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

[Andrew Herron]

On Sun, Sep 27, 2009 at 5:29 PM, Brian Wood <beww at beww.org> wrote:

> On Sunday 27 September 2009 09:49:17 Russ Van Winkle wrote:
> > On Wed, Sep 23, 2009 at 1:23 AM, Daniel Kristjansson
> >
> > <danielk at cuymedia.net> wrote:
> > > The signal strength and S/N numbers are uncalibrated with the Linux
> > > DVB drivers. As a  general rule with any one card on any one
> > > channel, a larger number is better and smaller number is worse.
> > > The S/N is log(random num) on some cards, a linear value on others,
> > > an uncalibrated S/N register read on other cards, another name for
> > > signal strength on other cards, and sometimes completely made up.
> > > The reason we report those values is for primarily of antenna
> > > adjustment purposes.
> > >
> > > -- Daniel
> >
> > Is there a resource listing which cards give good information?
> >
> > I recently added an antenna preamp to my rooftop antenna, and haven't
> > seen any improvement to the reported signal strength or S/N.  I'm
> > using the Technisat Airstar HD-5000 under kernel version 2.6.28.
> >
> > The wiki entry for this card
> > (http://www.mythtv.org/wiki/Technisat_AirStar_HD-5000) seems to
> > indicate the card should be giving useful information.
>
> 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.
>
> --
> Brian Wood
> beww at beww.org
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>

One of the things I notice, irrespective of the DVB tuner used, is that if I
have an equivalent STB hooked up as a benchmark the DVB card seems to always
be less sensitive and to deliver a less stable & lower quality picture using
exactly the same signal. Any insight into why this might be?

Andrew
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