Privacy set 6AA (GPTN) Diagram SA 5061

Made:
1937-1949

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Science Museum Group Collections
© The Board of Trustees of the Science Museum, London

Science Museum Group Collections
© The Board of Trustees of the Science Museum, London

Science Museum Group Collections
© The Board of Trustees of the Science Museum, London

Science Museum Group Collections
© The Board of Trustees of the Science Museum, London

Privacy set 6AA (GPTN) Diagram SA 5061, c.1937-1949.

The Secraphone was initially based on the so-called Privacy Set No. 6, also known as Frequency Changer No. 6, a fairly simple voice scrambler based on the principle of frequency inversion. It contained a valve-based balanced ring modulator with a reference frequency of 2500 Hz and was used in combination with a black GPO telephone set No. 394 with green handset. All the party at the other end of the line needed, was an identical Privacy Set with a similar No. 394 telephone.

This system was in use between 1937 and 1949 and was rather unsafe as it was based on a simple principle and had no key settings. By mirroring the frequency around 2500 Hz once again, an eavesdropper could recover the original voice data.

Details

Category:
Telecommunications
Object Number:
2004-184
credit:
BT Group plc