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DVB-T dongles based on theRealtek RTL2832U can be used as a cheap SDR (Software-defined radio), since the chip allows transferring the raw I/Q (In-phase / Quadrature) samples to the host, which is officially used for DAB/DAB+/FM demodulation.(More about history & discovery of rtl-sdr)

There is various software for DSP (Digital signal processing), spectral analysis and signal intelligence using RTL-SDR such asSDR#,gqrx andgnuradio.Apart from these, there islibrtlsdr which most of the user-level packages rely because of the reason that librtlsdr comes as a part of the rtl-sdr codebase.This codebase contains both the library itself and also a number of command line tools such as rtl_test, rtl_sdr, rtl_tcp, and rtl_fm for testing RTL2832 and performing basic data transfer functions.Therefore, librtlsdr is one of the major tools in RTL-SDR community.

Most of the technically questions RTL-SDR enthusiasts ask is about reading samples from device and processing raw I/Q samples. I wanted to answer that kind of questions and demonstrate a implementation of this withrtl_map project which is built on top of librtlsdr. Also, I went a step further and added graph feature that creates amplitude (dB) - frequency (MHz) graph usinggnuplot andFFT (Fast Fourier transform) algorithm. (fftw used for this approach) I can list other useful open-source projects as#1,#2 and#3.

Another purpose of this project is making a testing tool & frequency scanner application for security researches.

https://www.rtl-sdr.com/rtl_map-a-simple-fft-visualizer-for-rtl-sdr/

• libusb1.0 (development packages must be installed manually, seelibusb.info)
• gnuplot (must be installed manually, seeinstallation page)
• librtlsdr (cmake / automatic installation & linking)
• fftw3 (cmake / automatic installation & linking)

git clone https://github.com/orhun/rtl_map

cd rtl_mapmkdir buildcd buildcmake ../makesudo make installsudo ldconfig

gcc rtl_map.c -o rtl_map -lrtlsdr -lfftw3 -lm

-d, set device index (default: 0)-s, set sample rate (default: 2048000 Hz)-f, center frequency (Hz) [mandatory argument]-g gain (0 for auto) (default: ~1-3)-n number of reads (default: int_max.)-r, refresh rate for continuous read (default: 500ms)-D, don't show gnuplot graph (default: show)-C, continuously read samples (default: off)-M, show magnitude graph (default graph: dB)-O, disable offset tuning (default: on)-T, turn off terminal log colors (default: on)-h, show help message and exitfilename (a '-' dumps samples to stdout)

[k3@arch ~]$ rtl_map -f 88000000 -D capture.dat[01:00:26] INFO Starting rtl_map ~[01:00:26] INFO Found 1 device(s):[01:00:26] INFO #0: Generic RTL2832U OEMFound Rafael Micro R820T tuner[01:00:27] INFO Using device: #0[01:00:27] INFO Gain set to 14.Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 128.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6 [01:00:27] INFO Center frequency set to 88000000 Hz.[01:00:27] INFO Sampling at 2048000 S/s[01:00:27] INFO Reading samples...[01:00:27] INFO Done, exiting...

rtl_map -f 88000000 -D -

rtl_map -f 88000000 -D -C -n 10 -

[k3@arch ~]$ rtl_map -f 88000000[01:00:28] INFO Starting rtl_map ~[01:00:28] INFO Found 1 device(s):[01:00:28] INFO #0: Generic RTL2832U OEMFound Rafael Micro R820T tuner[01:00:28] INFO Using device: #0[01:00:28] INFO Gain set to 14.Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6 [01:00:29] INFO Center frequency set to 88000000 Hz.[01:00:29] INFO Sampling at 2048000 S/s[01:00:29] INFO Creating FFT graph from samples using gnuplot...[01:00:29] INFO Done, exiting...

rtl_map -f 88000000 -C -r 100

There is a common issue with cheap RTL-SDR receivers which iscenter frequency spike orcentral peak problem related to I/Q imbalance. This problem can be solved with a implementation of some algorithms. (For more details:#1,#2)

1. Implement I/Q correction
2. Find the maximum value of samples, show it on graph with a different color. Might be useful for frequency scanner.
3. Frequency scanner feature
4. Check correctness of min/max point calculation.
5. Check correctness of amplitude (dB) calculation.

• 820T2 tuner used for testing. Other RTL-SDR devices must be tested.

You can contribute to this project if you are a RTL-SDR enthusiast or researcher. Fork the repository and start coding.I hope some people on this planet will consider my TODO(s) and help me build the frequency scanner tool :)

GNU General Public License (v3)

Copyright (c) 2019-2023,orhun