Core Audio in iOS 6 - What You'll Learn

Core Audio in iOS 6
Chris Adamson • @invalidname
CocoaConf DC
March 23, 2013

Sides and code available on my blog:
http://www.subfurther.com/blog

Monday, March 25, 13

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What You’ll Learn

• What Core Audio does and doesn’t do
• When to use and not use it
• What’s new in Core Audio for iOS 6


Simple things should be simple,
complex things should be possible.
–Alan Kay


AV Foundation,
Media Player

–Alan Kay


AV Foundation,
Media Player

–Alan Kay
Core Audio


Core Audio

• Low-level C framework for processing
audio
• Capture, play-out, real-time or off-line
processing
• The “complex things should be possible”
part of audio on OS X and iOS


Chris’ CA Taxonomy
• Engines: process streams of audio
• Capture, play-out, mixing, effects
processing
• Helpers: deal with formats, encodings, etc.
• File I/O, stream I/O, format conversion,
iOS “session” management


Helpers: Audio File

• Read from / write to multiple audio ﬁle
types (.aiff, .wav, .caf, .m4a, .mp3) in a
content-agnostic way
• Get metadata (data format, duration,
iTunes/ID3 info)


Helpers: Audio File
Stream

• Read audio from non-random-access
source like a network stream
• Discover encoding and encapsulation on
the ﬂy, then deliver audio packets to client
application


Helpers: Converters

• Convert buffers of audio to and from
different encodings
• One side must be in an uncompressed
format (i.e., Linear PCM)


Helpers: ExtAudioFile

• Combine file I/O and format conversion
• Read a compressed file into PCM buffers
• Write PCM buffers into a compressed file


Helpers: Audio Session
• iOS-only API to negotiate use of audio
resources with the rest of the system
• Deetermine whether your app mixes with
other apps’ audio, honors ring/silent
switch, can play in background, etc.
• Gets notiﬁed of audio interruptions
• See also AVAudioSession


Engines: Audio Units

• Low-latency (~10ms) processing of
capture/play-out audio data
• Effects, mixing, etc.
• Connect units manually or via an AUGraph
• Much more on this topic momentarily…


Engines: Audio Queue
• Convenience API for recording or play-out,
built atop audio units
• Rather than processing on-demand and on
Core Audio’s thread, your callback provides
or receives buffers of audio (at whatever
size is convenient to you)
• Higher latency, naturally
• Supports compressed formats (MP3, AAC)


Engines: Open AL

• API for 3D spatialized audio, implemented
atop audio units
• Set a source’s properties (x/y/z
coordinates, orientation, audio buffer, etc.),
OpenAL renders what it sounds like to the
listener from that location


Engines and Helpers
• Audio Units • Audio File

• Audio Queue • Audio File Stream

• Open AL • Audio Converter

• ExtAudioFile

• Audio Session


Audio Units


Audio Unit

AUSomething


Types of Audio Units
• Output (which also do input)
• Generator
• Converter
• Effect
• Mixer
• Music


Pull Model

AUSomething


Pull Model

AUSomething
AudioUnitRender()


Pull Model

AUSomethingElse AUSomething


Buses (aka, Elements)

AUSomethingElse

AUSomething

AUSomethingElse


AUGraph

AUSomethingElse

AUSomething

AUSomethingElse


Render Callbacks
OSStatus converterInputRenderCallback (void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList * ioData) {
CCFWebRadioPlayer *player = (__bridge CCFWebRadioPlayer*) inRefCon;

// read from buffer
ioData->mBuffers[0].mData = player.preRenderData;

return noErr;
}

AUSomething

AUSomethingElse


AURemoteIO
• Output unit used for play-out, capture
• A Core Audio thread repeatedly and
automatically calls AudioUnitRender()
• Must set EnableIO property to explicitly
enable capture and/or play-out
• Capture requires setting appropriate
AudioSession category


Create AURemoteIO
CheckError(NewAUGraph(&_auGraph),
! ! "couldn't create au graph");
!
CheckError(AUGraphOpen(_auGraph),
! ! "couldn't open au graph");
!
AudioComponentDescription componentDesc;
componentDesc.componentType = kAudioUnitType_Output;
componentDesc.componentSubType = kAudioUnitSubType_RemoteIO;
componentDesc.componentManufacturer =
kAudioUnitManufacturer_Apple;
!
AUNode remoteIONode;
CheckError(AUGraphAddNode(_auGraph,
! ! ! ! ! ! &componentDesc,
! ! ! ! ! ! &remoteIONode),
! ! "couldn't add remote io node");


Getting an AudioUnit
from AUNode

! CheckError(AUGraphNodeInfo(self.auGraph,
! ! ! ! ! ! ! remoteIONode,
! ! ! ! ! ! ! NULL,
! ! ! ! ! ! ! &_remoteIOUnit),
! ! ! "couldn't get remote io unit from node");


AURemoteIO Buses

AURemoteIO


AURemoteIO Buses

AURemoteIO
bus 0
to output H/W


AURemoteIO Buses

AURemoteIO
bus 0 bus 0
from app to output H/W


AURemoteIO Buses

bus 1
from input H/W
AURemoteIO
bus 0 bus 0


AURemoteIO Buses

bus 1 bus 1
from input H/W to app
AURemoteIO
bus 0 bus 0


EnableIO
! UInt32 oneFlag = 1;
! UInt32 busZero = 0;
! CheckError(AudioUnitSetProperty(self.remoteIOUnit,
! ! ! ! ! ! ! ! ! kAudioOutputUnitProperty_EnableIO,
! ! ! ! ! ! ! ! ! kAudioUnitScope_Output,
! ! ! ! ! ! ! ! ! busZero,
! ! ! ! ! ! ! ! ! &oneFlag,
! ! ! ! ! ! ! ! ! sizeof(oneFlag)),
! ! ! "couldn't enable remote io output");
! UInt32 busOne = 1;
! ! ! ! ! ! ! ! ! kAudioOutputUnitProperty_EnableIO,
! ! ! ! ! ! ! ! ! kAudioUnitScope_Input,
! ! ! ! ! ! ! ! ! busOne,
! ! ! ! ! ! ! ! ! &oneFlag,
! ! ! ! ! ! ! ! ! sizeof(oneFlag)),
! ! ! "couldn't enable remote io input");


Pass Through

bus 1
from input H/W
AURemoteIO
bus 0
to output H/W


Connect In to Out
! UInt32 busZero = 0;
! UInt32 busOne = 1;
! CheckError(AUGraphConnectNodeInput(self.auGraph,
! ! ! ! ! ! ! ! ! remoteIONode,
! ! ! ! ! ! ! ! ! busOne,
! ! ! ! ! ! ! ! ! remoteIONode,
! ! ! ! ! ! ! ! ! busZero),
! ! ! "couldn't connect remote io bus 1 to 0");


Pass-Through with Effect

AUEffect

bus 1
from input H/W
AURemoteIO
bus 0
to output H/W


Demo: Delay Effect
New in iOS 6!


Creating the AUDelay
! componentDesc.componentType = kAudioUnitType_Effect;
! componentDesc.componentSubType = kAudioUnitSubType_Delay;
! componentDesc.componentManufacturer =
kAudioUnitManufacturer_Apple;
!
! AUNode effectNode;
! CheckError(AUGraphAddNode(self.auGraph,
! ! ! ! ! ! ! &componentDesc,
! ! ! ! ! ! ! &effectNode),
! ! ! "couldn't create effect node");
! AudioUnit effectUnit;
! CheckError(AUGraphNodeInfo(self.auGraph,
! ! ! ! ! ! ! effectNode,
! ! ! ! ! ! ! NULL,
! ! ! ! ! ! ! &effectUnit),
! ! ! "couldn't get effect unit from node");


The problem with effect
units
• Audio Units available since iPhone OS 2.0
prefer int formats
• Effect units arrived with iOS 5 (arm7 era)
and only work with ﬂoat format
• Have to set the AUEffect unit’s format on
AURemoteIO


Setting formats
! AudioStreamBasicDescription effectDataFormat;
! UInt32 propSize = sizeof (effectDataFormat);
! CheckError(AudioUnitGetProperty(effectUnit,
! ! ! ! ! ! ! ! ! kAudioUnitProperty_StreamFormat,
! ! ! ! ! ! ! ! ! busZero,
! ! ! ! ! ! ! ! ! &effectDataFormat,
! ! ! ! ! ! ! ! ! &propSize),
! ! ! "couldn't read effect format");
! ! ! ! ! ! ! ! ! kAudioUnitProperty_StreamFormat,
! ! ! ! ! ! ! ! ! busOne,
! ! ! ! ! ! ! ! ! &effectDataFormat,
! ! ! ! ! ! ! ! ! propSize),
! ! ! "couldn't set bus one output format");

Then repeat AudioUnitSetProperty() for input scope / bus 0

AUNewTimePitch

• New in iOS 6!
• Allows you to change pitch independent of
time, or time independent of pitch
• How do you use it?


AUTimePitch
! AudioComponentDescription effectcd = {0};
! effectcd.componentType = kAudioUnitType_FormatConverter;
! effectcd.componentSubType = kAudioUnitSubType_NewTimePitch;
! effectcd.componentManufacturer = kAudioUnitManufacturer_Apple;
!
! AUNode effectNode;
! CheckError(AUGraphAddNode(self.auGraph,
! ! ! ! ! ! ! &effectcd,
! ! ! ! ! ! ! &effectNode),
! ! ! "couldn't get effect node [time/pitch]");

Notice the type is AUFormatConverter, not AUEffect

AudioUnitParameters.h
// Parameters for AUNewTimePitch
enum {
! ! // Global, rate, 1/32 -> 32.0, 1.0
! kNewTimePitchParam_Rate! ! ! ! ! ! =
! 0,
! ! // Global, Cents, -2400 -> 2400, 1.0
! kNewTimePitchParam_Pitch! ! ! ! ! ! = 1,
! ! // Global, generic, 3.0 -> 32.0, 8.0
! kNewTimePitchParam_Overlap! ! ! ! ! ! = 4,
! ! // Global, Boolean, 0->1, 1
! kNewTimePitchParam_EnablePeakLocking! ! ! = 6
};

This is the entire documentation for the AUNewTimePitch parameters


AUNewTimePitch
parameters
• Rate: kNewTimePitchParam_Rate takes a
Float32 rate from 1/32 speed to 32x
speed.
• Use powers of 2: 1/32, 1/16, …, 2, 4, 8…
• Pitch: kNewTimePitchParam_Pitch takes
a Float32 representing cents, meaning
1/100 of a musical semitone


Pitch shifting

• Pitch can vary, time does not
• Suitable for real-time sources, such as
audio capture


Demo: Pitch Shift
New in iOS 6!


Rate shifting
• Rate can vary, pitch does not
• Think of 1.5x and 2x speed modes in
Podcasts app
• Not suitable for real-time sources, as data
will be consumed faster. Files work well.
• Sources must be able to map time
systems with
kAudioUnitProperty_InputSamplesInOutput


Demo: Rate Shift
New in iOS 6!


AUSplitter

AUSomethingElse

AUSplitter

AUSomethingElse

New in iOS 6!

AUMatrixMixer
AUSomethingElse

AUSomethingElse

AUSomethingElse AUMatrixMixer

AUSomethingElse

AUSomethingElse

New in iOS 6!

Audio Queues
(and the APIs that help them)


AudioQueue
• Easier than AURemoteIO - provide data
when you want to, less time pressure, can
accept or provide compressed formats
(MP3, AAC)
• Recording queue - receive buffers of
captured audio in a callback
• Play-out queue - enqueue buffers of audio
to play, optionally reﬁll in a callback


AudioQueue

2 1 0


Common AQ scenarios
• File player - Read from file and “prime”
queue buffers, start queue, when called
back with used buffer, refill from next part
of file
• Synthesis - Maintain state in your own
code, write raw samples into buffers during
callbacks


Web Radio
• Project from CocoaConfs Columbus,
Portland, and Raleigh
• Use Audio File Stream Services to pick out
audio data from a network stream
• Enqueue these packets as new AQ buffers
• Dispose used buffers in callback


Parsing web radio


Parsing web radio
NSURLConnection delivers
NSData buffers, containing audio
and framing info. We pass it to NSData NSData
Audio File Services. Packets Packets Packets Packets Packets


Parsing web radio

Packets Packets
Audio File Services calls us back
with parsed packets of audio data. Packets Packets Packets


Parsing web radio

Packets Packets
Audio File Services calls us back
with parsed packets of audio data. Packets Packets Packets

We create an AudioQueueBuffer
Packets Packets
with those packets and enqueue it Packets
2 Packets
1 0
for play-out.
Packets Packets


A complex thing!

• What if we want to see that data after it’s
been decoded to PCM and is about to be
played?
• e.g., spectrum analysis, effects, visualizers
• AudioQueue design is “ﬁre-and-forget”


AudioQueue Tap!

http://www.last.fm/music/Spinal+Tap

AudioQueueProcessingTap

• Set as a property on the Audio Queue
• Calls back to your function with decoded
(PCM) audio data
• Three types: pre- or post- effects (that the
AQ performs), or siphon. First two can
modify the data.
• Only documentation is in AudioQueue.h


Creating an AQ Tap
! ! // create the tap
! ! UInt32 maxFrames = 0;
! ! AudioStreamBasicDescription tapFormat = {0};
! ! AudioQueueProcessingTapRef tapRef;
! ! CheckError(AudioQueueProcessingTapNew(audioQueue,
! ! ! ! ! ! ! ! ! ! ! tapProc,
! ! ! ! ! ! ! ! ! ! ! (__bridge void *)(player),
! ! ! ! ! ! ! ! ! ! ! kAudioQueueProcessingTap_PreEffects,
! ! ! ! ! ! ! ! ! ! ! &maxFrames,
! ! ! ! ! ! ! ! ! ! ! &tapFormat,
! ! ! ! ! ! ! ! ! ! ! &tapRef),
! ! ! ! "couldn't create AQ tap");

Notice that you receive maxFrames and tapFormat. These do not appear to be settable.


AQ Tap Proc
void tapProc (void * inClientData,
! ! ! AudioQueueProcessingTapRef inAQTap,
! ! ! UInt32 inNumberFrames,
! ! ! AudioTimeStamp * ioTimeStamp,
! ! ! UInt32 * ioFlags,
! ! ! UInt32 * outNumberFrames,
! ! ! AudioBufferList * ioData) {
! CCFWebRadioPlayer *player =
(__bridge CCFWebRadioPlayer*) inClientData;
! UInt32 getSourceFlags = 0;
! UInt32 getSourceFrames = 0;
! AudioQueueProcessingTapGetSourceAudio(inAQTap,
! ! ! ! ! ! ! ! ! ! inNumberFrames,
! ! ! ! ! ! ! ! ! ! ioTimeStamp,
! ! ! ! ! ! ! ! ! ! &getSourceFlags,
! ! ! ! ! ! ! ! ! ! &getSourceFrames,
! ! ! ! ! ! ! ! ! ! ioData);
// then do something with ioData
// ...

So what should we do
with the audio?


So what should we do
with the audio?

Let’s apply our pitch-shift effect


Shouldn’t this work?

AUEffect


Shouldn’t this work?

AUEffect
AudioUnitRender()


AudioUnitRender()
• Last argument is an AudioBufferList, whose
AudioBuffer members have mData pointers
• If mData != NULL, audio unit does its
thing with those samples
• If mData == NULL, audio data pulls from
whatever it’s connected to
• So we just call with AudioBufferList ioData
we got from tap callback, right?


Psych!

• AQ tap provides data as signed ints
• Effect units only work with ﬂoating point
• We need to do an on-the-spot format
conversion


invalidname’s convert-
and-effect recipe
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList * ioData) {
CCFWebRadioPlayer *player = (__bridge CCFWebRadioPlayer*) inRefCon;

// read from buffer
ioData->mBuffers[0].mData = player.preRenderData;

return noErr;
}

AUConverter AUEffect AUConverter AUGenericOutput

Note: red arrows are ﬂoat format, yellow arrows are int


How it works

• AUGraph: AUConverter → AUEffect →
AUConverter → AUGenericOutput
• Top AUConverter is connected to a render
callback function


The trick!
• Copy mData pointer to a state variable and
NULL it in ioData
• Call AudioQueueRender() on output unit.
The NULL makes it pull from the graph.
• Top of the graph pulls on render callback,
which gives it back the mData we copied
off.


Yes, really
This is the rest of tapProc()
! // copy off the ioData so the graph can read from it
// in render callback
! player.preRenderData = ioData->mBuffers[0].mData;
! ioData->mBuffers[0].mData = NULL;
!
! OSStatus renderErr = noErr;
! AudioUnitRenderActionFlags actionFlags = 0;
! renderErr = AudioUnitRender(player.genericOutputUnit,
! ! ! ! ! ! ! ! &actionFlags,
! ! ! ! ! ! ! ! player.renderTimeStamp,
! ! ! ! ! ! ! ! 0,
! ! ! ! ! ! ! ! inNumberFrames,
! ! ! ! ! ! ! ! ioData);
! NSLog (@"AudioUnitRender, renderErr = %ld",renderErr);
}

Yes, really
This is the render callback that supplies data to the int→ﬂoat converter

! ! ! ! ! ! ! ! ! AudioUnitRenderActionFlags *ioActionFlags,
! ! ! ! ! ! ! ! ! const AudioTimeStamp *inTimeStamp,
! ! ! ! ! ! ! ! ! UInt32 inBusNumber,
! ! ! ! ! ! ! ! ! UInt32 inNumberFrames,
! ! ! ! ! ! ! ! ! AudioBufferList * ioData) {
! CCFWebRadioPlayer *player =
(__bridge CCFWebRadioPlayer*) inRefCon;
!
! // read from buffer
! ioData->mBuffers[0].mData = player.preRenderData;

! return noErr;
}


Demo: AQ Tap +
AUNewTimePitch
New in iOS 6!


Other new stuff


Multi-Route
• Ordinarily, one input or output is active:
earpiece, speaker, headphones, dock-
connected device
• “Last in wins”
• With AV Session “multi-route” category,
you can use several at once
• WWDC 2012 session 505


Utility classes moved
again
• C++ utilities, including the CARingBuffer
• < Xcode 4.3, installed into /Developer
• Xcode 4.3-4.4, optional download from
developer.apple.com
• ≧ Xcode 4.5, sample code project “Core
Audio Utility Classes”


Takeaways
• Core Audio fundamentals never change
• New stuff is added as properties, typedefs,
enums, etc.
• Watch the SDK API diffs document to ﬁnd
the new stuff
• Hope you like header ﬁles and
experimentation


Q&A
• Slides will be posted to slideshare.net/
invalidname
• Code will be linked from there and my blog
• Watch CocoaConf glassboard,
@invalidname on Twitter/ADN, or [Time
code]; blog for announcement
• Thanks!


Core Audio in iOS 6 - What You'll Learn

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