Alesis ALESIS M20 16.2 THE ONE CLOCK PRINCIPLE

SMPTE Synchronization Overview, Chapter 16

The CD player’s digital clock is uncontrolled. The M20 normally would automatically

synchronize to an incoming digital signal, but in this case it’s following the video which is

traveling at its own speed. Since a slave can’t follow two masters, the slight timing

discontinuities appear as clicks and pops as samples are lost. Depending on your equipment,

there may be several ways to solve this problem, but in any case the solution will involve the

following principle:

16.2 THE ONE CLOCK PRINCIPLE

To prevent synchronization problems, every piece of equipment in the system must

ultimately refer to the same clock. There may be only one master at a time, and all

others must follow it. If different types of clock are required by different pieces of

equipment, all slave clocks must be derived from or synchronized to one master

clock. For example, there must be exactly one frame of video for every SMPTE time

code frame, and exactly the right number of digital samples for each frame.

If you violate the one clock principle in your system design or operation, the consequences

will range from the severe (ticks and pops in the audio, sudden fast-forwards or jumping out

of record) to the less obvious (an increasing drift in the lock between different machines as

they travel).

For a system to operate smoothly, you need all your audio gear taking “sonic pictures” and

your video gear taking pictures at the same rate. Professional video houses achieve this by

having a black burst or house sync generator with a clean, steady wow-and-flutter-free video

signal that is distributed to all VCRs in a building. Several companies make these kinds of

generators, and for complex systems a unified video/word clock/AES black generator (such

as MOTU’s Digital Timepiece™) may be the best solution. Depending on your system, you

would connect house sync to the M20’s Video In or Word Clock In jacks.

A solid and steady video or digital reference by itself is not enough, because only SMPTE

time code carries the actual data of hours:minutes:seconds:frames required for a chase-lock

system. A common mistake is to generate “wild” time code, not referenced to the video or

digital signal. In other words, your time code generator must start its frame message at the

exact same time that the video signal is starting its scan of the frame and the first sample of

the digital audio is going to the converters.

Just because it’s time code doesn’t mean it’s going at the same rate as everything else in the

system. By common standards, it may seem right, but even small errors add up when you’re

trying to divide each second into 48,000 slices. This brings to light another important

principle: understanding the difference between location references and clock references.