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CIS Makhovik (the "flywheel") vocoder

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Makhovik is classified as "vocoder" butcan be used also for sending encryption of data,it is designed to operate in the UHF  but very often is found on the HF as a part of the AT-3004D modem. The manipulation speed is 1200 Baud and the modulation used is BPSK(pictures 1 and 2)and are easy obtained by SA in case of a good recording of the signal.

pic. 1 

pic. 2
For what concerns its structure, this is signal is quite difficult to study. A fairly complete analysis can be found here,as for me I willillustratewhat I havebeen able to find  and verify in regard to theaforementionedanalysis.
Looking at the sonogram we can see an initial long BPSK preamble that is re-inserted several times: reinsertions do not happen at regular time intervals and have different durations. The preamble has a period of 12.5ms and since the speed of the system (1200 Baud) and the used modulation (BPSK), 12.5ms makes a 15-bits string matrix:this idle sequence111101011001000the characteristic "sign" of Makhovik but can be transmitted with different polarity (pic. 4). 

pic. 3 - period of the preamble, 12.5 milliseconds

 pic. 4 - the characteristic 15 bit string can be sent in both the negative and positive polarity

The header following the preamble, and preceeding the data block, is composed of a 511 bits synchronization block followed by the insertion of the encryption key. The key is composed of eight strings of 30 bits (i.e.240 bits lenght) and each string is repeated 3 times so that the key weight is 720 bits(pic. 5). The x3 redundancy, as well as in KG-84 encryption, is probably used to improve the accuracy and realiability of the transmission.
pic. 5
Preambles and headers (sync blocks + keys) are easy recognizable inside the bit flow (pic. 6) as well in the sonogram of the signal (pic. 7).

pic. 6

pic. 7
As a concuding note I want to point out that the bitstream comes directly from SA demodulator and then we have to do with "on-the-air"symbols.
As said, an interisting and detailed analysis can be read in radioscanner  forum although it's in Russian language, anyway both the positive/negative idling sequence and the 720 bits key block are valid cluesfor its identification.
Thanks to KarapuZ for the precious help and tips.


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