Practical Audio Units Warren Moore @warrenm Bare-Metal Audio Programming for iOS 

Fundamentals of Digital Audio The Problem The sounds we hear every day are continuous wave phenomena. BUT Computers can’t handle continuous waveforms: they speak binary. 

Fundamentals of Digital Audio Sampling Audio waves in nature are continuous in time and space: 

Fundamentals of Digital Audio Sampling We sample at regular intervals (usually 44.1 kHz), then discretize to produce samples that are discrete in time and space. 

Fundamentals of Digital Audio Reconstruction The audio output hardware uses interpolation to reconstruct a continuous waveform to send to the speaker. 

Fundamentals of Digital Audio A Word on Audio Formats To store samples for playback, we have to agree on a format: • Data type (signed integer is typical) • Data size (16 bits is typical) • Sample rate (44.1 kHz for music, much less for voice) • Number of channels (1 = mono, 2 = stereo, etc.) • Interleaved or non-interleaved We won’t even bother talking about compression here. 

Fundamentals of Digital Audio It’s all just moving samples around. ? 

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Making Noise with iOS AVFoundation System Sound Services Audio Queue Services Audio Units Audio File Stream Services Audio Session Services Hardware Drivers Extended Audio File Services Audio File Services Audio Conversion Services High Level Low Level Audio Units 

Selecting an Audio API Pick your poison Need to play a brief sound clip or effect? • System Audio Services Need to play a longer audio file, stream from a network, or access the iPod Library? • AVFoundation (specifically, AVPlayer) Need to synchronize multiple audio files? • Audio Queue Services Need access to uncompressed samples (e.g., for custom effects or on-the-fly sound generation)? • Audio Units 

What are Audio Units? Self-contained processors of audio samples Take on one of several roles: • Input or output device (microphone, speaker) • Mixer (blend 2+ streams of audio together) • Effect (distort, echo, match tempo, pitch correction) Wired together to make Audio Unit Graphs Comprise a plug-in architecture on Mac OS. Less so on iOS. 

The RemoteIO Audio Unit Talking to hardware Input Element Output Element RemoteIO Audio samples to your application Audio samples from your application 

Example Audio Unit Graphs #1 - Playthrough AUGraph RemoteIO (Input Element) RemoteIO (Output Element) 

Audio Unit Graph Overview 1. Describe the units we want and add nodes for them. 2. Open the graph to create the nodes and their corresponding units. 3. Make connections between nodes. 4. Configure render callbacks for any sources of samples that aren’t other nodes in the graph. 5. Initialize the graph to finalize connections and make sure formats are mutually-compatible. Getting up and running 

Audio Unit Playthrough Step 1 of 4: Building a Graph AUGraph auGraph = NULL; NewAUGraph(&auGraph); AudioComponentDescription ioDescription = {0}; ioDescription.componentManufacturer = kAudioUnitManufacturer_Apple; ioDescription.componentType = kAudioUnitType_Output; ioDescription.componentSubType = kAudioUnitSubType_RemoteIO; AUNode ioNode = 0; AUGraphAddNode(auGraph, &ioDescription, &ioNode); 

Audio Unit Playthrough Step 2 of 4: Opening the Graph AudioUnit ioUnit = NULL; AUGraphNodeInfo(auGraph, ioNode, NULL, &ioUnit); AUGraphOpen(auGraph); 

Audio Unit Playthrough Step 3 of 4: Configuring the Stream Format & Enabling Input AudioStreamBasicDescription audioFormat = {0}; audioFormat.mSampleRate! ! ! = 44100.0; audioFormat.mFormatID! ! ! ! = kAudioFormatLinearPCM; audioFormat.mFormatFlags!! ! = kAudioFormatFlagIsFloat | ! ! ! ! ! ! ! ! ! ! ! ! kAudioFormatFlagIsPacked; audioFormat.mFramesPerPacket! = 1; audioFormat.mChannelsPerFrame!= 1; audioFormat.mBitsPerChannel! ! = 8 * sizeof(Float32); audioFormat.mBytesPerPacket! ! = sizeof(Float32); audioFormat.mBytesPerFrame! ! = sizeof(Float32); 

Audio Unit Playthrough Step 3 of 4: Configuring the Stream Format & Enabling Input AudioUnitSetProperty(ioUnit, ! ! ! ! ! ! ! kAudioUnitProperty_StreamFormat, ! ! ! ! ! ! ! kAudioUnitScope_Output, ! ! ! ! ! ! ! kInputBus, ! ! ! ! ! ! ! &audioFormat, ! ! ! ! ! ! ! sizeof(audioFormat)); AudioUnitSetProperty(ioUnit, ! ! ! ! ! ! ! kAudioUnitProperty_StreamFormat, ! ! ! ! ! ! ! kAudioUnitScope_Input, ! ! ! ! ! ! ! kOutputBus, ! ! ! ! ! ! ! &audioFormat, ! ! ! ! ! ! ! sizeof(audioFormat)); uint32_t enableInput = 1; AudioUnitSetProperty(ioUnit, ! ! ! ! ! ! ! kAudioOutputUnitProperty_EnableIO, ! ! ! ! ! ! ! kAudioUnitScope_Output, ! ! ! ! ! ! ! kOutputBus, ! ! ! ! ! ! ! &enableInput, ! ! ! ! ! ! ! sizeof(enableInput)); 

Audio Unit Playthrough Step 4 of 4: Making Connections AUGraphConnectNodeInput(auGraph, ioNode, 1, ioNode, 0); AUGraphInitialize(auGraph); AUGraphStart(auGraph); Input Element Output Element 

Playthrough Demo Fast app switching FTW 

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Multichannel Mixer Example Audio Unit Graphs #2 - Playing Back Multiple Audio Files AUGraph RemoteIO (Output Element) 

Audio Unit Graph Overview 1. Describe the units we want and add nodes for them. 2. Open the graph to create the nodes and their corresponding units. 3. Make connections between nodes. 4. Configure render callbacks for any sources of samples that aren’t other nodes in the graph. 5. Initialize the graph to finalize connections and make sure formats are mutually-compatible. Getting up and running 

Using the Multichannel Mixer Step 1 of 4: Describe and Instantiate AudioComponentDescription mixerDescription = {0}; mixerDescription.componentManufacturer = kAudioUnitManufacturer_Apple; mixerDescription.componentType = kAudioUnitType_Mixer; mixerDescription.componentSubType = kAudioUnitSubType_MultiChannelMixer; AUGraphAddNode(auGraph, &mixerDescription, &mixerNode); //AUGraphOpen(auGraph); AUGraphNodeInfo(auGraph, mixerNode, NULL, &mixerUnit); 

Using the Multichannel Mixer Step 2 of 4: Configure format and channel count UInt32 mixerElementCount = 2; AudioUnitSetProperty(mixerUnit, kAudioUnitProperty_ElementCount, kAudioUnitScope_Input, 0, &mixerElementCount, sizeof(mixerElementCount)); AudioUnitSetProperty(mixerUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 1, &audioFormat, sizeof(audioFormat)); 

Using the Multichannel Mixer Step 3 of 4: Set up render callbacks AURenderCallbackStruct callbackStruct; callbackStruct.inputProc = &InputRenderCallback; callbackStruct.inputProcRefCon = (__bridge void *)self; AudioUnitSetProperty(mixerUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 1, &callbackStruct, sizeof(callbackStruct)); 

Using the Multichannel Mixer Step 4 of 4: Write the render callback function static OSStatus InputRenderCallback( void *inRefCon, ! AudioUnitRenderActionFlags *ioActionFlags, ! const AudioTimeStamp *inTimeStamp, ! UInt32 inBusNumber, ! UInt32 inNumberFrames, ! AudioBufferList *ioData) { ! MyAudioUnitManager *graph = (__bridge id)inRefCon; // Fill audio buffers with samples, from whatever source. } 

Extended Audio File Services We want to play back compressed files (MP3, AAC, etc). Problem: Audio Units lack support for handling compressed formats. Solution: Extended Audio File Services, whose sole purpose is to efficiently decompress audio files into ready-to-consume samples. Playing compressed audio files 

Extended Audio File Services Example of (naive) usage ExtAudioFileOpenURL((__bridge CFURLRef)fileURL, &audioFile); ExtAudioFileSetProperty(audioFile, kExtAudioFileProperty_ClientDataFormat, sizeof(audioFormat), &audioFormat); // Later, in render callback... ExtAudioFileRead(audioFile, &frames, data); 

Multichannel Mixer Demo Featuring the world’s worst mashup 

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Putting Audio Units to Work for You Resources Novocaine: a basic wrapper for reading and writing audio data on iOS http://alexbw.github.com/novocaine/ The Amazing Audio Engine: a much more sophisticated object-oriented Audio Unit wrapper (coming real soon now) http://theamazingaudioengine.com WWDC Videos: Not just Audio Units! Also Session Services and all of AVFoundation Available in iTunesU to registered developers 

Questions Feedback, contact, etc. I am Warren Moore. I am @warrenm on and . Email me at [email protected]. 

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