-Synchronization in multimedia involves two concepts which are often confused: clock (or speed) and time (location in time). + Synchronization in multimedia involves two concepts which are + often confused: clock (or speed) and time (location + in time).
-A clock is the mechanism by which two systems tick simultaneously. -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 systems sample rate constant, regular and accurate. -Word clock is usually at the frequency of the sample rate - ie at 48KHz, its period is about 20μs. 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 also be derived from these clocks as well. + A clock determines the speet 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 â or timecode â on the other hand specifies an absolute relationship or position on a timeline e.g. 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 40ms.
-In the case of 48KHz and 25fps, there are 1920 audio samples per video frame.
+ 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 concept of clock and timecode is reflected in JACK and Ardour: + 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). -Within software, 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 decreases fidelity. + 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. + 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.
--NB. to make things confusing, there are possibilities to synchronize clocks using timecode. e.g. using mechanism called jam-sync and a Phase-Locked-Loop. +
+ 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. + 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.