Set AVAudioEngine Input and Output Devices

Question

I've been playing around with Apple's shiny new AVFoundation library, but so far I've unable to set the input or output devices (e.g. a USB sound card) used by an AVAudioEngine, and I can't seem to find anything in the documentation to say it's even possible.

Does anyone have any experience with this?

Equality

As a long shot, I tried casting the mainMixerNode of an AVAudioEngine to an AVAudioIONode which actually worked (for some reason?) - it's a start. – Equality

Answer 1

Ok, after re-reading the docs for the 10th time, I noticed AVAudioEngine has members inputNode and outputNode (not sure how I missed that!).

The following code seems to do the job:

AudioDeviceID inputDeviceID = 53; // get this using AudioObjectGetPropertyData
AVAudioEngine *engine = [[AVAudioEngine alloc] init];
AudioUnit audioUnit = [[engine inputNode] audioUnit];

OSStatus error = AudioUnitSetProperty(audioUnit,
                                      kAudioOutputUnitProperty_CurrentDevice,
                                      kAudioUnitScope_Global,
                                      0,
                                      &inputDeviceID,
                                      sizeof(inputDeviceID));

I borrowed the non-AVFoundation C code from the CAPlayThrough example.

Equality

When I try to do this for the output node I get the error required condition is false: numChannelsAggDevice >= numChannelsSubDevice. – Brynhild

Did you get the correct device ID using AudioObjectGetPropertyData, or just use 53? How many channels does your input device have? – Equality

I got the correct device ID and the return status was 0, but no output came out of the device. – Brynhild

Sorry if I'm stating the obvious, but did you change [engine inputNode] to [engine outputNode] (assuming you're setting the output device)? – Equality

I did indeed (after making that mistake the first time I tried) and still no luck. – Brynhild

I'm outta ideas besides ruling out any hardware/driver issues, but if you wanna post your code showing usage of your audio engine I'd be happy to take a squiz. – Equality

The code is rather large an complex unfortunately, I just came across this and tried it as it looked like it would help solve a problem I'm having and thought I'd see if there was something basic I'm doing wrong. Ah well, thanks for your help! – Brynhild

Thanks for the example! Been trying this out for a while now. Setting the output works, but somehow I can't get the input working. I keep getting the following error: required condition is false: [AVAudioEngineGraph.mm:1345:Initialize: (IsFormatSampleRateAndChannelCountValid(outputHWFormat))] Not sure what I'm doing wrong. Do you have any suggestions? – Poetics

@FreekSanders it's hard to say without seeing the rest of your code. If you create a new question and link to it, I'll take a look. – Equality

@benjineer thanks in advance. I've posted a shortened version of my code here: https://mcmap.net/q/907105/-swift-avaudioengine-changing-the-audio-input-device-for-macos/2500428 – Poetics

Tbh, this is ridiculous how bad Apple documentation and functionality for selecting devices is... I've been reading for a couple of days, and OMG. – Embouchure

Answer 2

Here's a complete, if somewhat rough, function which will play some audio for testing purposes (pick a different the file if you don't have GarageBand installed there, of course). To avoid hard-coding a device ID, it switches to your alert ("sound effects") device which you can set in System Preferences.

AVAudioEngine *engine = [[AVAudioEngine alloc] init];
AudioUnit outputUnit = engine.outputNode.audioUnit;

OSStatus err = noErr;
AudioDeviceID outputDeviceID;
UInt32 propertySize;

AudioObjectPropertyAddress propertyAddress = {
    kAudioHardwarePropertyDefaultSystemOutputDevice,
    kAudioObjectPropertyScopeGlobal,
    kAudioObjectPropertyElementMaster };
propertySize = sizeof(outputDeviceID);
err = AudioObjectGetPropertyData(kAudioObjectSystemObject, &propertyAddress, 0, NULL, &propertySize, &outputDeviceID);
if (err) { NSLog(@"AudioHardwareGetProperty: %d", (int)err); return; }

err = AudioUnitSetProperty(outputUnit, kAudioOutputUnitProperty_CurrentDevice, kAudioUnitScope_Global, 0, &outputDeviceID, sizeof(outputDeviceID));
if (err) { NSLog(@"AudioUnitSetProperty: %d", (int)err); return; }

NSURL *url = [NSURL URLWithString:@"/Applications/GarageBand.app/Contents/Frameworks/MAAlchemy.framework/Versions/A/Resources/Libraries/WaveNoise/Liquid.wav"];
NSError *error = nil;
AVAudioFile *file = [[AVAudioFile alloc] initForReading:url error:&error];
if (file == nil) { NSLog(@"AVAudioFile error: %@", error); return; }

AVAudioPlayerNode *player = [[AVAudioPlayerNode alloc] init];
[engine attachNode:player];
[engine connect:player to:engine.outputNode format:nil];

NSLog(@"engine: %@", engine);

if (![engine startAndReturnError:&error]) {
    NSLog(@"engine failed to start: %@", error);
    return;
}

[player scheduleFile:file atTime:[AVAudioTime timeWithHostTime:mach_absolute_time()] completionHandler:^{
    NSLog(@"complete");
}];
[player play];

Ferren

This is great, was getting errors passing a format to [engine connect] - passing nil does the trick – Equality