[Ardour-Users] Another newbie Question: Anyone having experiences with following setup: Ardour, Jack, Linux, Lexicon Omega (or other USB1-device) ?

[Chris Caudle]

On Mon, February 13, 2017 2:53 pm, Harry Rüter wrote:
> Latency housekeeping/Ardour can you tell more about that ?

You can use tools provided with jack or Ardour to measure the total delay
from output of jack software, through jackd, out through hardware, back
into the hardware input, back through jackd, and to the jack software. 
There is an assumption that output path and input path of the hardware
have symmetical delays, which may not be completely accurate but is
usually pretty close (under 1ms difference).  You can provide as an extra
parameter to jackd details on that delay time, and software can query
jackd to find the delay values set.
In that way Ardour can calculate that new audio input should be moved
earlier in time to account for the fact that what you heard played back
was delayed some number of milliseconds by transferring through jackd then
through the hardware.
The general term is called "latency compensation."

> My "problem" is, I think there could be problems when  doing
> multitracking.

When recording multiple tracks you should use the hardware monitoring
capability of the Lexicon audio  interface to make sure there is no
additional delay in hearing what you are playing.
You hear the playback of the first track, but if the delay value has been
set in jackd correctly, when your new input audio is recorded by Ardour,
Ardour marks that there should be e.g. 20ms difference between the
beginning of the first track playing and the new audio that corresponds to
the beginning arriving, so Ardour will move the new audio 20ms earlier.

You should be listening to the new audio you play using the hardware
monitoring of the Lexicon, so that there is not an additional in and out
delay of what you play.  That is really the only time that latency matters
for multitrack recording, if your audio interface does not support
hardware monitoring and you have to monitor what you play by sending to
jackd, then jackd sends back to the hardware output.  In that case the
additional 20ms of delay compared to when you hit a key or pluck a string
may be annoying.  Should not apply to your Lexicon.

> do I have to calibrate for my special hardware-latency ?

Yes, you have to start jackd, disconnect your audio interface from any
speakers, and connect the output of the audio interface back to the input
of the audio interface.  Make sure the hardware monitoring is set to 100%
playback, not mixing some of the direct input back into the output.
Ardour has a tool to measure the latency for you, or if you want to see
more details use jack_iodelay (see the manpage for jack_iodelay for
details; I assume at the moment you are using linux or OS X and have man
available. I think the help will be a different format for Windows).

-- 
Chris Caudle