[linux-dvb] Why Parameter 'INVERSION' is really needed?

handygewinnspiel at gmx.de handygewinnspiel at gmx.de
Tue Sep 16 19:31:21 CEST 2008


Hi,

By looking at the actual proposals for DVB API and trying to understand what happens there some questions arised to me. May be someone can give some answer on this items.

What is the *real need* for giving applications the possibility of I-Q-Inversion? Why this strange one is included in next API's?

If i understand this parameter correctly it swaps I and Q inputs of an qam capable receiver. But otherwise that means that somewhere in the reception chain some real mistake was made, either on hardware or driver side.

And if some inversion is needed it should be corrected inside the dvb frontend, since for such piece of hardware *always* this inversion is needed. Correcting this later on application level is terrible, since somebody may use hardware with different inversion settings inside the same application with the very same channel definition.

By looking on EN300468 nowhere 'inversion' is stated, channel definition is made by:

DVB-T
 * centre_frequency
 * bandwidth
 * priority
 * Time_Slicing_indicator
 * MPE-FEC_indicator
 * constellation
 * hierarchy_information
 * code_rate-HP_stream
 * code_rate-LP_stream
 * guard_interval
 * transmission_mode

DVB-C uses
 * frequency
 * FEC_outer
 * modulation
 * symbol_rate
 * FEC_inner

DVB-S/S2
 * frequency
 * orbital_position
 * west_east_flag
 * polarization
 * modulation_system
 * roll off
 * modulation_type
 * symbol_rate

Therefore my question: Is it possible to skip Inversion property in future API's at all? Or are other standards out there where this channel property explicitely stated?

Are these following parameters missing or only named misleading? If so, what are there names?
 * MPE-FEC_indicator (DVB-T)
 * FEC_OUTER (DVB-C)

Why was DVB-T Hierachy split into two different parameters, Alpha and Hierarchy? One Parameter seems to be enough? EN300468 says

000 non-hierarchical, native interleaver
001 α = 1, native interleaver
010 α = 2, native interleaver
011 α = 4, native interleaver
100 non-hierarchical, in-depth interleaver
101 α = 1, in-depth interleaver
110 α = 2, in-depth interleaver
111 α = 4, in-depth interleaver


Sorry for this huge bunch of questions,
Winfried
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