[linux-dvb] Re: [OT] Reality

[Gavin Hamill <gdh@acentral.co.uk>]

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.



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