[mythtv-users] HDHomerun signal strength required

[backuppc at sundquist.imapmail.org]

On Thur, Feb 21, 2008, 7:27 AM, beww at beww said:
> George Mari wrote:
>
> >
> > I'm not an electronics engineer but I'll basically be splitting the
> >> signal strength in half, right?
> >
> > Right.
> (Long-Winded, read only if interested)
>
> Right, sort of.
>
> You are actually reducing the level by half, plus a little "extra"
> reduction due to the non-perfect nature of a passive splitter (back
> match losses, coupling losses etc.).
>
> While this sounds drastic it is really not.
>
> TV signal levels are measured in Dbmv, with the zero Dbmv reference
> level being 1 millivolt across 75 ohms. This is nominally the level
> that a TV set requires to produce a decent picture, and is in fact the
> minimum level that the FCC requires cable systems to deliver to the
> subscriber. Most TV sets actually deliver a better picture when given
> something closer to +10 Dbmv, though you have to be careful to not
> overdrive the tuner, especially with multi-channel systems (the tuner
> mixer stage is seeing the signal on all channels, even the ones not
> being tuned at the time, thus you can tolerate a higher signal level
> from an antenna delivering 6 channels as opposed to a 50-channel cable
> system).
>
> It's actually more complex than that, the FCC rules also specify the
> maximum difference between any two adjacent channels, the maximum
> variation over a 24-hour period and a few other specs, but thinking of
> it as a minimum of 0 Dbmv is basically correct.
>
> For you old timers, Dbmv. was originally called a DbJ, reflecting it's
> use by the Jerrold Corporation prior to its adoption as an official
> standard. (Jerrold was named for its founder, Milton Jerrold Schapp,
> who later became the governor of Pennsylvania, they have since been
> bought out by General Instrument).
>
> Decibels are a logarithmic scale to avoid having to use many many
> zeros to describe the levels commonly encountered in the real world.
>
> So, you might think that if you have a level of +10 Dbmv. and put
> a two-way splitter in the line, each output would have a level of
> +5 Dbmv.
>
> Wrong, you would have a level of +7 (actually about +6.5 due to
> splitting losses, you would have +7 if you had a "perfect" splitter).
>
> Splitting a 0 Dbmv reference level would result in a level of -3
> nominally, about -3.5 in actuality.
>
> So you see that a power reduction of "half" results in a nominal
> reduction of the level by 3 db. If moving from +10 to +7 you will
> probably not notice any difference at all, but if moving from -10 to
> -13 you might see an increase in noise (in what was probably an
> already pretty noisy signal).
>
> So the results of reducing a signal by "half" range from unnoticeable
> to problematical, depending on where you are starting from.
>
> Bored? I thought so, but I am an electronics engineer :-)
>
> beww

Coincidently, I am also expecting delivery of my HDHR today, also.

I was wondering about the splitting, also.  I had read this page:

http://www.hdtvprimer.com/ANTENNAS/merging.html  which discussed similar
tech stuff, but in reverse (merging instead of splitting).

My question is this:

Right now I have a single pcHDTV 5500 card which works OK except when
the weather is bad and the weather is often bad.  I want to see if it
truly is a problem with poor reception with this card as the wiki
suggests.  Plus I need more than one tuner, so the HDHR is on the way.

But now I have three RF jacks to split into (one 5500 and two HDHR).
The splitters seem to come in 1:2 and 1:4 varieties.  Should I just use
a 1:4 splitter and leave one empty?  Would this cause "reflection"
problems as noted in the above link?  Would I need to put a terminating
resistor on this?

Another related question:  My daughter sometimes may want to play an old
VHS tape or two we have laying around.  I am not converting these all to
DVDs.   Our VHS player outputs to coax on NTSC channel 3.  I could have
the pcHDTV card (which does NTSC as well as ATSC) pick that up (right?).
Could I plug that into a 1:4 splitter, effectively making it a 2:3
splitter?  Another way to pose that question is "even though four of the
five F-plugs say `out' (see
http://rsk.imageg.net/graphics/product_images/pRS1C-2265543w345.jpg) are
all five in fact wired in parallel?"

I actually have a merger up in the attic of two directional UHF
antennas, so I am probably looking at all kinds of efficiency losses.
Should I consider a signal amplifer after the merge?  My DTV signal
strength could be better.

Sorry for asking what are basically not true myth-related questions, but
since this thread was already started...

J.S.