[linux-dvb] Tuner sensitivity - details not on linux dvb wiki?

Morgan Tørvolt morgan.torvolt at gmail.com
Sun Mar 4 22:56:58 CET 2007


> I thought the MT352 is a demodulator...  isn't the tuner a separate entity??
> For example my cards both use a MT352 demod and they also both have a Thomson
> 7579 tuner.... so it may be that your devices both had the MT352 but
> different tuners... here's what I discovered on the wiki
>
> http://www.linuxtv.org/wiki/index.php/Demodulator
>
> incidentally, was the misbehaving USB stick one of the ones listed between
> 33-42 in the following list??
>
> http://www.linuxtv.org/wiki/index.php/DVB_USB#Twinhan_DVB-T_USB2.0
>
> Viktor

The reception quality on a card is based on two things, the receiver
and the demodulator. After the demodulator, the signal is digital, so
there is not much more to be done other than running trough error
correction, which will allways be done the same way. On DVB-S, the
tuner is the most important part as virtually all muxes run with the
simple QPSK modulation. On DVB-T the modulation is quite complex. The
fact that a single frequency can be transmittet from different places
with the same data together with all the signals bouncing off
mountains and such gives the demodulator a much harder job than with
DVB-T. How the connection is made between frontend and demodulator can
also affect the signal alot since some ways will introduce more noise
on the way to the demod. How much that last thing affects performance
I cannot tell, but it is very individual for each setup. The
performance of these cheap frontends and demods will also vary quite a
bit, so luck has something to do with the performance also.

That being said, usually this type of equipment will have varying
performance, and as much as 3dB signal sensitivity difference is not
uncommon. Still, a bad version of one could be as good as a good
version of the other. It is impossible to tell for certain. One could
make a database where everyone with at least two tuners could enter
which ones they have and what the performance difference is, but I am
not sure how accurate that will be over time.

An inline variable attenuator is good for signal sensitivity testing
in many cases, but attenuating the signal also attenuates the noise,
so the carrier to noise level stays the same. That must be taken into
account when testing like that. If the difference between two card is
that one gets a better signal quality from a given carrier to noise
level, but the other has a higher gain preamp, the one with the better
preamp will win this test, but is not nessesarily the best card
anyway. Adding an external preamp will give the loser a higher signal
to work with so you get better signal quality, but the same could do
little for the test winner as it already has a strong enough signal
but is unable to take advantage of it. A white noise generator is very
expensive though, so that is in practise the only solution most people
have.

-Morgan-



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