[linux-dvb] Some thoughts and questions

Johannes Stezenbach js at linuxtv.org
Sun Sep 30 11:00:45 CEST 2007

On Sun, Sep 30, 2007 at 03:11:39AM +0400, Manu Abraham wrote:
> Johannes Stezenbach wrote:
> > On Sat, Sep 29, 2007, Manu Abraham wrote:
> >> ...
> > 
> > Instead of losing myself in the details of your questions,
> > some background info:
> > 
> >> 1) LNB drift
> > 
> That said, since we have different LNB LO drifts and the frontend driver doesn't know
> what the actual drift the LO is having, but that only a standard (vague ?) definition of
> the drift,

The actual drift is totally irrelevant for the zig-zag scan.
Zig-zag is a trial-and-error approach, and only needs to know
- the step size (derived from the demod's carrier capture range)
- max. the number of steps (derived from channel bandwidth)
- the time to wait between steps (derived from the demod's
  worst case time to lock on a signal)

Got it _now_?

> i would like to experiment a bit with the stb0899 with regards on the same.
> Since the STB0899 doesn't use the swzigzag from dvb_frontend.c, it is much easier to
> do it in the stb0899 driver.

dvb_frontend lacks a hook which would allow one to
implement auto-offset correction for the TU1216 DVB-T demod
(I mean the +/- n * 125kHz or +/- n * 166kHz offset from the frequency
given in the NIT used by some transmitters to reduce interference with
adjacent channels.)

If what you add for stb0899 would be usable for tu1216 etc. it
would be useful. You must not break the exisiting sw zig-zag though.

> (This is supposing on the basis that we are not fiddling around with the STV0299)

Why do you still try to single out stv0299 when I told you
that almost all existing DVB-S frontends use it?
(cx24116 being the one exception, as Georg Acher informed me.)

> Ok, now that i explained the test scenario, how would you prefer LNB drift be provided
> to the frontend ?
> The criteria would be thus, that the user be able to specify the drift for his LNB, since
> people have small LO drift values to large LO drift values. ie it is not fixed as it is originally
> thought to be.
> Do you think it would be the best if the user specifies the drift ? Or maybe in a user
> specific library, for example the lnb.c that one could be extended further.
> What's your opinion ?

Most people have no idea what drift their LNB has, nor should they
ever have to care about it.


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