Reading audio with Extended Audio File Services (ExtAudioFileRead)

Question

I am working on understanding Core Audio, or rather: Extended Audio File Services

Here, I want to use ExtAudioFileRead() to read some audio data from a file

Answer 1

mNumberBuffers == 2 when you're dealing with non-interleaved stereo. It is the number of audio channels when not interleaved.

Answer 2

Cocoa simply seems to not accept mNumberBuffers > 1. Which is a shame as it makes the whole structure useless. (I hope this is right...)

Answer 3

I also had the same problem with mNumberBuffers > 1,,... My work around involved creating my own internal buffer:

something like:

char buffer1[byteSize]; char buffer2[byteSize]; ......

you could also use pointers to make things easier,..like:

buffer[index][byteSize];

then you would have to iterate through them manually and fill them up on the main thread to avoid glitches in audio.

ExtAudioFileRead will only fill buffer[0] in the AudioBufferList, you could then have that pointing to different manually allocated buffers as the audio plays.

hope this helps.

Answer 4

I think you're misunderstanding the purpose of the mNumberBuffers field. It's typically 1 for mono and interleaved stereo data. The only reason you would set it to something else is for multi-track data where each channel is in a separate data buffer.

If you want to read a part of a file, you would set dataByteSize of the buffer to a reasonable size, and when you read the file, tell the API only to give you that many bytes, and loop over it.