1JACK_IODELAY(1) General Commands Manual JACK_IODELAY(1)
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6 jack_iodelay - JACK toolkit client to measure roundtrip latency
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9 jack_iodelay
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12 jack_iodelay will create one input and one output port, and then mea‐
13 sures the latency (signal delay) between them. For this to work, the
14 output port must be connected to its input port. The measurement is
15 accurate to a resolution of greater than 1 sample.
16 The expected use is to connect jack_iodelay's output port to a hardware
18 playback port, then use a physical loopback cable from the correspond‐
19 ing hardware output connector to an input connector, and to connect
20 that corresponding hardware capture port to jack_iodelay's input port.
21 This creates a roundtrip that goes through any analog-to-digital and
22 digital-to-analog converters that are present in the audio hardware.
23 Although the hardware loopback latency is the expected use, it is also
25 possible to use jack_iodelay to measure the latency along any fully
26 connected signal path, such as those involving other JACK clients.
27 Once jack_iodelay completes its measurement it will print the total
29 latency it has detected. This will include the JACK buffer length in
30 addition to any other latency in the signal path. It will continue to
31 print the value every 0.5 seconds so that if you wish you can vary
32 aspects of the signal path to see their effect on the measured latency.
33 If no incoming signal is detected from the input port, jack_iodelay
35 will print
36 Signal below threshold... .
38 every second until this changes (e.g. until you establish the correct
40 connections).
41 To use the value measured by jack_iodelay with the -I and -O arguments
43 of a JACK backend (also called Input Latency and Output Latency in the
44 setup dialog of qjackctl), you must subtract the JACK buffer size from
45 the result. The buffer size is determined by multiplying the number of
46 frames per period (given to the jackd backend by the -p or --period
47 option) by the number of periods per buffer (given to the jackd backend
48 by the -n or --nperiods option). Note that JACK2 will add an implicit
49 additional period when using the default asynchronous mode, so for
50 JACK1 or JACK2 in synchronous mode, the buffer size is n*p, but for
51 JACK2 in asynchronous mode the buffer size is (n+1)*p. Once the JACK
52 buffer size is subtracted from the measured latency, the result is the
53 "extra" latency due to the interface hardware. Then, if you believe
54 that the latency is equally distributed between the input and output
55 parts of your audio hardware (extremely likely), divide the result by
56 two and use that for input and output latency values. Doing this mea‐
57 surement will enable JACK clients that use the JACK latency API to
58 accurately position/delay audio to keep signals synchronized even when
59 there are inherent delays in the end-to-end signal pathways.
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62 Originally written in C++ by Fons Adriaensen, ported to C by Torben
63 Hohn.
641.9.12 December 2019 JACK_IODELAY(1)