Re: [linux-dvb] TDA10086 fails? DiSEqC bad? TT S-1401 Horizontal transponder fails

[Matthias Schwarzott <zzam@gentoo.org>]

On Freitag, 21. März 2008, Manu Abraham wrote:
> Matthias Schwarzott wrote:
> > On Freitag, 21. März 2008, Manu Abraham wrote:
> >> Hi Mathias,
> >>
> >> Matthias Schwarzott wrote:
> >>> Zarlink zl1003x datasheet (avail on net) tells this:
> >>> fbw = (alpha * symbol rate) / (2.0 * 0.8) + foffset
> >>>
> >>> where alpha is roll-off 1.35 for dvb-s and 1.20 for DSS
> >>>
> >>> The manual suggests to use highest possible bandwidth for aquiring a
> >>> lock. And after that read back the offset from the demod and adjust the
> >>> tuner then.
> >>
> >> There are some small differences between some of the demodulators. Most
> >> of the Intel DVB-S demods have a striking feature, which are found in
> >> few other demods only. This was seen on the Zarlink and Microtune
> >> devices, from where it originated from.
> >>
> >> Other vendors also have implementations similar to this such as Fujitsu
> >> and the newer devices from STM. This involves more complexity within the
> >> demodulator core.
> >>
> >> They are capable of doing Auto SR. ie, you request the maximum possible,
> >> the demod gives you a SR offset and you can re-adjust the BW filter on
> >> the tuner.
> >>
> >> This feature is also more popularly known as "Blindscan", where you need
> >> to just know the frequency of the signal only. This is the basic feature
> >> upon which Blindscan is built upon. Most demods can accomodate a SR
> >> tolerance of around +/-5% only, greater than which they will fail to
> >> acquire. Since the sampling frequency aka Nyquist sampling rate depends
> >> directly on the Symbol rate (SR) in which case you need to know the
> >> Symbol Rate, which is used to set up the tuner BW filter too.
> >
> > I meant not doing auto SR for demod, but just setting tuner to maximum BW
> > and programming demod as usual (with setting SR). And then read offset
> > freq. from demod (that is basically the full foffset).
>
> I followed you,
>
> You wanted to do:
>
> 1*) Set Tuner to max avail BW (BW directly proportional to SR and RO,
> nothing to do with frequency)
>
> 2*) You ask the tuner to tune to a frequency
>
> 3*) Request the demodulator to acquire at the Nyquist rate (SR involved)
>
> 4*) After acquisition and the transform applied internally by the demod,
> you get a frequency offset
>
>
> with the slight change to (1*) Auto SR devices and normal devices, just
> do the same thing altogether.
>
> > So we can calc the real needed bandwidth filter to get the signal through
> > or even retune to get the signal more near to zero-IF.
> > Maybe this even require a thread to follow drift.
>
> With what math will you calculate BW from Carrier frequency ? AFAIK,
> Bandwidth implies Symbols per Second or Symbol Rate and is independant
> of frequency.
>
> BW also known as Symbol Frequency according to Shannons Channel capacity
> theorem as in:
>
> http://en.wikipedia.org/wiki/Shannon-Hartley_theorem
> http://en.wikipedia.org/wiki/Channel_capacity
> http://en.wikipedia.org/wiki/Bit_rate
>
Ack, BW does not depend on frequency.

I just mean this:

Having a signal with BW=20MHz at some frequency. Then tuning to this frequency 
and setting tuner BW to 20MHz will let the signal pass fine.
Same for setting BW to maximum (around 35MHz).

BUT: If LNB or other components drifted away by 5MHz the signal will be cut 
off. Same yelds for larger steps of the tuner so tuned frequency != center 
frequency of signal.
So you need to either tune to another frequency if possible - or enlarge BW of 
tuner by freq. offset (here: 5MHz).


As tuning algo, we also can just start at maximum BW setting and decrease it 
until we reach signal BW + offset between signal center and tuned frequency.
Being too narrow here requires tracking offset changes to not loose lock.

Or a lot simpler: Just add a margin of maybe 5MHz to BW.

Regards
Matthias

-- 
Matthias Schwarzott (zzam)

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