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

Andreas Oberritter obi at linuxtv.org
Wed Sep 17 21:57:31 CEST 2008


wk wrote:
> Andreas Oberritter wrote:
>> If you find two devices which need different inversion settings in the
>> same network, then it's a driver bug, which can easily be corrected.
>>
>>   
> How can this be a drivers bug, if in the dvb api documentation no
> *default value* for inversion was ever defined?
> Seems to be a bug inside API (linux-dvb-api-1.0.0.pdf, see
> http://www.linuxtv.org/docs.php).

Common sense says that the default value is not inverted.

>> Spectral inversion depends on the transmitter, too. It can change
>> anytime a broadcaster decides to change it. It happens, although not
>> very often.
>>
>> Specifying the inversion parameter can speed up the tuning process,
>> especially for devices which don't support automatic swapping in
>> hardware. But I would not recommend to store this parameter in a service
>> list.
>>
>> If we decide to keep the parameter, we should probably use four options:
>>  INVERSION_OFF, INVERSION_ON, INVERSION_AUTO_OFF_FIRST,
>> INVERSION_AUTO_ON_FIRST, which matches the capabilities of most
>> demodulators. A typical application would then probably use only the
>> last two options, while the first two options would rather be used for
>> diagnostics and to read back the detected inversion.
>>
>>
>>   
> May be. But since the application cannot get any information about
> transmitters inversion only AUTO makes sense here, neither ON or OFF.
> No information field is reserved for inversion inside terrestrial
> delivery system descriptor and cable delivery system descriptor,
> so applications are not able to figure out useful settings from NIT.
> Please try to see this from the application side, not from hardware
> point of view.

Well, that's essentially what I wrote: Use AUTO in a typical
application. All frontend devices support it, either in software or in
hardware.

Regards,
Andreas



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