Understanding Remote I/O AudioStreamBasicDescription (ASBD)

Question

I need help understanding the following ASBD. It's the default ASBD assigned to a fresh instance of RemoteIO (I got it by executing AudioUnitGetProperty(..., kAudioUnitProperty_StreamFormat, ...) on the RemoteIO audio unit, right after allocating and initializing it).

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

The question is, shouldn't mBytesPerFrame be 8? If I have 32 bits (4 bytes) per channel, and 2 channels per frame, shouldn't each frame be 8 bytes long (instead of 4)?

Thanks in advance.

score 5 · Answer 1 · answered Sep 07 '12 at 23:37

The value of mBytesPerFrame depends on mFormatFlags. From 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.

score 0 · Answer 2 · answered Sep 03 '12 at 21:29

I believe that because the format flags specify kAudioFormatFlagIsNonInterleaved it follows that the size of a frame in any buffer can only be the size of a 1 channel frame. If this is correct mChannelsPerFrame is certainly a confusing name.

I hope someone else will confirm / clarify this.

Understanding Remote I/O AudioStreamBasicDescription (ASBD)

2 Answers2