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----
-layout: default
-title: On Clock and Time
----
-
-<p>
-  <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 <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="https://en.wikipedia.org/wiki/Word_clock" title="https://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 &mdash; 
-  at 48&nbsp;kHz, its period is about 20&nbsp;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 <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&nbsp;<abbr title="frames
-  per second">fps</abbr> with a period of
-  40&nbsp;ms.
-  In the case of 48&nbsp;kHz and 25&nbsp;fps, there are 1920 audio samples 
-  per video frame.
-</p>
-
-<p>
-  The concepts of clock and timecode are reflected in JACK and Ardour:
-</p>
-
-<p>
-  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>
-  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 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>
-  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>
-