Hi, Thomas Pinz schrieb: > Hi, > > On Sunday 11 March 2007 09:40, Trent Piepho wrote: >> In dvb-pll, the frequency offset, which is the IF frequency the tuner will >> output at, is set to 36.213333 MHz. It looks like this code was written >> about two years ago by Patrick as part of the reverse engineered cxusb >> driver. >> >> In saa7134-dvb, there is some totally different code from programming >> fmd1216 tuners, and it uses an IF frequency of 36.13 MHz. >> >> So, why the difference? > > Usually a (DVB-T) Demod has an capture area of +/- 1MHz relative to the > programmend IF and also in most Demods the IF could be moved with certain > register settings. > > I don't know the FMD1216, but in many designs you have a third element which > determines the IF frequency, the SAW filter between the tuner and the demod. > So depending on which SAW-filter you are using you choose also an certain IF. > Epcos for example sells filters with 35.1, 35.825, 36, 36.125, 43.75 MHz and > much more. So if you find a SAW-filter on your device, maybe have a look an > the datasheet and you will find the right IF. > For example, the X6872D which could be found in some DVB-T sticks, has a > center frequency of 36.125MHz. (And 6.9MHz bandwidth) > > If the IF is shifted around 100kHz bit to the center frequency of the > SAW-filter and you have a good reception, you wont see much degration in the > quality because there are some provision. But if the conditions will go > worser, this would make your quality degration a little bit more worse. > > Kind regards, > > Thomas Pinz > I wrote the code in saa7134-dvb. The FMD1216 has an integrated SAW filter for DVB-T, the data sheet says 36.13MHz. This is strange because the data sheet also recommends a tuning step size of 167 kHz (4 MHz / 24), so it should be 36.167MHz.... When i started writing it, there was no dvb-pll module. And the reason why i didn't move to dvb-pll yet is that this module has no means to control the RF AGC. Philips recommends different setting for analog and DVB-T. I don't know how big the impact on the performane is but i think its good to stay as close a possible to the recommendations of the manufactor. Hartmut _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
