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On Clock and Time

Synchronization in multimedia involves two concepts which are often confused: clock (or speed) and time (location in time).

A clock determines the speed at which one or more systems operate. In the audio world this is generally referred to as Word Clock. 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 Burst, Tri-Level and DARS. Sample rates can be derived from these clocks as well.

Time or timecode specifies an absolute position on a timeline, such as 01:02:03:04 (expressed as Hours:Mins:Secs:Frames). It is actual data and not a clock signal per se. The granularity of timecode is Video Frames and is an order of magnitude lower than, say, Word Clock which is counted in samples. A typical frame rate is 25 fps with a period of 40 ms. In the case of 48 kHz and 25 fps, there are 1920 audio samples per video frame.

The concepts of clock and timecode are reflected in JACK and Ardour:

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.

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.

To make things confusing, there are possibilities to synchronize clocks using timecode. e.g. using mechanism called jam-sync and a phase-locked loop.

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.