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Speech coding at 64 and 32 Kb/s is well developed and standardized. The next bit rate of interest is at 16 Kb/s. Although. standardization has yet to be made, speech coding at 16 Kb/s is fairly well developed. The existing coders can produce good quality speech at rates as low as about 9.6 Kb/s. At present the major research area is at 8 to 4.8 Kb/s. This work deals first of all with enhancing the quality andkcomplexity of some of the most promising coders at 16 to 9.6 Kb/s as well as proposing new alternative coders. For this purpose coders operating at 16 Kb/s and 12 to 9.6 Kb/s have been grouped together and optimized for their corresponding bit rates. The second part of the work deals with the possibilities of coding the speech signals at lower rates than 9.6 Kb/s. Therefore, coders which produce good quality speech at bit rates 8 to 4.8 Kb/s have been designed and simulated. As well as designing coders to operate at rates below 32 Kb/s. it is very important to test them. Coders operating at 32 Kb/s and above contain only quantization noise and usually have large signal to noise ratios (SNR). For this reason their SNR's may be used for comparison of the coders. However, for the coders operating at 16 Kb/s and below this is not so and hence subjective testing is necessary for true comparison of the coders. The final part of this work deals with the subjective testing of 6 coders, three at 16 Kb/s and the other three at 9.6 Kb/s.

Ultrasound is a well-established tool for medical imaging. It is non-invasive and relatively inexpensive, but the severe attenuation caused by propagation through tissue limits its effectiveness for deep imaging. In recent years, the ready availability of fast, inexpensive computer hardware has facilitated the adoption of signal coding and compression techniques to counteract the effects of attenuation. Despite widespread investigation of the topic, published opinions vary as to the relative suitability of discrete-phase-modulated and frequency-modulated (or continuous-phase-modulated) signals for ultrasonic imaging applications. This thesis compares the performance of discrete binary-phase coded pulses to that of frequency-modulated pulses at the higher imaging frequencies at which the effects of attenuation are most severe. The performance of linear and non-linear frequency modulated pulses with optimal side-lobe characteristics is compared to that of complementary binary-phase coded pulses by simulation and experiment. Binary-phase coded pulses are shown to be more robust to the affects of attenuation and non-ideal transducers. The comparatively poor performance of frequency-modulated pulses is explained in terms of the spectral characteristics of the signals and filters required to reduce side-lobes to levels acceptable for imaging purposes. In theory, complementary code sets like bi-phase Golay pairs offer optimum side-lobe performance at the expense of a reduction in frame rate. In practice, misalignment caused by motion in the medium can have a severe impact on imaging performance. A novel motioncompensated imaging algorithm designed to reduce the occurrence of motion artefacts and eliminate the reduction in frame-rate associated with complementary-coding is presented. This is initially applied to conventional sequential-scan B-mode imaging then adapted for use in synthetic aperture B-mode imaging. Simulation results are presented comparing the performance of the motion-compensated sequential-scan and synthetic aperture systems with that of simulated systems using uncoded and frequency-modulated excitation pulses.

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