manual/include/the-right-computer-system-for-digital-audio.html

<p>
  It is nice to think that one could just go and buy any computer, install a
  bit of software on it and start using it to record and create music. This
  idea isn't necessarily wrong, but there are some important details that it
  misses. Any computer that that can be bought today (since somewhere around
  the end of 2012) is capable of recording and processing a lot of audio data.
  It will come with a builtin audio interface that can accept inputs from
  microphones and/or electrical instruments; it will have a disk with a huge
  amount of space for storing audio files.
</p>

<p>
  However, when recording, editing and mixing music, it is generally desirable
  to have very little <dfn>latency</dfn> between the time a sound is generated
  and when it can be heard. When the audio signal flows through a computer,
  that means that the computer has to be able to receive the signal, process it
  and send it back out again as quickly as possible. And this is where it
  becomes very important <em>what</em> computer system is being used for this
  task, because it is <strong>absolutely not</strong> the case that
  <em>any</em> computer can do it well.
</p>

<p>
  Routing audio through a computer will always cause some delay, but if it is
  small, it will generally never be noticed. There are also ways to work in
  which the delay does not matter at all (for example, not sending the output
  from the computer to speakers).
</p>

<p>
  The latency that is typically needed for working with digital audio is in the
  1&ndash;5&nbsp;ms range. For comparison, if one is sitting 1&nbsp;m
  (3&nbsp;ft) from a set of speakers, the time the sound takes to reach the
  ears is about 3&nbsp;ms. Any modern computer can limit the delay to
  100&nbsp;ms; most can keep it under 50&nbsp;ms. Many will be able to get down
  to 10&nbsp;ms without too much effort. Attempting to reduce the latency on a
  computer that cannot physically do it will cause clicks and glitches in the
  audio, which is clearly undesirable.
</p>

<h2>Hardware-related Considerations</h2>

<table class="dl">
  <tr><th>Video interface</th>
  <td>Poorly engineered video interfaces (and/or their device drivers) can
  "steal" computer resources for a long time, preventing the audio interface
  from keeping up with the flow of data.</td></tr>
  <tr><th>Wireless interface</th>
  <td>Poorly engineered wireless networking interfaces (and/or their device
  drivers) can also block the audio interface from keeping up with the flow of
  data.</td></tr>
  <tr><th><abbr title="Universal Serial Bus">USB</abbr> ports</th>
  <td>When using an audio interface connected via USB, and sometimes even if
  not, the precise configuration of the system's USB ports can make a big
  difference. There are many cases where plugging the interface into one port
  will work, but using different USB port results in much worse performance.
  This has been seen even on Apple systems.</td></tr>
  <tr><th>Internal USB Hubs</th>
  <td>Ideally, all USB ports should connect directly to the main bus inside the
  computer. Some laptops (and possibly some desktop systems) come wired with an
  internal USB hub between the ports and the system bus, which can then cause
  problems for various kinds of external USB devices, including some models of
  audio interfaces. It is very difficult to discover whether this is true or
  not without simplying trying it out.</td></tr>
  <tr><th><abbr title="Central Processing Unit">CPU</abbr> speed control</th>
  <td>Handling audio with low latency requires that the processor keeps running
  at its highest speed at all times. Many portable systems try to regulate
  processor speed in order to save power&mdash;for low latency audio, this
  should be totally disabled, either in the BIOS or at the OS level.</td></tr>
  <tr><th>Excessive Interrupt Sharing</th>
  <td>If the audio interface is forced by the computer to share an interrupt
  line (basically a way to tell the CPU that something needs its attention)
  with too many other (or wrong) devices, this can also prevent the audio
  interface from keeping up with the flow of data. In laptops it is generally
  impossible to do anything about this. In many desktop systems, it is possible
  at the BIOS level to reassign interrupts to work around the problem.</td></tr>
  <tr><th><abbr title="System Management Interrupt">SMI</abbr>s</th>
  <td>SMIs are interrupts sent by the motherboard to tell the computer about
  the state of various hardware. They cannot safely be disabled, and they can
  take a relatively long time to process. It is better to have a motherboard
  which never sends SMIs at all&mdash;this is also a requirement for realtime
  stock trading systems, which have similar issues with latency.</td></tr>
  <tr><th>Hyperthreading</th>
  <td>This technology is becoming less common as actual multi-core CPUs become
  the norm, but it still exists and is generally not good for realtime
  performance. Sometimes this can be disabled in the BIOS, sometimes it cannot.
  A processor that uses hyperthreading will be less stable in very low latency
  situations than one without.</td></tr>
  <tr><th>Excessive vibration</th>
  <td>This doesn't affect the flow of data to or from the audio interface, but
  it can cause the flow of data to and from disk storage to become
  <em>much</em> slower. If a computer going to be used in an environment with
  loud live sound (specifically, high bass volume), make sure it is placed so
  that the disk is not subjected to noticeable vibration. The vibrations will
  physically displace the read-write heads of disk, and the resulting errors
  will force a retry of the reading from the disk. Retrying over and over
  massively reduces the rate at which data can be read from the disk. Avoid
  this.</td></tr>
</table>