Automatic equalisation of TV telephone channels

摘要

One possible way of implementing the TV telephone systemudis by transmitting the video signal over existing telephoneudcables. Not only is this attractive from an economical standpoint,udit also appears to be technically reasonable. However, there areudcertain basic problems to be considered, particularly in referenceudto baseband analogue transmission over the local subscriberudarea.udFor a start, it is expected that fixed equalisers willudbe required at regular intervals along the line to compensateudfor attenuation losses. These equalisers will have to beuddesigned on the basis of some average channel characteristic.udAs a result, there will be uncorrected losses owing to deviationsudfrom the average characteristic for the different TV telephoneudconnections. These and other channel imperfections will haveudto be compensated automatically.udThis thesis is concerned with the design and testing ofudan automatic equaliser to serve the above-mentioned purpose.udChapter One outlines the basic principles of operation, togetherudwith the performance criterion used to assess the degree ofudoptimality achieved. The mode of operation is discussed, asudwell as the different possible choices and arrangements of theudbasic equaliser function.udTo provide further insight into the process, a mathematicaludstudy is given in Chapter Two. This includes considering theudcontrol algorithm for tap adjustments, the conditions forudconvergence, and the choice of step size to be used. The analysisudis extended to cover adaptive operations, where the informationudfor updating the tap settings is not exactly available but mustudbe estimated on-line. In addition, ways of improving performanceudthrough modifying the basic equaliser structure (such as usinguda feedback configuration instead of the conventional feedforwardudarrangement) are also considered. udThe theoretical investigation is carried further inudChapter Three. The effect of noise on equaliser capability and performance is discussed, along with the need for auddefined standard of signal-to-noise ratio for acceptableudtransmission. This is followed by an evaluation of theudeffect of having a non-linearity in the feedback loop.udThe resulting system is studied, using some form of linearisedudanalysis. Some reference to stability requirements is alsoudgiven.udIn order to confirm some of the theoretical findingsudof the previous chapters, and to demonstrate the potentialudof automatic equalisation in practice, a real-timeudexperimental system was designed and assembled. ChapterudFour outlines the work involved and the problems encountered.udThe choice of suitable test signals and their method ofudgeneration are discussed in some detail.udAfter the experimental equaliser had been built,udvarious tests were conducted, using 500 yards of 4 lb P.I.U.T.udcable to represent the TV telephone channel. The testsudwere designed to provide an estimate of some of the criticaludparameters and an assessment of system capability. Theudnature of the tests, with the results obtained, are givenudin Chapter Five.udThe implications of the test results are discussed inudChapter Six. The areas requiring further investigationudare identified. A preliminary discussion of the problemsudinvolved in integrating the system into the existingudtelephone network is also included.udThe conclusion summarises the state of progress inudevaluating the practicability of the automatic mean squareudequaliser for TV telephone applications.