--- /dev/null
+
+<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 1–5 ms range. For comparison, if you are sitting 1 m
+ (3 ft) from your speakers, the time the sound takes to reach your
+ ears is about 3 ms. Any modern computer can limit the delay to
+ 100 ms. Most can keep it under 50 ms. Many will be able to get
+ down to 10 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 — 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 — 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>
+
projects / ardour-manual / blobdiff