<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
+ 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 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>
- 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.
+ 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, 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).
+ 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 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
+ The latency that is typically needed for working with digital audio is in the
+ 1–5 ms range. For comparison, if one is sitting 1 m
+ (3 ft) from a set of speakers, the time the sound takes to reach the
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.
+ 100 ms; most can keep it under 50 ms. Many will be able to get down
+ to 10 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>
-<dl class="wide-table">
- <dt>Video interface</dt>
- <dd>Poorly engineered video interfaces (and/or their device drivers) can
+
+<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</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>
+ 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 simply 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—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—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 (spinning) 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.If you find this hard to believe, check
+ out <a href="https://www.youtube.com/watch?v=tDacjrSCeq4">this
+ video</a> which shows the effects of merely shouting at your drives. This
+ is likely not an issue with contemporary SSD drives, which have no
+ spinning/head mechanisms.</td></tr>
+</table>
+
+<p>
+ Richard Ames presents a long (28
+ minute) <a href="https://www.youtube.com/watch?v=GUsLLEkswzE">video</a>
+ that is very helpful if you want to understand these issues in more
+ depth. It is a little bit Windows-centric, but the explanations apply to
+ all types of computers and operating systems.
+</p>
projects / ardour-manual / blobdiff