If I remember correctly, the original jack_delay didn't even show the "extra loopback latency" information nor the suggestion of using some number of frames for -I and -O arguments, so that might be just a false indication, but it is strange.
Still, you can calculate the extra loopback latency yourself and then divide by two to put those numbers in the -I and -O arguments.
Make sure to run jack_iodelay with latency I/O at zero (default).
When you make the virtual connections, you will see a message in the output of jack_iodelay:
"new playback latency" and "new capture latency". The numbers depend on jack settings. In my case:
Code: Select all
Signal below threshold...
Signal below threshold...
new playback latency: [2048, 2048]
Signal below threshold...
Signal below threshold...
new capture latency: [1024, 1024]
4075.994 frames 84.917 ms total roundtrip latency
extra loopback latency: 1003 frames
use 501 for the backend arguments -I and -O
As shown above, somehow, I have 2048 frames of playback latency and 1024 frames of capture latency, so my total round trip latency would be 2048 + 1024 = 3072 frames... if there were not the extra latency caused by the hardware.
Jack_iodelay measures 4076 frames, so I would calculate 4076 - 3072 = 1004 frames of extra loopback latency and I would use 502 for -I and -O. Jack_iodelay calculates 1003 and 501, which is practically the same.
In any case, this guide [1] recommends against using a simple cable for closing the loop on onboard cards. This is not mentioned in the guide you linked:
If you want to measure the latency of a cheap, integrated sound card that only has line-output and mic-input you cannot close the loop with a simple patch cable: both connectors may be mechanically compatible, but electrically they are not designed to work together, so attempting to connect them may harm your audio interface.
[1]
https://wiki.linuxaudio.org/wiki/jack_latency_tests
EDIT: My test is made with a USB audio card (Cakewalk UA-25 EX). An integrated card will probably give much less extra latency.