manual/include/on-clock-and-time.html

<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 speed 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 (Ardour does this internally if using the ALSA backend) 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 and Ardour use 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&mdash;Sample Rate Conversion), which is
  expensive in terms of CPU usage and may decrease 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>