-Synchronisation 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. +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. -
- -
-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 samples per video frame.
+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.
-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.
+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.
@@ -31,9 +25,9 @@ The concept of clock and timecode is 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). +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 harware side JACK uses the clock of the audio-interface. Synchronization of multiple interfaces requires hardware support to sync the clocks. +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.
@@ -45,4 +39,7 @@ Timecode is used to align systems already synchronized by a clock to a common po 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. ++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. +