[mythtv-users] Coax splitters - how painful are they?

R. G. Newbury newbury at mandamus.org
Sun Sep 30 20:59:38 UTC 2007

Todd wrote:
> "R. G. Newbury" <newbury at mandamus.org> writes:
>> Well I DO NOT FEAR a visit from the FAA....it would be the DoT up here 
>> in the 'frozen north' (that describes our bureaucracies not our weather!)
>> Seriously, I want to combine a cable feed and an antenna through 
>> combiner, and feed one coax to the TV. Either feed alone is fine and I 
>> can switch them, but through a (cheap) combiner I get nothing
>> watchable.
> Hi RG, 
> This occurs because it's quite a complex engineering issue.
> You're trying to frequency division multiplex two broadband RF
> signals.  > Nothing cheap is going to do it.  To do it right, you need
> to at least downconvert the OTA channels (which I believe would
> require one tuner per channel) and then modulate them up into a
> frequency slot that's unused on the cable feed.  The math and the
> physics are not pretty.  It actually might be worse than that-- you
> may need one radio (tuner) per station to pull this combining off
> properly.   

>> Both signals have already been split: the cable coax to feed the 
>> mythbox, and the antenna has already been boosted (by a medium-cheap) 
>> amp, and split three ways, 2 of which feed the HDHR unit,
>> This may not be the best setup.. I do not know. I do suspect that I need 
>> better splitters than the el-cheapo's I had in the bottom of a box of junk.
> I'd recommend dedicating a capture card to OTA, and a separate one to
> CATV.  Then, if you want to also feed the TV with these signals for
> watching them live, the right tool is a low tech co-ax switch.  Radio
> Shack (or equiv) sells remote controlled ones.  Or if you have a an
> a/v receiver or TV with multiple co-ax inputs that can be used as a
> more elegant switch.  But combining broadband RF requires multiplexers
> that cost thousands, best I can tell (they're used in large facility
> closed circuit tv surveillance).  

Sorry, but you didn't read the part you abstracted nor other parts of 
the posted question. The mythbox is FINE. I have 4 tuners: 2 analog, 2 
digital. I have ONE input to the TV and I want both the antenna and the 
cable on that feed.

I already HAVE a coax switch.

I AM lazy enough that I would like NOT to have to get up and cross the 
room to change the switch.

I am not trying to do 'frequency division multiplex'. These are RF 
signals. I understand that the differing levels and the possibility of 
overlap beat harmonics makes combining them unlikely to work WELL, but I 
  admit I do not understand why I get NO usable signal from the 
combiner. It's been a couple of ....{days|years|epochs|eons} since I 
exited from my last physics exam (Quantum Physics 340 iirc..)

Because of the technical side-effects I won't continue down that line.

I have not seen a remotely controlled coax switch at Radio Shack (now 
The Source in Canada). The idea of using an A/V receiver as a switch is 
an interesting one...but I cannot recall seeing any such receivers with 
a coax input, not that I have been looking. The 'youngest' of my stereo 
receivers is at least 25 years old now! In fact, the physical switches 
on the one at work are failing so I have to control it with the remote! 
Maybe it *is* time for a new receiver...


More information about the mythtv-users mailing list