<br><br><div class="gmail_quote">On Sun, Sep 27, 2009 at 5:29 PM, Brian Wood <span dir="ltr"><<a href="mailto:beww@beww.org">beww@beww.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div class="im">On Sunday 27 September 2009 09:49:17 Russ Van Winkle wrote:<br>
> On Wed, Sep 23, 2009 at 1:23 AM, Daniel Kristjansson<br>
><br>
> <<a href="mailto:danielk@cuymedia.net">danielk@cuymedia.net</a>> wrote:<br>
> > The signal strength and S/N numbers are uncalibrated with the Linux<br>
> > DVB drivers. As a general rule with any one card on any one<br>
> > channel, a larger number is better and smaller number is worse.<br>
> > The S/N is log(random num) on some cards, a linear value on others,<br>
> > an uncalibrated S/N register read on other cards, another name for<br>
> > signal strength on other cards, and sometimes completely made up.<br>
> > The reason we report those values is for primarily of antenna<br>
> > adjustment purposes.<br>
> ><br>
> > -- Daniel<br>
><br>
> Is there a resource listing which cards give good information?<br>
><br>
> I recently added an antenna preamp to my rooftop antenna, and haven't<br>
> seen any improvement to the reported signal strength or S/N. I'm<br>
> using the Technisat Airstar HD-5000 under kernel version 2.6.28.<br>
><br>
> The wiki entry for this card<br>
> (<a href="http://www.mythtv.org/wiki/Technisat_AirStar_HD-5000" target="_blank">http://www.mythtv.org/wiki/Technisat_AirStar_HD-5000</a>) seems to<br>
> indicate the card should be giving useful information.<br>
<br>
</div>Signal strength and the signal to noise ratio (or even the signal+noise/noise<br>
ratio), are not the same thing. Remember an amplifier amplifies the noise as<br>
well as the signal, it also adds a small amount of noise itself (the "noise<br>
figure" of the amplifier).<br>
<br>
The "signal strength" or "S/N" reported by a card is probably the BER (bit<br>
error rate), which is loosely related to the signal strength and the S/N<br>
ratio. A true S/N ratio would probably not be changed much by a preamp, in<br>
fact it would probably be very slightly worse, because of the amplifier<br>
noise, though it may help in reducing the effect of internal noise in the<br>
card, by raising the signal level further above such noise, so your results<br>
are not all that unusual.<br>
<br>
You really have to know what it is that's being measured and how. The card's<br>
report is intended to help with antenna aiming, and give an indication that<br>
the card is receiving enough RF to operate in it's desired signal level<br>
range. It's not a scientific measurement, that would require a spectrum<br>
analyzer or similar device.<br>
<br>
If you can find more info on what the card is actually reporting it would be<br>
helppful, but you are certainly correct that some cards give more useful<br>
information than others.<br>
<font color="#888888"><br>
--<br>
Brian Wood<br>
<a href="mailto:beww@beww.org">beww@beww.org</a><br>
</font><div><div></div><div class="h5">_______________________________________________<br>
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</div></div></blockquote></div><br>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?<br>
<br>Andrew<br clear="all"><br><br>