Alesis ALESIS M20 ONE Clock Principle, 16.2A House Sync, 16.2B Genlocked Time Code, 16-2

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:

16.2THE 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).

16.2A HOUSE SYNC

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.

16.2B “GENLOCKED” TIME CODE

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.

16-2 ALESIS M20 REFERENCE MANUAL 1.06