signal-processing - 了解远程 I/O AudioStreamBasicDescription (ASBD)

Question

我需要帮助理解以下 ASBD。这是分配给 RemoteIO 的新实例的默认 ASBD (我通过在 AudioUnitGetProperty(..., kAudioUnitProperty_StreamFormat, ...) 音频单元上执行 RemoteIO 得到它，就在分配和初始化它之后)。

Float64 mSampleRate        44100
UInt32  mFormatID          1819304813
UInt32  mFormatFlags       41
UInt32  mBytesPerPacket    4
UInt32  mFramesPerPacket   1
UInt32  mBytesPerFrame     4
UInt32  mChannelsPerFrame  2
UInt32  mBitsPerChannel    32
UInt32  mReserved          0

问题是，不应该 mBytesPerFrame是 8 ?如果每个 channel 有 32 位(4 字节)，每帧有 2 个 channel ，那么每帧不应该是 8 字节长(而不是 4 字节)吗？

提前致谢。

Answer 1

mBytesPerFrame的值取决于 mFormatFlags .来自 CoreAudioTypes.h:

Typically, when an ASBD is being used, the fields describe the complete layout
of the sample data in the buffers that are represented by this description -
where typically those buffers are represented by an AudioBuffer that is
contained in an AudioBufferList.

However, when an ASBD has the kAudioFormatFlagIsNonInterleaved flag, the
AudioBufferList has a different structure and semantic. In this case, the ASBD
fields will describe the format of ONE of the AudioBuffers that are contained in
the list, AND each AudioBuffer in the list is determined to have a single (mono)
channel of audio data. Then, the ASBD's mChannelsPerFrame will indicate the
total number of AudioBuffers that are contained within the AudioBufferList -
where each buffer contains one channel. This is used primarily with the
AudioUnit (and AudioConverter) representation of this list - and won't be found
in the AudioHardware usage of this structure.