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| wiki:jack_latency_tests [2010/12/03 11:43] – rgareus | wiki:jack_latency_tests [2013/06/02 15:45] – external edit 127.0.0.1 |
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| ===== Measuring roundtrip latency with jack_delay ===== | ===== Measuring roundtrip latency with jack_delay ===== |
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| [[http://www.kokkinizita.net/linuxaudio/|jack_delay]] is an utility developed by Linux DSP guru Fons Adriaensen to measure the roundtrip latency of a closed loop audio chain. | [[http://kokkinizita.linuxaudio.org/linuxaudio/|jack_delay]] is an utility developed by Linux DSP guru Fons Adriaensen to measure the roundtrip latency of a closed loop audio chain. |
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| jack_delay works by emitting some rather annoying tones, capturing them again after a roundtrip through the whole chain and measuring the difference in phase so it can estimate with great accuracy how long has the whole process taken. This is no theoretical estimation, jack_delay is a measuring tool that will give you the real deal. | jack_delay works by emitting some rather annoying tones, capturing them again after a roundtrip through the whole chain and measuring the difference in phase so it can estimate with great accuracy how long has the whole process taken. This is no theoretical estimation, jack_delay is a measuring tool that will give you the real deal. |
| The [[http://jackaudio.org|JACK]] Audio Connection Kit has a few parameters to configure the latency. However the settings are constrained by hardware (audio-device, CPU and bus-speed). Lower latencies increase the load on the computer-system (it needs to process the audio in smaller chunks which arrive much more frequently). If the system can not keep up: an //x-run// (short for buffer over-run and buffer under-run) occurs which usually results in audible clicks or dropouts. | The [[http://jackaudio.org|JACK]] Audio Connection Kit has a few parameters to configure the latency. However the settings are constrained by hardware (audio-device, CPU and bus-speed). Lower latencies increase the load on the computer-system (it needs to process the audio in smaller chunks which arrive much more frequently). If the system can not keep up: an //x-run// (short for buffer over-run and buffer under-run) occurs which usually results in audible clicks or dropouts. |
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| Low-latency is not always a feature you want to have. It comes with a couple of drawbacks: the most prominent is increased power-consumption because the CPS needs to process many small chunks of audio-data, it is constantly active and can not enter power-saving mode. Furthermore, if more than one application (sound-processor) is involved in processing the sound, the operating system performs a context-switch to run each of these for each audio-cycle which results in a much higher system-load and an increased chance of x-runs. | Low-latency is not always a feature you want to have. It comes with a couple of drawbacks: the most prominent is increased power-consumption because the CPU needs to process many small chunks of audio-data, it is constantly active and can not enter power-saving mode. Furthermore, if more than one application (sound-processor) is involved in processing the sound, the operating system performs a context-switch to run each of these for each audio-cycle which results in a much higher system-load and an increased chance of x-runs. |
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| Stable low-latency (≤10ms) on GNU/Linux can usually only be achieved by running [[https://rt.wiki.kernel.org/|realtime-kernel]] (insert links to ''rtirq'' and setup/config info here). | Stable low-latency (≤10ms) on GNU/Linux can usually only be achieved by running [[https://rt.wiki.kernel.org/|realtime-kernel]] (insert links to ''rtirq'' and setup/config info here). |
| ===== References ===== | ===== References ===== |
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| * [[http://www.kokkinizita.net/linuxaudio/|jack_delay]] | * [[http://kokkinizita.linuxaudio.org/linuxaudio/|jack_delay]] |
| * [[http://linuxaudio.org/mailarchive/lau/2009/4/2/153739|Thread on LAU - using jdelay]] | * [[http://linuxaudio.org/mailarchive/lau/2009/4/2/153739|Thread on LAU - using jdelay]] |
| * [[http://linuxaudio.org/mailarchive/lau/2009/4/2/153759|Fons UA-5 measurement]] | * [[http://linuxaudio.org/mailarchive/lau/2009/4/2/153759|Fons UA-5 measurement]] |
| | * A script to acquire the data using ''jack_delay'' can the found at ''%%git://rg42.org/latentor%%'' or directly: [[http://rg42.org/gitweb/?p=latentor.git;a=snapshot;sf=tgz|latentor.tgz]]. |
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| ===== Raw data ===== | ===== Raw data ===== |
