jack2/man/jack_iodelay.0

.TH JACK_IODELAY "1" "!DATE!" "!VERSION!"
.SH NAME
jack_iodelay \- JACK toolkit client to measure roundtrip latency
.SH SYNOPSIS
.B jack_iodelay
.SH DESCRIPTION
.B 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.
.PP
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.
.PP
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.
.PP
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.
.PP
If no incoming signal is detected from the input port, jack_iodelay
will print 
.PP
\fT Signal below threshold... .\fR
.PP
every second until this changes (e.g. until you establish the correct
connections).
.PP
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.
.SH AUTHOR
Originally written in C++ by Fons Adriaensen, ported to C by Torben Hohn.