Tried and Tested / Coaxial Cable

Dear Charlie,

I have inherited a client with a whole slug of video problems. Part of my problem is that he has been told that the solution to his problems is to replace all of the coaxial cable, in his system with fiber optic cable. This is based upon a friend of his using a hand held video meter to scan his cables. The entire test was based upon whether or not he was seeing a one volt peak to peak video signal (140 IRE) from each camera at the monitor. I'm not sure if the video output was checked at the camera first. My concern is that I have verified the coax in the system to be a proper copper, 75 ohm rated cable. The connections are a little funky, but overall there doesn't appear to be a cable problem. Although I would love to replace the coax with fiber (a good potential profit), I am not sure that it is the route to go. I think that my client will be pissed when I am done if all of the problems don't go away. At any rate, I know that there are various problems and different ways to trouble shoot them, but my client is stuck on his friends analysis of the cable and his recommendations to go fiber. Please give me a response to give my client so that I can persuade him to have the cable retested properly. It may prove that fiber is the best way to go. It might save my client a lot of money and make me a hero.

Sign me.... Tried and Tested in the Field.

 Dear Tried and Tested,

In response to your question about the various techniques available for trouble shooting cable. In specific, you ask if it is possible to do a proper cable annalist using a video signal (as produced by an existing camera) and a hand held video meter. You mentioned looking for a full 1 volt, peak to peak video signal as a reference or assurance of good transmission capabilities of the cable.

As you suspect, this is not a qualified field test of coaxial cable. In as much as I may be able to "read" the amplitude of the video signal, this does not qualify the transmission capabilities of the cable. Since the video signal transmits on a broad band width of frequency, it is possible to lose specific ranges of, or random independent frequencies within the video signal while maintaining a one volt, peak to peak, video amplitude. This phenomenon could be caused by cable breakdown, bad or improper splices connections, cable length, RF interferences, or a small host of other cable related problems. Loss of frequency within the video signal will cause a loss of resolution or quality within the video image itself. In a color image, frequency loss could be the difference between a color accurate image and a nice picture. In black and white, it could be the difference between a sharp image and one that looks OK to the human eye.

Because of the above, the only way to truly test a coaxial cable for transmission capabilities is to do a complete frequency scan within a ten mega-hertz bandwidth. This requires the use of a frequency generator as well as a frequency meter or oscilloscope. Anything less and your test will not offer accurate information that can be used to determine the validity of a coaxial cable.

Although I do agree that fiber optics is the best way to go for video transmission, I also agree with you that replacing video cable within a system for the sake of upgrading to fiber is not valid. Especially if based upon false or inaccurate information concerning the transmission capabilities of the coaxial cable. In conclusion and retrospect hand held video meters are excellent tools when used for the proper purposes. They cannot however, validate a video cable's ability to accurately carry the frequency base of the video signal.

I sincerely hope that the information provided is of some service to you. If there is anything else that I may do to help you in the future, please feel free to call.