manual/include/the-right-computer-system-for-digital-audio.html
Shamus Hammons dfec6899ef Initial cleanup of manual content.
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<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>