[mythtv-users] HDHomerun signal strength required

[Brian Wood]

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