<p>
- <dfn>Synchronization</dfn> in multimedia involves two concepts which are
- often confused: <dfn>clock</dfn> (or speed) and <dfn>time</dfn> (location
+ <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 <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
+ 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
+ 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 <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
+ 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
+ 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
+ In the case of 48 kHz and 25 fps, there are 1920 audio samples
per video frame.
</p>
</p>
<p>
- JACK provides clock synchronization and is not concerned with time code
+ 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
+ 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
+ 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
+ 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
+ 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
+ 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
+ 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
+ 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>