Core Audio: Don't Be Afraid to Play it LOUD! [360iDev, San Jose 2010]

Core Audio: Don’t Be
Afraid To Play It LOUD!
Chris Adamson
360iDev San Jose 2010
@invalidname

Previously…

Robert Strojan — “iPhone Audio Lab”
Monday, 12:30

"Easy" and "CoreAudio" can't be used in the
same sentence. CoreAudio is very powerful,
very complex, and under-documented. Be
prepared for a steep learning curve, APIs with
millions of tiny little pieces, and puzzling things
out from sample code rather than reading
high-level documentation.

–Jens Alfke, coreaudio-api list
Feb 9, 2009

• Problem domain is hard
• Performance is hard
• Low latency is hard
• Reusability is hard

• Doing without would suck
• Slowness would suck
• Latency would suck
• Non-reusability would suck

Last Exit Before Toll

• Alternatives to Core Audio:
• Media Player: plays audio from iPod
library
• AV Framework: Obj-C API for ﬁle
playback and recording

What They Missed

• Mixing
• Effects
• Streaming from/to network
• Lots of C

CA Style
• Procedural C
• Work with buffers of samples
• Most functions return an OSStatus
• You must check this, every time
• Heavily callback-oriented
• Heavily property-oriented

Example: Audio Session

• iPhone-only API
• Inspect and set audio hardware and
software properties
• Declare interaction with other audio
processes on the device (e.g., iPod)

#import-ant!
#import <AudioToolbox/AudioToolbox.h>

Initialize audio session

OSStatus setupAudioSessionErr=
! AudioSessionInitialize (
! ! ! NULL, // default run loop
! ! ! NULL, // default run loop mode
! ! ! MyInterruptionHandler, // interruption callback
! ! ! self); // client callback data
!
NSAssert (setupAudioSessionErr == noErr,
@"Couldn't initialize audio session");

C function call
!


! AudioSessionInitialize ( C function pointer
!


! Obj-C pointer

Return value

!

Set category property

UInt32 sessionCategory =
kAudioSessionCategory_PlayAndRecord;

setupAudioSessionErr =
AudioSessionSetProperty (
kAudioSessionProperty_AudioCategory,
sizeof (sessionCategory),
&sessionCategory); !

@"Couldn't set audio session property");


kAudioSessionCategory_PlayAndRecord; Property name
setupAudioSessionErr = (constant)




Size of property




&sessionCategory); ! Pointer to value

Is audio input available?
UInt32 ui32PropertySize = sizeof (UInt32);
UInt32 inputAvailable;

! AudioSessionGetProperty(
kAudioSessionProperty_AudioInputAvailable,
&ui32PropertySize,
&inputAvailable);

@"Couldn't get current audio input available prop");

Pointable size

&ui32PropertySize,
&inputAvailable);


Pointable variable
&ui32PropertySize,
&inputAvailable);



&ui32PropertySize,
&inputAvailable);
Address of size
and variable

Callback on a property

setupAudioSessionErr = AudioSessionAddPropertyListener (
! ! ! kAudioSessionProperty_AudioInputAvailable,
! ! ! MyInputAvailableListener,
! ! ! self);

@"Couldn't setup audio input prop listener");


! ! ! self);
Property to listen to


! ! ! self);
C function pointer


! ! ! self);
“Client data” pointer

CA style recap
• Lots of getting and setting properties
• Always check the return OSStatus
• Asychronous callbacks to C functions
• Look up function templates in docs
• “Context” or “user info” pointer can be
an Obj-C object

Engines Utilities

Audio Units

Engines Utilities

Open AL
Audio Units

Engines Utilities

Audio Queue Open AL
Audio Units

Engines Utilities

AV Fndtn
Audio Queue Open AL
Audio Units

Engines Utilities

AV Fndtn
Audio Queue Open AL
Audio Units Audio File

Engines Utilities

AV Fndtn
Audio Queue Open AL Audio Conversion

Engines Utilities

AV Fndtn Ext Audio File

Engines Utilities

Audio File Stream

Engines Utilities

Audio Session
Audio File Stream

iPhone Audio Engines

• Media Player
• AV Foundation
• Audio Queue Services
• OpenAL
• Audio Unit Services

Media Player

• Objective-C class MPMusicPlayerController
— Plays music from iPod library
• play, pause, stop. Properties for
currentPlaybackTime, volume, etc.
• No access to decoded samples or ﬁles

AV Framework
• Objective-C classes: AVAudioPlayer,
AVAudioRecorder, AVAudioSession
• Play from / record to ﬁles
• Handles compressed formats
• play, record, pause, stop. volume,
currentTime properties
• No access to decoded samples

Audio Queue

• C functions for playback, recording
• Callback-driven: you ﬁll or process audio
buffers periodically
• Plays compressed formats
• Latency is negotiable

OpenAL
• C API intended for gaming, designed to
resemble OpenGL
• Attach audio to “sources” in 3D space
• Listener hears sound from source relative
to their own position
• Uncompressed (PCM) only
• Can stream to a source (push model)

Audio Units
• Lowest publicly-accessed part of Core
Audio
• C API for audio processing
• Uncompressed (PCM) only, no ﬂoats
• Extremely low latency
• OpenAL and Queue implemented atop
units

So What’s An Audio
Unit?

Elements / Elements /
Buses Buses

AU “pull” model

• Each unit in the chain pulls audio data from
some source
• Another unit
• A callback to your code
• Last unit is typically an I/O unit

Effect I/O Unit
Unit (output)

I/O Unit Effect I/O Unit
(input) Unit (output)

I/O Unit Effect I/O Unit
(input) Unit (output)

Caveat: Not entirely accurate, for reasons to be
explained soon…

Units on iPhone OS
• I/O Units
• “Remote” I/O: Abstraction around
harware in/out
• Voice Processing I/O: Same as RemoteIO,
but with echo supression for VOIP
• Generic I/O: No hardware access; use for
software audio rendering

Units on iPhone OS

• Mixer Units
• Multichannel Mixer: Mixes multiple input
streams into one output stream
• 3D Mixer: Mixes inputs with more
control over panning, resampling. Used by
OpenAL

Units on iPhone OS
• Effect units
• iPod Equalizer: Provides same features as
iPod playback equalizer
• Converter units
• Converter: Converts between ﬂavors of
PCM, but not compressed formats

Missing Units
• Unit types absent from iPhone OS
• Generators
• Music devices (MIDI) and effects
• Many units found on Mac absent on iPhone
• Filters, reverb, compressor, varispeed,
other effects

RemoteIO

• Your app’s most direct access to audio
input/output hardware
• Used at the end of unit chains that play
audio
• Used at the beginning of chains that
capture audio

RemoteIO Buses

• Bus 0 is for H/W output
• Headphones, speakers, etc.
• Bus 1 is for H/W input
• Headset mic, phone mic, etc.

Bus 1 Bus 1
Remote
I/O
Bus 0 Bus 0

But!

• There can only be one RemoteIO unit!

Play Through steps
1. Enable recording via Audio Session
2. Get the Remote I/O unit
3. Set stream format
4. Connect input to output
5. Let ’er rip

1. Set Up Audio Session
for Recording

• Initialize the audio session
• Set a capture-enabling category
• Get/set other useful session properties

Initialize Audio Session
AudioSessionInitialize (
! ! ! NULL, // interruption callback
! ! ! NULL); // client callback data


Enable Recording
!
setupAudioSessionErr = AudioSessionSetProperty
! (kAudioSessionProperty_AudioCategory,
! sizeof (sessionCategory),
! &sessionCategory); !


Get HW sample rate
UInt32 f64PropertySize = sizeof (Float64);

setupAudioSessionErr = AudioSessionGetProperty
(kAudioSessionProperty_CurrentHardwareSampleRate,
! &f64PropertySize,
! &hardwareSampleRate);
!
@"Couldn't get current hardware sample rate");

NSLog (@"current hardware sample rate = %f",
hardwareSampleRate);

2. Get the Remote I/O
Unit

• Describe the unit you want
• Search until you ﬁnd it
• Enable its input and output properties

Describing a Unit

AudioComponentDescription audioCompDesc;
audioCompDesc.componentType = kAudioUnitType_Output;
audioCompDesc.componentSubType = kAudioUnitSubType_RemoteIO;
audioCompDesc.componentManufacturer =
kAudioUnitManufacturer_Apple;
audioCompDesc.componentFlags = 0;
audioCompDesc.componentFlagsMask = 0;

Discovering a Unit
AudioUnit remoteIOUnit = NULL;
AudioComponent rioComponent = AudioComponentFindNext
(NULL, &audioCompDesc);
OSErr setupErr = AudioComponentInstanceNew
(rioComponent, &remoteIOUnit);
NSAssert (setupErr == noErr,
@"Couldn't get RIO unit instance");

• If there can be more than one match,
iterate over AudioComponentFindNext,
passing in last match you found

Enable output property
UInt32 oneFlag = 1;
AudioUnitElement bus0 = 0;

setupErr =
! AudioUnitSetProperty (remoteIOUnit,
! ! ! ! ! ! kAudioOutputUnitProperty_EnableIO,
! ! ! ! ! ! kAudioUnitScope_Output,
! ! ! ! ! ! bus0,
! ! ! ! ! ! &oneFlag,
! ! ! ! ! ! sizeof(oneFlag));

@"Couldn't enable RIO output");

Enable input property
AudioUnitElement bus1 = 1;

setupErr = AudioUnitSetProperty(remoteIOUnit,
! ! ! kAudioOutputUnitProperty_EnableIO,
! ! ! ! ! kAudioUnitScope_Input,
! ! ! ! ! bus1,
! ! ! ! &oneFlag,
! ! ! ! ! sizeof(oneFlag));

@"couldn't enable RIO input");

3. Set stream
properties

• Deﬁne an AudioStreamBasicDescription
• Set it as the
kAudioUnitProperty_StreamFormat
• You will screw this up at least once

The ASBD
• Description of the common traits of an
entire audio stream
• Bitrate, channel count, format, format-
specific flags, etc.
• Not all fields apply to all formats
• On iPhone, Audio Units always work with
LPCM

Create an ASBD
AudioStreamBasicDescription myASBD;
memset (&myASBD, 0, sizeof (myASBD));

myASBD.mSampleRate = hardwareSampleRate;
myASBD.mFormatID = kAudioFormatLinearPCM;
myASBD.mFormatFlags = kAudioFormatFlagsCanonical;
myASBD.mBytesPerPacket = 4;
myASBD.mFramesPerPacket = 1;
myASBD.mBytesPerFrame =
myASBD.mBytesPerPacket * myASBD.mFramesPerPacket;
myASBD.mChannelsPerFrame = 2;
myASBD.mBitsPerChannel = 16;

Set RemoteIO’s stream
properties
• Declares format you want to read from
input bus, and write to output bus
• Set the kAudioUnitProperty_StreamFormat
property
• On RemoteIO’s output scope for bus 1
• On RemoteIO’s input scope for bus 0

Set bus 1 / output-
scope stream format
setupErr =
! ! ! ! ! ! kAudioUnitProperty_StreamFormat,
! ! ! ! ! ! kAudioUnitScope_Output,
! ! ! ! ! ! bus1,
! ! ! ! ! ! &myASBD,
! ! ! ! ! ! sizeof (myASBD));

@"Couldn't set ASBD for RIO on output scope / bus 1");

Set bus 0 / input-scope
stream format
setupErr =
! ! ! ! ! ! kAudioUnitProperty_StreamFormat,
! ! ! ! ! ! kAudioUnitScope_Input,
! ! ! ! ! ! bus0,
! ! ! ! ! ! &myASBD,
! ! ! ! ! ! sizeof (myASBD));

@"Couldn't set ASBD for RIO on input scope / bus 0");

4. Connect Input to
Output

• Create a structure describing the
connection
• Set it as a property on the destination
audio unit

Declare connection

AudioUnitConnection connection;

connection.sourceAudioUnit = remoteIOUnit;
connection.sourceOutputNumber = bus1;
connection.destInputNumber = bus0;

Set connection
property
setupErr =
! AudioUnitSetProperty(remoteIOUnit,
! ! ! ! ! ! kAudioUnitProperty_MakeConnection,
! ! ! ! ! ! bus0,
! ! ! ! ! ! &connection,
! ! ! ! ! ! sizeof (connection));

@"Couldn't set RIO connection");

5. Let ’er Rip

setupErr =! AudioUnitInitialize(remoteIOUnit);
@"Couldn't initialize RIO unit");

// in handleStartTapped:
OSStatus startErr = noErr;
startErr = AudioOutputUnitStart (remoteIOUnit);
NSAssert (startErr == noErr,
@"Couldn't start RIO unit");

Recap
• We created the Remote I/O unit
• Enabled input and output
• Created an ASBD to describe stream
format and set it on bus 1 output and bus 0
input
• Connected bus 1 output to bus 0 input
• Initialized and started the unit

The Grind

• Setting up the Remote IO unit and its
stream properties is something you’ll do all
the time
• Now let’s try something different

We can do more than
just connect…

Bus 1 Bus 1
Remote
I/O
Bus 0 Bus 0

Example 2: Play through
with render callbacks

Pulling audio
• A unit’s pull can be done one of several
ways:
• By being connected directly to another
audio unit
• By registering a “render callback” to
provide samples
• This is the key to all things cool…

Setting a render
callback
• Instead of connecting the audio units
kAudioUnitProperty_MakeConnection…
• Set the property
kAudioUnitProperty_SetRenderCallback
• Provide a function pointer to your own
function, and a “user info” object
• Any samples you provide get played

Render Callback steps
1. Enable recording via Audio Session
2. Get the Remote I/O unit
3. Set stream format
4. Set up callback function
5. Let ’er rip
You’ve already done
all these

Creating a render
callback
• Deﬁne a struct containing whatever data
your callback function will need
• Set up an AUCallbackStruct
• Set it as the
kAudioUnitProperty_SetRenderCallback
on bus 0
• Implement the callback function

Create your user data /
context

typedef struct {
! AudioUnit rioUnit;
} EffectState;

//...

EffectState effectState;

AUCallbackStruct

AURenderCallbackStruct callbackStruct;
callbackStruct.inputProc = MyAURenderCallback;
callbackStruct.inputProcRefCon = effectState;

Set callback property

setupErr =
! ! kAudioUnitProperty_SetRenderCallback,
! ! kAudioUnitScope_Global,
! ! bus0,
! ! &callbackStruct,
! ! sizeof (callbackStruct));

@"Couldn't set RIO render callback on bus 0");

Callback function
template

typedef OSStatus (*AURenderCallback) (
void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData
);

Create callback
function
OSStatus MyAURenderCallback (
! ! ! void *! ! ! ! ! ! ! inRefCon,
! ! ! AudioUnitRenderActionFlags *!ioActionFlags,
! ! ! const AudioTimeStamp *! ! ! inTimeStamp,
! ! ! UInt32! ! ! ! ! ! ! inBusNumber,
! ! ! UInt32! ! ! ! ! ! ! inNumberFrames,
! ! ! AudioBufferList *! ! ! ! ioData) {
!
EffectState *effectState = (EffectState*) inRefCon;
AudioUnit rioUnit = effectState->rioUnit;

Do something
OSStatus renderErr = noErr;

UInt32 bus1 = 1;
renderErr = AudioUnitRender(rioUnit,
! ! ! ! ! ! ! ! ioActionFlags,
! ! ! ! ! ! ! ! inTimeStamp,
! ! ! ! ! ! ! ! bus1,
! ! ! ! ! ! ! ! inNumberFrames,
! ! ! ! ! ! ! ! ioData);

NSAssert (renderErr == noErr, @”Couldn’t render”);

• AudioUnitRender() gets available samples
from a unit (either by copying from its
buffer or calling an upstream unit)

Almost there

• You now have access to raw samples, in
your render callback, by way of the ioData
pointer
• Anything you care to do with those
samples can now be played out to
hardware

Example 3: Play through
with gain effect

iPhone Audio Effects
• Generally need to be performed in render
callbacks
• On Mac OS X, you can create custom units
to encapsulate effects
• This was supposed to be in iPhone OS 3,
but doesn’t actually work
• Try AUPlugin.h See how far you get.

Gain effect

• Get gain value of 0.0 to 1.0 from UISlider
• Multiply each sample by this value

Render considerations

• Render callbacks are on a real-time thread,
with a hard deadline to return
• If you miss the deadline you get silence
• Render code must be highly performant

Bad Ideas for Callbacks
• File or network I/O
• Blocking threads
• Heavy use of Obj-C messaging
• malloc()
• Basically anything that’s potentially slow, of
indeterminate duration, and/or blocks

Good ideas for
Callbacks
• Pass a struct, rather than an Obj-C object,
as the user info object for your callback
• Let other threads do work for you and
leave their work somewhere that the
callback can just read it
• Note possible race conditions

Slider value in struct
typedef struct {
! float slider1Value;
} EffectState;

// set up callback state object
effectState.rioUnit = remoteIOUnit;
effectState.slider1Value = [slider1 value];
// set callback method
AURenderCallbackStruct callbackStruct;
callbackStruct.inputProc = MyAURenderCallback;
callbackStruct.inputProcRefCon = &effectState;

-(IBAction) handleSlider1ValueChanged {
! effectState.slider1Value = [slider1 value];
}

Read Slider Value in
Callback
EffectState *effectState = (EffectState*) inRefCon;
AudioUnit rioUnit = effectState->rioUnit;
float gain = effectState->slider1Value;

// Call AudioUnitRender() as before to copy samples
// into ioData

Apply Effect to Samples
// walk the samples
AudioSampleType sample = 0;
for (int bufCount=0; bufCount<ioData->mNumberBuffers; bufCount++) {
! AudioBuffer buf = ioData->mBuffers[bufCount];
! ! int currentFrame = 0;
! ! while ( currentFrame < inNumberFrames ) {
! ! ! // copy sample to buffer, across all channels
! ! ! for (int currentChannel=0;
currentChannel<buf.mNumberChannels;
currentChannel++) {
! ! ! ! memcpy(&sample,
! ! ! ! ! buf.mData + (currentFrame * 4) + (currentChannel*2),
! ! ! ! ! sizeof(AudioSampleType));
! ! ! ! sample = (float) sample * gain;
! ! ! ! memcpy(buf.mData + (currentFrame * 4) + (currentChannel*2),
! ! ! ! ! &sample,
! ! ! ! ! sizeof(AudioSampleType));
! ! ! }!
! ! currentFrame++;
! }
}

Example 4: More
Interesting Effects

Ring Modulator

R(t) = C(t) x M(t)

Ring Modulation

• Multiplication of two signals
• One is usually a sine wave
• Originally implemented as a ring-shaped
circuit

Modulate! Modulate!
• Ring modulator best
known as the
“Dalek” voice effect
on Doctor Who (circa
1963)
• Also used in early
electronic music

Building a ring
modulator

• Need to model the sine wave
• Multiply samples
• Write modiﬁed sample back to ioData

Sine wave state
typedef struct {
! float slider1Value;
! AudioStreamBasicDescription asbd;
! float sineFrequency;
! float sinePhase;
} EffectState;

// in setup method...
// 23 Hz according to “Doctor Who” fan page
// http://homepage.powerup.com.au/~spratleo/
Tech/Dalek_Voice_Primer.html

effectState.sineFrequency = 23;
effectState.sinePhase = 0;
effectState.asbd = myASBD;

Sine wave value
// AudioUnitRender() from RIO bus 1 here...
// memcpy() from buffer here...

float theta = effectState->sinePhase * M_PI * 2;
sample = (sin(theta) * sample);

// memcpy() to buffer here...

effectState->sinePhase += 1.0 /
(asbd.mSampleRate / sineFrequency);
if (effectState->sinePhase > 1.0) {
! effectState->sinePhase -= 1.0;
}

/* to play pure sine wave
sample = (sin (theta) * 0x7FFF);
*/

More Effects

• http://musicdsp.org/, et. al.

Example 5: Mixing with
Effects

Bus 1

Mixer Bus 0

Bus 0

Render
Callback
Bus 1

Remote
Mixer Bus 0 Bus 0
I/O

Bus 0

Remote
Bus 1
I/O

Render
Callback
Bus 1

Remote
Mixer Bus 0 Bus 0
I/O

Bus 0

Audio
Remote Unit
Render
Bus 1 Callback
I/O Render()

Create a mixer unit
AudioComponentDescription mixerDesc;
mixerDesc.componentManufacturer =
kAudioUnitManufacturer_Apple;
mixerDesc.componentFlags = 0;
mixerDesc.componentFlagsMask = 0;
mixerDesc.componentType = kAudioUnitType_Mixer;
mixerDesc.componentSubType =
kAudioUnitSubType_MultiChannelMixer;

AudioComponent mixerComponent =
AudioComponentFindNext(NULL, &mixerDesc);
setupErr = AudioComponentInstanceNew
(mixerComponent, &mixerUnit);
@"Couldn't get mixer unit instance");

Conﬁg mixer unit

• Set the stream property on its inputs
• Set the render callback or connection
properties on its inputs
• Bus 0 is RobotVoiceRenderCallback
• What shall we mix with?

Playing audio ﬁles in
units
• Can’t do I/O in render callback
• If compressed, we’d also need to convert
to PCM
• Have a separate thread read/convert, then
place PCM data in a ring buffer
• See CARingBuffer (C++) in /Developer/
Extras/PublicUtility

Cheat #1: Convert
AAC ﬁle to PCM

afconvert --data LEI16 Girlfriend.m4a Girlfriend.caf

Cheat #2: Read ﬁle into
RAM

• Obviously not recommended

Set up struct for ﬁle-
playing callback
typedef struct {
! void* audioData;
! AudioSampleType *samplePtr;
} MusicPlaybackState;

//...

MusicPlaybackState musicPlaybackState;

Open audio ﬁle

NSURL *songURL = [NSURL fileURLWithPath:
! ! [[NSBundle mainBundle] pathForResource: @"Girlfriend"
! ! ! ! ofType: @"caf"]];
AudioFileID songFile;
setupErr = AudioFileOpenURL((CFURLRef) songURL,
! ! ! kAudioFileReadPermission,
! ! ! 0,
! ! ! &songFile);
NSAssert (setupErr == noErr, @"Couldn't open audio file");

Get size of audio data
and malloc()
UInt64 audioDataByteCount;
UInt32 audioDataByteCountSize = sizeof (audioDataByteCount);
setupErr = AudioFileGetProperty(songFile,
! ! ! ! ! kAudioFilePropertyAudioDataByteCount,
! ! ! ! ! &audioDataByteCountSize,
! ! ! ! ! &audioDataByteCount);
NSAssert (setupErr == noErr, @"Couldn't get size property");

musicPlaybackState.audioData = malloc (audioDataByteCount);
musicPlaybackState.samplePtr = musicPlaybackState.audioData;

Read ﬁle into RAM

UInt32 bytesRead = audioDataByteCount;
setupErr = AudioFileReadBytes(songFile,
! ! ! ! ! false,
! ! ! ! ! 0,
! ! ! ! ! &bytesRead,
! ! ! ! ! musicPlaybackState.audioData);
@"Couldn't read audio data");

Get ASBD from ﬁle

AudioStreamBasicDescription fileASBD;
setupErr = AudioFileGetProperty(songFile,
! ! ! ! ! kAudioFilePropertyDataFormat,
! ! ! ! ! &asbdSize,
! ! ! ! ! &fileASBD);
NSAssert (setupErr == noErr, @"Couldn't get file asbd");

Provide samples to
mixer

Render
Callback
Bus 1

Mixer

Set up render callback
AURenderCallbackStruct musicPlayerCallbackStruct;
musicPlayerCallbackStruct.inputProc = MusicPlayerCallback;
musicPlayerCallbackStruct.inputProcRefCon =
&musicPlaybackState;
!
setupErr =
! AudioUnitSetProperty(mixerUnit,
! ! ! ! ! kAudioUnitProperty_SetRenderCallback,
! ! ! ! ! kAudioUnitScope_Global,
! ! ! ! ! 1,
! ! ! ! ! &musicPlayerCallbackStruct,
! ! ! ! ! sizeof (musicPlayerCallbackStruct));

@"Couldn't set mixer render callback on bus 1");

Create
MusicPlayerCallback()
MusicPlaybackState *musicPlaybackState = (MusicPlaybackState*) inRefCon;
! !
// walk the samples
AudioSampleType sample = 0;
for (int bufCount=0; bufCount<ioData->mNumberBuffers; bufCount++) {
! AudioBuffer buf = ioData->mBuffers[bufCount];
! // AudioSampleType* bufferedSample = (AudioSampleType*) &buf.mData;
! int currentFrame = 0;
! while ( currentFrame < inNumberFrames ) {
! ! // copy sample to buffer, across all channels
! ! for (int currentChannel=0; currentChannel<buf.mNumberChannels;
currentChannel++) {
! ! ! sample = *musicPlaybackState->samplePtr++;
! ! ! memcpy(buf.mData + (currentFrame * 4) + (currentChannel*2),
! ! ! ! &sample,
! ! ! ! sizeof(AudioSampleType));
! ! }!
! ! currentFrame++;
! }
}
return noErr;

Connect mixer to RIO

Remote
Mixer Bus 0 Bus 0
I/O

Connect mixer to RIO
AudioUnitConnection connection;
connection.sourceAudioUnit = mixerUnit;
connection.sourceOutputNumber = bus0;
connection.destInputNumber = bus0;
!
setupErr =
kAudioUnitProperty_MakeConnection,
kAudioUnitScope_Input,
bus0,
&connection,
sizeof (connection));

@"Couldn't set mixer-to-RIO connection");

Bonus!

• Multichannel mixer unit has volume
parameter on each of its inputs

Adjusting mixer input
volume
-(IBAction) handleMicSliderValueChanged {
! OSStatus propSetErr = noErr;
! AudioUnitParameterValue sliderVal = [micSlider value];
! propSetErr = AudioUnitSetParameter(mixerUnit,
! ! ! ! ! ! kMultiChannelMixerParam_Volume,
! ! ! ! ! ! 0,
! ! ! ! ! ! sliderVal,
! ! ! ! ! ! 0);
NSAssert (propSetErr == noErr,
@"Couldn't set mixer volume on bus 0");
}

Takeaways

• Core Audio is hard
• Core Audio lets you do things that are
freaking awesome

Since you’ll need help
• coreaudio-api at lists.apple.com
• Apple developers post here every day
• stackoverﬂow.com
• devforums.apple.com
• Look for Michael Tyson’s post on Remote
I/O unit

Ask me?

• [Time code]; blog
• http://www.subfurther.com/blog
• @invalidname
• invalidname [at] gmail [dot] com

Core Audio: Don't Be Afraid to Play it LOUD! [360iDev, San Jose 2010]

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