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Teltron SP-612
Secure voice frequency inverter - under construction

The SP-612 was a secure voice encryption unit, based on the principle of frequency inversion, developed and built by Teltron in München (Germany) around 1975. The design is based on an FX-204 single-chip frequency inverter [2] and was mainly sold to the German police and other law enforcement agencies. It is also known as a voice scrambler or frequency inverter [1].
 
The device measures 17 x 10 x 3 cm and was pretty small for the era. It was usually mounted in a metal bracket that was fixed inside, say, a police car. The SP-612 was held in place by two springs that locked onto the pins at both sides of the front panel. This way the device could easily be removed for repair or maintenance.

A small sliding 'door' at the left of the front panel, gives access to a single-digit code settings with up and down buttons. The SP-612 offered very limited security as it only had 16 possible code settings (0-9 and A-F).
  
SP-612 voice scrambler - front panel

The device was further 'weakened' by the fact that it only provides frequency inversion and not time-domain scrambling as in later devices, such as the SP-850. Eavesdroppers could easily recorver the clear speech by using a freely adjustable frequency inverter.
 
SP-612 voice scrambler SP-612 voice scrambler - front panel Code selector uncovered Locking pins Mounted in car bracked Removing the SP-612 from the car backed Front panel - code selector visible Front panel - code selector covered

 
Controls
Connecting and operating the SP-612 was relatively simple. The device was connected to the wiring of the (police) car and the existing radio via the sliding bracked that mated with the 15-pin connector at the rear of the unit. Once installed in the bracket, the handset would be connected to the 5-pin 270° DIN socket at the front panel. A toggle switch is ued to select between SECURE and CLEAR modes and two indicators, a red and a yellow one, show the present state of the unit.


The SP-612 offers 16 different security codes to be selected with a single-digit selector that is hidden behind the sliding 'door' at the left. Sliding this door to the right, reveals the selector. It can be set to 0-9 or A-F. Once the code is set, the door is placed in the leftmost position again.
 
Interior
The SP-612 is housed in a slim-line aluminium enclosure that consists of a frame and two case shells. Each case shell is held in place by two screws at either side. After removing the screws and taking away the two shells, the well-organised interior is exposed as shown in the images below.
 
The device contains one large PCB with all parts mounted on one side, plus a smaller one for the controls and connections. The larger PCB is mounted horizontally and occupies most of the interior. The design is based on the FX-204 single-chip frequency inverter made by CML, that is prominently visible at one of the edges.

The SP-612 is well-built and neatly organised, using only first class components. All ICs are socketed, and the solder side of the PCB is freely accessible, making it a service friendly device.
  
Interior

 
Interior Interior - top view FX-204 single-chip frequency inverter DIP-switches Another DIP-swtich Connector detail Front panel detail Code selector detail

 
References
  1. Helmut 'Jim' Meyer, Teltron SP-612 voice scrambler - THANKS !
    Devices kindly donated by Jim Meyer, June 2014.
    Personal correspondence, July/August 2015.

  2. Consumer Microcircuits Ltd (CML), FX204 VSB 1 Frequency Inverter
    Publication D/204/6, November 1986, Provisional Issue.

  1. VSB = Variable Split Band.

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Crypto Museum. Last changed: Thursday, 20 August 2015 - 10:12 CET.
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