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) _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
