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 or digital-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 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.
.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 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.
.SH AUTHOR
Originally written in C++ by Fons Adriensen, ported to C by Torben Hohn.