MacOS recording audio - chunking 4096 frames

Ken Thomases ken at
Wed Jan 11 19:43:54 CST 2012

On Jan 10, 2012, at 7:36 AM, Joerg-Cyril.Hoehle at wrote:

> looking at mmdevapi/tests/capture logs, I found 2 issues
> 1. the 5 buffers initially sent are all returned with 0 bytes
> So I added a loop to return empty buffers to the audio input queue,
> such that GetBuffer returns data when GetCurrentPadding shows it.
> But why were these buffers returned at all?

Don't know off-hand.  I haven't looked, but could we be calling AudioQueuePrime() for the capture case?  We shouldn't be and who knows what it does.

Alternatively, are we calling AudioQueueFlush() or AudioQueueStop() after enqueuing the buffers?

> 2. Using a ~500ms mmdevapi duration, Wine gave MacOS CoreAudio 5 buffers
>  of ~4399 frames each. However, MacOS solely returned 4096 frames.
> Is anybody aware of such chunking?

Again, not off-hand, but it doesn't surprise me that it would have an internal buffer size and prefer to return whole buffers.  See, for example, the device property kAudioDevicePropertyBufferFrameSize.


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