Re: [Discuss-gnuradio] Resampling audio using rational resampler doesn't work for multimon-ng

[Kevin Reid]

On Sat, May 18, 2019 at 12:07 AM Albin Stigö <address@hidden> wrote:

You need to use a low pass filter (taps) in you resampler. Look at the spectrum after the resampler and this will be obvious to you. A lowpass filter with a cutt off at 3-4kHz or so should do it.

This cannot be the problem, because rational_resampler automatically calculates a filter if taps are not specified:

https://github.com/gnuradio/gnuradio/blob/4e4f44c726556496c420bceb87ccafe6843916f7/gr-filter/python/filter/rational_resampler.py#L107

(I also checked the GRC file, and the fractional bandwidth is also None (auto-set) despite appearing as "0" in the screenshot.)

Unfortunately, I don't have any ideas what the problem is. For what it's worth, I have successfully integrated multimon-ng into a flow graph (https://github.com/kpreid/shinysdr/blob/master/shinysdr/plugins/multimon.py) but the code is very differently structured so it would be hard to transfer the knowledge into GRC.

As a next troubleshooting step I would recommend adding GUI sinks at various parts of the flow graph so that you can check the signal integrity. (For an example, consider adding after the float-to-short a short-to-float and QT GUI Sink — to ensure the signal hasn't gotten lost in quantization noise and just needs to be amplified before converting to short, as well as checking that the bandwidth/resampling is good.)