c++ - 从 IAudioClient::GetMixFormat 解释 WAVEFORMATEXTENSIBLE

Question

我正在学习 Windows 核心音频 API。使用这个 Rendering a Stream例如，我发现 IAudioClient::GetMixFormat返回模棱两可的 WAVEFORMATEXTENSIBLE看起来像这样:

Format
  wFormatTag          = WAVE_FORMAT_EXTENSIBLE (65534)
  nChannels           = 2
  nSamplesPerSec      = 48000
  nAvgBytesPerSec     = 384000
  nBlockAlign         = 8
  wBitsPerSample      = 32
  cbSize              = 22
Samples  // this is a union
  wValidBitsPerSample = 32
  wSamplesPerBlock    = 32
  wReserved           = 32

因为 Format.cbSize 是 22，所以这是整个结构；没有 SubFormat GUID 来提供有关示例的更多信息。这看起来很奇怪，因为格式标记是 WAVE_FORMAT_EXTENSIBLE，我认为这意味着您必须依赖 Subformat GUID 来确定实际格式。

我可以看到有两个 channel 的 32 位样本。如何确定这些样本是整数还是浮点值？

[core-audio标签显然是针对 Apple API 的。 Windows Core Audio 是否有不同的标签？]

Answer 1

22 是 WAVEFORMATEX 之后的附加数据的大小数据:

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.

如果您查看 WAVEFORMATEXTENSIBLE 的定义，它以 WAVEFORMATEX 开头，然后包含额外的数据:

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

WAVEFORMATEX 数据后至少有 22 个字节。

SubFormat 字段包含您要查找的格式值。