Can anyone explain video synchronization?

[tonykunda]

Hey Guys,
I’m really having a hard time wrapping my head around video sync. I’ve heard terms like “Genlock” “Black Burst” and “Time Base Correctors”, but I don’t have a clue how all these things tie together.

The reason I ask:
Our church currently has 3 Sony BRC-300 cameras, a cheese-ball scan converter for the signal from our EasyWorship computer, and a Panasonic WJ-MX50 Mixer. The MX50 only has 4 inputs and we are looking to expand. I know someone who is willing to donate a GVG 110, but I understand that this switcher requires all the sources to be synchronized unlike the MX50, which has an internal frame synchronizer. I know that the BRC-300’s have a sync input, but where does this sync signal come from and how would I sync the cheesy scan converter, which doesn’t have a sync input. Also, how do I distribute the sync signal to all the devices? I’ve looked all over the Internet and I can’t seem to find anything that describes how to get your video synchronized.

I’m really a “n00b” when it comes to video signal flow, so any help you could give would be a grand help!
Thanks!

[Doug Jennings]

Well genlock and black burst serve the same function, to sync or keep multiple sources in time to a switcher (like the GVG).

Some switchers generate their own black burst or genlock signal but higher end units do not. A separate black burst generator or sync generator is required. Videotek is one brand but there are many.

Most sync or BB generators have multiple outputs but if not the signal can be run through distribution amplifiers like a normal composite video signal.

As far as the scan converter goes you'd need to either invest in a higher end unit that can by genlocked or invest in a frame synchronizer to provide the necessary timing adjustments so that the switcher can see it properly.

Personally I'd look into a high end Extron scan converter that is genlockable that way you'll get the best possible image and not have to introduce another box. A standalone frame sync might well be cheaper, though. You'll have to weigh the benefits for yourself.

I think I covered what you listed as your main questions but I'm sure there are plenty of folks on this forum that know more than I do.

Take care.

[waynehoskins]

A video signal is a sequence of 30 frames (ish) per second, scanned one line at a time, 525 horizontal lines in sequence per frame. The only definitive timing signal in the NTSC video is the vertical blanking interval, where the beam shuts off and jumps from the lower right corner to the upper left, ready to start painting the next frame.

To seamlessly cut or transition between sources requires that they be synchronized, painting the same part of the picture at the same time. Genlock or Reference Video is a signal that the video source device uses to lock its internal clock to, so its video is made in time with the reference. Switcher Black or Color Black or Black Burst is a common reference signal.

You can imagine N strips of motion picture film laid out next to each other, and to change between them you have to align the frames between them.

[Lucas]

I only had this explained to me yesterday so here goes.

Genlock is used to sync all camera's and devices to the same frame rate.
EG, at this precise second in time camera 1, 2 and 3 are all displaying frame 1, the next frame shown accross all cameras is frame 2.

The reason for this is so when you cut or fade between video sources you don't get glitches oo artifacts on the screen.

Usually the you sync the signal on a per camera basis, or possibly the CCU allows you to adjust it.

[kbob]

totally non-tech visual:

Think of lining up two film clips-on actual film, the stuff with the sprockets?-to fade together? Line them up so they start of the frame and the edge of the sprocket holes are "in sync" with one another, and the picture won't seem to jump when you cross cut the strips. If you don't, you'll get a glitch.

That's an image most people can picture....

[Brent Bauman]

I hate to be the bearer of bad news...

The BRC-300 cameras do have a Genlock input... unfortunately the cameras have NO TIMING adjustment capabilities... apparently someone at Sony was sleeping through that meeting

Normally pro cameras have a genlock input but they typically have a Subcarrier and H Phase adjustment... these are the two adjustments required in order to synchronize the cameras in the kind of application you have described.

Because the BRC-300's do not allow for these adjustments there are few options available (using frame synchronizers is one).

For the system you have described (using the GVG-110 switcher) a frame sync would be required in order to time each input.

[cjlowe]

Originally Posted by Brent Bauman

The BRC-300 cameras do have a Genlock input... unfortunately the cameras have NO TIMING adjustment capabilities...

WHAT?! Unless these cameras output SDI, that's CRAZY bad design.

As for what genlock/etc actually IS... it's a "reference signal" which makes sure all the cameras are outputting the same part of the signal at the same time, so your TV won't "jump" when you switch between cameras.

I actually had to use the BARS output of a DVCAM deck as my genlock reference once... ew.

[waynehoskins]

Eek! (both DVCAM bars and the no timing adjustment .. more so the no timing adjustment)

[tonykunda]

Thanks for the info thus far! It's really cleared up the abstract picture of sync in my head. Now, on to the specific problem:

If I understand Brent correctly, although the BRC-300's are "sync-able" in theory, they really aren't. (Makes me wonder why Sony even bothered putting the Sync input on them!?!) What a shame, I've really come to like the BRC's— a significant upgrade from the nasty security cameras my predecessor put in!

Taking this into account, would it be better to pursue a newer switcher that has internal frame sync, such as the Panasonic AVHS300G or the GVG Indigo? (I would much rather have the Indigo, but I don't think that my senior pastor will like the price tag on it!)

[Brent Bauman]

A couple of options... I know that the BRC-300's do have optional output modules (including a Serial Digital Interface - SDI option). The advantage of using this - coupled with an SDI capable switcher is the timing issues are significantly reduced...

Another option - as you have indicated - would be to use a newer switcher with internal frame sync capabilities...

A third option would be to find some frame synchronizers (one per video input) and time the entire system to a reference signal.

Incidentally you will see various terminology used to refer to the gen-lock signal...

CVBS - Composite Video with Blanking and Sync. (you may hear this referred to as Color Video with Burst and Sync, and other things... basically all referring to the same type of signal).

Video Black - essentially CVBS where the composite video happens to be a black image.

Color bars - again CVBS but in this case instead of black the video happens to be an image made up of color bars.

Reference - just another word for CVBS. Reference is kind of a loose way of saying - whatever you want to use as a reference source... connect it here.

Any old video - not often, but sometimes the installer might use a single video feed (say from camera one) as the reference video source.

Most of the time the designer would choose to use a sync generator... often a rack-mounted unit that provides a stable source of video (CVBS) that may be distributed (using distribution amplifiers or a loop-through method - or both) to whatever equipment needs to be sync'd.

Other video sources (such as a VCR/VTR or even a satellite receiver or DVD player) could be used as a source of composite video (with blanking and sync) but this is not recommended. Usually some effort is made to ensure that the reference signal (in whatever form you have it) is ALWAYS there. In broadcast facilities (and even on mobile production trucks) it is common to have a back up sync generator and an automatic changeover unit (ACO) in order to ensure an un-interrupted timing source.

One of the reasons to NOT use a VCR for a reference signal is... what happens when the tape stops... most of the time you will lose video... and therefore your reference signal.

That's not to say that I haven't ever had to do this... in an emergency any sync'll do.

Also... the video output from a VCR is relatively unstable... this instability is exactly what a Time Base Corrector (or TBC) was designed to correct. It slightly adjusts the timing reference of the video signal to match it's reference input (genlock) in order to stabilize the video so you could use it in a switcher system.


Hope this helps.

[cjlowe]

Just a thought,

Maybe BRC-300s are meant to plug into a CCU and the CCU has the timing adjustment on it? I know SONY have a habit of hiding some of the controls (e.g. master pedestal) and only making them accessible via a CCU.

[Brent Bauman]

Originally Posted by cjlowe

Just a thought,

Maybe BRC-300s are meant to plug into a CCU and the CCU has the timing adjustment on it? I know SONY have a habit of hiding some of the controls (e.g. master pedestal) and only making them accessible via a CCU.

Well... there is an optical multiplex unit available for this camera... really only a way of using fiber optic cable to get the digital signals from the camera (using an optical card) to a control room location. Once you get the digital signal to the BRU-300 (the optical unit) the unit can be outfitted with an analog video output card... but I am not sure that it has any timing adjustments. I don't believe so.