使用 AVAudioSequencer 将 MIDI 发送到第三方 AUv3 乐器

Question

I'm having trouble controlling third-party AUv3 instruments with MIDI using AVAudioSequencer (iOS 12.1.4, Swift 4.2, Xcode 10.1) and would appreciate your help.

What I'm doing currently:

• Get all AUs of type kAudioUnitType_MusicDevice .

• Instantiate one and connect it to the AVAudioEngine .

• Create some notes, and put them on a MusicTrack .

• Hand the track data over to an AVAudioSequencer connected to the engine.

• Set the destinationAudioUnit of the track to my selected Audio Unit.

So far, so good, but...

When I play the sequence using AVAudioSequencer it plays fine the first time, using the selected Audio Unit. On the second time I get either silence, or a sine wave sound (and I wonder who is making that). I'm thinking the Audio Unit should not be going out of scope in between playbacks of the sequence, but I do stop the engine and restart it again for the new round. (But it should even be possible to swap AUs while the engine is running, so I think this is OK.)

Are there some steps that I'm missing? I would love to include code, but it is really hard to condense it down to its essence from a wall of text. But if you want to ask for specifics, I can answer. Or if you can point me to a working example that shows how to reliably send MIDI to AUv3 using AVAudioSequencer , that would be great.

Is AVAudioSequencer even supposed to work with other Audio Units than Apple's? Or should I start looking for other ways to send MIDI over to AUv3?

I should add that I can consistently send MIDI to the AUv3 using the InstrumentPlayer method from Apple's AUv3Host sample, but that involves a concurrent thread, and results in all sorts of UI sync and timing problems.

EDIT: I added an example project to GitHub: https://github.com/jerekapyaho/so54753738

It seems that it's now working in iPadOS 13.7, but I don't think I'm doing anything that different than earlier, except this loads a MIDI file from the bundle, instead of generating it from data on the fly.

If someone still has iOS 12, it would be interesting to know if it's broken there, but working on iOS 13.x (x = ?)

Answer 1

In case you are using AVAudioUnitSampler as an audio unit instrument, the sine tone happens when you stop and start the audio engine without reloading the preset. Whenever you start the engine you need to load any instruments back into the sampler (eg a SoundFont), otherwise you may hear the sine. This is an issue with the Apple AUSampler, not with 3rd party instruments.

Btw you can test it under iOS 12 using the simulator.