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

[Brian Wood]

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