When you have complain by a customer about a very poor picture generated by a security camera on a closed circuit television (CCTV). There number of factor to cause attenuation and picture distortion.
Upon inspection of the security camera, we found that the CCTV system consisted of National Television System Committee (NTSC) standard video generated and displayed in black-and-white. The main monitoring point of the CCTV system was a warehouse and a weather-proofed housed box security camera located on a 15-ft high mounted atop the building’s roof.
The security camera was connected to a security DVR in a security office some 200 ft away via a 75 ohm Siamese coaxial and power in one cable, which was routed down into the office by means of a pipe type of entry. Power to the security camera was provided via a 12V low-voltage DC link on the same Siamese cable; both the monitor and camera low-voltage DC supplies were simply plugged into a wall outlet convenient to the operator at the guard’s office.
Symptom
The picture quality on the video monitor looked as though contrast control was turn all the way one direction so that there was no contrast at all; the picture looked washed-out and was barely visible, which was a nearly light gray.
Testing procedures carried out
Fisrt step in solving this kind of problems is to start the diagnostics from step one, which is the security camera. Our first task was to go up to the security camera at the roof and see the picture quality and video signal looked like as it exited the security camera. Tyou can perform this test with a mobile monitor or any mobile display that you can connect directly to the camera. his test was much aided by the fact that our handheld, 50 MHz bandwidth, solid state, digitizing oscilloscope with LCD display had an internal battery pack and did not require any AC power for operation.
First Step. 1, the coaxial cable was disconnected at the camera and a BNC style fitting was installed. This fitting was equipped with a 75 ohm terminator resistor on one leg. Then, we connected our handheld oscilloscope into the remaining open end of the BNC connector. The result was a healthy NTSC composite video signal high quality picture. Conclusion: the camera was clearly putting out a good signal, which was about 1.8V across the 75 ohm termination.
The BNC connector was removed and the 75 ohm coaxial cable was reconnected.
Second test
The 2nd test was made at the DVR System end of the cable and right at the point where the cable was connected to the security dvr. Again, the BNC was used, but this time no 75 ohm termination resistor was used with it since the BNC was attached to both the DVR and the cable. The result of this test was that almost no video signal could be seen on our handheld monitor screen.
We then changed the monitor’s vertical scale the Conclusion: the signal loss was occurring along the 75 ohm cable path, or was it?
Security DVRs have been seen to “load down” a signal due to an internal failure on its input circuit; as such, we didn’t want to rule this possibility out. A quick test with the BNC and the 75 ohm termination resistor in place of the video monitor quickly ruled out this possibility; the signal was essentially. We really could conclude that the signal’s loss was occurring along the 75 ohm cable.
3rd test. We placed our handheld monitor into the BNC at the junction of the cable and security DVR. This allowed us to take a relative dB measurement reading, using the original zero-level as the reference. Here we see that the DC loss exists. This loss represents a voltage loss ratio of 4.84:1, or a signal loss of nearly 5V for every volt put into the cable!
How much signal attenuation should you expect on a 200-ft long, 75 ohm coaxial cable? A quick look at the coaxial cable manufacturer’s Master Catalog gave us the approximate answer: around 2 dB of loss at 10 MHz for 200 ft of RG-59/U type cable as used in CATV applications. The whole attenuation chart is shown in the accompanying table below.
Oh yes, since the manufacturer’s information was provided only in dB form, what would we have done if we didn’t understand dB and weren’t working in terms of dB on our handheld monitor? You guessed it. We would have had no idea what was “normal” and what was not on a coaxial siamese cable run of the type being investigated. All we would have had was some guesswork, which is not a very good way to go in most cases.
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Video signal used in CCTV equipment is normally a one volt peak-to-peak signal and is impedance sensitive to 75 ohms for ideal video reproduction at the monitor. If these environments are not met, then the video will degrade in quality and performance. Distribution Amplification