[linux-dvb] Re: DVB-S with CI card to buy

[Ragnar Sundblad <ragge@nada.kth.se>]

--On den 12 juni 2004 01:13 +0200 Kenneth Aafløy <ke-aa@frisurf.no> wrote:

> You did read that my post had nothing to do about voltage levels, as that
> is  as easy as riding a bike.
It now comes back to me: The voltage levels were part of the
problem in my case. I should have mentioned that. I found that
out after having been measuring on the tuner tuning cap voltage
and found that a bit low. To try the theory out, I connected a
variable power supply to the 12 V.
That is why I decided to try with a new psu.

> > My new PSU had newer capacitors and maybe filters out noise
> > better. That might be the entire thing.
> Exactly my point.
So, at second thought, that wasn't the entire thing, but it
might have helped.

Still, selling cards that don't accept a little ripple or
11.9 V on the 12 V in a computer is really really bad
manners, and I'd say not that far from fraud.
I checked the ripple with a (high bw) scope, it was not
much and quite normal for a typical psu.

> > They should probably have put an amp between the tuner output
> > and the demodulator input, I guess the best would have been
> > as part of the filter that sits in between.
> I would belive that there is already an amp in there.
Believe what you want, there isn't one. I have checked out
this (quite simple) circuit, and measured it in action.
It is only a few passive componentes. The sample-and-hold A/D
in the demodulator generates great spikes through the filter
all the way back to the tuner on the baseband feed.
Is is just a plain old crap design, or if you prefer, a non
engineered design. If you don't beleive me, why don't you buy
one of those cards and check it out for yourself.

> > > It sounds like you are getting the frequency of the power grid out on
> > > the composite output, at least in this context.
> >
> > I you mean power grid as in the 230 V power that is absolutely
> > not the case, the lines/waves are of entirely different frequencies.
> > I have spoken to two different persons that have both a DVB-T FF
> > and some other DVB-x FF card, and both had this problem on the
> > DVB-T card and obviously used their other cards for video out.
> > The problem most probably comes from the power distribution
> > on the card.
> And where does that come from?
Not from the power grid anyway. From the components on the
FF DVB-T card most probably, since the other FF cards doesn't
seem to have the same problem. What is the problem with that?

The power in most electronics is noisy, especially if it
contains logic as computers do to a very high degree.
That is why you have to filter the power to sensitive parts,
as this analog part are. There is, still, nothing strange or
magic about that, just common electronics engineering.

In addition, you have to all over the design see to it that
all the power is decoupled enough to not induce to much
ripple, especially with logic, the digital parts don't like
too much noice either.
Obviously, someone has screwed something up on the FF DVB-T
cards, and it is probably in the power distribution on
the card, thought I haven't looked into that.

> > DVB-C and T is in the 50-900 MHz range, DVB-S is in the 1000-1800,
> > that really doesn't matter to much to the design in this case.
> So we are still in the plenty of mhz area, hmm?
Yes, so what? There is nothing new or secret or magic about
that either, it has been done for many years even in home
equipment.

> Oh, so the tuner (the sensitive part) is in a tin-can and the rest is
> outside?
The sensitive part for reception is not the tuner alone, it is
the tuner and demodulator combination. They typically have to
be well integrated, and fed with stable, specially filtered
power to work well. On those cards, nothing of this seems to
be true.

If you buy a combined tuner and demodulator as on the DVB-S
cards you leave much of this work to the module manufacturer,
they often even have their own power system so that you can
feed them with a single or at least just a few voltages that
sometimes doesn't even have to be to well filtered.

> > (Is there any card on the market that actually uses parts
> > that haven't been discontinued for at least a year?
> > Where do they get the parts?)
> Hu, what are we talking about here? The Hauppauge thread recently seen on
> this  board?
I don't understand what thread you are referring to.

All cards I have seen have part that you can't get the specs
for, not only because the manfucaturers like to keep them
secret, but because they are discontinued since years and
the manufacturers don't want to give out the specs for
old parts.

> > All in all, the DVB cards I have bought are just not well
> > engineered. They have caused me a lot of trouble, and every
> > now and then they act up in one way or another. The system
> > isn't reliable at all.
> Are you absolutly sure that this statement is accurate?
Yes. I have spent ours measuring signals on them too.
I have made about everything I can think of to make them work
reliably. They just won't. I have maybe gone from one failed
tuning in 3 to one failed per week or so, but that isn't reliable.

> I've been
> operating my  DVB-S card for over a year without hardware flaws, and if
> you'd like it, I  could seek some advise from friends who are running DVB
> with MythTV and other  transmission standards than DVB-S.
Yes, those of you who use DVB-S often seem to be more succesful,
just as I said. If you know anyone that have improved the design
of the DVB-T cards, I am all ears.

> > I wouldn't call different PCI problems on mother boards rare,
> > but the system tends to often work anyway.
> Would you say that PCI transfer errors are rare or ?
Different PCI chips can make all kinds of weird things that
are more or less in the grey margins of the specs, and if you
are unlucky you get a combination that don't work.
There are all from electrical (less common, actually), to
timing, to burst transfer timing problems.
Most often, those problems won't show up, but when they do
they often are very tricky to find. It is often easier to try
replacing one the involved parts.

I don't understand what you are trying to get out of this
discussion? You have obviously never seen or examined the
cards I have, and it seems you haven't had any reason to
closely examine yours either since your card works to your
satisfaction.
I was only saying that filtering power lines in an electronic
device is just common engineering, not any magic.

/ragge