[Mplayer-dvb] Re: How to do a central DVB-S Server for a community network ?

[Stephen Davies]

On Thu, 3 Oct 2002, ChristianHJW wrote:

> "Stephen Davies" <steve at daviesfam.org> schrieb im Newsbeitrag
> news:Pine.LNX.4.21.0210031057520.1008-100000 at bob.daviesfam.org...
> 
> > One thing that you are missing I think is that a single DVB receiver card
> > is capable of grabbing more than one station at a time - in principle all
> > the stations in a single multiplex (ie muxed together on the same
> > frequency) can be handled at the same time.  So I don't think you'll need
> > as much hardware as you imagine.
> 
> I had no idea ! its hard to believe also, because this would result in huge
> streams coming out of one card ? i can receive 120 TV and 300 radio stations
> with my Pinnacle DVB-S card, and AFAIK the Astra DVB-S will all use the same
> transmitting frequency ?
> 
> If we estimate a TV station to be 3000 kbps and every radio station 240
> kbps, this sums up to 450 mbit/s ?? Probably a bit too much for a simple PCI
> slot :-) ?

The DVB card tunes one transponder at a time.  The card receives an MPEG2
transport stream at something like 35MBit/sec.  Within that total will be
data for various channels all muxed up together.  Typically say between 4
and 10 different programmes.

If you look at www.lyngsat.com you can see how it works.

As an example: on Astra 2A, 11.72GHz is a multiplex containing:
BBC1 England, BBC2 England, BBC News 24, BBC Four, CBBC, Cbeebies, BBC
Choice, BBC One Northern Ireland, BBC Radio Cymru, BBCi

So one DVB-S card could grab any one or more of these
channels.  Practically, dvbstream can give you up to four of them.
 
> So, to be able to render about 40 TV stations, how many cards were
> reallistically necessary ?

Well - that depends on how the channels you want are distributed across
the transponders.

You could use www.lyngsat.com to look at the satellite you are interested
in and figure it out.

> 
> > For your servers you probably want the cheaper "Budget" DVB-S cards -
> > which have the advantage of access to the entire transport stream as
> > transmitted.  On the other hand, these cards don't support CAMs - perhaps
> > you need that for your application.
> 
> CAM ? sorry, you mean CIMs, for PayTV ? no, thats not necessary, and also
> not planned ..

Hmm - make sure that the content is really available "Free to Air" - ie
unecrypted.  Note that "Free to View" doesn't mean the same thing...

> Why would we have to use DVB cards on the client side ? wouldnt any normal
> MPEG2 decoder card work fine also, if we convert the transport streams to
> normal MPEG first ?

With rtpfeed, the DVB card is used as a hardware mpeg2 decoder.

The mpeg2 stream on dvb-s is "normal" - so in principle windows software
or hardware cards will handle it.  But I for one don't know the details.

> Of course, playing on PC and outputting via TV out was the easiest option,
> but i am thinking of a solution where no interaction with PC was necessary
> to watch TV, like a radio remote 'calling' the wanted TV station on the PC,
> MPEG2 decoder card ( even more than one, max 4 ) outputting to TV out, and
> signal being converted to an antenna signal ( an old VCR can do this job )
> and distributed in the whole house via normal antenna cable, so every TV in
> the house can receive those 1, 2, 3 or 4 different 'internal' TV channels
> same time, everyhwere in the house. TVs had only max. 4 different UHF
> channels to be tuned to ( like ch 26, 30, 34, 38 ) and the radio remotes
> connected to the PC could be used to decide what TV station is rendered to

You are asking for one machine to decode 4 mpeg2 streams
concurrently.  That's a big ask for a software decoder.  Probably is
possible using hardware decoders.

> Legal side was no problem at all, as there was no PayTV decoded and
> distributed, but only freely available stations.

I suspect that you are kidding yourself that you are entitled to
redistribute just because its transmitted publicly.  But I am not
qualified to say for sure about that.

Regards,
Steve