[mythtv-users] Understanding HD Antennas ( was HDHomeRun and jumpy video)
beww at beww.org
Sun Sep 16 17:52:49 UTC 2007
Jon Boehm wrote:
> How does reducing the input level sometimes help things? It seams to me
> that I'm STUCK with noise that is on the line. Once noise gets into the
> signal its impossible to get out. When you attenuate the signal the
> noise get promotionally attenuated giving the same signal-to-noise
> ratio. And again, amplifying the signal proportionally amplifys the
> noise again giving the same signal-to-noise ratio. So how to attenuator
> and amps help in noise situations?
Note that I said "sometimes".
People often assume that the signal level from an antenna feed is the
same on all channels, and this is rarely the case.
Take a simplified example, two channels, one very strong and one quite weak.
You might think the answer to the problem is to amplify the signal, but
that will amplify both channels. The high-level channel may well be
strong enough to cause garbage in the amplifier (third-order products
and other garbage), and this can sometimes manifest itself as "noise",
especially to an untrained observer.
Even if the high-level channel is not hot enough to cause problems in
the preamp, the output of the amplifier will be very high on the
high-level channel, possibly strong enough to cause problems in the TV
tuner input stage (mixer or, in rare cases, an RF amplifier).
So, in a case like this attenuating the signal before the preamp or TV
set will actually result in a better perceived picture.
Lots of things cause "noise" in a signal, and thermal noise caused by
low signal level is only one of them.
If the problem is actually true "noise" then your analysis is
essentially correct (though there are noise "coring" and other reduction
systems), but in a lot of cases the "noise" is actually something else.
Sometimes a strong out-of-band signal, like a nearby FM transmitter, can
cause mixing products in a tuner or amp that look like "noise", and
selectively attenuating (or "trapping") such a signal can help.
Sometimes it helps to trap the entire FM band out of the signal before
it hits the amplifier, especially in urban areas with a lot of FM signals.
It is best to try and get all the channels at the same level, or as
close as you can, before hitting an amplifier, but this requires being
able to selectively control the level on individual channels, not
something most consumer devices can do.
So my statement that sometimes reducing the level can help is correct,
but it is not a terribly common situation.
The only way to know what is really going on is with a spectrum analyzer
or similar device, not found in most consumer closets.
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