jack_iodelay man page on Mageia

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JACK_IODELAY(1)						       JACK_IODELAY(1)

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 mea‐
       sures 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 correspond‐
       ing 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 mea‐
       surement 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.

1.9.9.5				 October 2013		       JACK_IODELAY(1)
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