Get On The Audiobus (CocoaConf Columbus, September 2013)

Get on the Audiobus
Chris Adamson • @invalidname
CocoaConf Columbus • September 2013
Slides & code will be posted to the CocoaConf
Glassboard, and announced on my Twitter & app.net
Sunday, September 29, 13

Roadmap
• How audio works on iOS
• What Audiobus is and how it connects apps
• Adopting Audiobus in your audio app

Audio on iOS
• Each app is responsible for its own audio
• No access to audio to/from other apps
• Apps use the Audio Session API to interact
with the system
• See also AVAudioSession in AV
Foundation

Audio Session
• Allows inspection of hardware properties
(sampling rate, hardware latencies) and
negotiation of access to system audio
resources
• Audio “category” declares what your app does
with audio
• This affects things like whether you mix with
other apps’ audio, honor ring/silent, can play
in background, etc.

Audio Categories
• Ambient
• Solo Ambient
• Playback
• Record
• Play and Record
• Audio Processing
• Multi-Route

Audio Engines
• How your app interacts with audio
hardware, i.e., captures or produces sound
• OpenAL (play-out only)
• Audio Queue
• Audio Units

AURemoteIO
AURemoteIO
Bus 0: audio out
Bus 1: audio in

Mixing between apps
AURemoteIO AURemoteIO

Mixing between apps
AURemoteIO AURemoteIO
!

Every App is an Island
• Only awareness of other apps’ audio is
value of
kAudioSessionProperty_OtherAudioIsPlaying
property
• No access to what other apps are playing
audio, how loud it is, etc.

Which means…
• You can’t record audio from one app in
another app
• Production apps can’t specialize; have to
provide everything (instruments, ﬁlters/
effects, recording) that you’d ever need

Enter Audiobus

Audiobus
• Standalone app that coordinates inter-app
audio
• Currently on 50% off sale ($4.99)
• Only works with apps that adopt the
Audiobus API
• 300 and counting!

What Audiobus Is
• Audiobus is an app for users to coordinate
audio across supported apps
• User decides which apps are the
inputs, effects, and outputs

What Audiobus Isn’t
• Audiobus is not a general-purpose system-
level audio capture (like Audio Hijack on
Mac)
• Audiobus cannot get audio from or send
audio to an arbitrary app
• Apps must adopt the Audiobus SDK and
register with the Audiobus website

How the heck does it
even work?
Considering that inter-app communication
is nearly impossible on iOS…

Secret Sauce!
• Audiobus began with MIDI “System Exclusive” (SysEx)
messages, deﬁned as being arbitrary blobs of data
unique to a given MIDI device
• Originally meant for synths to exchange waveforms,
patches or other software/ﬁrmware upgrades, etc.
• MIDI messages available to all interested apps via Core
MIDI
• Later switched to Mach Ports, which Core MIDI is
implemented atop

Audiobus Concepts
• Apps take on roles based on their
relationship to Audiobus
• Inputs produce audio
• Outputs receive audio
• Filters receive from inputs and send to
outputs
• Points of connection are called ports

http://developer.audiob.us/doc/index.html

Basic Audiobus
Integration
• Decide if you’re an input, output, or ﬁlter
• Decide if you can work with the Remote
IO unit or Audiobus’ port API
• Adopt the Audiobus SDK to connect to
Audiobus at runtime
• Register at audiob.us

An Audiobus
Integration Case Study

Audiobus Web Radio
• Web Radio app developed as in-class
exercise for all-day Core Audio class
• Coming to CocoaConfs Boston & Atlanta
• Uses Audio File Stream to receive packets
of MP3 and play them with Audio Queue
012
Packets
Packets
Packets
Packets
Packets
Packets

LPCM or GTFO
• Audiobus ports and AURemoteIO only
work with uncompressed LPCM audio
• Web radio app is dealing in MP3 or AAC
• Conversion to LPCM happens inside the
Audio Queue

Ofﬂine Queues
012
AURemoteIO
Packets
Packets
Packets
Packets
Packets
Packets
AudioQueueOfﬂineRender()

Libraries
• Download the Audiobus SDK from
developer.audiob.us
• Add libAudiobus.a and the Audiobus
headers to your project
• Add Accelerate,AudioToolbox,
QuartzCore, CoreGraphics, and Security
frameworks to your project

Enable background audio
• Add “audio” to the app’s “Required
Background Modes” if it’s not already
present
• All Audiobus-enabled apps must
participate in backgrounding, since they
must be able to be running when
Audiobus is in foreground

Create a launch URL
scheme
• You must have a URL scheme for your app
that ends in “.audiobus” for Audiobus to be
able to launch you
• Add this to the target’s “URL Types”

Get Audiobus API key
• For apps registered with iTunes Connect,
submit your App Store URL or ID
• For unpublished apps or tinkering, register
for a temporary ID, good for 14 days
• This requires dropping the Info.plist from
your app bundle (not from project!)

Audiobus API Keys
• Audiobus app gets a master list of known
keys from a server every 30 minutes
• For temporary IDs, click the link from the
developer page on the device that you’re using
Audiobus on to register your App ID
• e.g., audiobus-registry://
developer.audiob.us/tempreg?t=0ff37
• The dev page can mail you the link

Set Audiobus-compatible
behaviors
• Audio Session category must be playback
or play-and-record
• Must also set the mix-with-others property
on the audio session
• Often do both these things in the
AppDelegate

UInt32 audioCategory = kAudioSessionCategory_MediaPlayback;
AudioSessionSetProperty(kAudioSessionProperty_AudioCategory,
!! ! ! ! ! ! ! sizeof(audioCategory),
!! ! ! ! ! ! ! &audioCategory);
UInt32 allowMixing = YES;
AudioSessionSetProperty(kAudioSessionProperty_OverrideCategory
MixWithOthers,
! ! ! ! ! ! ! ! ! sizeof (allowMixing),
! ! ! ! ! ! ! ! ! &allowMixing);
Note:Audio Session is deprecated in iOS 7
Modern apps can use the AV Foundation equivalents

SetupYour App’s Audio
System
• Your app’s audio infrastructure needs to be up
and running before you connect to Audiobus
• If you’re going to send audio via the
ABAudioBusAudioUnitWrapper, you’ll need
to initialize your AURemoteIO
• For the web radio app, I send an
NSNotiﬁcation once the player class starts
playing

Instantiate
ABAudiobusController
• ABAudiobusController is your app’s
connection to Audiobus; probably gets held
as a strong property somewhere
• init method takes the launch URL and your
API key
• Obviously, these must match what you
registered on audiob.us

self.audiobusController = [[ABAudiobusController alloc]
! ! ! ! ! ! initWithAppLaunchURL:
[NSURL URLWithString:@"audiobuswebradio.audiobus://"]
! ! ! ! ! ! apiKey:AUDIOBUS_API_KEY];

Create ABOutputPort
(and Audio Unit wrapper)
• ABOutputPort sends audio to Audiobus
(via ABOutputPortSendAudio() function)
• If you use a RemoteIO unit for your
output, the ABAudiobusAudioUnitWrapper
will make these calls for you

CCFViewController *vc =
(CCFViewController*) self.window.rootViewController;
ABOutputPort *output = [self.audiobusController
! ! ! ! ! ! addOutputPortNamed:@"Audio Output"
! ! ! ! ! ! title:NSLocalizedString(@"Main App Output", @"")];
self.audiobusAudioUnitWrapper = [[ABAudiobusAudioUnitWrapper alloc]
! ! ! ! ! ! initWithAudiobusController:self.audiobusController
! ! ! ! ! ! ! ! audioUnit:vc.player.remoteIOUnit
! ! ! ! ! ! ! ! output:output
! ! ! ! ! ! ! ! input:nil];

Ports
• Apps that don’t use the Audio Unit
Wrapper use ports directly instead
• ABOutputPortSendAudio() for senders
(inputs and ﬁlters)
• Block-based callback or poll with
ABInputPortReceive() for receivers (ﬁlters
and outputs)

Filters & Outputs
• If you produce audio output based on
input, you need to tell the ABInputPort, so
that the signal isn’t doubled in Audiobus.

As for iOS 7…

Sherlocked?
Embrace & Extend!
From: michael@audiob.us
Date: June 19, 2013
Subject: Important Information Regarding Audiobus, iOS 7 and Inter-App
Audio
iOS 7 introduces many new features, including Apple’s own Inter-App
Audio framework which we’re planning to incorporate into Audiobus so you
don’t have to. For details and further discussion, we highly recommend
checking out our thread on the Apple developer forums:
https://devforums.apple.com/thread/191197

Closing Thoughts
• Audiobus is approachable for developers
already working at the Audio Unit level
• Adding Audiobus will get it seen by users
who’ve proven willing to pay for good apps
(we love music app users!)
• Future-proofed for iOS 7

Q&A
Slides & code will be posted to the CocoaConf Glassboard,
and announced on my Twitter & app.net (@invalidname)

Get On The Audiobus (CocoaConf Columbus, September 2013)

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