Other Subjects Studied

Radios - HF and RF

Various types of military rado relay equipment in the frequency range 50-5000Mhz, with power outputs from 100mW to 50W and facilities for simplex voice operation or for use with telephone equipments were developed. A 100 channel short-range radio system used a Random Access Discreet Address System (RADA) operating at 165 Mhz, where messages were transmitted in short bursts enabling the subscriber to speak to any other by selecting a particular pulse sequence was also developed. Back to list

Clansman Tactical Net Radio

A digital radio was developed with improved perfomance due to its high standard of frequency accuracy and control by the use of a micro-miniaturised digital synthesiser used in conjunction with a separate crystal reference oscillator.

A range of accessories, including nickel-cadmium secondary batteries, battery-charging equipment, audio gear, control harness (used to control and operate a number of radios in a vehicular installation), and antennas were developed along with the radios. Back to list

Speech Communication in intense acoustic noise

An array of loudspeakers was used to generate an acoustic field in a reverberant enclosure. The cockpit noise level due to jet engines and turbulent air flow was pre-recorded and used to set up an acoustic field in the chamber. The intercom situation was simulated by placing microphones inside and outside the flying helmet of a dummy to compare the noise level at the entrance to the ear canal and in the cockpit. Headphones were placed on a dummy typifying one of a crew of listeners normally used in intelligibility testing of all or part of the aircraft intercom system. The signals picked up by the miniature microphones at the dummy-listeners headphones measured the degree of noise interference which was experienced. Back to list

Objective measurements on transducers

A small anechoic chamber and associated electronic gear was used to measure the perfomance of the transducers used to assess speech communication links and develop new audio gear for the services. Back to list

Experimental Store-and-Forward systems

In the 1970's SRDE designed and constructed a vehicle-mounted computer controlled message switch, capable of handling the telegraph traffic of an Army field communication system. The equipment mounted in a 3-ton 4- wheel drive vehicle was successfully used by the Army in BAOR. The development of the equipment and computer program was carried out at SRDE in associaction with the Military Data Systens Division of Elliott Bros. Back to list

Reliability and fault investigation of integrated circuits

SRDE was responsible for the design and development of several successful integrated circuits for the CLANSMAN project. These ranged from relatively low density analogue circuits to high density complex digital circuits. The fault investigation programme covered:

  1. Visual inspection of the chip surface for mechanical damage caused during assembly and incomplete etching of the interconnection pattern.
  2. Examination of the diffusion areas within the chip. This identified faults associated with the mask imperfections as well as misplaced diffusions due to blemishes in the photo-resist masking process.
  3. Measurements of the sheet resistivity of diffused resistors, breakdown voltages and other parameters of transistors and diodes. The identification of phantom components latent in the diffusion process and in the topography of the chip.
  4. Visual inspection of the chip surface using optical and scanning electron microscopes. Back to list

R.F Radiation and interference

SRDE was concerned with the development and application of testing techniques and the production of R.F. Interference Standards. This led to the development of improved techniques for:

Reduction of acoustic noise in power sources

This work was undertaken to produce a specification for the non-detectability of the noise emitted by military equipment at specified ranges. The investigations included enclosing power sources with sound-proofing materials and the quietening of a gas turbine set. Techniques were developed for carrying out objective noise measurements accurately and repeatably on power supply equipment as used by the Army and the correlation of such measurements with subjective assessments in typical user environments. Back to list

Lightweight radio relay at X-band.

A compact microwave line-of-sight radio link for voice and data communication was developed. Its narrower beamwidth gave greater security, and the high frequency a larger channel carrying capacity and it could be operated at a low power levels if required.

The smaller size enabled it to be mounted in a Landrover or vehicle already carrying a UHF installation. The dificulty with aligning a SHF antenna was considerably reduced by modifications to the standard telescope mast and could be set in complete darkness. The receiver of the radio used a Gunn diode as a local oscillator and the mechanical tuning of the of the microwave cavity was obtained by the penetration of a shutter into the microwave cavity, its rate of penetration being controlled to give a linear tuning. The temperature coefficient of the oscillator was compensated by an invar probe inside the cavity. Back to list

Speech Studies

To make the most economical use of available transmission bandwidths the fundamental structure and the information bearing elements contained in speech was determined with a machine "PAT" (Parametric Artificial Talker) designed and built at SRDE. By changing the voltages applied to the input, sounds resembling human speech were synthesised and displayed as two and three-dimensional speech patterns. When the voltages were varied to a pre-set pattern PAT uttered complete phrases and by varying the control signals comparatively slowly and passing them through a narrow band filter it was shown that speech could be transmitted with a bandwith of less than 100 Hz. Back to list

An automatic equaliser

When digital data is passed over communication channels not specifically designed for data transmission (such as speech circuits, or H.F. radio circuits) the dispersive nature of the channel circuits distorts the detected data in such a way as to produce severe inter-symbol interference even when operating at very modest bit rates.

An automatic equaliser was constructed to provide an adjustable characteristic that was equal and opposite to the dispersion characteristics of a communication channel and in the process making the overall characteristic virtually distortionless. The binary coded data was transmitted with a lower error rate and higher bit speed.

The apparatus was able to be patched to perform the functions of both preset and adaptive types (those which require an initial test-signal procedure and those which continuously adapt to ever changing conditions as over H.F. radio). Back to list

Digital coding of speech

SRDE developed methods for transmitting messages in digital form through military communication networks. The advantages obtained were:

  1. The transmission was practically error-free over radio links.
  2. The switching centres were compact and reliable and were able to be computer controlled.
  3. Provide standardised communication channels for voice, telegraph, facsimile or digital data.

SRDE devised coding methods for converting speech into digits and back again using the minimum number of binary digits (bits) per second in the transmission link for an acceptable speech quality. Lowering the bit rate reduced the bandwidth and the cost of the radio equipment.

To maintain the speech quality when the bit rate was reduced a different coding method was used. In a tactical system where the number of subscribers to any radio link was relatively large and the radio links short, an inexpensive coder was used with a high bit rate. In a strategic system with a smaller number of subscribers and longer radio links a lower bit rate was used with a more complex coder. The speech coders developed were:

  1. Instantaneously Companded PCM - 42kilobits/sec.
  2. Syllabically Adaptive Deltamodulation- 19.2kilobits/sec.
  3. Syllabically Adaptive PCM - 9.6kilobits/sec.
  4. Channel Vocoder - 2.4 kilobits/sec.

Coder A was used is systems where transmission over the radio relay links was digital but the circuit switching was carried out on the decoded (analogue) speech at each switching centre. A larger bit rate was used to give a speech quality good enough to tolerate the progressive addition of quantisation noise due to repeated re-coding. The dynamic range of the coder was made large enough to accommodate successive attenuations at the analogue junctions.

In coders B, C and D, the bit rate was reduced to the lowest practicable value consistent with retaining a serviceable speech quality for each of the three coding systems. Back to list

Automatic Testing

Automatic testing used "system" tests with an analytical approach to give the required test information from the analysis of a limited number of actual measurements. Costly test hardware was traded for computer software. The need for test-points and electrical access was greatly reduced in many types of equipment. Repetitive waveforms were analysed by computer methods with mathematical models of the waveforms in question, such as voltage-time functions provided to the computer. The model was built up by a quantising process, in which the amplitude of the waveform was "sampled", digitised and stored at known, close time intervals. The problem with dealing with high frequencies was overcome by sampling at manageable rates, the complete model was built up from samples from successive waveforms, placed in their correct time-relationship as though relative to a single waveform.

The Transfer Function Test Methods included impulse testing, and Random noise and Cross-Correlation techniques. A mathematical model describing the relation between input and output signals for a given electrical network was derived, and departures from specification by a computer method, were made from appropriate measurements. Circuits were characterised by the use of impulse methods where, a step function was applied to the circuit and the input waveform was measured. The same result was achieved by the injection of random noise and the application of correlation methods. Back to list

Electromagnetic compatibility

The co-siting of radio equipments occurs whenever a user requires access to a number of radio networks. He may be in command of one net, a subscriber to another net and also requires access to the Trunk Communication System. The overall compatibility of these and other systems was analysed by SRDE and the EMC Analysis Facility of the British Aircraft Corporation using a computer based network model.

R.F. filters were designed at SRDE with a dual role of reducing the levels of spurious signals and the wide-band noise from the transmitter, and of increasing the receiver protection against internally generated intermodulation Products and against non-linear blocking. The effects of intermodulation were significantly reduced when:

  1. Each transmitter had a separate aerial.
  2. The number of transmitters in a complex was kept a minimum by the use of time-division multiplex systems.
  3. All vehicles were kept at least 60 ft from the central aerial complex.
  4. The frequency assignment took full account of intermodulation products. Back to list

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