[mythtv-users] Best place to insert a CATV amp

Bill Bill at explosivo.com
Fri Jan 26 21:33:18 UTC 2007

On Fri, 26 Jan 2007 14:18:09 -0700
Brian Wood <beww at beww.org> wrote:

> On Jan 26, 2007, at 1:44 PM, Bill wrote:
> >
> > Hello all,
> >
> > I cannot seem to find a good answer anywhere on this, so maybe  
> > someone here can help.
> >
> > Q:  In a line with several splits, where is the best place to add  
> > an amplifier?
> >
> > I just hooked up my newly recreated mythtv backend.  It has a  
> > PVR350 and a PVR500 in it.  At first I was getting pretty icky  
> > picture, so I hooked up a powered amp and it got better.  But not  
> > as good as it should.
> >
> > Pretty much my arangement is like so:  Sorry for it being  
> > confusing, but I am unsure how to portay it.  I've noted where I  
> > have considered putting it by inserting (option #).  I can also get  
> > an amp with multiple outputs which may be the way to go.
> >
> > I *believe* I have near perfect cable signal coming in (not too  
> > strong / not too weak) - but then again they did not seem to be the  
> > sharpest of people...
> >
> > -> STREET
> >
> > Cable Modem:
> > (ends here)
> >
> > TV: (Option 1)
> >
> > -> TV
> > -> TV
> > -> TV
> > -> TV -> (Option 2) -> 3 WAY SPLITTER
> >
> > -> TV
> > -> PVR350
> > -> (Option 3) -> PVR500 (Which has a 2 WAY built in I believe)
> >
> > Basically the question comes down to is it better to boost the  
> > signal higher and let it drop down as it goes through, or let it  
> > drop down, then boost it.
> >
> > As I understand it, you get poor quality when it is too high or too  
> > low... so should I work out the drops so it always stays as close  
> > to the starting point?
> Admit it, you wrote that question just to bait me, right ?
> As usual there is no simple "one size fits all" answer. In general it  
> is better to amplify prior to the splits but there are several  
> factors that can change that.
> Your statement about poor quality if the signal is too high *or* too  
> low is quite correct. Too high a signal going through an amplifier or  
> mixer (including one in your TV set) and you run afoul of the  
> "composite triple beat" (commonly known as "cross-mod") spec. Too low  
> a signal and you run into trouble with the signal-to-noise (actually  
> signal plus noise to noise) spec.
> In fact, the "theoretical cacade-ability" of an amplifier is when  
> those two levels become the same, leaving absolutely no "window"  
> within which to meet the specs.
> So you want to be sure that the input to your amplifier is as high as  
> it can be to keep the S/N good without going out of spec on the CTB.  
> This optimum level will depend on many things, including how many  
> channels are being carried.
> But another problem is that the level received from the cable company  
> is constantly changing due to variations in the temperature of the  
> coax cable, and the changes are not "even" since cable has more  
> attenuation at higher frequencies. If you drive your amp at the limit  
> of its input and the level then increases when the sun goes down you  
> will have CTB problems.
> The cable companies use AGC amplifiers to try and even out the  
> temperature variations but the cable from the last AGC amp to your  
> house is not compensated. Obviously these changes will be more  
> pronounced with an overhead drop as opposed to an underground one.
> Without a signal level meter you really have no idea what the levels  
> of the incoming channels are, and those are expensive items not owned  
> by most consumers (though I do have one, I am a "nutcase"). In any  
> case the incoming levels are not totally within your control, though  
> you can reduce them with an attenuator you have to be careful that  
> you use one that can pass the low-frequency return signal if you have  
> a cable modem or the like. The daily and seasonal variations are  
> pretty much out of your control.
> So you really have to just use common sense and empirically determine  
> the best setup. Start by putting the amp before your splitters,  
> though if you have a cable modem you might have to put it after the  
> directional coupler whose tap feeds that, unless your amp has a  
> return path.
> There's a good chance this will work for you, Make sure you use a  
> good quality push-pull amplifier designed for cable signals. The  
> single-ended amps designed for antenna signals are not satisfactory -  
> for one thing the second harmonic of the low band will fall in the  
> mid-band and cause beats.
> If this doesn't work well for you may have to just experiment.
> Even this is not a "good" answer, I've left out a lot and simplified  
> much to make it readable here.
> Other options are to prevail on the cable company to get more signal  
> from them, eliminating the need for an amp. That really depends on  
> their attitude and how well you get along with them.


First off thanks for all of those words.  Thats a lot of explaination, which helps, so thank you for going through all the trouble!

I will try to get it in right after the cable modem split (that occurs outside).


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