Realtime audio processing without output

Question

I'm looking on this example http://teragonaudio.com/article/How-to-do-realtime-recording-with-effect-processing-on-iOS.html and i want to turn off my output. I try to change: `kAudioSessionCategory_PlayAndRecord` to `kAudioSessionCategory_RecordAudio` but this is not working. I also try to get rid off: ``` if(AudioUnitSetProperty(*audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 1, &streamDescription, sizeof(streamDescription)) != noErr) { return 1; } ``` Becouse i want to get sound from microphone but not playing it. But not matter what i do when my sound get to renderCallback method there is a -50 error. When audio is automatically play on output everything works fine... Update with code: ``` using namespace std; AudioUnit *audioUnit = NULL; float *convertedSampleBuffer = NULL; int initAudioSession() { audioUnit = (AudioUnit*)malloc(sizeof(AudioUnit)); if(AudioSessionInitialize(NULL, NULL, NULL, NULL) != noErr) { return 1; } if(AudioSessionSetActive(true) != noErr…

Accepted Answer

If you want only recording, no playback, simply comment out the line that sets renderCallback:

```
AURenderCallbackStruct callbackStruct;
callbackStruct.inputProc = renderCallback; // Render function
callbackStruct.inputProcRefCon = NULL;
if(AudioUnitSetProperty(*audioUnit, kAudioUnitProperty_SetRenderCallback,
   kAudioUnitScope_Input, 0, &callbackStruct,
   sizeof(AURenderCallbackStruct)) != noErr) {
  return 1;
}
```

Update after seeing code:

As I suspected, you're missing input callback. Add these lines:

```
// at top:
#define kInputBus 1

AURenderCallbackStruct callbackStruct;
/**/
callbackStruct.inputProc = &ALAudioUnit::recordingCallback;
callbackStruct.inputProcRefCon = this;
status = AudioUnitSetProperty(audioUnit,
                              kAudioOutputUnitProperty_SetInputCallback,
                              kAudioUnitScope_Global,
                              kInputBus,
                              &callbackStruct,
                              sizeof(callbackStruct));
```

Now in your recordingCallback:

```
OSStatus ALAudioUnit::recordingCallback(void *inRefCon,
                                        AudioUnitRenderActionFlags *ioActionFlags,
                                        const AudioTimeStamp *inTimeStamp,
                                        UInt32 inBusNumber,
                                        UInt32 inNumberFrames,
                                        AudioBufferList *ioData)
{
    // TODO: Use inRefCon to access our interface object to do stuff
    // Then, use inNumberFrames to figure out how much data is available, and make
    // that much space available in buffers in an AudioBufferList.

// Then:
    // Obtain recorded samples

OSStatus status;

ALAudioUnit *pThis = reinterpret_cast<ALAudioUnit*>(inRefCon);
    if (!pThis)
        return noErr;

//assert (pThis->m_nMaxSliceFrames >= inNumberFrames);

pThis->recorderBufferList->GetBufferList().mBuffers[0].mDataByteSize = inNumberFrames * pThis->m_recorderSBD.mBytesPerFrame;

status = AudioUnitRender(pThis->audioUnit,
                             ioActionFlags,
                             inTimeStamp,
                             inBusNumber,
                             inNumberFrames,
                             &pThis->recorderBufferList->GetBufferList());
    THROW_EXCEPTION_IF_ERROR(status, "error rendering audio unit");

// If we're not playing, I don't care about the data, simply discard it
    if (!pThis->playbackState || pThis->isSeeking) return noErr;

// Now, we have the samples we just read sitting in buffers in bufferList
    pThis->DoStuffWithTheRecordedAudio(inNumberFrames, pThis->recorderBufferList, inTimeStamp);

return noErr;
}
```

Btw, I'm allocating my own buffer instead of using the one provided by AudioUnit. You might want to change those parts if you want to use AudioUnit allocated buffer.

Update:

How to allocate own buffer:

```
recorderBufferList = new AUBufferList();
recorderBufferList->Allocate(m_recorderSBD, m_nMaxSliceFrames);
recorderBufferList->PrepareBuffer(m_recorderSBD, m_nMaxSliceFrames);
```

Also, if you're doing this, tell AudioUnit to not allocate buffers:

```
// Disable buffer allocation for the recorder (optional - do this if we want to pass in our own)
flag = 0;
status = AudioUnitSetProperty(audioUnit,
                              kAudioUnitProperty_ShouldAllocateBuffer,
                              kAudioUnitScope_Input,
                              kInputBus,
                              &flag,
                              sizeof(flag));
```

You'll need to include CoreAudio utility classes

Realtime audio processing without output

Solution

Problem