Fix for Airspy MIXER gain

[Sylvain Munaut]

Hi,

> So I was having a problem with my GOES LRIT receiver in GNU Radio. In
> GQRX the SNR was 7dB and in GNU Radio was 4dB regardless of the setting.
> Then I noticed that the Mixer (BB Gain) wasn't working at all. Checked
> over the internet and there was this issue:

First GQRX uses gnuradio, so I assume you meant GRC in the sentence above.


> So I made some changes and here is the working version:
> https://github.com/osmocom/gr-osmosdr/pull/8 (I know its a mirror, but
> the patch is also attached)

That patch is just not correct. It would map two distinct gain control
to the same gain stage ...

BB gain is not MIX gain, there is no BaseBand gain stage on the airspy.

To set the gain stage in GNURadio for the airspy you need to use the
named gain stages setting API but because this API uses run-time
discovery of available gain stages, and that's not easily doable in
GRC using only the 'gui' stuff because you need to actually _call_ a
function on the gr-osmosdr block to update it. That's doable using the
function probe in theory.

The 3 "boxes"  ( RF / IF / BB ) that are by default in the GRC
property are just historic shortcuts from before it supported a full
named gain API.

The best solution would just to have a new GRC block (or two, one Qt,
one WX) that automatically discovers all the gain stage at run-time
and makes a slider for each of them just like osmocom_fft does since
it's often what people want to do. And this way it auto adapts to
whatever SDR is detected at run time which is kind of the point of
gr-osmosdr.

Or several entry box in GRC each time with the name of gain stage to
control and the value to set for the case where you want manual
control.
Something like "Gain 1 Name:" "Gain 1 Value"  "Gain 2 Name" " Gain 2 Value" ...


Cheers,

    Sylvain