add support for IF bandwidth filter setting to rtlsdr and soapysdr #48
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This should help with noisy environment by improving rejection of nearby strong signals.
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Digging through rtlsdr source I stumbled upon this API (supported since 2016 in both rtlsdr and soapysdr) and traced it down to
https://lists.osmocom.org/pipermail/osmocom-sdr/2015-April/000043.html
The video from the linked message suggests it can improve SNR ratio by narrowing the default IF filter to reject unwanted signals even if they are within the sampling rate bandwidth, thereby avoiding/reducing ADC overload with strong unwanted adjacent signals.
My dumpvdl2 receivers are already in a rather quiet environment (i.e. in the middle of nowhere) so my testing doesn't show a significant improvement (although the error rate seems to be lower with this patch), but at least this suggests this patch "does no harm".
The selected bandwidth is hardware dependent, not all hardware supports it so failure to set bw is ignored. The target bw is set to max_freq - min_freq plus a margin of 2*SYMBOL_RATE (probably not that relevant given the filter roll off isn't likely to be very steep, but let's be cautious anyway).
I have added support for rtlsdr and soapysdr drivers since I am able to test those (and did so), I have not touched the other SDR inputs.
Further relevant info can be found here
HTH