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Use cases
See also: Input formats
To use Redsea with RTL-SDR you will need to first install the rtl-sdr package.
It should include the FM decoder tool rtl_fm.
The full command to decode RDS live via rtl_fm, on an FM station at 87.9 MHz, is:
rtl_fm -M fm -l 0 -A std -p 0 -s 171k -g 20 -F 9 -f 87.9M | redsea -r 171k
If you did not run make install you should replace redsea with the full path to
the executable, for instance ~/redsea/src/redsea.
For Raspberry Pi 1 it's necessary to change -A std to -A fast. This way
more CPU cycles will be left to redsea.
Note that rtl_fm will tune the receiver a bit off-center; this is normal and
is done to avoid the DC spike. See the rtl_fm FAQ for more about this
behavior.
csdr can be used to demodulate streams from other radios on the command line, like AirSpy:
airspy_rx -f 87.9 -a 2500000 -g 9 -r /dev/stdout |\
csdr convert_i16_f | csdr fir_decimate_cc 10 0.05 HAMMING |\
csdr fmdemod_quadri_cf | csdr convert_f_i16 |\
redsea -r 250k
Or HackRF:
hackrf_transfer -f 87900000 -s 10000000 |\
csdr convert_i16_f | csdr fir_decimate_cc 40 0.0125 HAMMING |\
csdr fmdemod_quadri_cf | csdr convert_f_i16 |\
redsea -r 250k
Or any IQ file at baseband:
csdr convert_u8_f < iqfile.iq | csdr fir_decimate_cc 40 0.0125 HAMMING |\
csdr fmdemod_quadri_cf | csdr convert_f_i16 |\
redsea -r 250k
For the IQ file, make sure you choose the first conversion command according to the sample format of your IQ file. Also, make sure that the decimation ratio produces the correct sample rate.
You can even use csdr to shift another signal to baseband before decimation. The csdr readme has examples for frequency shifting.
It's easy to decode audio files containing a demodulated FM carrier. Note that the file must have around 128k samples per second or more. 171k will work fastest, because it doesn't require resampling internally.
redsea -f multiplex.wav
If your sound card supports recording at high sample rates (192 kHz) you
can also decode the MPX output of an FM tuner or RDS encoder, for instance
with this sox command:
rec -t .s16 -r 171k -c 1 - | redsea -r 171k
If this doesn't work, try higher sample rates, especially 192 kHz.
By default, the raw MPX input is assumed to be 16-bit signed-integer single-channel samples at 171 kHz.
The --feed-through option echoes the input signal back to stdout. This lets you use both the original signal and
the decoded RDS via different streams.
For example, the signal can be listened to using sox, while RDS groups are printed to stderr:
rtl_fm -M fm -l 0 -A std -p 0 -s 171k -g 20 -F 9 -f 87.9M |\
redsea -r 171k --feed-through |\
play -t .s16 -r 171k -c 1 -
Or you can use stereodemux to listen in stereo:
rtl_fm -M fm -l 0 -A std -p 0 -s 171k -g 20 -F 9 -f 87.9M |\
redsea -r 171k --feed-through |\
demux -r 171k |\
play -t .s16 -r 171k -c 2 -
However, this only works with raw MPX input for now.
This command writes a hex-format outputfile with a timestamp in its file name.
rtl_fm -M fm -l 0 -A std -p 0 -s 171k -g 40 -F 9 $@ | \
redsea -x -r 171k | tee `head -12l | tail -1l | \
cut -d" " -f1`_`date +%F`_`date +%k`-`date +%M`-`date +%S`_$2Hz.spy