Abstract
The past two decades have witnessed rapid growth and development within the telecommunications industry. This has been primarily fuelled by the proliferation of digital mobile communication applications and services which have become commonplace and easily within the financial reach of businesses and the general public. Current research trends, involving integration and packetisation of voice, video and data channels into true multimedia communications, promise a similar technological revolution in the next decade. One of the key design issues of the new high quality multimedia services is a requirement for very high data rates. Whilst the available bandwidth in wire based terrestrial network is a relatively cheap and expandable resource, it becomes inherently limited in satellite or cellular radio systems. In order to accommodate ever growing numbers of subscribers whilst maintaining high quality and low operational costs, it is necessary to maximise spectral efficiency and reduce power consumption. This has given rise to the rapid development of signal compression techniques, which in the speech transmission domain are known as speech coding algorithms. The research carried out for this thesis has mainly focused on the design and development of low bit rate narrowband and wideband speech coding systems which utilise a variable rate approach in order to improve their perceptual quality and reduce their transmission rates. The algorithms subsequently developed are based on the existing vocoding schemes, whose rigid fixed rate structure is a major limitation to achieving higher quality and lower rates. The variable rate schemes utilise the time-varying characteristics of the speech signal which is classified according to the developed segmentation algorithms. Two main schemes were developed, a variable bit rate with an average as low as 1.35 kbps and a variable frame rate with an average of 2.1 kbps, both achieving or even surpassing the subjective quality of the existing vocoding standard at 4.15 kbps. Wideband speech exhibits characteristics which are not embodied within narrowband speech and which contribute to the superior perceived quality. A very high quality wideband vocoder operating at rates (fixed and variable) below 9 kbps is presented in this thesis, whereby particular attention is paid to preserving the information in higher frequencies in order to maximise the attainable quality.