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

On Sun, 02 May 2004 19:44:43 GMT, ITM wrote:

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...

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?

In this case they are transmitting a signal over more than 40ft of
cable.

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.

--
Cheers
Dave. pam is missing e-mail

On Sun, 02 May 2004 21:36:30 +0100 (BST), "Dave Liquorice"
wrote:

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?

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??

ITM wrote:
On Sun, 02 May 2004 21:36:30 +0100 (BST), "Dave Liquorice"
wrote:

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?

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??

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?

--
Dr. Craig Graham, Software Engineer
Advanced Analysis and Integration Limited, UK. http://www.aail.co.uk/

On Sun, 2 May 2004 23:09:40 +0100, "Craig Graham"
wrote:

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?

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?

"ITM" wrote in message
...
On Sun, 2 May 2004 23:09:40 +0100, "Craig Graham"
wrote:

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?

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?


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.


---
http://www.basecuritysystems.no-ip.com

Outgoing mail is certified Virus Free.
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On Sun, 2 May 2004 23:09:40 +0100, Craig Graham wrote:

You say power is live/neutral. Are you sure it's not DC?

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

If it's DC, then one of those supply wires will be the ground

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

Alternatively, is that audio wire really audio or could it be video
ground? Do you definitely have audio on those cameras?

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

--
Cheers
Dave. pam is missing e-mail

On Sun, 2 May 2004 23:09:40 +0100, "Craig Graham"
wrote:

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?

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???

"ITM" wrote in message
...
On Sun, 2 May 2004 23:09:40 +0100, "Craig Graham"
wrote:

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?

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???

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.


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http://www.basecuritysystems.no-ip.com

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