RTL_FM on the fly updates

tom at tswartz.net tom at tswartz.net
Tue Jan 8 00:36:56 UTC 2019

Hey Mark:

I'd happily be corrected, if it proves otherwise but I just gave it a shot locally.

My setup was:
rtl_tcp streaming over ethernet to a LAN host running GQRX 2.11.5

Wide FM worked as expected, tuned up to a local broadcast FM station no issues.
NFM also worked, and I received a local ham repeater on 2m band.

I did notice that the gain values were different in GQRX's GUI when
switching between rtl_tcp and rtl_sdr drivers.
Perhaps that subtle difference in gain values were what you had seen?
I was not using Hardware AGC in either instance, but I set the values
via the slider.
rtl_tcp reported: `set gain 372` (and other values) when I adjusted the
GQRX gain sliders.

Perhaps someone else with better knowledge of the agc/gain settings
can speak to why you may have been seeing a difference with the AGC enabled.

Tom Swartz

On Mon, Jan 07, 2019 at 06:55:13PM -0500, Mark Russell wrote:
> At first I thought rtl_tcp was the way to go, but after looking at the
> code, it doesnt seem to support wide band FM, which led me to rtl_fm
> instead. 
> Also, when using rtl_tcp as a server, I found my signal strengths were
> weaker when connecting to the rtl_tcp server with GQRX.  I changed gqrx
> back to use the USB dongle directly and then i click the hardware agc
> checkbox - which gives me better signal strength.  
> Is that not typical behavior? Maybe I'm not configuring something
> correctly?
> Thanks :)
> On Mon, Jan 7, 2019, 18:16 <tom at tswartz.net wrote:
>     Any reason that you couldn't use rtl_tcp instead?
>     It's pretty common to run rtl_tcp on a remote RPi server,
>     then use a different, local computer to host GQRX.
>     Squelch, frequency changing, and other features should be
>     available via that protocol presently.
>     --
>     Tom Swartz
>     On Mon, Jan 07, 2019 at 05:01:37PM -0500, Mark Russell wrote:
>     > Good evening -
>     > I am trying to make modifications to rtl_fm so that I can change
>     > frequencies, gains, modulation types, and squelch values on the fly
>     (and
>     > thus avoiding a restart). 
>     >
>     > Its unclear to me what functions need to be called in order to apply
>     > changes to the sdr device so that I can do the above. The rtl_fm
>     code isnt
>     > documented so I dont have much to go off of...
>     >
>     > GQRX doesnt run reliably on any RPi3's I've tried so I hoping to use
>     a
>     > modified rtl_fm in its place. Any help would be appreciated.
>     > Thanks :)
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