RTL_FM Squelch Issue

[Taylor_425]

Hi Everyone,

Just wondering if anyone has a solution for an issue that I am having with the linux RTL_FM program.
I am running it on an RPi 3B - Raspian with an RTL-SDR Blog V3

Here is my RTL_FM command:

sudo /usr/local/bin/rtl_fm -d 1 -M fm -f 148.53M -g 15 -p -3 -l 80 -s 12k -E dc -E pad

No for the problem......When I have the squelch set to " -l 80 " there is no hissing and everything is silent when there is no audio. But when i have an audio transmission you can hear the transmission but it is choppy and cuts out randomly. Now if i set the squelch to " -l 50 " you can hear hissing continuously but audio transmissions come across with no choppyness and sound perfect.

Just wondering if anyone has a fix so the audio is silent until a transmission and then have the transmission not choppy at all

Ill try and upload an example from my records asap

Any help will be greatly appreciated!

 

[DRL-XM43]

Sounds like the value 80 is a bit too much try something like 72, depending on background noise play with the number as required.

 

[Taylor_425]

See thats what i figured, but ive played around with it extensively and -l 76 there is no hissing, -l 75 there is hissing....the transmisson choppyness contiues till i bring the -l to 65

 

[DRL-XM43]

Can you look at your target frequency with something like GQRX and see if you have extreme background noise signals -- if so perhaps you can reposition something so the background levels get minimal then you can play with gain and squelch to achieve a good balance,

Also your gain looks a little light but raising it will raise the noise floor too so your best shot is to lower the noise floor perhaps by playing with antenna position or see if you have a lot of interference from surrounding gadgets and the like. Until you look at that slice of the spectrum with a scope like GQRX you are in the dark about where you stand with settings.

 

[Taylor_425]

Ill have to try that....i just find it strange that even with the noise floor, im able to squelch it out to silence, but then normal transmissions cut in and out while they are broadcasting....its like for some reason it thinks there is nothing being transmitted for a fraction of a second

 

[Ubbe]

I guess there's no spectrum display available in Linux. Can you try a Windows computer and SDR# and take a look at the spectrum what's going on at your location and to try out different gain values for tuner and IF to dial in the correct values as the squelch are dependent of them.

/Ubbe

 

[DRL-XM43]

Can you look at your target frequency with something like GQRX and see if you have extreme background noise signals -- if so perhaps you can reposition something so the background levels get minimal then you can play with gain and squelch to achieve a good balance,

Also your gain looks a little light but raising it will raise the noise floor too so your best shot is to lower the noise floor perhaps by playing with antenna position or see if you have a lot of interference from surrounding gadgets and the like. Until you look at that slice of the spectrum with a scope like GQRX you are in the dark about where you stand with settings.

If you cannot run GQRX to see the spectrum dispay can you run something like

osmocom_fft -a rtl=0 -v -f 410.2125e6 -s 2.4e6 -g 40
(put your own frequency)

That will also give you a spectrum display.

If you can visualize the piece of the spectrum you are dealing with it helps to understand a lot about what is what.

Have you corrected your ppm?

 

[Taylor_425]

Ok ill have to try that, the problem is this is in a remote location, I can remote into it and change settings but I cant easily travel to it. I have set the PPM correctly after taking awhile waiting for fire calls as this is a small department i monitor.

Here I have upload 3 audio files to google driver for you to listen to.

First one here is with the squelch disabled, you get the hissing background noise, but the transmission sound perfect:
No Squelch Good Transmission

Second one here is with the squelch set to -l 76 which is where the hissing stops but the transmissions are choppy:
Squelch with Bad Audio

Lastly this one is where the squelch is set to -l 75 , the hissing starts again but the transmission issue continues:
Squelch, Hissing and Transmission Issue

 

[slicerwizard]

Squelch logic in rtl_fm is defective.

 

[Taylor_425]

Im starting to see that........

Anyone have any alternative software i can try instead of rtl_fm....all i want to do is monitor this dispatch freq and broadcast it to broadcastify

 

[DRL-XM43]

Im starting to see that........

Anyone have any alternative software i can try instead of rtl_fm....all i want to do is monitor this dispatch freq and broadcast it to broadcastify

I suppose you cannot run GQRX??? , I don't know anything about RPi 3B but it would do it.

 

[DRL-XM43]

I suppose you cannot run GQRX??? , I don't know anything about RPi 3B but it would do it.

I checked around the GQRX forum and saw a post which may apply?

"Immediate success with the 2-11 GQDX version on my new (and first) PI3B+"