Matthias Schwarzott wrote: > 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. I guess this is what you mean: >> 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 instead of 2*) you have an offsetted center freq (fc). In this case, what happens is that, your tuner is not at the center and in fact will waste bandwidth. In such a circumstance, you want to compensate for this in 1*) by choosing an excess bandwidth. This will help, to acquire a LOCK. Things do look well.Now, we have a larger problem at hand. The whole point of about bandwidth limiting is to avoid excess bandwidth, since that excess bandwidth is not really accounted for. The demodulator might not be able to "walk through" the excess bandwidth. In such cases, you will just see this as a higher BER (when locked) or taking a longer time for a "demod LOCK" to acquire. Or even worser, for the steps which are at the end of the spectrum, and supposing that there exists a narrow channel adjacent to this slot, Your demod will have a tendency to LOCK to the adjacent slot or to your required slot, over which you will have little control over. This is a problem that people regularly come up with, with most of our drivers. It's a hard to sort out problem. The proper solution for this problem, would be to apply the deviation to center freq. to 2*) rather than (increasing bandwidth) to 1*). While holding BW the same. (Bandwidth remains the same, it is that which your frequency is offsetted. Move the center frequency around, rather than increasing the BW to compensate for the freq. offset) NOTE: With swzigzag in dvb_frontend, it assumes 1*) and 2*) are the same. ie drift is added directly to the frequency offset. Also, when you fix this bug, many frontends will be broken and will not tune. Applies only to satellite delivery, AFAICT. Regards, Manu _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
