From: "Gavin Hamill" <[EMAIL PROTECTED]> > On Saturday 15 May 2004 05:24, Jerico Webmail wrote: > > I'm also interested to see how many broadcasters make the > > transition to 8PSK over satellite for DTH. > > I didn't even think there were any other options than QPSK - > I stand corrected :)
The DVB-S standard (ETSI EN 300 421) specifies only QPSK modulation. Strangely enough, the DVB-SI standard (ETSI EN 300 468) provides for other modulation schemes (8PSK and 16-QAM) to be indicated in the satellite_delivery_system descriptor. >From what I understand, 8PSK does not work well with the existing Viterbi + Reed-Solomon FEC methods. That's why DVB-S2 will use 8PSK in conjunction with new FEC methods, LDPC + BCH, yielding a 30% bandwidth increase at the same SNR. Unfortunately, the final publication of the DVB-S2 standard (ETSI EN 302 307) is still a bit off: http://webapp.etsi.org/WorkProgram/Report_WorkItem.asp?WKI_ID=19738 The full schedule now says publication will be on March 31st, 2005... > Yep, and some cablecos are really packing it in at 128QAM... You can't compare cable and satellite systems. Satellite is ~30MHz bandwidth per channel (transponder), but with a very weak signal arriving at the receiver. That's why you use high symbol rates and a low number of bits per symbol there. On cable, your channel bandwidth is limited to 6, 7 or 8MHz, but you get a much stronger signal to the receiver. Thus, low symbol rates (bandwidth with a NyQuist roll-off factor of 15%, i.e. bandwidth / 1.15 is the maximum possible symbol rate) and a high number of bits per symbol is used (typically QAM-64 with 6 bits per symbol). > Out of curiousity, what kind of additional bandwidth is 8PSK giving? QPSK is 2 bits per symbol, 8PSK is 3 bits per symbol, i.e. 50% more. But you're unlikely to see the full increase in real life, 30% is the number that's given as a real life figure for DVB-S2. > I'm quite sure the '15Mbps' is purely for show, 15Mbps may just be the limit, DVB services usually use VBR, that's why a static rate cannot be given. From my observations, European DVB television services typically use 2-7Mbps VBR MPEG-2 streams. Regards,� -- Robert Schlabbach e-mail: [EMAIL PROTECTED] Berlin, Germany -- Info: To unsubscribe send a mail to [EMAIL PROTECTED] with "unsubscribe linux-dvb" as subject.
