> In response to Steven Toth's suggestion regarding figuring out what > the various units are across demodulators, I took a quick inventory > and came up with the following list. Note that this is just a first > pass by taking a quick look at the source for each demodulator (I > haven't looked for the datasheets for any of them yet or done sample > captures to see what the reported ranges are). > > Could everybody who is responsible for a demod please take a look at > the list and see if you can fill in the holes? > > Having a definitive list of the current state is important to being > able to provide unified reporting of SNR. As I used to work with satellite signals on an earth station, and was responsible for the development of measurement techniques, I thought I should join in here for some hopefully revelaing info. I am guessing here of course, but I believe that there is no real SNR measurement in any of the tuners available for computers. It is quite easy to calculate SNR from BER, and even some professional satellite modems in the 10kUSD price range and up uses the received BER to calculate SNR. The received BER is calculated from how many bit errors the FEC fixes. On a QPSK, a SNR of 16 would generally mean that the time between biterrors is so long that it is near impossible to calculate a SNR from the ber unless you run an average of multiple days, or even weeks. I remember doing a test for a week on a satellite mux without getting a single biterror. This means that you might never get a reading above 16 on the SNR. Looking at the curves on wikipedia (http://en.wikipedia.org/wiki/Image:PSK_BER_curves.svg), you can assume that with an EbNo of 12 db, on a 27500MS 3/4 QPSK mux, the average raw biterror will arrive every ( 27500 * 2 * (3/4) * (188/204) = 38015*10^6 Mb/s ) 10^8 / 38015*10^6 = 0.0026 s. If we then use EbNo at 13dB, the snr is 10^-10, at EbNo at 14 it is 10^-13. At this point, you have one bit-error each 260s on the same mux. Suddenly the internal SNR calculation algorithm has a zero element, and it gets impossible to calculate SNR. Conclusion is, that no matter what you do on most tuners, you will not get more than roughly 16-17dB SNR on QPSK signals. If they say that they will give a range og 0-30dB, then the higher numbers will most probably never happen. On other modulations this highest number is different of course, and on lower bandwidth signals, this number is reduced somewhat as a ber of 10^-9 will happen less frequently with fewer bits per second coming trough the receiver. Hope this made any sense (and was correct). -Morgan- _______________________________________________ linux-dvb mailing list linux-dvb@xxxxxxxxxxx http://www.linuxtv.org/cgi-bin/mailman/listinfo/linux-dvb
