[linux-dvb] Re: [OT] Reality

["Robert Schlabbach" <robert_s@gmx.net>]

From: "Gavin Hamill" <gdh@acentral.co.uk>
> 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: robert_s@gmx.net
Berlin, Germany




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