Question

In AudioStreamBasicDescription struct:

struct AudioStreamBasicDescription {
    Float64 mSampleRate;
    UInt32  mFormatID;
    UInt32  mFormatFlags;
    UInt32  mBytesPerPacket;
    UInt32  mFramesPerPacket;
    UInt32  mBytesPerFrame;
    UInt32  mChannelsPerFrame;
    UInt32  mBitsPerChannel;
    UInt32  mReserved; 
};

if we know nChannelsPerFrame and mBitsPerChannels, we can calculate mBytesPerFrame like this:
mBytesPerFrame = mBitsPerChannels * mChannelsPerFrame / 8 (correct me if I’m wrong)

I believe that all of the fields exist for some reason. What is it for mBytesPerFrame?

Answer 1

Yes. The field can be used to aid in describing packing and alignment of LPCM sample data/frames. That’s also mandatory for describing some external LPCM representations.

For example, you can specify a 20 bit sample size, but an implementation may favor to represent/transfer/store that using 3 bytes, or 24 bits (ignoring 4 bits per sample).