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Two co-channel signal sources were fed to a PYE Whitehall radio to asses the disruption caused by co-channel interference with varying signal and interference levels, modulation and frequency offsets. At the end of the work there was no advantage in using FM and capture effect to reduce co-channel interference. It was found that the use of FM made the harmful effects of the co-channel interference considerable worse than experienced with AM.
Further work was conducted using a test receiver where it was seen that capture effect improved with increased deviation (wider bandwidth) and with the deviation levels used in FM broadcast there was a pronounced capture effect of a few dB.
A multi-modulation evaluation and comparison of quasi-synchronous Pilot Carrier ssb at 150 MHz (1985)
At this time the Radio Agency were heavily involved with the determination of future technology for land mobile radio and pilot carrier SSB was a contender in the competing technologies. After conducting their investigations the Radio Agency loaned the Directorate a pilot carrier SSB mobile radio developed by PYE telecommunications. The aim of the work was to see if SSB would work successfully within a three station quasi-synchronous system and to determine what parameters would be required.
An experimental three station system was rigged and set up at Bovingdon, Stanmore and Sandridge running in parallel with the existing AM system which by then had been converted to run on the 150 MHz band. The ssb transmitters were made locally using conventional technology and connected in parallel with the existing AM system. Transmitter combining was used to provide the same antennas for each system.
The results for the ssb system proved very promising since there was a complete absence of the disruptive intercarrier beating normally associated with QSAM and audio base band phasing was found to be very tolerant, so much so that an audio phase shift of 180 degrees was indiscernible in ssb whilst introducing significant distortion with AM. The ability of SSB to work satisfactorily with very weak signals was also very impressive.
Despite these very positive findings ssb radios were not developed further since PYE did not continue with the manufacture of VHF ssb mobile radios, principally because the mode was not adopted within land mobile radio.
Marconi Advanced Scrambler trials (1992)
As part of a central procurement initiative the Home Office called for competitive operational trials to be undertaken to research a variety of speech scrambler systems. The trials were completed in the late 1980’s with the GEC - Marconi Secure Systems scrambler (MASC) being the recommended speech scrambler for UK police forces.
The operational scrambler trials were limited to UHF personal radios but there was some interest in deploying them in VHF AM radios, however there was little experience of the device in quasi-synchronous AM systems and there were some investigations into how the device might work using the radio test area. The results were generally positive but the uptake of speech scramblers was limited given the digital encryption to be implemented with the forthcoming Airwave system.
Investigative support for Bristol University and trial of DSP modulation (1992)
The Directorate was supporting research by Bristol University to use DSP to generate analogue and digital voice modulation. The intention was to create spectrally efficient digital voice modulation which would fit within a nominal 12.5 kHz or 25 kHz bandwidth. The trial was heavily supported by the laboratories and staff at Bristol University who modified standard PYE transmitters for digital modulation. Tests were conducted from the Bovingdon site using single site working. At the conclusion of the trial the advantages of using DSP were clearly demonstrated albeit the silicon technology of the time was too slow to allow sufficient bit rates to ensure the best audio quality.
Line Linked Repeater (1993)
The facilities at Bovingdon were used for making adaptations of existing fire service handheld radios into an experimental UHF line linked repeater for use in sub-surface environments. The experimental equipment was subsequently successfully trialled at various locations in London and the South East. Following the trials Messrs Cytron Ltd put together a prototype system which was also tested successfully.
Preliminary trials of Motorola TETRA base stations and mobiles (1994)
This work was conducted as a joint exercise between BT and Motorola to demonstrate the use of TETRA in a wide area coverage environment. Motorola base stations were installed at the sites with digitally implemented line linking. There followed a program of trials, tests and measurements to prove the coverage and cell handovers as a precursor to Airwave.
Audio intelligibility comparisons between analogue FM and TETRA (1996)
There were concerns that the quality of the digital speech provided by TETRA would be worse when compared to FM or AM.
An assessment was made of the intelligibility of the TETRA codec by comparing recovered audio from simple AM FM and TETRA radio systems. A series of Dynamic Rhyme Tests were conducted with the help of the Institute of Sound and Vibration research at Southampton University.
At the conclusion of the work it could be seen that TETRA vocoded speech scores were very similar to those obtained using AM and FM.
Acknowledgement: Peter Bridgeman
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