102 lines
5.7 KiB
HTML
102 lines
5.7 KiB
HTML
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<p>
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It is nice to think that one could just go and buy any computer, install a
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bit of software on it and start using it to record and create music. This
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idea isn't necessarily wrong, but there are some important details that it
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misses. Any computer that that can be bought today (since somewhere around
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the end of 2012) is capable of recording and processing a lot of audio data.
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It will come with a builtin audio interface that can accept inputs from
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microphones and/or electrical instruments; it will have a disk with a huge
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amount of space for storing audio files.
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</p>
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<p>
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However, when recording, editing and mixing music, it is generally desirable
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to have very little <dfn>latency</dfn> between the time a sound is generated
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and when it can be heard. When the audio signal flows through a computer,
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that means that the computer has to be able to receive the signal, process it
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and send it back out again as quickly as possible. And this is where it
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becomes very important <em>what</em> computer system is being used for this
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task, because it is <strong>absolutely not</strong> the case that
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<em>any</em> computer can do it well.
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</p>
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<p>
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Routing audio through a computer will always cause some delay, but if it is
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small, it will generally never be noticed. There are also ways to work in
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which the delay does not matter at all (for example, not sending the output
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from the computer to speakers).
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</p>
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<p>
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The latency that is typically needed for working with digital audio is in the
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1–5 ms range. For comparison, if one is sitting 1 m
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(3 ft) from a set of speakers, the time the sound takes to reach the
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ears is about 3 ms. Any modern computer can limit the delay to
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100 ms; most can keep it under 50 ms. Many will be able to get down
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to 10 ms without too much effort. Attempting to reduce the latency on a
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computer that cannot physically do it will cause clicks and glitches in the
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audio, which is clearly undesirable.
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</p>
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<h2>Hardware-related Considerations</h2>
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<table class="dl">
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<tr><th>Video interface</th>
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<td>Poorly engineered video interfaces (and/or their device drivers) can
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"steal" computer resources for a long time, preventing the audio interface
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from keeping up with the flow of data.</td></tr>
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<tr><th>Wireless interface</th>
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<td>Poorly engineered wireless networking interfaces (and/or their device
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drivers) can also block the audio interface from keeping up with the flow of
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data.</td></tr>
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<tr><th><abbr title="Universal Serial Bus">USB</abbr> ports</th>
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<td>When using an audio interface connected via USB, and sometimes even if
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not, the precise configuration of the system's USB ports can make a big
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difference. There are many cases where plugging the interface into one port
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will work, but using different USB port results in much worse performance.
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This has been seen even on Apple systems.</td></tr>
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<tr><th>Internal USB Hubs</th>
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<td>Ideally, all USB ports should connect directly to the main bus inside the
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computer. Some laptops (and possibly some desktop systems) come wired with an
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internal USB hub between the ports and the system bus, which can then cause
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problems for various kinds of external USB devices, including some models of
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audio interfaces. It is very difficult to discover whether this is true or
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not, without simply trying it out.</td></tr>
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<tr><th><abbr title="Central Processing Unit">CPU</abbr> speed control</th>
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<td>Handling audio with low latency requires that the processor keeps running
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at its highest speed at all times. Many portable systems try to regulate
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processor speed in order to save power—for low latency audio, this
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should be totally disabled, either in the BIOS or at the OS level.</td></tr>
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<tr><th>Excessive Interrupt Sharing</th>
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<td>If the audio interface is forced by the computer to share an interrupt
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line (basically a way to tell the CPU that something needs its attention)
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with too many other (or wrong) devices, this can also prevent the audio
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interface from keeping up with the flow of data. In laptops it is generally
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impossible to do anything about this. In many desktop systems, it is possible
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at the BIOS level to reassign interrupts to work around the problem.</td></tr>
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<tr><th><abbr title="System Management Interrupt">SMI</abbr>s</th>
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<td>SMIs are interrupts sent by the motherboard to tell the computer about
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the state of various hardware. They cannot safely be disabled, and they can
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take a relatively long time to process. It is better to have a motherboard
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which never sends SMIs at all—this is also a requirement for realtime
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stock trading systems, which have similar issues with latency.</td></tr>
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<tr><th>Hyperthreading</th>
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<td>This technology is becoming less common as actual multi-core CPUs become
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the norm, but it still exists and is generally not good for realtime
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performance. Sometimes this can be disabled in the BIOS, sometimes it cannot.
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A processor that uses hyperthreading will be less stable in very low latency
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situations than one without.</td></tr>
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<tr><th>Excessive vibration</th>
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<td>This doesn't affect the flow of data to or from the audio interface, but
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it can cause the flow of data to and from disk storage to become
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<em>much</em> slower. If a computer going to be used in an environment with
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loud live sound (specifically, high bass volume), make sure it is placed so
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that the disk is not subjected to noticeable vibration. The vibrations will
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physically displace the read-write heads of disk, and the resulting errors
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will force a retry of the reading from the disk. Retrying over and over
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massively reduces the rate at which data can be read from the disk. Avoid
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this.</td></tr>
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</table>
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