VoIP limiting the number of frames in rendercallback

Question

I am currently developing a VoIP application and one of the libraries I am using requires me to send the frames in the input call back. The requirements is that I must send a sample count which is defined as the number of samples in a frame. This callback will get called whenever the microphone takes in new samples.

Valid numbers are (sample rate) * (audio length) / 1000. Where audio length can be 2.5, 5, 10, 20, 40 or 60 millseconds.

Before using kAudioUnitProperty_MaximumFramesPerSlice to limit my inNumberFrames to 960, it was bringing in consistently around 1115 inNumberFrames. So I decided to limit it by setting that property. However when I do, it brings the inNumberFrames count down to 512??? Am I approaching this problem the wrong way?

static OSStatus inputRenderCallBack(void *inRefCon,
                                    AudioUnitRenderActionFlags  *ioActionFlags,
                                    const AudioTimeStamp    *inTimeStamp,
                                    UInt32 inBusNumber,
                                    UInt32 inNumberFrames, //512
                                    AudioBufferList *ioData)
{
    AudioBufferList bufferList;
    bufferList.mNumberBuffers = 1;
    bufferList.mBuffers[0].mNumberChannels = kNumberOfChannels;
    bufferList.mBuffers[0].mData = NULL;
    bufferList.mBuffers[0].mDataByteSize = inNumberFrames * sizeof(SInt16) * kNumberOfChannels;

    OCTAudioEngine *engine = (__bridge OCTAudioEngine *)(inRefCon);
    OSStatus status = AudioUnitRender(engine.ioUnit,
                                       ioActionFlags,
                                       inTimeStamp,
                                       inBusNumber,
                                       inNumberFrames,
                                       &bufferList);
    NSError *error;
    [engine.toxav sendAudioFrame:bufferList.mBuffers[0].mData
                     sampleCount:kSampleCount //960
                        channels:kNumberOfChannels //2
                      sampleRate:[engine currentAudioSampleRate] //24000
                        toFriend:engine.friendNumber
                           error:&error];

    return status;
}

What is the reason you are trying to limit the number of frames going in? Most of these issues are better addressed using a circular buffer. Check out this [question](http://stackoverflow.com/questions/30691684/whats-the-reason-of-using-circular-buffer-in-ios-audio-calling-app/30698791#30698791). — dave234, Jun 16 '15 at 12:01
I am trying to see if I can avoid the circular buffer situation in the first place. I already am using one for receiving audio, since it's necessary when the audio is being pulled for playback. — cvu, Jun 16 '15 at 16:25
Why avoid the circular buffer? They're cheap, and solve your problem. — dave234, Jun 16 '15 at 16:28
But wouldn't that mean I have to stuff in a circular buffer during the callback, then have another thread that constantly checks to see if there is enough frames in that circular buffer to consume? Sounds like a mess but guess I'll give it a try. — cvu, Jun 16 '15 at 17:06
It is messier for sure, but that is the nature of multithreaded apps. I believe it is necessary if you require a specific frame count. When you get used to them they're fine. The hardware always wants base 2 frame counts. 1115 frames sounds odd. 1024 is the default. You can get it as low as 128, but it will almost always be a base 2 number at the remoteIO. Bottom line is that the inNumberFrames is not in your control, so you have to give yourself a cushion to work with. If you can do your work on the audio thread you can add to the head, and pull from the tail in render callback too. — dave234, Jun 16 '15 at 17:27
Yep that's what I did! Add to the head, and consumed the tail in the same input callback thread. To avoid buffer overflows, I called `sendAudioFrame` more than once depending on how many frames are in the buffer. Thanks a lot! — cvu, Jun 16 '15 at 18:10
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/80700/discussion-between-cvu-and-dave). — cvu, Jun 16 '15 at 19:27

score 1 · Accepted Answer · answered Jun 16 '15 at 07:29

You can (and should) use kAudioUnitProperty_MaximumFramesPerSlice to specify the maximum number of samples per frame, not the preferred number; please refer to Apple's Technical Q&A QA1533 and QA1606. To set the preferred number of samples per frame, use the setPreferredIOBufferDuration:error: method of AVAudioSession. For example, if the sample rate is 32,000, you can use:

NSError *error = nil;
NSTimeInterval bufferDuration = 0.008;
[session setPreferredIOBufferDuration:bufferDuration error:&error];

to set your frame length to 256, because 32,000 * 0.008 = 256.

Note, however, that the setPreferredIOBufferDuration:error: method sets the preferred number, not the exact number. You can suggest a frame length to the operating system, but sometimes, due to hardware limits, you may not get exactly what you want, but something close to it instead.

VoIP limiting the number of frames in rendercallback

1 Answers1