iOS Audio Units - Connecting with Graphs? - ios

Question

I've jumped off the deep end, and have decided to figure out low-latency audio on iOS using Audio Units. I've read as much documentation (from Apple and forums galore) as I can find, and the overall concepts make sense, but I'm still scratching my head on some concepts that I need help with:
I saw somewhere that AU Graphs are deprecated and that I should instead connect Audio Units directly. I'm cool with that... but how? Do I just need to use the Connection property of an Audio Unit to connect it to a source AU, and off I go? Initialize and Start the Units, and watch the magic happen? (cause it doesn't for me...)
What's the best Audio Unit setup to use if I simply want to grab audio from my mic, do some processing to the audio data, and then store that audio data without sending it out to the RemoteIO speaker, bus 0 output? I tried hooking up a GenericOutput AudioUnit to catch the data in a callback without any luck...
That's it. I can provide code when requested, but it's way too late, and this has wiped me out. If there's know easy answer, that's cool. I'll send any code snippets at will. Suffice it to say, I can easily get a simple RemoteIO, mic in, speaker out setup working great. Latency seems non-existant (at least to my ears). I just want to do something with the mic data and store it in memory without it going out to the speaker. Eventually hooking in the eq and mixer would be hip, but one step at a time.
FWIW, I'm coding in Xamarin Forms/C# land, but code examples in Objective C, Swift or whatever is fine. I'm stuck on the concepts, not necessarily the exact code.
THANKS!

Answer 1

Working with audio units without a graph is pretty simple and very flexible. To connect two units, you call AudioUnitSetProperty this way :
AudioUnitConnection connection;
connection.sourceAudioUnit = sourceUnit;
connection.sourceOutputNumber = sourceOutputIndex;
connection.destInputNumber = destinationInputIndex;
AudioUnitSetProperty(
destinationUnit,
kAudioUnitProperty_MakeConnection,
kAudioUnitScope_Input,
destinationInputIndex,
&connection,
sizeof(connection)
);
Note that it is required for the units connected this way to have their Stream Format set uniformly and that it must be done before their initialization.

Answer 2

Your question mentions Audio Units, and Graphs. As said in the comments, the graph concept has been replaced with the idea of attaching "nodes" to an AVAudioEngine. These nodes then "connect" to other nodes. Connecting nodes creates signal paths and starting the engine makes it all happen. This may be obvious, but I am trying to respond generally here.
You can do this all in Swift or in Objective-C.
Two high level perspectives to consider with iOS audio are the idea of a "host" and that of a "plugin". The host is an app and it hosts plugins. The plugin is usually created as an "app extension" and you can look up audio unit extensions for more about that as needed. You said you have one doing what you want, so this is all explaining the code used in a host
Attach AudioUnit to an AVaudioEngine
var components = [AVAudioUnitComponent]()
let description =
AudioComponentDescription(
componentType: 0,
componentSubType: 0,
componentManufacturer: 0,
componentFlags: 0,
componentFlagsMask: 0
)
components = AVAudioUnitComponentManager.shared().components(matching: description)
.compactMap({ au -> AVAudioUnitComponent? in
if AudioUnitTypes.codeInTypes(
au.audioComponentDescription.componentType,
AudioUnitTypes.instrumentAudioUnitTypes,
AudioUnitTypes.fxAudioUnitTypes,
AudioUnitTypes.midiAudioUnitTypes
) && !AudioUnitTypes.isApplePlugin(au.manufacturerName) {
return au
}
return nil
})
guard let component = components.first else { fatalError("bugs") }
let description = component.audioComponentDescription
AVAudioUnit.instantiate(with: description) { (audioUnit: AVAudioUnit?, error: Error?) in
if let e = error {
return print("\(e)")
}
// save and connect
guard let audioUnit = audioUnit else {
print("Audio Unit was Nil")
return
}
let hardwareFormat = self.engine.outputNode.outputFormat(forBus: 0)
self.engine.attach(au)
self.engine.connect(au, to: self.engine.mainMixerNode, format: hardwareFormat)
}
Once you have your AudioUnit loaded, you can connect your Athe AVAudioNodeTapBlock below, it has more to it since it need to be a binary or something that other host apps that aren't yours can load.
Recording an AVAudioInputNode
(You can replace the audio unit with the input node.)
In an app, you can record audio by creating an AVAudioInputNode or just reference the 'inputNode' property of the AVAudioEngine, which is going to be connected to the system's selected input device(mic, line in, etc) by default
Once you have the input node you want to process the audio of, next "install a tap" on the node. You can also connect your input node to a mixer node and install a tap there.
https://developer.apple.com/documentation/avfoundation/avaudionode/1387122-installtap
func installTap(onBus bus: AVAudioNodeBus,
bufferSize: AVAudioFrameCount,
format: AVAudioFormat?,
block tapBlock: #escaping AVAudioNodeTapBlock)
The installed tap will basically split your audio stream into two signal paths. It will keep sending the audio to the AvaudioEngine's output device and also send the audio to a function that you define. This function(AVAudioNodeTapBlock) is passed to 'installTap' from AVAudioNode. The AVFoundation subsystem calls the AVAudioNodeTapBlock and passes you the input data one buffer at a time along with the time at which the data arrived.
https://developer.apple.com/documentation/avfoundation/avaudionodetapblock
typealias AVAudioNodeTapBlock = (AVAudioPCMBuffer, AVAudioTime) -> Void
Now the system is sending the audio data to a programmable context, and you can do what you want with it.
To use it elsewhere, you can create a separate AVAudioPCMBuffer and write each of the passed in buffers to it in the AVAudioNodeTapBlock.