Feedback, April 2009
Apr 1, 2009 12:00 PM
Comments from readers about wasteful spending and HD over triax
Wasteful spending
Dear editor:
I just pulled the February issue from my mailbox and read your editorial, “The $650 million DTV converter box.” Here's my reaction: Yes! Yes! Yes! Yes! Yes!
Thank you, Brad, for shedding some light on a tiny corner of this gargantuan catastrophe and for speaking out against it. What a mess! This is definitely a case where the cure is worse than the disease. I just hope we can survive it. And I'm not talking about the recession/depression; I'm talking about the recovery plan.
Thanks for not being afraid to contradict big brother. I wish we could place clones of you throughout the media.
Jeff Perry
A new fan of Brad Dick
HD over triax
Dear editor:
I am currently writing a final year dissertation for Television Production (BSc Hons) on outside broadcast, and one section of the project covers signal feeds from video through to audio. Below is an excerpt from the “High definition over triax” article that was written by Robert P. Seidel of CBS and published in your August 2003 issue:
“Connecting the cameras to the OB truck is achieved through the use of a triax cable, or fiber-optic cable is sometimes used for a high-definition feed. Triax cable is composed of two insulated copper wires, which the data signals from the camera are sent along to the CCU and additionally power is also sent to the camera. Although high-definition signals can be sent along a triax wire, there are several issues that arise due to the fact that it was not originally designed to carry the high-bandwidth signal of a HD feed. However, the earlier need for a cable that would support super slo mo filming sent from a standard-definition cable that used a large amount of data means that HD feeds are just about able to fit into a triax cable's limit. Some problems have been posed by this which were brought to light when CBS tested sending HD signals over triax in 2002. The issues reported included nonsymmetrical overshoots on rising and falling edges, excessive ringing following transitions and unequal color component delays on the order of 25ns.”
I would be grateful if you could explain the following terms: “nonasymmetrical overshoots on rising and falling edges” and “excessive ringing.” I understand the color component delays to an extent as the red part of the signal showing at the top of the screen in the tests due to the delay of information. However, I am not fully sure about what the other two phrases mean and would not want to assume their definition. Thank you for your time, and I await your reply.
James Rice
University of Central Lancashire
England
Robert P. Seidel responds:
Thank you for your interest in my article. The term “nonsymmetrical overshoots on rising and falling edges” means that the transition from peak black to peak white is not producing a square wave function, which softens the sharpness of the image's edge. The undershoot from peak white to peak black is not the same shape as the overshoot, which means the transmission system is not linear.
“Excessive ringing” means that after a transition from, say, white to black, the black video just after the transition will vary in amplitude. In other words, it will not be black, but rather varying shades of black. This has the effect of reducing the sharpness of the image.
Good luck with your dissertation!
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