rf modulation on CATV

[msgchef]

I just finished designing a channel plus RF video distribution system for my new home being built and decided to check in with CP tech support. The Techie floors me by saying that most cable systems use all the frequencies and therefore I wouldn't find an empty channel to modulate for my four inputs. He says the work around is a dual rg6 system with a set top box at each tv location to accept one feed of modulation and the other for the cable box feed. Is this correct? It certainly doesn't sound it. Isn't that what the whole system is all about? He says no problem if I use sat tv, however, I doubt I have the sightlines for sat. any ideas?

 

[AverageJoe]

He's probably right that most cable systems use all available channels up to their design limits (typically around 750 to 860 MHz). I've inserted additional channels for security cameras, etc., but instead of using a dual cable I filtered out unwatched channels and combined modulators to the existing cable. It can get a bit expensive to do it without interference (channel elimination filters aren't cheap), but it keeps the signals compatible with the downstream set top boxes (If you're only subscribing to their basic service it gets a bit easier).

 

[msgchef]

He said those individual filters run $100 ea, that is a bit much. Is he right about the sat, that they don't use all the channels thus allowing and few to be used.

 

[AverageJoe]

Satellite providers use different frequencies so there would be plenty of channel locations available to insert your own. You would probably just need to get some diplex filters to insert the modulators into their coax, and also at the other end to strip off the channels before the connection to the receivers. Your satellite installer should be able to suggest channels and point you to a filter that's compatible with their system.

 

[bandphan]

Using a notch filter and knocking out a channel you dont use much, maybe home shopping network, as oppose to low/high pass filters works well. I do it for security cameras all the time. Is it digital cable?

 

[msgchef]

Yes it is digital cable. How do you determine the frequency of the particular channel and then to buy the right notch filter? Choosing a never watched channel makes a lot of sense.

 

[AverageJoe]

Well, if it's 100% digital, any channel you pick to delete will probably remove 6 or 8 program services, and would be hard to tell which one's unless your local cable company tells you.

Today, it would not be very common for a cable system to have no analog channels to pick from (It'll be a different story in the future, though).

If you have a TV with an ordinary tuner, connect the cable to it and see what channels you get. As bandphan mentioned, shopping channels are a common choice, as cable operators get a little incentive to keep them in the lower channels, and that puts the inserted channel in a convenient place. Channel Deletion Filters are typically sold by channel number instead of frequency, so that makes it a little easier.