Automatic equalisation of TV telephone channels
Creators: name: Lee, TT
Publisher: name: University Of Tasmania
Year: 2023

description: One possible way of implementing the TV telephone system is by transmitting the video signal over existing telephone cables. Not only is this attractive from an economical standpoint, it also appears to be technically reasonable. However, there are certain basic problems to be considered, particularly in reference to baseband analogue transmission over the local subscriber area. For a start, it is expected that fixed equalisers will be required at regular intervals along the line to compensate for attenuation losses. These equalisers will have to be designed on the basis of some average channel characteristic. As a result, there will be uncorrected losses owing to deviations from the average characteristic for the different TV telephone connections. These and other channel imperfections will have to be compensated automatically. This thesis is concerned with the design and testing of an automatic equaliser to serve the above-mentioned purpose. Chapter One outlines the basic principles of operation, together with the performance criterion used to assess the degree of optimality achieved. The mode of operation is discussed, as well as the different possible choices and arrangements of the basic equaliser function. To provide further insight into the process, a mathematical study is given in Chapter Two. This includes considering the control algorithm for tap adjustments, the conditions for convergence, and the choice of step size to be used. The analysis is extended to cover adaptive operations, where the information for updating the tap settings is not exactly available but must be estimated on-line. In addition, ways of improving performance through modifying the basic equaliser structure (such as using a feedback configuration instead of the conventional feedforward arrangement) are also considered. The theoretical investigation is carried further in Chapter Three. The effect of noise on equaliser capability and performance is discussed, along with the need for a defined standard of signal-to-noise ratio for acceptable transmission. This is followed by an evaluation of the effect of having a non-linearity in the feedback loop. The resulting system is studied, using some form of linearised analysis. Some reference to stability requirements is also given. In order to confirm some of the theoretical findings of the previous chapters, and to demonstrate the potential of automatic equalisation in practice, a real-time experimental system was designed and assembled. Chapter Four outlines the work involved and the problems encountered. The choice of suitable test signals and their method of generation are discussed in some detail. After the experimental equaliser had been built, various tests were conducted, using 500 yards of 4 lb P.I.U.T. cable to represent the TV telephone channel. The tests were designed to provide an estimate of some of the critical parameters and an assessment of system capability. The nature of the tests, with the results obtained, are given in Chapter Five. The implications of the test results are discussed in Chapter Six. The areas requiring further investigation are identified. A preliminary discussion of the problems involved in integrating the system into the existing telephone network is also included. The conclusion summarises the state of progress in evaluating the practicability of the automatic mean square equaliser for TV telephone applications.