If you’ve played piano, you are proably familiar with the metronome.

Or perhaps you have played in a rock band, you’ve realized first hand the value of a good drummer.

Or maybe you’ve worked on your car.  You are likely well aware of the importance of the timing belt.

In any of those cases,  if you lose the “sync,” things start falling apart.

Same in video . . .

Ever been in an edit session and have a transition effect between two images jump around or change color?

Or perhaps you have you experienced the frustration of trying to make a dub from one source to another but the picture keeps rolling?

Or how about trying to output a show from a Final Cut Pro or Avid session and you can’t make a proper edit to start the layback?

What is likely happening in those situations is a lack of sync between the devices involved.

As you probably know, NTSC video signal typically runs at 30 frames a second. (ok, it actually 29.97, but let’s keep this simple for now.)  But just because two signals run at the same steady frame rate doesn’t mean that they can mix smoothly together.

My good friend and engineering expert Chris Vazquez likens it to one of those 4 x 400 meter sprint races one sees on the Olympics.  Runners have to be “in sync” for the handoff to go smoothly.

So likewise, to have a smooth transition between two or more video devices, such as in a dissolve between two cameras covering a live event, with a character generator added on top, the signals of each device have to be on the same clock.  In other words, they are “genlocked.” http://en.wikipedia.org/wiki/Genlock

What is commonly used for video reference in professional setups is an external sync generator.  It is set to put out a black signal that feeds to all the devices in the facility or particular setup.  That black signal, often referred to as ”house reference” becomes the master drummer. (further good info on this subject: http://tinyurl.com/yz9n8pp)

These range from the large sync generators for a facility

to the rack size for a small production company:

to a small one for an edit suite or field setup:

As explained in this well-written web article http://tinyurl.com/ykgcu8m “Genlock performs four main functions: vertical, horizontal, frame, and color synchronization. These features are all normal parts of a standard composite video signal, which allows a TV or monitor to display an image properly.”

Now you might be thinking that you know of situations where there was no reference put into a record tape machine.

And you would be correct.  Some higher end devices do have internal sync generators, and if that is not the case, most devices will adjust to the sync of the incoming video signal if nothing else is available.  But these are not ideal remedies since that type of sync is certainly not “system wide.”

There are devices that have “frame sync generators,” devices that act like a drill sargeant that force all incoming signals, no matter what clock they are on, to get forcibly reconstituted to a particular clock.  That may sound ideal, but there are disadvantages to the use of such devices, one being the possible introduction of a delay to a signal.

Still, frame syncs are popular in switchers used in remote live situations (like the battlefield or a remote location) where having problems with sync is just not an acceptable situation.

But returning to the main point, most professional video setups and facilities will utilize a black burst sync generator as their house reference.

Ah, but you are thinking, what about SD and HD?

Good question.  Yes, even though both are on 30 fps, remember from above that there are elements other than time that need to be kept in sync, such as color.  The SD signal and HD signal carry color information differently.  And since high end HD decks often provide internal downconversion (HD to SD) or upconversion (SD to HD) features, you’ll understand why on such machines there are usually inputs for both SD and HD sync.

I welcome your comments below.

If you enjoyed this article, you may want to check out some of the perenially-popular tech articles written by Chris Vazquez for this site, like, “Is the Generation Loss In Digital Betacam?” http://tinyurl.com/6yd6gn

Contact me, David Ryan, for your media replication and related multimedia needs.  301-217-0000 x104 or dryan@videolabs.net I follow Thomas Friedman’s “world is flat” model http://tinyurl.com/yhxfrkt, and I work with clients worldwide.  I would enjoy having the opportunity to be of service to you.

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