[mythtv-users] OT: OTA HD

Brian Wood beww at beww.org
Wed Dec 24 01:43:18 UTC 2008

On Tuesday 23 December 2008 18:30:31 David Brodbeck wrote:
> Travis Tabbal wrote:
> > I had not considered the interference issue before I posted about setups
> > like this. You might be able to use a diplexer, those are used in
> > satelite/OTA installs. I have no idea how they would behave with CATV
> > setups though.
> A diplexer won't work.  It works for satellite because satellite and OTA
> TV use different frequency bands, making them easy to separate.  Some of
> the cable frequencies overlap the OTA frequencies, so that trick doesn't
> work for mixing OTA and cable.

Quite correct, a diplexer separates signals of different frequencies. This is 
what allows you to talk and listen at the same time with your cell phone, 
though those have a 45Mhz. separation in the signal frequencies, to make 
duplex operation cheaper. Diplexers for two-way radios that use 4.5Mhz. 
separation are big, bulky and costly.

A channel elimination filter, or band-stop filter, might do what the OP wants, 
but a high-quality one is extremely expensive. Microwave Filter Corp. makes 
some nice ones, but they are custom built-to-order, and you could buy several 
nice MythTV systems for what they cost. I have seen cavity type diplexes that 
look like several garbage cans (cavity resonators).

Best answer is to not try to combine the cable and OA signals at all, just too 
risky, and use an A/B switch, coaxial relay or other very high isolation 

Assuming you need to have Myth control the switching device, there are 
controllable A/B switches, or you can use coaxial relays made by Dow-Key, 
justmake sure to terminate any unused inputs.

beww at beww.org

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