Phase manipulation of speech using FIR digital filters
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Three related investigations involving the fields of FIR digital filters, phase manipulation of speech, and speech coding via bandwidth compression are reported.
The first investigation is aimed at providing a means of generating the impulse response coefficients of a non-linear phase FIR digital filter. Existing methods of designing linear-phase filters are discussed and compared from a defined common comparison base.
The methods available for designing non-linear phase filters are examined. An existing linear phase design method is extended to the non-linear phase case and shown to be useful. The required impulse response length in the presence of non-linear phase is studied. Particular emphasis is placed on "random phase" filters and their generation because they are required by the second investigation.
The second investigation examines in detail the ramifications of phase randomising a speech signal. The analytic zero representation of speech which forms the underlying base on which the discussion, and answers, are based is elucidated. The technique of using a non-linear phase FIR filter is shown to be feasible and as a minimum, offers at least the same level of performance as a very early reported technique. Significant differences in the behaviour of male and female speech is demonstrated.
The third and final investigation reports some early and incomplete experiments on a radically different approach to achieving band width compression and expansion of a signal. The technique is referred to as "phase unwrapping". It is based on the application of a linear phase FIR digital filter in an adaptation of the traditional convolution relation. The motivation and validity of the basic idea is outlined and justified via application of the procedure to simple sinusoids and one experiment using real speech. The fundamental problem to be overcome is identified and the basis of a possible means of solution indicated.