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