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 Portland • August 2013
Friday, August 16, 13

2. Friday, August 16, 13

3. Roadmap
• How audio works on iOS
• What Audiobus is and how it connects apps
• Adopting Audiobus in your audio app
Friday, August 16, 13

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
Friday, August 16, 13

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.
Friday, August 16, 13

6. Audio Categories
• Ambient
• Solo Ambient
• Playback
• Record
• Play and Record
• Audio Processing
• Multi-Route
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7. Audio Engines
• How your app interacts with audio
hardware, i.e., captures or produces sound
• OpenAL (play-out only)
• Audio Queue
• Audio Units
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8. AURemoteIO
AURemoteIO
Bus 0: audio out
Bus 1: audio in
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9. Mixing between apps
AURemoteIO AURemoteIO
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10. Mixing between apps
AURemoteIO AURemoteIO
!
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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, what it is, etc.
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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
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13. Enter Audiobus
Friday, August 16, 13

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
• 250 and counting!
Friday, August 16, 13

15. Demo
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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
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17. 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
Friday, August 16, 13

18. How the heck does it
even work?
Considering that inter-app communication
is nearly impossible on iOS…
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19. Secret Sauce!
• Audiobus uses 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
Friday, August 16, 13

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
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21. http://developer.audiob.us/doc/index.html
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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
Friday, August 16, 13

23. An Audiobus
Integration Case Study
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24. 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
Friday, August 16, 13

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
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26. Offline Queues
012
AURemoteIO
Packets
Packets
Packets
Packets
Packets
Packets
AudioQueueOfflineRender()
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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
Friday, August 16, 13

28. Friday, August 16, 13

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 be running when
Audiobus is in foreground
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30. Friday, August 16, 13

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”
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32. Friday, August 16, 13

33. Get Audiobus API key
• For published apps, 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!)
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34. Friday, August 16, 13

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
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36. Friday, August 16, 13

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
Friday, August 16, 13

38. UInt32 audioCategory = kAudioSessionCategory_MediaPlayback;
AudioSessionSetProperty(kAudioSessionProperty_AudioCategory,
! ! ! ! ! ! ! ! sizeof(audioCategory),
! ! ! ! ! ! ! ! &audioCategory);
UInt32 allowMixing = YES;
AudioSessionSetProperty(kAudioSessionProperty_OverrideCategory
MixWithOthers,
! ! ! ! ! ! ! ! ! sizeof (allowMixing),
! ! ! ! ! ! ! ! ! &allowMixing);
Friday, August 16, 13

39. 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
Friday, August 16, 13

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
Friday, August 16, 13

41. self.audiobusController = [[ABAudiobusController alloc]
! ! ! ! ! ! initWithAppLaunchURL:
[NSURL URLWithString:@"audiobuswebradio.audiobus://"]
! ! ! ! ! ! apiKey:AUDIOBUS_API_KEY];
Friday, August 16, 13

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
Friday, August 16, 13

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];
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44. Demo
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45. Demo
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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)
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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.
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48. As for iOS 7…
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49. Friday, August 16, 13

50. 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
Friday, August 16, 13

51. 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
Friday, August 16, 13

52. Q&A
Slides & code will be posted to the CocoaConf Glassboard,
and announced on my Twitter & app.net (@invalidname)
Friday, August 16, 13