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 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 multiplying 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 distributed 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
       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 Adriaensen, ported to C by Torben Hohn.

1.9.21                                             2024-04-06                                    JACK_IODELAY(1)