[linux-dvb] Re: DVB questions

[Stephen Davies <steve@daviesfam.org>]

On Thu, 31 Jan 2002, Dennis Noordsij wrote:

> What exactly is the difference between DVB-T and DVB-C ?

DVB-T and DVB-C use different modulation etc at the "low level" to
optimize them for the different environments.  I suppose, also, that the
tuner on a DVB-C card covers a wider frequency range than in necessary for
DVB-T.

The cards aren't interchangeable.

> Technically I have cable at home, but it is no different than an
> antenna would be, except a few more channels. The VCR and TV have no
> clue wether it's cable or antenna.

Indeed, in the analogue world the cable uses ordinary PAL format like
broadcasts.  Often the cable system uses different frequencies etc, but
either that's not the case in Finland, or your TV and VCR tune the
extended range.



> I can buy a so-called digibox to watch DVB using cable. Is that what
> the DVB-C refers to, or some other kind of connection designed for
> DVB?

That digibox may or may not be a DVB-C receiver.  Also - even if your
cable's digital services ARE DVB-C, its possible that the channels are
encrypted and so cannot be received using a free-to-air card like the
Hauppage Nova cards.

> With a DVB-T card, can I just use the tv antenna (although it is a
> cable :) and receive DVB channels with it? (they are carried) (I'm in
> Finland) Sorry if that's a stupid question, but they're not cheap (not
> even the budget card) so I want to be sure.

I suppose its possible that your cable TV feed could possibly include
channels encoded with DVB-T.  That could happen if your provider captures
DVB-T off-air and puts it on your cable.

But if your provider sells a digital TV package its more likely that they
are using DVB-C (or, perhaps, something proprietary).  In either case, a
DVB-T card won't do.

> Now, because I don't need an actual TV-out, will a 700 MHz Athlon be fast 
> enough to do live viewing? (it can decode a DVD fine, but I understand the 
> stream coming from the DVB-T card needs to be processed first before 
> something like MPlayer will understand it).

My system is an Athlon 1.13GHz and uses around 35% or so of its throughput
to do this.  So I'd judge that a 700MHz machine will be enough.  How much
CPU does DVD decoding take on your machine?

> Most important is to be able to process and 
> view the incoming stream using something like MPlayer in real time and 
> preferably with audio and video in sync :)

Well - it does that for me.

> Second important is to be able to 
> save the stream in a format that mencoder (mpeg2->divx) can work with to 
> archive shows with the divx codec.

You can dump the DVB to disk as an mpeg2 program stream.  mencoder can
then process that directly.  Your box won't be fast enough to mencode to
divx in real time I don't think.

> I imagine OSD can be done the same way MPlayer now does it, because the movie 
> is playing in an XVideo accelerated window I can overlay some text myself, 
> maybe not alpha blended but hey :) and it will show up fine on tv.

You won't be able to use VDR itself.  But it should be easy to do what you
say: open a window "on top" of the XV window.  Make the window background
the same as mplayer's chromakey colour and it will become transparent...

Using directfb you could in principle do alpha-blended overlays too.

Regards,
Steve




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