ios audio unit remoteIO playback while recording

Question

I have been charged to add VOIP into an game (cross-platform, so can't use the Apple gamekit to do it).

For 3 or 4 days now, i'm trying to get my head wrap around audio unit and remoteIO... I have overlooked tens of examples and such, but every time it is only applying a simple algorithm to the input PCM and play it back on the speaker. According to Apple's documentation in order to do VOIP we should use kAudioSessionCategory_PlayAndRecord.

UInt32 audioCategory = kAudioSessionCategory_PlayAndRecord;

        status = AudioSessionSetProperty(kAudioSessionProperty_AudioCategory, 
                                         sizeof(audioCategory), 
                                         &audioCategory);
        XThrowIfError(status, "couldn't set audio category");

1) But it seems (to me) that playAndRecord will always play what coming from the mic (or more excatly the PerformThru callback // aurioTouch), am I wrong ?

I have the simplest callback, doing nothing but AURender

static OSStatus PerformThru(
                            void                        *inRefCon, 
                            AudioUnitRenderActionFlags  *ioActionFlags, 
                            const AudioTimeStamp        *inTimeStamp, 
                            UInt32                      inBusNumber, 
                            UInt32                      inNumberFrames, 
                            AudioBufferList             *ioData)
{
OSStatus err = AudioUnitRender(THIS->rioUnit, ioActionFlags, inTimeStamp, 1, inNumberFrames, ioData);
    if (err) 
       printf("PerformThru: error %d\n", (int)err);
    return err
}

From that callback I'm intending to send data to the peer (Not directly of course, but data will come from it)...

I do not see how I can play different output than the input, except maybe with 2 units, one recording, one playing, but it doesn't seems to be what Apple intended to (still accroding to the documentation).

And of course, I cannot find any documentation about it, audio unit is still pretty much un-documented...

Anyone would have an idea on what would be the best way to do it ?

Answer 1

I have not used VOIP or kAudioSessionCategory_PlayAndRecord. But if you want to record/transmit voice picked up from the mic and play back incoming data from network packages: Here is a good sample which included both mic and playback. Also if you have not read this doc from Apple, I would strongly recommend this.

In short: You need to create an AudioUnits instance. In it, configure two callbacks: one for mic and one for playback. The callback mic function will supply you the data that was picked up from the mic. You then can convert and transmit to other devices with whatever chosen network protocol. The playback callback function is where you supply the incoming data from other network devices to play back.

Answer 2

You can see this simple example. It describes how to use remote IO unit. After understanding this example, you should watch PJSIP's audio driver. These should help you implementing your own solution. Best of luck.