Modulator (running CCTV) ruining my TV Signal - please help!

[Johnsmith]

Dear all

I'm hoping someone out there might be able to help me. I've done as much reading around as I can (as well as experiment with different set ups) and have not been able to find a solution on my own.

Apologies if this comes across a bit novice-like. I've tried to explain it as best I can.

IN BRIEF:

I've set up my aerial (via a loftbox) so that I have an aerial inputs in nearly every room of the house.

But a modulator I'm using (for a small CCTV unit) is ruining the signal I send round to the rest of my house. The CCTV is black & white and has no sound and I'm quite happy with that.

MORE DETAIL:

Here's how the system I've installed works:

1. I have a good aerail on the roof (approx 1 year old) which puts out a good, strong, signal.

2. The 'main'/'primary' aerial lead runs from the roof to the ground floor.

3. At this point, on the ground floor, I connect in the CCTV unit. This has a scart, so I have to modulate the signal back so that I can shoot it back up to the loft.

4. Once back in the loft, I connect this lead to my Loftbox.

5. At that point, the loftbox sends aerial leads back down to the various rooms in the house. This means that each room also sees the CCTV picture. A bit grainy but great to have.

PROBLEM:

The problem is that, once I attach the CCTV modulator, the signal becomes much worse. This was bad enough on analogue TV. But now that I've got Freeview, I'm missing some of the main channels (itv1, Ch4).

If I remove the CCTV/modulator unit from the system, the picture on all the rooms being fed is crystal clear again (both on analogue and freeview).

The modulator I'm using is a £40 jobbie from Maplins (I did try using another one too, but that made no difference).

Does anyone have any ideas on:

1. How I can improve the signal (and stop the modulator doing this)?

2. Recommend any product that might help? I don't mind blowing the budget on a much better modulator - but from what I've read, they all do pretty much the same thing, don't they? If you can recommend one, I'd be interested to hear.

Sorry if this is a bit rambling!

Any help would be really appreciated.

MODERATOR - apologies if I've put this in the wrong forum. Please feel free to move it.

 

[aptsys]

How are you combining the TV signal with the output of the modulator? What channel have you set the modulator to? Keep it away from any of the MUX's frequencies.

 

[Spark123]

The first thing I would try as aptsys says is to try shifting the channel the modulator outputs on away from those of the TV stations. I have used an old VCR in the past to convert the composite signal (CCTV) to one which a TV aerial input can use without problem, but saying that I don't have freeview!

 

[electronicsuk]

If adjusting the RF channel on the modulator doesn't help then you'll probably need a channel filter inline with the RF output, this will attenuate all channels/frequencies either side of a desired channel. They are a relatively specialist item (you could try http://www.taylorbros.co.uk) and are used on large commercial MATV systems.

You'll need to inform the supplier at the time of order which channel you'd like the filter set to, so be sure to find a space in the spectrum that's not occupied by existing digital or analogue broadcasts.

 

[mattylad]

Also if you are splitting the signal with a splitter to put it into the loftbox (I dont know how many inputs it has) you would bebetter doing it with an inductive splitter rather than a resistive one.

 

[RF Lighting]

Loft boxes have a dedicated input for a modulated CCTV signal. Did you connect to that?

 

[TicklyT]

Maybe an attenuator in the CCTV feed to the amp would do the job. It could be the CCTV signal is a few orders of magnitude greater than the signal from the aerial.

Years ago (in the days when Sinclair Spectrum PCs were new ) I found connecting the modulator in the computer to a splitter/combiner swamped the TV signal. Introducing a 30 dB pad into the computer's output restored the signal to the TV, and allowed the signal from the Spectrum to be displayed.

30 dB is a lot of attenuation - something like 1/1000th of the original signal level!