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.