4224 photo and copy below courtesy E-Bay seller

The encryption method used is audio encryption, so the 4224 can be used with most military radio equipment. Normally the encryptor is simply installed between the the audio input of the transceiver and the handset.

Encryption is achieved by a process of time division multiplexing and frequency dispersion both under the control of a complex keystream generator. The original audio signal is thus rearranged in both time and frequency domain while producing encrypted audio output within the original 2 kHz bandwidth. Synchronizing data is also included in this bandwidth.
 

Vintage: 1990
Unit weight: 3 kg.

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Sept 11/05