iOS麦克风捕获LPCM在8 kHz的采样率下不起作用

Question

I am using AudioComponentInstance audioUnit to record microphone on iOS, Liniar PCM, with various sample rates (configurable from app). The weird part is, it is working fine for 11025, 22050 and 44100, but is not working for 8000. It does not provide any error or something, I just don't receive anything in the callback. Also, tested with 9600 and it is not working.

Does anybody know a reason for this? I start the session with

status = AudioUnitInitialize(audioUnit);
status = AudioOutputUnitStart(audioUnit);

If needed I can provide more (all) the code, but I don't think this is related to the code.

UPDATE

After some more testing I found out that the size of the buffer I use in the recording callback was a problem. In the callback I call

    status = AudioUnitRender([instance audioUnit],
                             ioActionFlags,
                             inTimeStamp,
                             inBusNumber,
                             inNumberFrames,
                             entry->bufferList);

It seems the inNumberFrames is 186 or 185, and I created a buffer with 128. But now it is stil failing 4 out of 5 times with status -50. I set the buffer size to 256, 512, 186 but the behaviour is the same. The outcome is that the recording is distorted, as frames are lost.

Answer 1

Well, found out the answer myself. For some obscure reasons, 186 frames reported to be ready by the callback was one too many. After I replaced the value with 185 in the call of AudioUnitRender() it worked like a charm. It seems there is a bug and somethimes CoreAudio reports an extra frame as ready.

For higher frequencies the problem did not exist, as the number of frames was always constant and power of 2 (256, 512, 1024)