Video signal from CCTV cameras??

[ITM]

A few years ago I had a 2-camera CCTV system installed, with both cameras fed through RF modulators so that I could view each on a different channel on the TV. The "guts" of this setup was a complex of cross-wiring in a project box:

- a pair of daisy-chained RF modulators

- power supplies to the cameras and modulators

- a capacitor

- another component which could have been a transistor (??!)

Due to problems with this setup, and overheating componentry, I have dismantled it and tried to simplify it with the use of more standard connectors into my video system. Before I begin, I wanted to check that I had correctly understood the wiring of the existing cameras...

Each camera is linked to the junction box via a 4-core cable. Two of these are for the power supply. The other two, I presumed, were video and audio. I put some phono plugs on these two, and connected them to the composite & audio inputs on my TV to check them out. The signal appears to be almost non-existent, however - I can just about make out the outline of an image on the screen, so I'm sure that I have the video/audio cables correctly identified, but the signal is so weak as to be un-usable.

Having no knowledge of CCTV systems, I was wondering whether anyone could provide information on the video/audio output from them? Would they usually need significant amplification before being fed into a TV? In this case they are transmitting a signal over more than 40ft of cable. The cameras themselves are very small, and presumably pretty cheap.

Given the contents of the previous setup, there didn't appear to be any obvious signal amplification, so I'm curious to know how it worked!

Any advice would be very gratefully received... TIA Ian

[Dave Liquorice]

Did you also pickup the Ov (ground) for the camera and connect it to the shell of the phono or just the "video" wire to the pin?

I wouldn't expect as much loss as you are reporting, loss of HF and reduction in level maybe but not to hardly visible levels.

[ITM]

Only the video wire to the pin. There was no ground - only power live/neutral and (presumably) audio (4 wires in all).

What would be your guess as to the likeliest reason for the weakness of signal??

[Craig Graham]

There has to be a ground. The only reason you're getting anything at all is because it's finding a very poor return path somewhere.

You say power is live/neutral. Are you sure it's not DC? If it's DC, then one of those supply wires will be the ground and you have to bring that out to both the audio and video shells. If you get horrible hum, just do it to either audio or video but not both. Alternatively, is that audio wire really audio or could it be video ground? Do you definitely have audio on those cameras?

[ITM]

Yes it's DC power - so can/should I take the ground from the power supply and connect it to the grounds on the video and audio phono plugs?

[Dave Liquorice]

The OP mentions power supplies for the cameras and modulators, so the chances are it is DC rather than the mains...

And easy to check from the old layout, he wants the one that connected to the screening boxes of the modulators.

Or it could be balanced video but unlikely from a cheap camera they tend to just generate normal unbalanced 75ohm baseband video.

[BigWallop]

If you've only put one wire to the centre pin of the phono plug then you'll need a ground (zero volts) path from the power supply to the outer casing of the phono plug to re-create a complete system circuit again. Every electron needs to flow around complete circuits to make things work. You couldn't have a formula one race if the cars didn't go round on a complete track, that's why these tracks are called circuits. The cars have to go all the way round and come back to where the started from to have completed the circuit, and same goes for every electron in the circuits you're trying to make with your wiring scheme.

[ITM]

Strange...I connected the ground from the power supply to the shell of the plug and still get no clear video picture. I'm pretty sure that it was the correct cable, as this one was originally connected to the outside casing of the modulator.

Is there anything else that I could've done wrong???

[BigWallop]

If you have the wire that was connected to outer casing on the modulator and now connected it to the centre pin of the phono plug, then you've definitely got the wrong wires connected.

[ITM]

OK I've finally cracked it - the wiring was correct, but one of the ground connections was loose in the connector block.

Many thanks for all your help - and for your patience with a novice!

[BigWallop]

It works out a lot cheaper if you know the basics and then ask in here as well. :-))