Speech ScramblerIn the early 1990s, a secure mobile phone system came into service for the first time, when the Thames Valley Police started using a new ‘speech scrambler’ system, following research directed by Professor Joe McGeehan at the University of Bristol Centre for Communications Research. Until this time, it was possible for unauthorised listeners to scan calls from both mobile and fixed telephones. This meant the police and law enforcement services had been compromised by eavesdropping, posing a threat to their own safety and the effectiveness of their operations.

The scrambler started a new era of higher police security, confidentiality and operational protection, and the encryption techniques involved allowed for the expansion of commercial and personal mobile phone usage to the degree we see today.

Background difficulties with mobile communications:

Throughout the 1970s and early eighties, research had focused on increasing the quality and reliability of wireless voice communications. These analogue systems were subject to amplitude and phase distortion from the multipath fading. As the signal travelled to the receiver, it would become weaker and suffer from multiple reflections and defractions, making the speech sound very distorted, and data transmission almost impossible. However, with the onset of the oil crisis in 1974, and increasingly high transport costs, there was a demand to make improvements. Mobile communications could significantly reduce the cost, say, of running a fleet of vans or trucks, but, currently, had very limited use through the police, fire and ambulance services, utilities and taxis. Private mobile radios were also installed in some high value executive cars at around £2,000 per unit.

In 1975, Prof McGeehan applied for a grant for £9,500 from the then Science Research Council to study mobile radio, looking how to manipulate the spectrum of the received signal so it would reach the recipient without fading or loss of quality. In 1980, this work resulted in an improved AM transportable mobile radio, manufactured by Mobira-Oy (part of Nokia), which was adopted throughout the Securicor fleet of vans and various private mobile radio networks.

Home office trials: 

However, there were existing problems with the privacy and security of calls made from analogue phones and although this may have been inconvenient for business users, particular issues were arising within the police force and other government agencies. It was easy and cheap for criminals to tune into messages and plan accordingly. How could this be prevented, while still offering the essential mobile communications benefits? Scrambling techniques, therefore, had to:

  • Prevent eavesdropping effectively
  • Deliver quality recovered (unscrambled) speech to the receiver
  • Have only a small time delay, as it was crucial the first syllable of a message was not lost
  • Have low bandwidth expansion and low battery drain
  • Incorporate the highest levels of encryption security
  • Be easy to produce, and be affordable

In the late eighties, the Home Office called for competitive trials to be undertaken to research a variety of speech scrambler systems. Prof McGeehan’s research led to the invention of a prototype which satisfied all the above criteria. This was then developed by GEC-Marconi Secure Systems into the Marconi Advanced Scrambler (MASC) system. From the five options tendered, the Home Office selected MASC as the only recommended speech scrambler for UK police forces, and it started to be rolled out across the country.

Speech ScramblerHow does it work?

The device makes speech private by mixing up the format of bandwidth frequencies as the speech travels between receivers so it cannot be understood by an eavesdropper. The speech is then decoded using an encryption process at the receiver end. This unscrambles it and allows the authorised listener to hear high quality voice communication without a significant time delay.

Prof McGeehan’s team used an existing digital signal processor (DSP) to create the scrambling process. This allowed for fast, precise and repeatable frequency manipulation, and very accurate reconstruction of the voice signal.

Prof McGeehan recalls: “I realised there was a great need to improve the security of communications and spotted that existing technology, with the addition of new novel engineering solutions, could provide the answer. Then, as now, the challenge was to find a commercial partner willing to take a risk, and support the work and its commercial exploitation.”

Speech Scrambler in service

By 1992, the police were using the scrambler in active service. The new mobile device was no more than a couple of inches in length and could be incorporated into the officer’s uniform radio. Although Terrestrial Trunked Radio (TETRA) digital mobile radio systems started to be introduced in Europe during the 1990s, they were expensive to roll-out and the speech scrambler allowed forces to convert to digital gradually, as costs came down. However, the scrambler continues to be used in parts of the world where digital systems are not fully developed.