- Synchronization in multimedia involves two concepts which are - often confused: clock (or speed) and time (location + Synchronization in multimedia involves two concepts which are + often confused: clock (or speed) and time (location in time).
- A clock determines the speet at which one or more systems - operate. In the audio world this is generally referred to as + 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 + 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.
- Time or timecode specifies an absolute position on a timeline,
- such as 01:02:03:04 (expressed as Hours:Mins:Secs:Frames). It is
+ 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
+ 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
+ In the case of 48 kHz and 25 fps, there are 1920 audio samples
per video frame.
- 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.
- 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.
- To make things confusing, there are possibilities to synchronize clocks - using timecode. e.g. using mechanism called jam-sync and a + 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 + 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.