[Jack-Devel] Using USB sound card with Jack

[Chris Caudle]

On Tue, January 23, 2018 10:12 am, Benny Alexandar wrote:
> Any documentation on how to use the jack_delay for measuring ?

$ man jack_iodelay

NAME
       jack_iodelay - JACK toolkit client to measure roundtrip latency

SYNOPSIS
       jack_iodelay

DESCRIPTION
       jack_iodelay  will  create  one  input and one output port, and
then measures the latency
       (signal delay) between them. For this to work, the output port must
be connected  to  its
       input port. The measurement is accurate to a resolution of greater
than 1 sample.

       The  expected  use  is to connect jack_iodelay's output port to a
hardware playback port,
       then use a physical loopback cable from the corresponding hardware
output connector to an
       input  connector,  and  to connect that corresponding hardware
capture port to jack_iode‐
       lay's input port. This creates a roundtrip that goes through  any 
analog-to-digital  and
       digital-to-analog converters that are present in the audio hardware.

       Although  the  hardware  loopback latency is the expected use, it
is also possible to use
       jack_iodelay to measure the latency along any fully connected
signal path, such as  those
       involving other JACK clients.

       Once  jack_iodelay  completes  its  measurement  it  will  print
the total latency it has
       detected. This will include the JACK buffer length in addition to
any  other  latency  in
       the  signal  path.  It  will continue to print the value every 0.5
seconds so that if you
       wish you can vary aspects of the signal path to see their effect on
the measured latency.

       If no incoming signal is detected from the input port, jack_iodelay
will print

        Signal below threshold... .

       every second until this changes (e.g. until you establish the
correct connections).

       To use the value measured by jack_iodelay with the -I and -O
arguments of a JACK  backend
       (also  called Input Latency and Output Latency in the setup dialog
of qjackctl), you must
       subtract the JACK buffer size from the result. The buffer size is
determined by multiply‐
       ing  the  number  of  frames per period (given to the jackd backend
by the -p or --period
       option) by the number of periods per buffer (given to the jackd 
backend  by  the  -n  or
       --nperiods  option).   Note  that JACK2 will add an implicit
additional period when using
       the default asynchronous mode, so for JACK1 or JACK2 in synchronous
mode, the buffer size
       is  n*p,  but  for  JACK2 in asynchronous mode the buffer size is
(n+1)*p.  Once the JACK
       buffer size is subtracted from the measured latency, the result is 
the  "extra"  latency
       due  to the interface hardware. Then, if you believe that the
latency is equally distrib‐
       uted between the input and output parts of your audio hardware
(extremely likely), divide
       the  result  by two and use that for input and output latency
values. Doing this measure‐
       ment will enable JACK clients that use the JACK latency API to
accurately  position/delay
       audio  to keep signals synchronized even when there are inherent
delays in the end-to-end
       signal pathways.

AUTHOR
       Originally written in C++ by Fons Adriaensen, ported to C by Torben
Hohn.

1.9.10                                     August 2017                    
      JACK_IODELAY(1)


-- 
Chris Caudle