[linux-dvb] Some thoughts and questions

Wolfgang Wegner wolfgang at leila.ping.de
Sat Sep 29 13:23:56 CEST 2007


On Sat, Sep 29, 2007 at 04:18:31PM +0400, Manu Abraham wrote:
> Johannes Stezenbach wrote:
> > 
> >> 1) LNB drift
> > 
> > - LNBs have a constant error plus a temperature drift
> >   (e.g. +/-1MHz error, +/-3Mhz drift for a temperature range
> >   of -40 ... +60 °C -- cheap no name equipment usually worse)
> This is the old LNB, the one's we use are generally based on PLL's have some 15 - 20k drift

hmm.... I am not sure how old our LNBs (at work) are, but there and with
my LNBs at home (around 2 years old) I have measured offsets of around
1-2 MHz.

> > - sw zig-zag is by no means stv0299 specific and is used by
> >   (almost?) all DVB-S demod drivers
> hmm, i didn't mean swzigzag, but as you see from that discussion, 
> it was the drift that i am looking at

Is it really a difference? I am not sure if I understand it correctly,
but IMHO this is or could be handled as the same issue.

For low symbol rates, the problem is that you can not get the frontend
locked to the transponder if the offset (probably being caused by drift
or simply constant frequency offset because of the cheap crystal in
frontend PLL and/or LNB) is too high. You can only overcome this by

> My point being to have zigzag specific to the demod, since each device of the 
> devices which implements zigzag does it in a different way.
> The computation being different

Why is the computation different? I did not look into it yet, but at
the moment I can only see stepsize and transponder bandwidth (symbol rate
in case of DVB-S) as the parameters the algorithm depends on.

> > IIRC Andrew de Quincey spent significant time optimizing the
> > zig-zag code and the parameters for various frontends.
> >
> I do remember the time where he spent so much time optimizing the
>  swzigzag for the STV0299.

It seems my thinking is too easy above, I will have to look into the
code to get enlightenment, I hope. :-)

> You have any cases of broadcasters doing it in a non-standard way ?
> ie inverted transmission ?

I do not remember the transponders, but there are some - not on
Astra/Hotbird, though. Maybe I can give an example on monday.


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