[mythtv-users] Signal strength reported by tuner card?

[Russ Van Winkle]

On Sun, Sep 27, 2009 at 11:29 AM, Brian Wood <beww at beww.org> wrote:

> Signal strength and the signal to noise ratio (or even the signal+noise/noise
> ratio), are not the same thing. Remember an amplifier amplifies the noise as
> well as the signal, it also adds a small amount of noise itself (the "noise
> figure" of the amplifier).
>
> The "signal strength" or "S/N" reported by a card is probably the BER (bit
> error rate), which is loosely related to the signal strength and the S/N
> ratio. A true S/N ratio would probably not be changed much by a preamp, in
> fact it would probably be very slightly worse, because of the amplifier
> noise, though it may help in reducing the effect of internal noise in the
> card, by raising the signal level further above such noise, so your results
> are not all that unusual.
>
> You really have to know what it is that's being measured and how. The card's
> report is intended to help with antenna aiming, and give an indication that
> the card is receiving enough RF to operate in it's desired signal level
> range. It's not a scientific measurement, that would require a spectrum
> analyzer or similar device.
>
> If you can find more info on what the card is actually reporting it would be
> helppful, but you are certainly correct that some cards give more useful
> information than others.

Good point on the signal strength vs. S/N numbers.  I suppose it would
have been more accurate to say I saw little *change* in the reported
numbers, rather than *improvement*.  I'm also seeing continued
dropouts on the most distant signal, approx. 40 miles away according
to tvfool.com (http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d8ecd67c9d4d898).
 By "most distant" I'm referring only to the top five signals on that
chart, with 'only digital' selected, i.e. the only ones giving a
line-of-sight signal path.

The three strongest signals come from very nearly 180 degrees apart
(two to the northwest share a transmitter site), with two weaker (more
distant) signals from about 20 degrees off that axis (further north).
I'd hoped to receive them all with one directional antenna, and not
have to cope with a rotor or combining antennas.  Not having access to
a spectrum analyzer or other professional equipment, I originally
aimed the antenna last fall by the simple expedient of attaching the
feed to an analog TV set and watching the picture.  I may have
inadvertently moved the antenna while installing the external
component of the preamp despite taking precautions to avoid doing so.

I added the preamp to (hopefully) compensate for line loss due to a
fairly long cable run from the antenna to the backend with the tuner
card.  At this point I'm unsure if I need to re-aim the antenna, or if
the newly installed preamp isn't functioning.  (Or both.)

-- 
Russ D. Van Winkle