28 August 2015

CIS-20 in idle state

The main features of the waveform, as reported in radioscanner.ru, are: k = 3/5, 20 channels + 1 pilot, channel modulation pi/4 DQPSK, speed 75 Baud and channel step 120 Hz.
In this sample the modem was heard when in idle state: modulation switched to BPSK (4 positions in the absolute constellation) but same manipulation speed and channel separation (75 Baud and 120 Hz). Notice the resampling at its native frequency (7200 Hz as shown here). ACF in idle state is 200 mSec.
AT-3104D waveform (idle mode) OFDM parameters
AT-3104D waveform (idle mode) ACF
The recordered signal does not have a good quality and modulation is a bit confused, so I isolated a single channel to confirm the relative constellation (BPSK).
AT-3104D waveform (idle mode) single channel inspection
 As other characteristic, at least in this sample, is the carrier #13 that is not modulated

According to this post in radioscanner CIS-20 is a new version of CIS-12 so probably sourced from the same AT-3104 modem (or the older AT-3004D). Both the two modems were designed to have many different devices plugged into them to form the modem’s input – most often teleprinters unique to the different military services and civilian agencies that use them, and also, vocoders and speech encipherment systems, but all are transported in the same fashion – using one of these 2 waveforms.

1 comment:

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