IOS: how to reprogram audio (PCM data) using Audio Unit at runtime?

Question

How can I reprogram the audio (PCM data) using the Audio Unit while running / in real time?

I have an Audio Unit setup as follows.

- (void) setUpAudioUnit {
    OSStatus status;
    AudioComponentInstance audioUnit;
    AudioComponent inputComponent;
    AudioComponentDescription audioComponentDescription;
    AudioStreamBasicDescription audioStreamBasicDescription;

    // Describe audio component
    audioComponentDescription.componentType = kAudioUnitType_Output;
    audioComponentDescription.componentSubType = kAudioUnitSubType_VoiceProcessingIO;
    audioComponentDescription.componentFlags = 0;
    audioComponentDescription.componentFlagsMask = 0;
    audioComponentDescription.componentManufacturer = kAudioUnitManufacturer_Apple;

    // Get component
    inputComponent = AudioComponentFindNext(NULL, &audioComponentDescription);

    // Get audio units
    status = AudioComponentInstanceNew(inputComponent, &audioUnit);
    checkStatus(status);

    // Enable IO for recording
    UInt32 flag = 1;
    status = AudioUnitSetProperty(audioUnit,
                                  kAudioOutputUnitProperty_EnableIO,
                                  kAudioUnitScope_Input,
                                  kInputBus,
                                  &flag,
                                  sizeof(flag));
    checkStatus(status);

    // Enable IO for playback
    status = AudioUnitSetProperty(audioUnit,
                                  kAudioOutputUnitProperty_EnableIO,
                                  kAudioUnitScope_Output,
                                  kOutputBus,
                                  &flag,
                                  sizeof(flag));
    checkStatus(status);

    // Describe format
    audioStreamBasicDescription.mSampleRate         = AUDIO_SAMPLE_RATE;
    audioStreamBasicDescription.mFormatID           = kAudioFormatLinearPCM;
    audioStreamBasicDescription.mFormatFlags        = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    audioStreamBasicDescription.mFramesPerPacket    = AUDIO_FRAMES_PER_PACKET;
    audioStreamBasicDescription.mChannelsPerFrame   = AUDIO_CHANNELS_PER_FRAME;
    audioStreamBasicDescription.mBitsPerChannel     = AUDIO_BITS_PER_CHANNEL;
    audioStreamBasicDescription.mBytesPerPacket     = AUDIO_BYTES_PER_PACKET;
    audioStreamBasicDescription.mBytesPerFrame      = AUDIO_BYTES_PER_FRAME;

    // Apply format
    status = AudioUnitSetProperty(audioUnit,
                                  kAudioUnitProperty_StreamFormat,
                                  kAudioUnitScope_Output,
                                  kInputBus,
                                  &audioStreamBasicDescription,
                                  sizeof(audioStreamBasicDescription));
    checkStatus(status);


    /* Make sure we set the correct audio category before restarting */
    UInt32 audioCategory = kAudioSessionCategory_PlayAndRecord;
    status = AudioSessionSetProperty(kAudioSessionProperty_AudioCategory,
                                     sizeof(audioCategory),
                                     &audioCategory);

    checkStatus(status);


    status = AudioUnitSetProperty(audioUnit,
                                  kAudioUnitProperty_StreamFormat,
                                  kAudioUnitScope_Input,
                                  kOutputBus,
                                  &audioStreamBasicDescription,
                                  sizeof(audioStreamBasicDescription));
    checkStatus(status);


    // Set input callback
    AURenderCallbackStruct callbackStruct;
    callbackStruct.inputProc = recordingCallback;
    callbackStruct.inputProcRefCon = (__bridge void *)(self);
    status = AudioUnitSetProperty(audioUnit,
                                  kAudioOutputUnitProperty_SetInputCallback,
                                  kAudioUnitScope_Global,
                                  kInputBus,
                                  &callbackStruct,
                                  sizeof(callbackStruct));
    checkStatus(status);

    // Set output callback
    callbackStruct.inputProc = playbackCallback;
    callbackStruct.inputProcRefCon = (__bridge void *)(self);
    status = AudioUnitSetProperty(audioUnit,
                                  kAudioUnitProperty_SetRenderCallback,
                                  kAudioUnitScope_Global,
                                  kOutputBus,
                                  &callbackStruct,
                                  sizeof(callbackStruct));
    checkStatus(status);

    // 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_Output,
                                  kInputBus,
                                  &flag,
                                  sizeof(flag));


}

And the sound settings are as follows.

kOutputBus 0
kInputBus 1
AUDIO_SAMPLE_RATE 44100
AUDIO_FRAMES_PER_PACKET 1
AUDIO_CHANNELS_PER_FRAME 1
AUDIO_BITS_PER_CHANNEL 16 
AUDIO_BYTES_PER_PACKET 2
AUDIO_BYTES_PER_FRAME 2

I get PCM data from write callback as

audioBufferList->mBuffers[0].mData

SO, how can I reprogram PCM data from 44.1 kHz to 8 kHz and vice versa? I have a lot of googled, but I have not found any sample code or direct instructions for this.

Found this thread, but none of them gives clear instructions.

• Which built-in AudioUnit can reconfigure audio?
• Change AUGraph Sampling Rate on iOS

Any sample code or information is welcome.

Answer 1

. , - , . , AudioStreamBasicDescription (sysASBD), , - . , 8K sampleRate : ReomoteIO (mic) → converter → your8Kprocessing → converter → RemoteIO (out).

.

AudioComponentDescription convDesc;
convDesc.componentType = kAudioUnitType_FormatConverter;
convDesc.componentSubType = kAudioUnitSubType_AUConverter;
convDesc.componentFlags = 0;
convDesc.componentFlagsMask = 0;
convDesc.componentManufacturer = kAudioUnitManufacturer_Apple;

ASBDin ASBDout

UInt32 sizeASBD = sizeof(AudioStreamBasicDescription);
AudioStreamBasicDescription ioASBDin;
AudioStreamBasicDescription ioASBDout;
AudioUnitGetProperty(remoteIO, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 1, &ioASBDin, &sizeASBD);
AudioUnitGetProperty(remoteIO, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 0, &ioASBDout, &sizeASBD);

, , , ASBD ASBD , . , 8K.

AudioStreamBasicDescription asbd8K;

AudioComponentInstance converter44To8;
AudioUnitSetProperty(converter44To8,kAudioUnitProperty_StreamFormat,kAudioUnitScope_Input,0,& ioASBDin,sizeof(AudioStreamBasicDescription));
AudioUnitSetProperty(converter44To8,kAudioUnitProperty_StreamFormat,kAudioUnitScope_Output,0,&asbd8K,sizeof(AudioStreamBasicDescription));


AudioComponentInstance converter8To44;
AudioUnitSetProperty(converter8To44,kAudioUnitProperty_StreamFormat,kAudioUnitScope_Input,0,&asbd8K,sizeof(AudioStreamBasicDescription));
AudioUnitSetProperty(converter8To44,kAudioUnitProperty_StreamFormat,kAudioUnitScope_Output,0,& ioASBDout,sizeof(AudioStreamBasicDescription));