Resource issues when trying to record 2 FM broadcast stations

[Martin Lefkowitz]

I  haven't spent a whole lot of time working with gnuradio.  I'm not a 
DSP engineer.  I am however a software engineer that has dealt with 
signals so not completely a laymen, but I am out of my comfort zone.

I  had successfully created this project about 4 years ago using Marcus 
Leech's Stereo FM Radio implementation and grafting it into the front 
design of this  the multiple station receiver design. Described here

https://forums.radioreference.com/threads/multiple-fm-station-receiver.341340/#lg=_xfUid-2-1556650274&slide=0 


I recorded 2 pirate radio stations at a 4 day music festival. Started 
the recording on Thursday.  Stopped it on Sunday.  The sample rate was 
2.4M.  I created an Heir block that had the appropriate parts of Marcus 
Leech's Stereo FM radio in it.  and replaced everything from the xlating 
FIR to the multiply const in the second block figure above with the heir 
block, and added some WXQT blocks to handle the gui for 2 stations like 
wav sinks, file names, etc.  It was successful, but one thing that did 
happen was I needed to go from a Acer 720 (celeron) with Gallium 
installation to an (older) I5 laptop.

I let that project sit for a couple of years.  The first thing I did 
when I got back to it was upgrade everything.  Big mistake... Nothing 
worked after that.  I got grc 3.10 running with radioconda.  I swapped 
Marcus's implementation out for Zan's fm_stereo_demod_clean

https://forums.radioreference.com/threads/multiple-fm-station-receiver.341340/#lg=_xfUid-2-1556650274&slide=0

http://zansprojects.blogspot.com/2019/05/stereo-fm-receiving-with-rtl-sdr-and.html

In this new implementation I needed to create a series of variables to 
handle the design issue of having the signal be a multiple of 48k for 
the audio sink to be the correct speed.  In particular the decimations 
in the rational resampler.This worked, but when I changed the sample 
rate to widen the bandwidth to receive 2 stations further apart on the 
FM band than 1Mhz I ran into performance issues.  Basically with my 
setup the CPU load is about 90% with a sample rate of 1.24 Mhz.

I was able to do this because of the great work done by the people 
mentioned above, but figuring out how to deal with the performance 
issues is kind of beyond the disciplines I am familiar with.  I am 
wondering if anybody has any suggestions on how to get 
fm_stereo_demod_clean to have better performance.  I did not see an 
updated version of the Marcus Leech design so I figured there was 
something blocking that update, but maybe that is incorrect. I am also 
wondering if I could get a more powerful laptop/NUC and make it work 
with brute force by throwing money at the project.  I would love to hear 
any suggestions from people with more grc and dsp experience, or which 
path I should take now.


Thanks in advance


Marty