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

Brian Wood beww at beww.org
Fri Jan 26 21:18:09 UTC 2007

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...
> 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.

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