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.