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

<p>
  It would be nice to think that you 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 wrong, but there some important details that it
  misses.
</p>
<p>
  Any computer that you can buy 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 or electrical instruments. It will have a disk with a huge
  amount of space for storing audio files.
</p>
<p>
  When you are recording, editing and mixing music, you generally want to
  work with very little <dfn>latency</dfn> between the time that
  a sound is generated and when you can hear it. 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 fast as
  possible.<br> 
  And that is where it becomes very important <em>what</em> computer system
  you have, because it is <strong>absolutely not</strong> the case that any
  computer can do this job well.
</p>
<p>
  Routing audio through a computer will always cause some delay, but if it
  is small, you will generally never notice it. 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 you want for working with digital audio is typically in
  the 1&ndash;5&nbsp;ms range. For comparison, if you are sitting 1&nbsp;m 
  (3&nbsp;ft) from your speakers, the time the sound takes to reach your
  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. If you try to reduce the delay
  on a computer that cannot meet your goal, you will get clicks and
  glitches in the audio, which is clearly extremely undesirable.
</p>

<h2>Hardware-related Considerations</h2>
<dl class="wide-table">
  <dt>Video interface</dt>
  <dd>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</dd>
  <dt>Wireless interface</dt>
  <dd>Poorly engineered wireless networking interfaces (and/or their device
  drivers) can also block the audio interface from keeping up with the flow
  of data</dd>
  <dt><abbr title="Universal Serial Bus">USB</abbr> ports</dt>
  <dd>If you are using an audio interface connected via USB, and sometimes
  even if you are not, the precise configuration of your 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.
  </dd>
  <dt>Internal USB Hubs</dt>
  <dd>Ideally, you'd like your USB ports to all 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.</dd>
  <dt><abbr title="Central Processing Unit">CPU</abbr> speed control</dt>
  <dd>Handling audio with low latency requires that your 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, you want this totally disabled, either in the BIOS or at the OS
  level.</dd>
  <dt>Excessive Interrupt Sharing</dt>
  <dd>If your audio interface is forced by your computer to share an
  interrupt line (basically a way to tell the CPU that something needs
  its attention) with too many, or the wrong, other 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.</dd>
  <dt><abbr title="System Management Interrupt">SMI</abbr>s</dt>
  <dd>SMIs are interrupts sent by the motherboard to tell the computer
  about the state of various hardware. They cannot safely be disabled,
  but they can also 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.</dd>
  <dt>Hyperthreading</dt>
  <dd>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 you can disable this in the BIOS,
  sometimes you cannot. A processor that uses hyperthreading will be
  less stable in very low latency situations than one without.</dd>
  <dt>Excessive vibration</dt>
  <dd>This doesn't affect the flow of data to/from the audio interface,
  but it can cause the flow of data to/from your disk storage to become
  <em>much</em> slower. If you are going to use a computer in an
  environment with loud live sound (specifically, high bass volume),
  make sure to place it so that the disk is not subject to noticeable
  vibration. The vibrations will physically displace the head-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.</dd>
</dl>