Advanced Signal Processing
Covers the many aspects of modern signal processing systems from the transduction unit through to the display.
Inspec keywords: cryogenics; power amplifiers; radar transmitters; optical fibre communication; underwater acoustic communication; charge-coupled devices; MESFET integrated circuits; diversity reception; sonar; signal processing; acousto-optical correlation; comparators (circuits); very high speed integrated circuits; transmitting antennas; digital filters; display instrumentation; Ada; satellite communication
Other keywords: logarithmic receiver; VLSI architecture; communication receiver; diversity technique; high throughput sonar processor; cryogenic device; integrated optical technique; satellite communication; Ada; optical fibre system; high power amplifier design; acousto optic correlator; scan converter; GaAs IC amplifier; receiver array technology; underwater acoustic sensor; GaAs MESFET comparator; digital filter; display ergonomics; radar transmitter; advanced image understanding; CCD processor; VLSI array processor; transmitter aerial; advanced signal processing; very high performance integrated circuit
Subjects: Display technology; Optical communication; Radio links and equipment; Digital signal processing; Radar equipment, systems and applications; Amplifiers; Optical, image and video signal processing; Sonar and acoustic radar; Semiconductor integrated circuits; Digital filters; Signal processing and detection; Digital filters
I concentrate on the basic function to be performed by a radiating device, whether it be aerial, antenna, array or transducer, and to consider its effect upon a system whose function is to gain information. Most of this chapter concentrates on transmitter antenna applications in radar and radio communications. However, where there is relevant equivalence in other fields, such as sonar, it is discussed so as to draw the reader's attention to the similarities, or lack of them.
The definition of “active sonar” is applied to systems with a wide range of operating parameters. Sonar power amplifiers operate in environments ranging from large shipborne equipments housed in electronics cabinets to small, self contained expendable sonobuoys. Duty cycles of operation can range from high-resolution sonars, which deliver short, high frequency pulses with low duty cycle up to systems which are required to operate continuously, often over a very wide bandwidth. Additionally, control sophistication ranges from simple single-frequency generators to modules that deliver complex signals to closely defined tolerances in amplitude over a range of frequency. This chapter presents an overview of the application areas and their specific requirements. A more detailed examination of active surveillance-sonar parameter leads to a number of options for power amplifier design. Finally, the principles developed are demonstrated in design examples for power amplifiers.
A radar transmitter may be regarded both as a power converter and as a signal synthesiser or amplifier. The latter viewpoint is of primary interest. However, the performance attainable in this regard is usually constrained by conflicting considerations of power conversion performance, so this aspect is examined first.
In sonar, we are concerned with the detection of targets against a background of unwanted signals. These unwanted signals may be noise in several forms, or reverberation. The task of the receiving system designer is to ensure an optimal system response to the wanted signal and minimal response to the unwanted signal. The receive array can play an important part in this process. Sonar suffers from noise problems due to its use of sound waves whose influence on the receiving sensor may be indistinguishable from that due to other mechanical sources. Discrimination against vibration for example may be best achieved in the sound sensor itself rather than in the signal processing. This chapter is concerned with the very early stages in the sonar detection process.
The renaissance of optical technology as a result of the development of coherent optics and optical fibres has led to a new class of transducers based upon the microphonic nature of optical fibres. Prominent among these sensors is the optical fibre hydrophone. Meanwhile, less radical approaches to the drawbacks of conventional piezoelectric hydrophones, involving a better understanding of materials behaviour, have resulted in the development of a variety of composite materials formulations. New fabrication techniques have provided the means for new configurations using conventional materials, thus reducing constraints in the design of transducers. Recent developments in these areas are described in this chapter together with some discussion on techniques and applications.
This chapter discusses the radiowaves that arrive in communication receivers. In all except very simple transmission conditions, radio communication links are subjected to conditions in which energy can travel from the transmitter to the receiver via more than one path. This "multipath" situation arises in different ways depending upon the application, for example in MF and HF skywave transmissions it arises because of reflections from two or more ionospheric layers, or from two-hop propagation. In mobile radio, it arises because of reflection and scattering from buildings, trees and other obstacles along the path.
This chapter gives a brief overview of monolithic microwave amplifiers on GaAs with particular emphasis on low-noise preamplifiers for radars and communications and wide-band amplifiers for electronic countermeasures and instrumentation. Two particular design techniques have been examined - conventional passive matching and feedback, with examples of MMICs produced being given.
This chapter is concerned primarily with the use of integrated optical techniques in acousto-optic receivers. More specifically, we consider devices in which a light beam is confined in a waveguide deposited on the surface of a suitable planar substrate (typically a piece of single-crystal lithium niobate) and interacts with a surface acoustic wave (SAW) launched by a specially designed interdigital electrode structure.
Several different types of logarithmic amplifiers are used in receivers. The logarithmic circuits under consideration here are high-frequency types intended for operation at video frequencies up to 50 MHz and at receiver intermediate frequencies up to 1 GHz. At these frequencies, the logarithmic characteristic has to be approximated using multi-stage circuits. Three well-established approximation techniques are examined. Unfortunately logarithmic amplifiers of this type are not easily examined analytically because of the difficulties in dealing with both the non-linear aspects and the approximation techniques involved. For this reason, the techniques are discussed in general, simple terms. Complete logarithmic amplifiers are commercially available. They are commonly based on discrete-component hybrid construction and achieve high levels of performance. As systems become more complex, demands on space and weight increase, the ability to tailor construction of receivers to specific equipment requirements becomes more important. However, designing and building discrete-component logarithmic amplifiers is a complex and difficult task. Specialised integrated circuits (ICs) are available and these simplify the task of design and construction of amplifiers to meet individual requirements. The use of ICs provides the advantages of reproducibility, small size, greatly reduced component count and enhanced reliability. The general types of logarithmic ICs both current and future, are indicated.
A complete breadboard four-channel assembly of programmable beam-steering and signal processing has been tested at sea. Preliminary experiments have confirmed successful operation of computer-controlled beam-steering, signal amplitude conditioning, and active and passive processing under conditions representing a fully integrated system. Having demonstrated the viability of such an assembly, work is now progressing on a second version, adequately engineered, to stand the rigours of a comprehensive sea trials programme and operate reliably for long periods of time. As far as the future role of the c.c.d. in compact sonars is concerned, this is open to speculation. The c.c.d. development work summarised in this paper is the result of a six year programme of design, evaluation and improvement aimed at the sonar application. Present designs are considered to be close to the limits of what can be achieved in charge-coupled technology, and it remains to be seen whether equivalent digital devices, with improved stability and dynamic range, will eventually appear as a result of future programmes of work in VHSIC/VLSI technology. In the meantime it is felt that the existing c.c.d.-based system referred to above will continue to provide a very useful research tool in investigating the application of programmable spatial and signal processing to improve the performance of the small-array compact sonar.
Echolocation is a primary sense for orientation and navigation in bats, dolphins and some birds. A variety of highly effective systems has evolved to serve individual needs and it should be possible to learn much by studying their operation, but the task presents great problems both in biology and in physical analysis.
This chapter addresses the principles, architectures and performance of acousto-optic correlator systems. Two specific applications which might benefit from these devices are considered, namely spread-spectrum signal processing and lidar ambiguity-function processing.
A novel GaAs MESFET comparator for high speed and medium resolution ADC is described. These comparators do not require any reference voltages. Threshold levels are built into comparators themselves. The com parators and coding logic for flash converter type 2and 3-bit ADC were designed and fabricated in monolithic form. The ICs were operated at an effective sampling rate of 1.0 GHz. A two-stage comparator design will improve the sensitivity and provide 4-bit ADC resolution. Sample-and-hold circuits are being developed at various laboratories. Master-slave flip-flops can be used for latches. Therefore, a single-chip monolithic 3or 4-bit GaAs FET ADC for gigahertz sampling rate operation can be fabricated in the near future. Further work is required to study the errors and temperature effects before incorporating these ADCs into real systems.
This chapter provides an overview of the various design methods available for the realisation of digital logic circuits used in signal processing with particular emphasis on silicon structures. The traditional methods of designing logic systems by deriving an optimum interconnection of basic logic modules (such as the SN74 series) using a mix of intuitive and theoretical techniques are no longer applicable to today's technol ogy. The major reason for this lies in the complexity of the systems which we are now capable of designing and the need to realise these designs in terms of integrated circuit technology designing in silicon.
VHPIC, information technology and VLSI technology are of key importance to military and civil equipment and systems, but it is a highly demanding area, in investment, expertise and new thinking. It will be very beneficial to exploit commonality between applications, both civil and military. On the military side cost reduction through use of VHPIC may be as important as increased per formance. Improved design tools to give an order of magnitude reduction in design costs will be crucial. Effective continuing collaboration between companies, to share the research task and to exchange technology, design tools and VHPIC circuits, will be very important to success.
The Very High Speed Integrated Circuits (VHSIC) Program of the United States Department of Defense will have very significant impact on present and future digital signal processing capabilities. Just as English forces used the longbow at the battle of Crecy in 1346 to multiply their effectiveness, so must modern Western military forces multiply their effectivness through the use of advanced technology, particularly electronics technology. The VHSIC Program will preserve and extend the Western lead in deployed electronics technology, providing improvements in signal processing capability for surveillance, com munications, target classification and several other appliction areas where technology can overcome numerical advantages of potential adversaries.
In a single chapter it is possible only to highlight a few aspects of digital filters. The topics included are selected in order to illustrate relationships between linear systems theory and digital filter structures, some of the frequency-domain design methods and the trade-offs available for implementation.
This chapter discusses those display types which are, or which potentially could be, used for graphics display applications within the fields of radar and sonar, and as text displays in these and other fields. In addition to the pre-eminent c.r.t., various flat panel technologies will be discussed flat panel displays having a particular attraction in confined spaces and for mounting in desk positions.
Sidescan sonar is somewhat similar to the mechanically scanned system except that scanning is achieved by moving the sonar bodily through the water, usually by towing the system behind the ship, while the beam points broadside to the direction of motion. In this way a swarf is swept out on one side of the ship. By using what is in effect two systems it is possible to view on both sides of the ship at the same time. An example of a very large sidescan sonar developed by the Institute of Oceanographic Science is called Project Gloria. This operates at a frequency of about 6.5 kHz, has a beamwidth of 2 degrees, a maximum range of about 20 km and a range resolution of 10 metres.
Ergonomics concerns the design of equipment for human use at work. With simple equipment it may be feasible to correct design deficiencies on the basis of operational experience, but in complex systems this approach is too costly, cumbersome, time-consuming, disruptive and ineffectual and it becomes essen tial to apply ergonomic principles during the specification and design of the system rather than to attempt to remedy ergonomic deficiencies after the system is in being. Modern systems with extensive software may seem to encourage once again system modification in response to operational experience, especially if the facility to modify in this way is claimed beguilingly to permit flexibility or adaptability, but such changes may lead to unwelcome human error unless the associated skills and learning can be transferred intact.
Simulation, in one form or another, provides the equipment developer with the means of selecting the best design approach. The recent advances in the field of signal processing with the much improved processing electronics, software languages and development tools offer the promise of radically different solutions to a wide range of civil and defence requirements. Typically, the complex electronics based control systems that are at the heart of many avionics, plant control and weapon systems rely on the successful implementation of the numerous real-time processing tasks. Further, there is a general desire on the part of the customer for such equipment to have minimal human involvement in the operation and maintenance of the equipment. It is therefore evident that software based, multi processing, electronic designs will increasingly take on the burden of this much sought after capability.The rapid advance of software based processing systems has encouraged the replacement of human control and decision making by machines. Where this involves high cost, human safety or use in extreme environments the system performance has to be rigorously established. In these cases simulators will be needed and their planned use to support multiple functions during the life of the equipment is sensible and economic.
The performance of the next generation of operational sonars must be improved to counter the effects of target noise reduction and anechoic coatings. Improved performance dictates the use of large-area sensor arrays, with many more processing channels and "smarter" processing algorithms such as adaptive or high-resolution processing at the front end of the system and image processing methods at the display end. In addition, operational systems require fault tolerance, comprehensive online monitoring and maintenance facilities, reconfigurability and improved development aids. An increase in processing throughput of two or three orders of magnitude over current systems is necessary to provide these functions. Advances in semiconductor and integrated-circuit technology promise significant improvements in device performance over the next decade, but it is unlikely that these alone will provide the necessary increase in systems performance. Comparable improvements are required in the areas of algorithm development and system architecture if the capabilities of the advanced technologies currently being developed in various national VHPIC and VHSIC programmes are to be fully exploited. This chapter briefly reviews some aspects of recent developments in these fields and their impact on digital signal processing systems in future sonar applications.
A single-mode optical fibre is an extremely high bandwidth, very low attenuation transmission medium. For signal processing applications, the large information bandwidth which may be modulated onto an optical carrier implies that time delay-bandwidth products of the order of 106 may be realised. This is some two orders of magnitude higher than the TB product which may be achieved using conventional delay line media. Signal bandwidths of perhaps 100 GHz may be contemplated, since the use of the very high (optical) carrier frequency implies very small fractional bandwidth occupancy. An optical fibre delay line signal processor requires that these advantages be exploited via some suitable technology. The exploitation process may take several forms. At its simplest, the data may be (usually intensity) modulated onto the optical carrier and launched into a conventional fibre optic trans mission system. The receiver will then combine predetermined fractions of this data with the input data and retransmit the rest to form a multiple tapped delay line system. The tapping and combining network is, in this case, entirely electronic and this system is distinguished from an identical electrical network only in the use of the fibre delay line which may offer advantages concerning low radiated signal strengths and compact format. Of more interest is an all-optical delay line signal processor in which the tapping and recombining are effected in the optical domain. The speed of this processor is not electronically limited, and most of the discussion in this chapter investigates the properties of this class of processor.
The continuing high demand for global, regional and domestic communications is being satisfied by a variety of satellite systems of ever increasing capability and complexity. It was not until 1963 that rockets sufficiently powerful to launch satellites into geostationary orbit were available, and SYNCOM II was the first to lie on the geostationary arc 35,800 km above the equator. INTELSAT (the international telecommunications satellite consortium) commenced operations in the mid 60's. INTELSAT provides multichannel trunking facilities for international carriers as well as leasing transponders to individual nations for regional or domestic systems. WESTAR, RCA, SATCOM, Satellite Business Systems (SBS) and EUTELSAT are examples of domestic/regional systems. INMARSAT provides ship/shore communications in the Atlantic, Pacific and Indian Ocean regions, and a specialized search and rescue system exists in the form of SARSAT. High-quality signals are trans mitted over satellites to cable network link-ends, and more recently, directly to consumers in their homes. Direct-broadcasting should arrive in the U.K. after the launch of UNISAT, and should that system not materialize, it will be one of the services available from OLYMPUS.
The GEC Rectangular Image and Data ("GRID" see footnote) processor has the typical SIMD architecture exhibited by such machines as the UCL CLIP4 (Duff, 1980), ICL DAP (Reddaway, 1973) and NASA MPP (Batcher 1980). It has been developed to provide a powerful and flexible system suitable for image and signal processing applications. The major system components have been designed in custom VLSI, giving the GRID a considerable size advantage over existing parallel array processors. This chapter describes both the hardware and software aspects of the GRID system and presents some of the algorithm work relating to the system's image and signal processing capabilities.
This chapter discusses the future signal processing technology required for radar systems. Although signal processing can be considered to begin on the radio frequencies right at the antenna array, we confine our attention to post-r.f. processing, commencing with digital processing and beamforming for the antenna array and following the data stream to the fully-processed plot or track outputs. We use the phrase “very high performance integrated circuits” (VHPIC) without reference to any particular technology programme. Similarly, we do not refer to particular applications of radar, because we consider that all radars, whether on the battlefield, airborne, or for long-range ground-based air defence, will tend towards multi-mode operation, requiring flexible, coherent processing, adaptive antenna processing and false-alarm control.
This chapter describes the building block methodology whereby VLSI circuits are identified which support an architectural concept designed to provide commonality across a wide range of signal processing functions such as beamforming, filtering and spectrum analysis and common to many sonar applications. In addition the architecture is designed so that future sonar systems may be realised with modules (function units comprised of VLSI circuits) configured using simple guidelines and computer-aided design methodology to minimise the design-resource requirements. This is in contrast to the "modular functional element" architectural concepts proposed by Koral, which combined LSI building blocks at a much lower level.
Ada is a new programming language developed under contract for the US Department of Defense. It was procured and designed as, initially an alterna tive, and eventually a replacement, for the excessively large number of languages and dialects in use in the US defence industry. Its intended field of application is “embedded computer systems” by which is understood systems where the computer is a component rather than a tool in its own right. Such systems include radars, communications equipment, missile-guidance systems, fire con trol and navigation aids. From 1st January 1984, all new USDoD projects of this kind are required to use Ada as the programming language.
This chapter describes some of the essential functions required for analog and digital signal processing. Cryogenic circuits can fulfil all of the required functions for signal processing, often with more than an order of magnitude increase in bandwidth capability over that which is available from existing room-temperature circuits.
RSRE are sponsoring the development of a fast and powerful signal processor based on the architecture of the ICL Distributed Array Processor (DAP). A number of application areas exist where a suitable signal processor is critical to continued success. Possible examples may be found in the fields of image processing, antenna array processing and multi-mode pulse Doppler radars. This latter application is being used as a 'pace setter' to establish the design and performance criteria which must be met. It is believed that the variety of tasks, the data rates, the computational load and programmability required for this application will demand an array processor solution which will be suitable for the many other applications. This chapter describes the evolution of the ICL DAP from its original implementation in MSI technology, when it was incorporated into the architecture of a large mainframe computer, to the ruggedised 'MIL' DAP currently under development in LSI technology. This will function as a compact stand-alone signal processor suitable for the multi-mode radar. It should perhaps be mentioned however that the DAP is not the only solution and in the UK individual radar manufacturers are pursuing their own architectures, which also show great potential.
Image understanding is concerned with the important area of interpreting the output of a sensor and initiating an appropriate response to the perceived scenario. Representation of knowledge derived from human expertise coupled with suitable problem solving strategies provides a method for achieving advanced image understanding in complex and unconstrained scenes.
Some day, machines that recognise speech will be commonplace. People will talk to computers, typewriters, toys, TV sets, household appliances, cars, door locks and wrist-watches. Before looking at the signal processing and pattern processing that will be the basis of the next generation of automatic speech-recognition systems, we consider the nature of the signal that these devices will have to deal with.
Image processing has left the labs for the field. Computer vision has gone through feasibility demonstration in a number of applications - visual inspection,automatic assembly and robotics and is ready for real-time cost effective implementations. This is especially true of military applications, in real-time scene analysis for the detection, recognition and tracking of targets in video from real-time (TV-like) sensors. Real-time implementation provides an exceptional challenge because of the extremely high data rates (in excess of 20 MHz), which result in computational throughputs of several thousand mega operations per second (MOPS). Even harder to realize are the requirements of low cost and size of the hardware integrated with the sensors.
