I’ll kick things off with the first thing I tried; Class compliant audio interface.
Setup: Digitone OS 1.30
USB Settings: USB/Audio
Audio Settings in AUM/Audiobus: 48KHz/256 frames buffer size.
Connected directly to 2018 iPad Pro 11" using a USB Type-C to USB Type-A adapter. iPad OS 13.5.1
Scenario: Hosting one AUv3 synth in AUM or Audiobus. Synth has hold on keys to produce a constant drone.
Issue: Audio randomly stops streaming to Digitone even though the meters clearly show audio coming out of the AUv3 synth. Killing and restarting AUM/Audiobus reconnects the audio. When the audio stops, it does not play over the iPad speakers suggesting the driver crashed or a cable was bumped to break the connection. There is simply no audio. The issue seems to happen when I just let the drone do its thing and don’t interact with the iPad. As though some sort of keepalive is missing.
While both AUM and Audiobus have a means to keep the screen active and audio in the background, I have tried disabling screen lock and Do Not Disturb to no effect. The behavior happens at random but it happens without fail and within 10 minutes or less. Usually within 5 minutes.
This same behavior does not occur with a 2nd Gen. Scarlett 2i4. I have not tested with the Analog Heat MKII.
If others can replicate this behavior then perhaps there is an issue. I can’t prove out iPad OS as the culprit but it does not happen with at least one other class compliant interface.
Update 1: Performed a second round of tests.
Scenario: Hosting Tal-U-No-LX v2 in AUM. Sequenced from Digitone.
Results: Audio stopped at 10m15s. Recovered audio by unplugging and replugging USB cable from the back of the Digitone. Audio then stopped again at 3m43s
Update 2: Audio can be restored by restarting the host app, unplugging and replugging the USB cable, or by changing Settings => System => USB Config from USB Audio/MIDI to USB MIDI and back. With the exception of restarting the host app, the latter two methods don’t require that the synth be stopped and started again. It will still produce levels on the meters and restart audio once connected again.
Update 3: Tested with powered USB hub. Same results. Audio stream stops within 10 minutes or less.