WORD  wBitsPerSample;
WORD  cbSize;

XAudio2 supports WAVE_FORMAT_PCM, WAVE_FORMAT_IEEE_FLOAT, WAVE_FORMAT_ADPCM, WAVE_FORMAT_WMAUDIO2, and WAVE_FORMAT_WMAUDIO3. On Xbox One, it also supports WAVE_FORMAT_XMA2.

WAVEFORMAT

The simplest kind of wave format is a partial version of WAVEFORMATEX (it's only 14 instead of 18 bytes) that was the original polymorphic header for audio. This structure is largely considered deprecated in favor of WAVEFORMATEX, and is only used as part of the structure definition for some older formats.

WORD  wFormatTag;
WORD  nChannels;
DWORD nSamplesPerSec;
DWORD nAvgBytesPerSec;
WORD  nBlockAlign;

PCMWAVEFORMAT

The next simplest form is another partial WAVEFORMATEX version using an older structure (it's only 16 bytes instead of 18 bytes). This is commonly used to encode PCM format data using WAVE_FORMAT_PCM or WAVE_FORMAT_IEEE_FLOAT, although it's recommended you use a complete WAVEFORMATEX instead.

WAVEFORMAT wf;
WORD       wBitsPerSample;

XAudio2 only supports wBitsPerSample values of 8, 16, 24, or 32 for WAVE_FORMAT_PCM.

XAudio2 only supports a wBitsPerSample of 32 for WAVE_FORMAT_IEEE_FLOAT.

Generally for handling legacy .wav files, you allow the header format structure to be only 16 bytes long for the WAVE_FORMAT_PCM or WAVE_FORMAT_IEEE_FLOAT tags and just assume cbSize is 0.

ADPCMWAVEFORMAT

A common compressed encoding for audio data is ADPCM which is indicated with WAVE_FORMAT_ADPCM.

WAVEFORMATEX    wfx;
WORD            wSamplesPerBlock;
WORD            wNumCoef;
ADPCMCOEFSET    aCoef[];

XAudio2 only supports Microsoft ADPCM which specifically contains 32 extra bytes (MSADPCM_FORMAT_EXTRA_BYTES) in the wfx.cbSize. It has a wSamplesPerBlock of 4 (MSADPCM_BITS_PER_SAMPLE), and uses 7 (MSADPCM_NUM_COEFFICIENTS) fixed coefficient pairs:

{ 256, 0 }, { 512, -256 }, { 0, 0 }, { 192, 64 }, { 240, 0 }, { 460, -208 }, { 392, -232 }

WAVEFORMATEXTENSIBLE

A more complicated version of WAVEFORMATEX is this extensible structure indicated by WAVE_FORMAT_EXTENSIBLE with a cbSize of 22 for a total of 40 bytes. It encodes the format as a GUID allowing for custom audio formats to be easily encoded.

WAVEFORMATEX    Format;
union {
    WORD wValidBitsPerSample;
    WORD wSamplesPerBlock;
    WORD wReserved;
} Samples;
DWORD           dwChannelMask;
GUID            SubFormat;

The most common use of this format is for encoding multi-channel audio since it provides an explicit dwChannelMask. The standard WAVE_FORMAT_ tags are encoded in a WAVEFORMATEXTENSIBLE using 'well-known' GUIDs of the following form:

{ formatTag, 0x0000, 0x0010, 0x80, 0x00, 0x00, 0xAA, 0x00, 0x38, 0x9B, 0x71 }

Here's example code for finding the standard format tag that handles WAVEFORMATEXTENSIBLE:

inline uint32_t GetFormatTag(const WAVEFORMATEX* wfx)
{
    if (wfx->wFormatTag == WAVE_FORMAT_EXTENSIBLE)
    {
        if (wfx->cbSize < (sizeof(WAVEFORMATEXTENSIBLE) - sizeof(WAVEFORMATEX)))
            return 0;

        static const GUID s_wfexBase = { 0x00000000, 0x0000, 0x0010, 0x80, 0x00, 0x00, 0xAA, 0x00, 0x38, 0x9B, 0x71 };

        auto wfex = reinterpret_cast<const WAVEFORMATEXTENSIBLE*>(wfx);

        if (memcmp(reinterpret_cast<const BYTE*>(&wfex->SubFormat) + sizeof(DWORD),
            reinterpret_cast<const BYTE*>(&s_wfexBase) + sizeof(DWORD), sizeof(GUID) - sizeof(DWORD)) != 0)
        {
            return 0;
        }

        return wfex->SubFormat.Data1;
    }
    else
    {
        return wfx->wFormatTag;
    }
}

XMA2WAVEFORMATEX

The Xbox One supports a custom hardware-compressed audio format known as XMA and is indicated using a custom variant of WAVEFORMATEX with the WAVE_FORMAT_XMA2 (0x166). It has a wfx.cbSize value of 34 for a total of 52 bytes.

WAVEFORMATEX wfx;
WORD  NumStreams;
DWORD ChannelMask;
DWORD SamplesEncoded;
DWORD BytesPerBlock;
DWORD PlayBegin;
DWORD PlayLength;
DWORD LoopBegin;
DWORD LoopLength;
BYTE  LoopCount;
BYTE  EncoderVersion;
WORD  BlockCount;

Older versions of the Xbox supported XMA v1 indicated with WAVE_FORMAT_XMA (0x165).