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_iodelay's
       input port. This creates a roundtrip that goes through any analog-to-digital  or  digital-
       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 period length in addition to any other latency in the
       signal  path.  It  will continue to print the value every 0.5 seconds or so 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 period size from the result. Then, if you believe that  the  latency  is
       equally  distributed  between the input and output parts of your audio hardware (extremely
       likely), divide the result by two and use that for  input  and/or  output  latency  value.
       Doing  this  measurement  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 Adriensen, ported to C by Torben Hohn.