[linux-dvb] Some thoughts and questions

Manu Abraham abraham.manu at gmail.com
Sat Sep 29 12:53:42 CEST 2007


Felix Hi,

Felix Domke wrote:
> Hi,
> 
>>>>>  - Inversion might happen on up- and downconversion, depending on what
>>>>> frequency situation you have.
>>>>>  - The SatelliteDeliverySystemDescriptor does not specify Inversion.
>>>> AFAICS, Inversion isn't a part of the transport.
>>> Why not? It's part of it like the frequency, isn't it? 
>> What i meant is that it does not depend on the transport, but the hardware.
>> LNB + tuner <-> demod wiring
>> Whereas frequency is not.
> Some transponders have an inversed frequency spectrum already when
> broadcasted (before downconversion).
> 
> There is no way for the broadcaster to specify this.

What do you mean by a inverted spectrum when broadcast ?
AFAICS, Inversion happens after there is a Phase shift to extract the I and Q signals

The QPSK modulator block diagram as described here on page 6 http://web.syr.edu/~syu05/report%20for%20software%20radio.pdf
can be helpful.

Suppose the broadcaster does some stupid thing. eventually there is no 
separate I and Q signal for inversion to happen.

Now, there are two places where this can happen, afaict.

* LNB LO for by the down conversion mixer
* Tuner I and Q inputs wired in an inverted way.

Manu





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