TL;DR need help getting overdub sync in SooperLooper
Background
I've been using Ubuntu Studio 22.04 very happily since October 2022. However now I'm trying to dive deeper into latency fine-tuning without success. I'm using SooperLooper, connected in Carla / jack / ALSA with a USB interface. Because all of this is so great out of the box, I'm able to run jack at 64 / 2 and it almost feels real in terms of timing. But there is a little delay and that's what I'm trying to tune.
Troubleshooting
A command line tool called jack_iodelay tells me that jack -I and -O values should be both set to 32, and that I'm getting a 64 sample loopback delay. (SIDENOTE: jack_iodelay always suggests half the buffer no matter how many frames per buffer). So I set Studio Controls input and output latency accordingly. My test is to record four drumstick clicks in SL, then record a new track of stick clicks played to track 1, I'll usually play along until I hear sync in the headphones (direct monitoring the mic track), then record one measure, redoing it if loop play sounds off. I feel that one generation of this is within the Haas window. However, if I silence track 1, and repeat the process playing to track 2, then silence track 2 and play to track 3, etc, until there are 6 tracks, then play track 1 and 6 together, there is an audible delay (track 6 is late). Recording a bit of this output in audacity and measuring peak-to-peak, the accumulated error is something like 1500-2300 samples (yes, that variance is just how human I am!).
Where to apply dubious latency compensation values
That suggests that I should be adding another 300-460 samples of compensation. Is that reasonable? Is jack_iodelay the right tool to use, and are the Studio Controls latency parameters the right place to enter the i/o delay values? I should add that SL is set to automatically obtain latency from jack. Alternatively, I tried entering half the buffer size in SL's latency i/o settings, and the track 1 to 6 delay was greater. Also, Studio Controls documentation seems to only go up to Ubuntu 20.04, are there any newer docs?
