Do Audio Queue Services buffers need to be even multiples of packet size?

Question

I'm trying to use Audio Queue Services to play mp3 audio that is being delivered from an external process. I am using NSTask and NSOutputHandle to get the output from the command - that part works fine. I'm using Audio File Stream Services to parse the data from the command - that seems to work as well. In my Audio File Stream Services Listener function, I'm not sure what to do with the packets that come in. It would be great if I could just throw them at the audio queue but apparently it doesn't work that way. You're supposed to define a series of buffers and enqueue them on the audio queue. Can the buffers correspond to the packets or do I have to somehow convert them? I'm not very good at C or pointer math so the idea of converting arbitrary-sized packets to non-matching-sized buffers it kind of scary to me. I've read the Apple docs many times but it only covers reading from a file, which seems to skip this whole packet/buffer conversion step.

Answer 1

You should be able to configure the AudioQueue such that the buffer sizes match your packet sizes. Additionally, the AudioQueue will do the job of decoding the mp3 - you shouldn't need to do any of your own conversions.

Use the inBufferByteSize parameter to configure the buffer size:

OSStatus AudioQueueAllocateBuffer (
    AudioQueueRef inAQ,
    UInt32 inBufferByteSize,
    AudioQueueBufferRef *outBuffer
);

Answer 2

If your packets are all different sizes, you can use AudioQueueAllocateBuffer to allocate each buffer with that custom size before filling it, and free it instead of re-queueing it after use by the audio queue callback.

For less memory management (which impacts performance), if you know the max packet size, you can allocate a buffer that big, and then only partially fill that buffer (after checking the packet size to make sure it fits). There's a parameter, mAudioDataByteSize, for the amount with which each buffer is actually filled.