JACK_IODELAY(1)JACK_IODELAY(1)NAMEjack_iodelay - JACK toolkit client to measure roundtrip latency
SYNOPSISjack_iodelayDESCRIPTIONjack_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 December 2012 JACK_IODELAY(1)