On Sat, May 15, 2004 at 02:36:24PM +0200, Robert Schlabbach wrote: > > 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.
I'm not sure what DVB-SI is (is the one pertaining to CAMs and encryption?) but from reading this list over the last two years, it's become apparent that there are quite a few little discrepancies such as this, and as a result new 'standards' by different manufacturers seem to emerge... > > 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), Ahhh of course! That was the missing link - I completely forgot that a transponder has much wider transmisson bandwidth.. > 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). And the pieces gradually fall into place - that explains why DVB-C symbol rates are always in the region of high-6000s.. > 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. Yes, even our main national broadcaster is varying in higher end of that range. Indeed it's about the only one I can't stream over 802.11b WLAN - all the others just about fit into the 5Mbps max throughput I can achieve. Given that DVB-T in the UK is mostly using QAM16 and FEC3_4, the encoding bitrates can't be too high.. It's interesting to note that the reason we dropped from using QAM64 / FEC2_3 was to 'improve the picture quality' (reduce UNCs in fringe areas) but the lower available bitrate means each station is now transmitting a lower quality picture. Again, so much for progress... Thanks for the info - always interesting reading :) Cheers, Gavin. -- Info: To unsubscribe send a mail to [EMAIL PROTECTED] with "unsubscribe linux-dvb" as subject.
