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Telsy TS-500
Time domain voice scrambler

The TS-500 was a rather simple mobile voice encryption unit, developed in 1976 by Telsy in Italy. It is the first in a range of secure voice products that were released after the company started in 1971. It uses time domain voice scrambling and is also known as the OEM variant Teltron SP-810.
 
The image on the right shows the front panel of a typical TS-500 unit. It only has one switch, that is used to select between clear and encrypted voice. No other controls are present.

The cryptographic key is no more than a small PCB with a labyrinth of tracks that is inserted into a slot at the front of the unit. In the image, the key-card has been extracted from its slot. The card is symmetric (i.e. it can be inserted upside down) and is extremely easy to copy. Furthermore, it allows only a limited number of combinations to be made.
  
Telsy TS-500 voice encryption unit

Despite the simplicity of the key card, the actual electronics are rather advanced considering the unit's age. The TS-500 contains two PCBs that are easily revealed by removing the cover. They are mounted back-to-back, so that the components are immediately visible. The Logic Board contains all digital components, whilst the Audio Board contains the analog components.

At the heart of the system are two FX-209 Adaptive Delta Modulation chips, manufactured by CML in the UK. One is used as an encoder and the other one is the decoder. Analog speech is digitized with a delta demodulator, after which it is scrambled in the time domain and converted back into an analogue signal by means of a delta modulator. The TS-500 was succeeded in 1978 by the TDS-2000 series duplex voice encryption devices that offered 2-dimensional scrambling.
 
TS-500 unit Telsy TS-500 voice encryption unit Key card Removing the key card Logic board Audio board Audio Board detail Connections at the rear

 
References
  1. Consumer Microcircuits Limited (CML), FX-209 Datasheet
    Retrieved June 2013.

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