I have measured hardware latency using the Ardour6 “Audio/MIDI Setup” dialog multiple times (once per day or so) and I got differing results (differs about 12ms=600 samples). That means that after the calibration, latency compensation works perfect, but when hardware latency changes, the old correction values are wrong and my recordings are out of sync. At the moment I cannot reliably reproduce the problem; it just happens from time to time.
I thought hardware latency is (roughly) constant. Do you have an idea what could possibly cause this big deviation?
I use a Focusrite Scarlett 6i6 1st gen USB audio interface with a thinkpad X250 laptop. I use Linux Mint 19.3, but with an realtime kernel (which is not strictly necessary for me, because I use direct monitoring anyway).
I use alsa (not jack), 48kHz, 1024 samples, 2 periods. I configure all this within Ardour6, using the Audio setup dialog.
I check the settings from time to time by routing Ardour’s click over a cable and then recording it. When playing, the recording should be in perfect sync with Ardour’s click. With a fresh calibration, this is the case.
My measurements differ by about 600 samples@48KHz=12ms. I.e. the hardware input and output latency settings differ by 300 each.