245 lines
10 KiB
HTML
245 lines
10 KiB
HTML
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<p>
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<a
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href="http://en.wikipedia.org/wiki/Latency_%28audio%29"><dfn>Latency</dfn></a>
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is a system's reaction time to a given stimulus. There are many factors that
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contribute to the total latency of a system. In order to achieve exact time
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synchronization all sources of latency need to be taken into account and
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compensated for.
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</p>
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<h2>Sources of Latency</h2>
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<h3>Sound propagation through the air</h3>
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<p>
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Since sound is a mechanical perturbation in a fluid, it travels at
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comparatively slow <a href="http://en.wikipedia.org/wiki/Speed_of_sound">speed</a>
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of about 340 m/s. As a consequence, your acoustic guitar or piano has a
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latency of about 1–2 ms, due to the propagation time of the sound
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between your instrument and your ear.
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</p>
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<h3>Digital-to-Analog and Analog-to-Digital conversion</h3>
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<p>
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Electric signals travel quite fast (on the order of the speed of light),
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so their propagation time is negligible in this context. But the conversions
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between the analog and digital domain take a comparatively long time to perform,
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so their contribution to the total latency may be considerable on
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otherwise very low-latency systems. Conversion delay is usually below 1 ms.
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</p>
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<h3>Digital Signal Processing</h3>
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<p>
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Digital processors tend to process audio in chunks, and the size of that chunk
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depends on the needs of the algorithm and performance/cost considerations.
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This is usually the main cause of latency when you use a computer and one you
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can try to predict and optimize.
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</p>
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<h3>Computer I/O Architecture</h3>
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<p>
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A computer is a general purpose processor, not a digital audio processor.
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This means our audio data has to jump a lot of fences in its path from the
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outside to the CPU and back, contending in the process with some other parts
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of the system vying for the same resources (CPU time, bus bandwidth, etc.)
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</p>
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<h2>The Latency chain</h2>
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<img src="/images/latency-chain.png" title="Latency chain" alt="Latency chain" />
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<p>
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<em>Figure: Latency chain.</em>
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The numbers are an example for a typical PC. With professional gear and an
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optimized system the total roundtrip latency is usually lower. The important
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point is that latency is always additive and a sum of many independent factors.
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</p>
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<p>
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Processing latency is usually divided into <dfn>capture latency</dfn> (the time
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it takes for the digitized audio to be available for digital processing, usually
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one audio period), and <dfn>playback latency</dfn> (the time it takes for
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In practice, the combination of both matters. It is called <dfn>roundtrip
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latency</dfn>: the time necessary for a certain audio event to be captured,
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processed and played back.
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</p>
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<p class="note">
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It is important to note that processing latency in a jackd is a matter of
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choice. It can be lowered within the limits imposed by the hardware (audio
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device, CPU and bus speed) and audio driver. Lower latencies increase the
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load on the system because it needs to process the audio in smaller chunks
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which arrive much more frequently. The lower the latency, the more likely
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the system will fail to meet its processing deadline and the dreaded
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<dfn>xrun</dfn> (short for buffer over- or under-run) will make its
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appearance more often, leaving its merry trail of clicks, pops and crackles.
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</p>
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<p>
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The digital I/O latency is usually negligible for integrated or
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<abbr title="Periphal Component Interface">PCI</abbr> audio devices, but
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for USB or FireWire interfaces the bus clocking and buffering can add some
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milliseconds.
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</p>
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<h2>Low Latency usecases</h2>
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<p>
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Low latency is <strong>not</strong> always a feature you want to have. It
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comes with a couple of drawbacks: the most prominent is increased power
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consumption because the CPU needs to process many small chunks of audio data,
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it is constantly active and can not enter power-saving mode (think fan-noise).
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Since each application that is part of the signal chain must run in every
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audio cycle, low-latency systems will undergo<dfn>context switches</dfn>
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between applications more often, which incur a significant overhead.
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This results in a much higher system load and an increased chance of xruns.
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</p>
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<p>
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For a few applications, low latency is critical:
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</p>
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<h3>Playing virtual instruments</h3>
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<p>
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A large delay between the pressing of the keys and the sound the instrument
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produces will throw-off the timing of most instrumentalists (save church
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organists, whom we believe to be awesome latency-compensation organic systems.)
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</p>
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<h3>Software audio monitoring</h3>
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<p>
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If a singer is hearing her own voice through two different paths, her head
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bones and headphones, even small latencies can be very disturbing and
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manifest as a tinny, irritating sound.
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</p>
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<h3>Live effects</h3>
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<p>
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Low latency is important when using the computer as an effect rack for
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inline effects such as compression or EQ. For reverbs, slightly higher
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latency might be tolerable, if the direct sound is not routed through the
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computer.
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</p>
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<h3>Live mixing</h3>
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<p>
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Some sound engineers use a computer for mixing live performances.
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Basically that is a combination of the above: monitoring on stage,
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effects processing and EQ.
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</p>
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<p>
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In many other cases, such as playback, recording, overdubbing, mixing,
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mastering, etc. latency is not important, since it can easily be
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compensated for.<br />
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To explain that statement: During mixing or mastering you don't care
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if it takes 10ms or 100ms between the instant you press the play button
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and sound coming from the speaker. The same is true when recording with a count in.
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</p>
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<h2>Latency compensation</h2>
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<p>
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During tracking it is important that the sound that is currently being
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played back is internally aligned with the sound that is being recorded.
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</p>
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<p>
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This is where latency-compensation comes into play. There are two ways to
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compensate for latency in a DAW, <dfn>read-ahead</dfn> and
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<dfn>write-behind</dfn>. The DAW starts playing a bit early (relative to
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the playhead), so that when the sound arrives at the speakers a short time
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later, it is exactly aligned with the material that is being recorded.
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Since we know that play-back has latency, the incoming audio can be delayed
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by the same amount to line things up again.
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</p>
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<p>
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As you may see, the second approach is prone to various implementation
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issues regarding timecode and transport synchronization. Ardour uses read-ahead
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to compensate for latency. The time displayed in the Ardour clock corresponds
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to the audio-signal that you hear on the speakers (and is not where Ardour
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reads files from disk).
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</p>
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<p>
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As a side note, this is also one of the reasons why many projects start at
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timecode <samp>01:00:00:00</samp>. When compensating for output latency the
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DAW will need to read data from before the start of the session, so that the
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audio arrives in time at the output when the timecode hits <samp>01:00:00:00</samp>.
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Ardour3 does handle the case of <samp>00:00:00:00</samp> properly but not all
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systems/software/hardware that you may inter-operate with may behave the same.
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</p>
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<h2>Latency Compensation And Clock Sync</h2>
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<p>
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To achieve sample accurate timecode synchronization, the latency introduced
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by the audio setup needs to be known and compensated for.
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</p>
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<p>
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In order to compensate for latency, JACK or JACK applications need to know
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exactly how long a certain signal needs to be read-ahead or delayed:
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</p>
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<img src="/images/jack-latency-excerpt.png" title="Jack Latency Compensation" alt="Jack Latency Compensation" />
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<p>
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<em>Figure: Jack Latency Compensation.</em>
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</p>
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<p>
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In the figure above, clients A and B need to be able to answer the following
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two questions:
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</p>
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<ul>
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<li>
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How long has it been since the data read from port Ai or Bi arrived at the
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edge of the JACK graph (capture)?
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</li>
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<li>
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How long will it be until the data writen to port Ao or Bo arrives at the
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edge of the JACK graph (playback)?
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</li>
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</ul>
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<p>
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JACK features an <abbr title="Application Programming Interface">API</abbr>
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that allows applications to determine the answers to above questions.
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However JACK can not know about the additional latency that is introduced
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by the computer architecture, operating system and soundcard. These values
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can be specified by the JACK command line parameters <kbd class="input">-I</kbd>
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and <kbd class="input">-O</kbd> and vary from system
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to system but are constant on each. On a general purpose computer system
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the only way to accurately learn about the total (additional) latency is to
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measure it.
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</p>
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<h2>Calibrating JACK Latency</h2>
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<p>
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Linux DSP guru Fons Adriaensen wrote a tool called <dfn>jack_delay</dfn>
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to accurately measure the roundtrip latency of a closed loop audio chain,
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with sub-sample accuracy. JACK itself includes a variant of this tool
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called <dfn>jack_iodelay</dfn>.
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</p>
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<p>
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Jack_iodelay allows you to measure the total latency of the system,
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subtracts the known latency of JACK itself and suggests values for
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jackd's audio-backend parameters.
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</p>
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<p>
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jack_[io]delay works by emitting some rather annoying tones, capturing
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them again after a round trip through the whole chain, and measuring the
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difference in phase so it can estimate with great accuracy the time taken.
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</p>
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<p>
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You can close the loop in a number of ways:
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</p>
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<ul>
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<li>
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Putting a speaker close to a microphone. This is rarely done, as air
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propagation latency is well known so there is no need to measure it.
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</li>
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<li>
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Connecting the output of your audio interface to its input using a
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patch cable. This can be an analog or a digital loop, depending on
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the nature of the input/output you use. A digital loop will not factor
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in the <abbr title="Analog to Digital, Digital to Analog">AD/DA</abbr>
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converter latency.
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</li>
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</ul>
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<p>
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Once you have closed the loop you have to:
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</p>
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<ol>
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<li>Launch jackd with the configuration you want to test.</li>
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<li>Launch <kbd class="input">jack_delay</kbd> on the commandline.</li>
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<li>Make the appropriate connections between your jack ports so the loop is closed.</li>
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<li>Adjust the playback and capture levels in your mixer.</li>
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</ol>
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