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