Quantisation mechanisms in multi-protoype waveform coding

Prototype Waveform Coding is one of the most promising methods for speech coding at low bit rates over telecommunications networks. This thesis investigates quantisation mechanisms in Multi-Prototype Waveform (MPW) coding, and two prototype waveform quantisation algorithms for speech coding at bit rates of 2.4kb/s are proposed. Speech coders based on these algorithms have been found to be capable of producing coded speech with equivalent perceptual quality to that generated by the US 1016 Federal Standard CELP-4.8kb/s algorithm. The two proposed prototype waveform quantisation algorithms are based on Prototype Waveform Interpolation (PWI). The first algorithm is in an open loop architecture (Open Loop Quantisation). In this algorithm, the speech residual is represented as a series of prototype waveforms (PWs). The PWs are extracted in both voiced and unvoiced speech, time aligned and quantised and, at the receiver, the excitation is reconstructed by smooth interpolation between them. For low bit rate coding, the PW is decomposed into a slowly evolving waveform (SEW) and a rapidly evolving waveform (REW). The SEW is coded using vector quantisation on both magnitude and phase spectra. The SEW codebook search is based on the best matching of the SEW and the SEW codebook vector. The REW phase spectra is not quantised, but it is recovered using Gaussian noise. The REW magnitude spectra, on the other hand, can be either quantised with a certain update rate or only derived according to SEW behaviours

A delta–sigma-modulated speed control system for induction motors

The electronic control techniques used in static frequency changers differ considerably, depending on the type of system considered and the particular specifications and requirements. Mark-space ratio control and pulse-width modulation are among techniques already well-established for use with induction motors to provide a variable-speed a.c. drive. The project is concerned with the applicability of delta-sigma modulation as the basis of a novel method of electronic speed control. The techniques involved are well-known in communication systems, where they are extensively used in the transmission of speech signals. With a sine wave at the input, a delta–sigma modulator produces an output quantised in both voltage and frequency, which enables power control to be achieved from a fixed d.c. supply using only a single power controller. [Continues.

Development of a Full-Field Time-of-Flight Range Imaging System

A full-field, time-of-flight, image ranging system or 3D camera has been developed from a proof-of-principle to a working prototype stage, capable of determining the intensity and range for every pixel in a scene. The system can be adapted to the requirements of various applications, producing high precision range measurements with sub-millimetre resolution, or high speed measurements at video frame rates. Parallel data acquisition at each pixel provides high spatial resolution independent of the operating speed. The range imaging system uses a heterodyne technique to indirectly measure time of flight. Laser diodes with highly diverging beams are intensity modulated at radio frequencies and used to illuminate the scene. Reflected light is focused on to an image intensifier used as a high speed optical shutter, which is modulated at a slightly different frequency to that of the laser source. The output from the shutter is a low frequency beat signal, which is sampled by a digital video camera. Optical propagation delay is encoded into the phase of the beat signal, hence from a captured time variant intensity sequence, the beat signal phase can be measured to determine range for every pixel in the scene. A direct digital synthesiser (DDS) is designed and constructed, capable of generating up to three outputs at frequencies beyond 100 MHz with the relative frequency stability in excess of nine orders of magnitude required to control the laser and shutter modulation. Driver circuits were also designed to modulate the image intensifier photocathode at 50 Vpp, and four laser diodes with a combined power output of 320 mW, both over a frequency range of 10-100 MHz. The DDS, laser, and image intensifier response are characterised. A unique method of measuring the image intensifier optical modulation response is developed, requiring the construction of a pico-second pulsed laser source. This characterisation revealed deficiencies in the measured responses, which were mitigated through hardware modifications where possible. The effects of remaining imperfections, such as modulation waveform harmonics and image intensifier irising, can be calibrated and removed from the range measurements during software processing using the characterisation data. Finally, a digital method of generating the high frequency modulation signals using a FPGA to replace the analogue DDS is developed, providing a highly integrated solution, reducing the complexity, and enhancing flexibility. In addition, a novel modulation coding technique is developed to remove the undesirable influence of waveform harmonics from the range measurement without extending the acquisition time. When combined with a proposed modification to the laser illumination source, the digital system can enhance range measurement precision and linearity. From this work, a flexible full-field image ranging system is successfully realised. The system is demonstrated operating in a high precision mode with sub-millimetre depth resolution, and also in a high speed mode operating at video update rates (25 fps), in both cases providing high (512 512) spatial resolution over distances of several metres

Experiments Relevant to the Development of Laser Interferometric Gravitational Wave Detectors

The development of gravitational wave detectors has been in progress for approximately twenty-five years. As yet there has been no clear evidence for the successful detection of such propagating fluctuations in the curvature of spacetime, but the prospects seem good that detectors of sufficient sensitivity to detect gravitational waves of astrophysical origin can be constructed in the near future. The most promising form of detector is the long baseline laser interferometer, and prototypes are being developed at a number of sites around the world. A 10 metre prototype is currently being developed in Glasgow. This thesis is an account of work based on the Glasgow prototype. After an elementary introduction to the theoretical foundations of gravitational waves, various sources of gravitational radiation, the nature of their emitted signal and their strengths are considered. Suitable detectors and their possible sensitivities are reviewed. Noise sources which could limit the sensitivity of laser interferometer detectors and the constraints which these place on the design of the detector are discussed. Since the test masses in an interferometer detector must be freely suspended as pendulums, yet their orientation must be accurately controlled to maintain correct alignment of the optical cavities forming the interferometer, an active orientation control system was developed and installed on the Glasgow prototype. This system provides a high degree of positional and angular stabilisation at low frequencies while leaving the test mass essentially free at high frequencies. Some of the potential limitations and noise sources are noted and their magnitudes calculated. A digital recording system was designed and used to record data from the prototype detector at Glasgow. The effects of the detector's response are analysed and techniques to recover the gravitational wave signal from the recorded data are described. The analysis of some data recorded with this system is then reported. The pulse statistics of the interferometer are analysed and the implications for searches for millisecond pulses of gravitational waves are discussed. The results of a search for periodic signals in the detector output are presented. Various sources of contamination which may be present in the detector output are identified, limitations of the recorded data are noted, and techniques which may be used to reduce the importance of these effects are described