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wiki:jack_latency_tests [2010/12/02 11:12] – lgarrido | wiki:jack_latency_tests [2014/03/18 11:39] (current) – [Interpretation and Analysis] lgarrido | ||
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I once watched this guy in an audio forum talk about how he got less latency if he launched jackd from the command-line, | I once watched this guy in an audio forum talk about how he got less latency if he launched jackd from the command-line, | ||
- | So, leaving aside buzzwords, hype and urban legends, what is actually latency? [[http:// | + | So, leaving aside buzzwords, hype and urban legends, what is actually latency? [[http:// |
In the audio world, latency is the time elapsed between an action that produces sound and the actual perception of that sound by a listener. | In the audio world, latency is the time elapsed between an action that produces sound and the actual perception of that sound by a listener. | ||
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There are many factors that contribute to the total latency of a given system. Some of the more relevant are: | There are many factors that contribute to the total latency of a given system. Some of the more relevant are: | ||
- | * Sound propagation | + | |
* Your acoustic guitar or piano has a latency of about 1-2 ms, due to the propagation of the sound between your instrument and your ear . | * Your acoustic guitar or piano has a latency of about 1-2 ms, due to the propagation of the sound between your instrument and your ear . | ||
* At a large concert venue if you are far away from the stage the sound will travel faster through the path " | * At a large concert venue if you are far away from the stage the sound will travel faster through the path " | ||
- | * Digital-to-Analog and Analog-to-Digital conversion. Electric | + | |
- | * Digital Signal Processing. Digital | + | |
- | * Computer I/O Architecture. A computer is a general purpose processor, not a digital audio processor. This means our audio data has to jump a lot of fences in his path from the outside to the CPU and back, contending in the process with some other parts of the system vying for the same resources (CPU time, bus bandwidth, etc.) Thanks to the combined efforts of kernel, audio driver and jackd smart developers you are in position to tune your system a bit more towards the digital audio processing task, but don't expect miracles. Remember you can use your computer also to write documents, surf the net, save some lemmings... Polyvalence comes at a cost. | + | |
===== Does latency really, really matter? ===== | ===== Does latency really, really matter? ===== | ||
- | Less than most people think, in our opinion. There are only a few use-cases | + | Less than most people think, in our opinion. There are only some situations |
- | * Playing virtual instruments. A large delay between the pressing of the keys and the sound the instrument produces will throw-off the timing of most instrumentalists (save if they are church organists, whom we believe are awesome latency-compensation organic systems.) | + | |
- | * Software audio monitoring. If a singer is hearing her own voice through two different paths, her head bones and headphones, large latencies can be disturbing. | + | |
- | * Live-effects. | + | |
- | * Live-mixing (case example needed). | + | |
- | In most other cases (such as playback, recording, overdubbing, | + | In many other cases (such as playback, recording, overdubbing, |
- | In the end, it is not that much the size, but how well you use it. | + | At the end of the day, it is not so much its size, but how well you use it. |
===== The smaller the better? ===== | ===== The smaller the better? ===== | ||
TODO | TODO | ||
- | ===== Capture latency | + | ===== Capture latency, playback latency, roundtrip latency, huh? ===== |
- | TODO | + | |
+ | When you are talking about latencies it is important to be precise about where and how are they measured. | ||
+ | |||
+ | At the end of the day, what really matters is the latency perceived by the person who is using the system. But this latency is itself comprised of smaller, partial latencies. The ones that usually contribute more to the total are usually: | ||
+ | |||
+ | * **Air propagation latency**. | ||
+ | * **Converter latency**. | ||
+ | * **Processing latency**. | ||
+ | * **Digital I/O latency**. | ||
+ | |||
+ | There is not much we can do about the first two other than using headphones or sitting near the loudspeaker and buying quality gear. | ||
+ | |||
+ | Processing latency is usually divided into capture latency and playback latency: | ||
+ | |||
+ | * **Capture latency**: the time necessary for the digitized audio to be available for digital processing. Usually it is one audio period. | ||
+ | |||
+ | * **Playback latency**: the time necessary for the digitized audio to be processed and delivered out of the processing chain. At best it is one audio period. | ||
+ | |||
+ | But this division is an implementation detail of no great interest for the user, since one has no control over it. What really matters is the combination of both, what we could call the **processing roundtrip latency**: the time necessary for a certain audio event to be captured, processed and played back. | ||
+ | |||
+ | It is important to note that **processing latency in a jackd is a matter of choice**: we can lower it as much as we want, within the limits imposed by the audio driver. But the lower it is, the more likely our system will fail to meet it, so the dreaded xrun will make its appearance more often, leaving its merry trail of clicks, pops and crackles. | ||
+ | |||
+ | The digital I/O latency is usually negligible for integrated or PCI audio devices but, as we'll see later, it can be important for USB or FireWire interfaces. | ||
===== Measuring roundtrip latency with jack_delay ===== | ===== Measuring roundtrip latency with jack_delay ===== | ||
- | TODO | + | |
+ | [[http:// | ||
+ | |||
+ | 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. | ||
+ | |||
+ | You can close the loop on a number of ways: | ||
+ | |||
+ | * Putting a speaker close to a microphone. This is rarely done, as air propagation latency is well known so there is no need to measure it. | ||
+ | * Connecting the output of your audio interface to its input using a patch cable. This can be an analog or a digital loop, depending on the nature of the input/ | ||
+ | |||
+ | If you want to measure the latency of a cheap, integrated sound card that only has line-output and mic-input you cannot close the loop with a simple patch cable: both connectors may be mechanically compatible, but electrically they are not designed to work together, so attempting to connect them may harm your audio interface. If you are curious, anyway, you can [[http:// | ||
+ | |||
+ | {{: | ||
+ | |||
+ | {{: | ||
+ | |||
+ | Some notes: | ||
+ | * The black plug goes into the mic outlet, so the 100 ohm resistor is in parallel with it. | ||
+ | * The mic ring carries a DC voltage to polarize electret microphones, | ||
+ | |||
+ | Once you have closed the loop you have to: | ||
+ | - Launch jackd with the configuration you want to test. | ||
+ | - Launch jack_delay. | ||
+ | - Make the appropriate connections between your jack ports so the loop is closed. | ||
+ | - Adjust the playback and capture levels in your mixer. | ||
+ | |||
+ | If everything goes according to plan, jack_delay should start to emit messages indicating the roundtrip delay measured both in milliseconds and in audio frames (there are SR audio frames in one second, where SR is the sample rate.) | ||
+ | |||
+ | < | ||
+ | $ jack_delay | ||
+ | capture latency | ||
+ | playback_latency = 2048 | ||
+ | Signal below threshold... | ||
+ | Signal below threshold... | ||
+ | Signal below threshold... | ||
+ | 3104.419 frames | ||
+ | 3104.416 frames | ||
+ | 3104.415 frames | ||
+ | 3104.417 frames | ||
+ | </ | ||
===== Adjusting latency ===== | ===== Adjusting latency ===== | ||
TODO | TODO | ||
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The [[http:// | The [[http:// | ||
- | 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, | + | 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, |
Stable low-latency (≤10ms) on GNU/Linux can usually only be achieved by running [[https:// | Stable low-latency (≤10ms) on GNU/Linux can usually only be achieved by running [[https:// | ||
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- It seems that for USB devices JACK // | - It seems that for USB devices JACK // | ||
- The latency reported by JACK is inconsistent. | - The latency reported by JACK is inconsistent. | ||
+ | |||
+ | The first issue was explained by ALSA developer Clemens Ladisch in [[http:// | ||
Looking closer at these two issues: | Looking closer at these two issues: | ||
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Alas, total latency measurement can not tell which it is. | Alas, total latency measurement can not tell which it is. | ||
- | |||
===== References ===== | ===== References ===== | ||
- | * [[http://www.kokkinizita.net/ | + | * [[http:// |
* [[http:// | * [[http:// | ||
* [[http:// | * [[http:// | ||
+ | * A script to acquire the data using '' | ||
===== Raw data ===== | ===== Raw data ===== |