---
<p>
-Synchronisation in multimedia involves two concepts which are often confused: <strong>clock</strong> (or speed) and <strong>time</strong> (location in time).
+ <dfn>Synchronization</dfn> in multimedia involves two concepts which are
+ often confused: <dfn>clock</dfn> (or speed) and <dfn>time</dfn> (location
+ in time).
</p>
<p>
-A <em>clock</em> is the mechanism by which two systems <em>tick</em> simultaneously.
-In the audio world this is generally referred to as <a href="http://en.wikipedia.org/wiki/Word_clock" title="http://en.wikipedia.org/wiki/Word_clock">Word_clock</a>.
-It does not carry any absolute reference to a point in time: A clock is used to keep a systems sample rate constant, regular and accurate.
-Word clock is usually at the frequency of the sample-rate - ie at 48KHz, its period is about 20μs. Word Clock is the most common 'sample rate' based clock but other clocks do exist such as Black and Burst, Tri-Level and DARS. Sample rates can also be derived from these clocks as well.
+ A <dfn>clock</dfn> determines the speet at which one or more systems
+ operate. In the audio world this is generally referred to as
+ <a href="http://en.wikipedia.org/wiki/Word_clock" title="http://en.wikipedia.org/wiki/Word_clock">Word Clock</a>.
+ It does not carry any absolute reference to a point in time: A clock is
+ used to keep a system's sample rate regular and accurate.
+ Word clock is usually at the frequency of the sample rate —
+ at 48 kHz, its period is about 20 μs. Word Clock is the most
+ common sample rate based clock but other clocks do exist such as Black and
+ Burst, Tri-Level and DARS. Sample rates can be derived from these clocks as well.
</p>
<p>
-Time – or <em>timecode</em> – on the other hand specifies an absolute relationship or position on a timeline e.g. <code>01:02:03:04</code> (expressed as Hours:Mins:Secs:Frames). It is actual data and not a clock-signal per se.
-The granularity of timecode is <strong>Video Frames</strong> and is an order of magnitude lower than, say, Word Clock which is counted in <strong>samples</strong>. A typical frame-rate is 25 fps with a period of 40ms.
-In the case of 48kHz and 25fps, there are 1920 samples per video frame.
+ Time or <dfn>timecode</dfn> specifies an absolute position on a timeline,
+ such as <code>01:02:03:04</code> (expressed as Hours:Mins:Secs:Frames). It is
+ actual <em>data</em> and not a clock <em>signal</em> per se.
+ The granularity of timecode is <dfn>Video Frames</dfn> and is an order of
+ magnitude lower than, say, Word Clock which is counted in
+ <dfn>samples</dfn>. A typical frame rate is 25 <abbr title="frames
+ per second">fps</abbr> with a period of
+ 40 ms.
+ In the case of 48 kHz and 25 fps, there are 1920 audio samples
+ per video frame.
</p>
<p>
-An interesting point to note is that LTC (Linear Time Code) is a Manchester Encoded, Frequency Modulated signal that carries both 'Clock' and 'Time'. It is possible to extract absolute position data and speed from it.
+ The concepts of clock and timecode are reflected in JACK and Ardour:
</p>
<p>
-The concept of clock and timecode is reflected in JACK and Ardour:
+ JACK provides clock synchronization and is not concerned with time code
+ (this is not entirely true, more on jack-transport later).
+ On the software side, jackd provides sample-accurate synchronization
+ between all JACK applications.
+ On the hardware side, JACK uses the clock of the audio-interface.
+ Synchronization of multiple interfaces requires hardware support to sync
+ the clocks.
+ If two interfaces run at different clocks the only way to align the
+ signals is via re-sampling (SRC - Sample Rate Conversion), which is
+ expensive in terms of CPU usage and may decreases fidelity if done
+ incorrectly.
</p>
<p>
-JACK provides clock-synchronization and is not concerned with time-code (this is not entirely true, more on jack-transport later).
-Within software, jackd provides sample-accurate synchronization between all JACK applications.
-On the harware side JACK uses the clock of the audio-interface. Synchronization of multiple interfaces requires hardware support to sync the clocks.
-If two interfaces run at different clocks the only way to align the signals is via re-sampling (SRC - Sample Rate Conversion) - which decreases fidelity.
+ Timecode is used to align systems already synchronized by a clock to
+ a common point in time, this is application specific and various
+ standards and methods exist to do this.
</p>
-<p>
-Timecode is used to align systems already synchronized by a clock to a common point in time, this is application specific and various standards and methods exist to do this.
+<p class="note">
+ To make things confusing, there are possibilities to synchronize clocks
+ using timecode. e.g. using mechanism called <dfn>jam-sync</dfn> and a
+ <dfn>phase-locked loop</dfn>.
</p>
<p>
-NB. to make things confusing, there are possibilities to synchronize clocks using timecode. e.g. using mechanism called <em>jam-sync</em> and a Phase-Locked-Loop.
+ An interesting point to note is that LTC (Linear Time Code) is a
+ Manchester encoded, frequency modulated signal that carries both
+ clock and time. It is possible to extract absolute position data
+ and speed from it.
</p>
-
projects / ardour-manual-diverged / blobdiff