AM Demodulator RF bandwidth setting in presets not being applied

[brendanthebig]

Re Version 3.9.1... When loading presets with AM Demodulator, I have noticed that the RF bandwidth setting is not being applied. It works ok when making manual adjustments. I have tried to re-save the preset after making bandwidth changes but problem still persists.

[f4exb]

I also noticed the same with the SSB demod. It may be due to changes in sample rate handling in which case it was introduced in 3.9.0

[brendanthebig]

I haven't noticed the problem with SSB demod myself. AM demod had no issues in 3.9.0 for me.

[f4exb]

There is definitely a problem with the sample rate used in the interpolator:
https://github.com/f4exb/sdrangel/blob/master/plugins/channelrx/demodam/amdemod.cpp#L200

It was already there in 3.9.0:
d476736

It also affects SSB demod and SSB mod in a similar way.

Fix is easy and will be part of next release.

[brendanthebig]

Hi Edouard, V3.10.0 has fixed the issue with RF bandwidth on AM demod but there is still an issue with SSB demod. It seems that the high cut-off frequency being applied is about half of what is selected.

Update :- It seems also to be dependant on the freq shift in ssb demod, for example, correct high cutoff freq is applied when freq shift is + or - 20KHz, 60KHz, 100KHz etc (ie 40KHz steps).

[f4exb]

The frequency shift has an influence on how many halfband decimators are used each stage halving the baseband sample rate before feeding the samples to the demod (this is the "channelizer" step). This can indicate that there is still a problem with the final interpolator (decimator actually) handling in the demod.

The change that was introduced and caused the first problem was to modify the interpolator differentially (only when there are changes in sample rate) but this can work only if it is initialized once in the constructor and that's obviously missing. It is missing in all plugins by the way but may or may not be a problem depending on the typical sample rate that may or may not trigger an initial change.

[f4exb]

There is an additional problem with the SSB demod filtering that I think has been there for ages. The demod decimator filter is set at half the required RF bandwidth. This is fine for double sideband signals when what is meant by RF bandwidth is the total double sideband bandwidth which is the case for all other demods. I suppose the decimator settings for the SSB were just copied and pasted and that was a long time ago...

[brendanthebig]

Working well in V3.10.1...Good job Edouard...