Using an rtl-sdr to listen to fm radio
A few months back I picked up an rtl-sdr from rtl-sdr.com for about
$20. Today, I finally got around to using it. Setup was pretty
straight forward, and tuning parameters so it could pick up fm radio
took a few tries. This was ultimately what I developed:
#!/bin/sh
# play the radio using rtl-sdr and sox
station="89.1"
if [ $# -ge 1 ]
then
station="$1"
fi
rtl_fm -M wbfm -f ${station}e6 -s 150k -r 75k \
| play -t raw -r 75k -e signed-integer -b 16 -c 1 -V1 -
rtl_fm is bundled with the rtl-sdr software available from osmocom.org
and is available in most repos.
see
http://kmkeen.com/rtl-demod-guide/ for usage from the author.
-M wbfm : wideband fm which is what most commercial stations use.
-f : declare a sepcific frequency in Hz. As fm radio is in the MHz
range, we add e6 so calling with a particular station e.g.
96.3 or 89.1 is translated appropriately
-s : sample rate. Rate at which to sample the radio.
commenter JOJOPI says: bandwith of the modulation of a
stereo WBFM signal is 75kHz and the HF-bandwith of the
signal is 150kHz. Therefore we use 150k here.
-r : resample rate. Again, due to the information JOJOPI provided,
we use 75k here.
play is a tool bundled with sox, the swissarmy knife of audio
processing. Some of the flags are self explananatory, -t is type
of audio and -r is the sample rate.
-e : type of encoding. Output from rtl_fm clarifies that output is
16-bit signed.
-b : number of bits, 4 or 8 bit are old school, 16 is common, 24
for professionals (see sox(1) for more info).
-c : number of channels, in this case, meaning mono - not stereo
(which would be -c 2) etc.
-V : how verbose, 1 meaning pretty quiet, otherwise output is
cluttered with harmless info.
- : read from stdin (ofc). rtl_fm outputs to stdout by default or
a file maybe specified. Ever wanted to record a radio
station? Now you can!
I run this script on my headless raspberry pi to listen to perfect
fm radio. The rtl-sdr is much more capable than just fm radio and
can be used for am, police scanner, airplanes, satellites, and
pretty much anything else that communicates over radio. osmocom.org
has some great information, as does rtl-sdr.com
