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Teltron SP-601
Secure voice frequency inverter - this page is a stub

The SP-601 was a secure voice encryption unit, based on the principle of frequency inversion, also known as a frequency domain speech scrambler, developed by Teltron in München (German) in 1972. The device was intended for use by the police and other law enforcement agencies and was suitable for existing analogue VHF/UHF 2-way radios. It had 6 different scrambling codes.

The image on the right shows a typical SP-601. It is housed in an aluminium enclosure with two rigs at the top, allowing it to be mounted in a vehicle bracket. Radio and power are wired at the rear. The handset is connected at the front.

Unlike later voice scramblers, which used a single-chip solution, the SP-601 is built with discrete components, spread over two PCBs. Generally speaking, voice scrambling is not (and never was) very secure. An eavesdropper could simply reverse the audio spectrum once more, to make the secret conversation audible again.
  

With just 6 available codes (i.e. frequency mirroring points), the interceptor has little trouble finding the correct one. In fact, in the days when police scanners were popular, some listeners got so experienced that after a while their brain would 'translate' the scrambled speech 'on the fly' without any technical means. The SP-601 was succeeded by the SP-612 which was based on a single-chip solution and offered 16 scrambling codes, but even this was not secure enough.

Documentation
  1. Leaflet, Sprachverschleierungsgerät SP 601 für Zweiweg-Funkgeräte
    Teltron GmbH, München, Date unknown (German). 1

  2. Handbook, Sprachverschleierungsgerät SP 601 für Zweiweg-Funkgeräte
    Teltron GmbH, München, 31 January 1972 (German). 1
  1. Documentation kindly supplied by Jim Meyer [1].

References
  1. Helmut 'Jim' Meyer, Handbook and leaflet of SP-601 - THANKS !
    Documentation kindly donated by Jim Meyer, June 2015.
    Personal correspondence, July/August 2015.
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Crypto Museum. Last changed: Thursday, 20 August 2015 - 10:27 CET.
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