I have an app (iPhone / iPad) that currently uses miniaudio and I'd to transition to a pure CoreAudio solution, and I cannot for the life of me get it to work. I want to set up a remoteIO with microphone and speaker callbacks, so that I get a callback when the microphone (USB microphone at 384kHz, mono) has samples for me, and I get a callback when the speakers (49kHz, stereo) need more samples. It should be insanely simple. It's Objective C as I have never got round to Swift and can't see it happening this late in life, but that shouldn't change things. So if anyone can tell me what I'm doing wrong here it would be GREATLY appreciated. My playbackCallback is never being fired, only the recordCallback. `-(void) launchRemoteIOandSleep { NSError *error; [ [AVAudioSession sharedInstance] setPreferredIOBufferDuration:(1024.0f / 48000.0f) error:&error ]; OSStatus status; AudioComponentInstance audioUnit; // set up callback structures - different sanity clauses to identify in breakpoints renderCallBackHandle sanityClause666; sanityClause666.remoteIO = audioUnit; sanityClause666.sanityCheck666 = 666; renderCallBackHandle sanityClause667; sanityClause667.remoteIO = audioUnit; sanityClause667.sanityCheck666 = 667; // set up audio formats AudioStreamBasicDescription audioFormatInput; FillOutASBDForLPCM(audioFormatInput,384000.0,1,16,16,0,0,0); AudioStreamBasicDescription audioFormatOutput; FillOutASBDForLPCM(audioFormatOutput,48000.0,2,16,16,0,0,0); // set up callback structs AURenderCallbackStruct callbackStructRender; callbackStructRender.inputProc = playbackCallback; callbackStructRender.inputProcRefCon = &sanityClause666; AURenderCallbackStruct callbackStructRecord; callbackStructRecord.inputProc = recordCallback; callbackStructRecord.inputProcRefCon = &sanityClause667; // grab remoteIO AudioComponentDescription desc; desc.componentType = kAudioUnitType_Output; desc.componentSubType = kAudioUnitSubType_RemoteIO; desc.componentManufacturer = kAudioUnitManufacturer_Apple; desc.componentFlags = 0; desc.componentFlagsMask = 0; AudioComponent component = AudioComponentFindNext(NULL, &desc); // Get audio unit status = AudioComponentInstanceNew(component, &audioUnit); checkStatus(status); // Enable IO for both recording and playback // this enables the OUTPUT side of the OUTPUT bus which is the speaker (I thnk ... ) UInt32 flag = 1; status = AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Output, kOutputBus, &flag, sizeof(flag)); checkStatus(status); // this enables the INPUT side of the INPUT bus which is the mic (I thnk ... ) flag = 1; status = AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, kInputBus, &flag, sizeof(flag)); checkStatus(status); // Apply format - INPUT bus of OUTPUT SCOPE which is my samples into remoteIO status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, kInputBus, &audioFormatOutput, sizeof(audioFormatOutput)); checkStatus(status); // Apply format - OUTPUT bus of INPUT SCOPE which is where I pick up my samples from mic status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, kOutputBus, &audioFormatInput, sizeof(audioFormatInput)); checkStatus(status); // set output callback status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Output, kInputBus, &callbackStructRender, sizeof(callbackStructRender)); checkStatus(status); // Set input callback status = AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_SetInputCallback, kAudioUnitScope_Input, kOutputBus, &callbackStructRecord, sizeof(callbackStructRecord)); checkStatus(status); // Disable buffer allocation for the recorder flag = 0; status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_ShouldAllocateBuffer, kAudioUnitScope_Input, kInputBus, &flag, sizeof(flag)); // Initialise status = AudioUnitInitialize(audioUnit); checkStatus(status); status = AudioOutputUnitStart(audioUnit); checkStatus(status); [ self waitForAudioStabilisation ]; while (1) sleep(2); }`