Mysterious Vertical Roll

[Malko]

Hey,

I have come into a situation where I'm using someone else's old switcher/broadcast setup and design. After rewiring certain components and replacing the near dead switcher with an old Panasonic MX50, I was finally able to hook up a feed from the booth that runs our live screens to the switcher so that we can present Scripture live with our webcast and save on the edit time later.

The problem is what you will see in this video file. Note that the image should be completely still.



The BNC run is long and goes through some walls, more than likely running parallel with power. Any suggestions as to what I can do to remedy this issue?

 

[mikkowilson]

Hum bars.

Normally caused by a ground loop - various components of your system plugged into separate power sources. Can also be interference from power lines traveling next to the video lines.

One quick test: run the cameras on battery power. That might fix it. Though looks like that's a computer source.

The best solution is a "Hum buster" (AKA: Hum Eliminator, or Isolation Transformer) - It's a device that breaks the electrical connection while passing the signal through to prevent exactly the interference you see.

Example: http://www.bhphotovideo.com/c/produ...nics_VIT_75_VIT_75_Isolation_Transformer.html

Go through the system and unplug (and then re-connect) each video & audio cable until the hum goes away. Start with the longest. - Chances are it's just the source in question.
Assuming it's just one souce, you'll just need a 1-channel hum buster to clean it up. If it's multiple signals (like multiple cameras coming in from a remote locatoin, all with hum, then you might need a multi-channel unit.)


- Mikko

 

[Malko]

Mikko,

Thank you so much! Yes, it's only the computer feed that has that hum - the cameras are run on triax and the CCUs are located near the switcher. We were so happy to get a switcher with frame sync (our other switcher was half broken, but ultimately our blackburst unit failed so that's why we got another switcher) to then find that the feed looked like that. Now we're heading in the right direction!

 

[mikkowilson]

Yup, Triax cameras are powered (of course) by the CCU (base station) - running on the same power as the switcher (& other control room gear). So hey don't have that issue.

A single channel video hum-buster should cure your problem.

- Mikko

 

[NoDice]

isnt that "roll" caused from a difference in framerates?

 

[mikkowilson]

Nope.

If it was a vertical timing issue (incorrect frame-rate(s)), you'd see the vertical interval as the frames rolled around.

This is simple interference. Basically the frequency of the power line is very close to the frequency of the video, so it makes the bar across almost in sync .. however it scrolls because video is 29.97fps, whereas power is 60Hz, which divides into 30. So that .03fps difference causes the interference bar to roll up through the frame.

- Mikko