Audiobus is an iOS app that allows other apps to work together as an audio-processing toolchain: play your MIDI keyboard into one app, run it through filters in other apps, and mix it in a third. All in real-time, foreground or background. That such a thing is possible on the locked down iOS platform is remarkable enough, but what's even more remarkable is that hundreds of audio apps have added Audiobus support in the few months since its debut, including Apple's own GarageBand. In this session, we'll take a look at the Audiobus SDK and see how to create inputs, outputs, and filters that can be managed by the Audiobus app to process audio in collaboration with other apps on the device.

1. Get on the Audiobus
Chris Adamson • @invalidname
CocoaConf Boston • October, 2013
Slides & code will be posted to the CocoaConf Glassboard,
and announced on my Twitter & app.net (@invalidname)

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

4. 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

5. 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.

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

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

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

9. Mixing between apps
AURemoteIO
AURemoteIO

10. Mixing between apps
AURemoteIO
!
AURemoteIO

11. 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.

12. 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

13. Enter Audiobus

14. 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!

15. Demo

16. 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

17. What Audiobus Isn’t
• Audiobus is not a general-purpose systemlevel 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

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

19. 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

20. 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

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

22. 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

23. An Audiobus
Integration Case Study

24. Audiobus Web Radio
• Web Radio app developed as in-class exercise
for Thursday's all-day Core Audio class
• Will also be at CocoaConf Atlanta
• Uses Audio File Stream to receive packets of
MP3 and play them with Audio Queue
Packets
Packets
2
Packets
Packets
Packets
1
0
Packets

25. 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

26. Offline Queues
Packets
Packets
2
Packets
1
Packets
Packets
0
Packets
AURemoteIO
AudioQueueOfflineRender()

27. 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

29. 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 keep running when
Audiobus is in foreground

31. 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”

33. 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!)

35. 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

37. 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

38. 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

39. Setup Your 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

40. 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

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

42. 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

43. 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];

44. Demo

45. Demo

46. 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)

47. 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.

48. As for iOS 7…

50. iOS 7 Inter-App Audio

51. 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

52. 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

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