Where to buy a twin RF modulator?

[ITM]

Does anyone know where I can buy a twin RF modulator - i.e. two a/v inputs, with RF pass-through and preferably wideband channel choice?

I want to use it for a pair of CCTV cameras which I want to be able to view on various TV's around the house. I was originally thinking of daisy-chaining a pair of modulators together, but my existing AV setup is very extensive (Tivo, Sky Plus, VCR, distribution around the house...) so I'm concerned about the signal loss. I've already had problems with signal loss after introducing new RF equipment into the loop.

TIA for any tips.

[BigWallop]

I think you'll find it difficult to get an RF modulator to send out two different station signals from the one box, but you might be able to find a modulator with two camera inputs and one RF output, which will then switch between the two cameras or show both in a "picture in picture" format, but these will be slightly more expensive.

[Owain]

"ITM" wrote | Does anyone know where I can buy a twin RF modulator - i.e. two a/v | inputs, with RF pass-through and preferably wideband channel choice?

try uk.tech.digital-tv and wait for Bill to come around; also see his website:

ebay.co.uk under tvaerialguy

no 3093791615 sounds like what you want for the modulator (shame he hasn't got two of 3093791602) and maybe 3093791617 for the filter-leveller.

| I've already had problems with signal loss after introducing new | RF equipment into the loop.

You don't need to daisy-chain them, just adjust the signals to the correct level on each channel and then combine them eg

UHF Broadcast TV ---------------> notch filters | if needed to remove | clashes with in-house V channels UHF in to Sky and Tivo | | -----| | Sky Box UHF out - channel pass filter | and level adjust -----------| | Tivo UHF out - channel pass filter | and level adjust --------------| | CCTV UHF out - channel pass filter | and level adjust --------------| | CCTV UHF out - channel pass filter | and level adjust --------------| | COMBINER ----> TVs

(or somethign like that anyway)

Owain

[Ian Stirling]

It's probably simpler to get a signal combiner, and simply plug both modulators into it, before feeding to the rest of the house.

[Owain]

"Owain" wrote | You don't need to daisy-chain them, just adjust the signals to the correct | level on each channel and then combine them eg | | | UHF Broadcast | | | V | UHF in to Sky and Tivo

Well, duh. Sky doesn't need a UHF in, I was thinking of a Freeview box.

Owain

[BigWallop]

Might be worth a visit to the Antiference web site:

have quite a good range of products that might be of help or give some ideas.

[ITM]

Uh-oh...this is looking a little more complicated than I thought...!

I hadn't anticipated the need for channel pass filters and level adjusters...could someone please explain this?

I've just come across this, which looks like what I thought I needed:

looks a little expensive, but from what you're all saying, will I also need something in addition to this??

Thanks for all the help.

[Andy Hall]

Although the channels need to be chosen carefully to avoid interference of the modulator harmonics with the TV signals.

This involves checking the channels in use and avoiding those that are

3,5,7 and 9 channels away from any terrestrial signal.

.andy

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[ITM]

All of my existing channels are in the range 23-50. If I use the twin RF modulator from CPC, and put the cameras on channels 53 and 56 then in theory there should be no problems, right?

In this case, would I have any need for channel pass filters or level adjusters??

[Andy Hall]

Sorry, but it isn't as easy as that.

Apart from avoiding being on the TV channels, you also need to avoid being on (N + 5) and (N + 9) away from them as well or you will get interference.

This means looking up the exact channels in use by the terrestrial transmitter(s) and avoiding those that are 5 and 9 channels away, as well as any that are adjacent.

Channelising filters will help with adjusting everything to equal level.

.andy

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[ITM]

Sorry I've never come across a channelising filter...presumably this would need to fitted in line with the output RF from the twin modulator? A Google search didn't find any such beast - could you possibly direct me to a URL for one of these? What would happen if I didn't use a filter?

[Andy Hall]

Yes.

Normally you use a single input levelling filter on the signals coming from the antenna and a multiple input one on the inputs from the local modulators. Then you combine the two outputs together through a splitter and amplify the result via a distribution amplifier to feed the TV sets.

What would happen if I didn't use a filter?

You will see a variation in picture quality across the different signal sources and may see interference.

.andy

To email, substitute .nospam with .gl

[Owain]

"ITM" wrote | >You don't need to daisy-chain them, just adjust the signals to the | >correct level on each channel and then combine them eg | >(or somethign like that anyway) | Uh-oh...this is looking a little more complicated than I thought...! | I hadn't anticipated the need for channel pass filters and level | adjusters...could someone please explain this?

As Andy said, you need to avoid 3/5/7/9 channels to existing channels. As well as the 5 'normal' terrestrial channels, you will probably also have digital multiplexes (making 10 or 11 channels in use) and possibly also fainter 'copies' of the normal terrestrial channels from more distant transmitters. Once the 3/5/7/9 channel spacing is taken into account there end up being a shortage of spare channels.

You also want to avoid having multiple sources of the same channel being passed through different devices, and wildly differing levels between channels.

So you put channel pass filters to kill anything unwanted at source and adjust the levels of the wanted channels, if needed, before distributing round.

| I've just come across this, which looks like what I thought I needed: |

| It looks a little expensive,

Especially when you could get item no 3094296967 for a tenner at the moment (3 modulators) accompanied by excellent technical support :-) or you might just as well buy two of these

| but from what you're all saying, will I | also need something in addition to this??

That depends on the channels allocated and their signal levels, in your area (and just how good a job you want to do).

Owain