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CROWD36 is the name given to a mode used for Russian diplomatic communications by hobbyists. It actual name is believed to be the "SERDOLIK" long range radio-system. It is a MFSK (Multi Frequency Shift Keying) system with 36 possible tones sent a speed of 40 baud.
Unlike most of Rivets modes CROWD36 decoding isn't automatic and requires the user to to input some information to aid the calibration of the program. All of the CROWD36 broadcasts I have seen so far start with what appears to be a synchronisation sequence made up of four tones as you can see in this spectrogram ..
When it is CROWD36 decoding mode Rivet hunts for four tones (tone#1,tone#2,tone#3,tone#4). Where tone#1 = tone#3 and tone#2 = tone#4 plus tone#1 minus tone#3 is less than 100 Hz. Rivet then calibrates itself on tone#1 which it takes to be the tone number specified by the user. You set this value by clicking on the "Options" menu "Set CROWD36 High Sync Tone" and entering a number between 0 and 33. The broadcasts I have seen either use tone 24 or tone 31 for this value but it does need some experimentation and I would recommend that you record CROWD36 broadcasts (as WAV files , mono and 8000 Hz sample rate) then you can process that file in Rivet trying different values.
In the past CROWD36 transmitted encrypted information in the form of 5 digit numbers then later moved to online encryption. These days the mode seems to be being used for link setup information with the actual traffic being sent in another mode if the conditions are suitable. So frequently the only decoded traffic is lists of repeating characters such as below ..
There is a lot of work need doing to Rivet's CROWD36 decoder module and if anyone can provide me with any further information to help I would be very grateful.
There is a WAV file recording of a CROWD36 signal on the Rivet sound samples page here.