[linux-dvb] Re: Spectral Inversion (was Re: Re: patch for scan (-t bitfield select))

["Robert Schlabbach" <robert_s@gmx.net>]

From: "Holger Waechtler" <holger@convergence.de>
> no, e.g. the L64781 and the tda10021 report no inversion. Some revisions
> of the TT DVB-C card too. The reason for the inversion is probably that
> the hardware designer swapped the I/Q wires between tuner and
demodulator.

AFAIK, there is no such thing as "I/Q wires", only a (typically
differential) IF signal between the tuner and the demodulator. From my
understanding, the spectral inversion is generated in the tuner - You _add_
the IF centre frequency to the desired target frequency to calculate the
PLL frequency. If I understand the downconversion process correctly, the
PLL frequency is _subtracted_ from the input frequency:

input target frequency - (target frequency + IF) = -IF

Thus, the target frequency ends up _spectrally inverted_ at the IF, and the
demodulator has to perform a spectral inversion to recover the original
signal.

Or is my understanding completely wrong...?

Regards,
--
Robert Schlabbach
e-mail: robert_s@gmx.net
Berlin, Germany



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