various radio projects using gnuradio and rtl-sdr.
commrx is a communications receiver with AM, WFM, NFM, USB, LSB reception.
![commrx screen] (https://github.com/unclejed613/gnuradio-projects-rtlsdr/blob/master/Screenshot%20from%202017-01-17%2018-23-22.png)
scanner is a police scanner. it's very crude in that it tunes the SDR stick to the frequency of interest rather than picking signals out somewhere within the bandwidth. scanner also can use a list of presets like a police scanner. the list is editable, and there are two python scripts to manage the frequency list, one a GUI, and the other using a curses interface.
scrambler is a gnuradio implementation of the old fashioned analog "ring modulator" voice scrambler. it uses a balanced modulator and low pass filters to do "speech inversion" where the baseband audio is low pass filtered, run through a balanced modulator which uses a 3khz tone as the "carrier". this produces an output where the portion of audio spectrum below 3khz is a mirror image of the audio input spectrum. also present in the modulator output is a frequency shifted copy of the input spectrum, and this is low pass filtered out, leaving only the "inverted" audio. this is by no means a secure form of communications. back in the 60's through the 80's, it was used for deterring only casual listeners using police scanners from eavesdropping. on vhf frequencies the inverted audio would be transmitted by a standard FM system. transmitting inverted audio on an HF SSB radio had the obvious weakness of receiving it using LSB instead of USB, and getting the original audio without having to do anything. there was a privacy system in use on ship-to-shore that made listening more difficult, and that was to warble the 3khz oscillator, which had the effect of frequency shifting the scrambled audio to the point where decoded audio was almost unintelligible. the trick to decpding the audio would have been to discover what the method was for synchronizing the warbling.