Good point. Following was missing:
patch@patchbox:~ $ jack_connect jack_delay:out system:playback_1
patch@patchbox:~ $ jack_connect jack_delay:in system:capture_1
Now the measurement is working. I was a bit surprised with the result though. I measured the 64 samples 48kHz 2 periods configurations and got:
235.286 frames 4.902 ms total roundtrip latency
extra loopback latency: 43 frames
use 21 for the backend arguments -I and -O ??
I was expecting roughly 2 * 64 / 48000 + ~1ms (CODEC) =~ 3.7ms. jackd is running on sync mode so I assumed no extra buffering period:
patch@patchbox:~ $ ps -aux | grep jack
(...) /usr/bin/jackd -t 2000 -R -P 95 -d alsa -d hw:pisound -r 48000 -p 64 -n 2 -X seq -s -S
How should I interpret this: is the Pisound CODEC taking 3.3ms latency? Or is jackd somehow introducing additional buffering, given that it reports 43 frames of extra latency (235.286 - 43 ~= 192 frames expected? )?