Echo Control Techniques in the Telephone Network

M.Eng. Project, October 1979

Supervisors: L. Mason and P. Kabal

Conventional echo suppressors offer inadequate performance on circuits exhibiting long propagation delay and/or low echo return loss (ERL), due to an increased incidence of double-talk and annoying echo. Echo cancellation is an alternative technique, with the potential of overcoming the echo problem on low ERL circuits and maintaining echo control during double talk. This report describes the design of an echo canceller with an adaptive filter. A description of the echo control problems encountered in telephone network is first made, then the echo suppressor device is described and its disadvantages mentioned. Finally, the description of an echo canceller is made. The proposed device uses nonuniform coding which reduces its cost and size. In addition, it works well not only during double talk but also during phase roll, which are among the most adverse circuits conditions an echo canceller may encounter. As an important fact, when satellite trunks are equipped with echo cancellers, the quality of the circuit equals that of domestic terrestrial circuits.

Adaptive Transform Coding of Speech

M.Eng. Thesis, July 1979

Supervisor: P. Kabal

This thesis investigates adaptive transform coding of speech signals. The thesis addresses the problem of adaptive quantization of the transform coefficients. An all-pole estimate of the signal energy spectrum results in a quantization strategy which outperforms previously reported techniques. Additional perceptual improvements are obtained by pre-emphasizing the input signal to better reproduce high frequency formants and by windowing the input to reduce block boundary discontinuities. Results from computer simulations of this coding technique are presented. At 16 kb/sec the proposed scheme yields high quality speech. At the rate of 9.6 kb/sec, the coded speech is completely intelligible but contains a slight warbling sound.

A Baseband Data Transmission System Using Delay Modulation Coding

M.Eng. Project, 1979

Supervisor: P. Kabal

Fractional Tap Spacing Equalizers for Data Transmission

M.Eng. Thesis, March 1979

Supervisor: P. Kabal

This thesis presents a study of the theory of convectional
and Fractional Tap Spacing Equalizers and outlines their relative benefits and
drawbacks. Two special cases of Fractional Tap Spacing Equalizers are emphasized
in this work: the *T*/2-Tap Spacing Equalizer and a new type of equalizer,
called a Hybrid Transversal Equalizer, in which the tap spacing is either *T*
or *T*/2 (where 1/*T* is the data source symbols rate). A mathematical
analysis of these equalizers is carried out and some new results are derived. To
support the mathematical analysis, a computer program was used to compare the
performance of these models of equalizers and the results are analysed.

Optimal Quantizers in Linear Predictive Encoding of Speech

M.Eng. Thesis, March 1978

Supervisor: P. Kabal

Thee have been many attempts in the past to reduce the transmission rate for a digital representation of a speech waveform. One technique for achieving this goal is a parametric representation using linear prediction, in which the parameters of that model are quantized before being transmitted. The purpose of this thesis is to study the effects of quantization. First, linear prediction methods in analysis, pitch extraction and synthesis are reviewed. Different distance measures and fidelity criteria are introduced. Then, for the reflection coefficients of linear prediction, schemes like inverse sine quantization and one which minimizes the expected spectral deviation bound, are discussed in detail. Finally, because these coefficients are mutually dependent, a decorrelation procedure is applied, and for the set of parameters obtained in this way, a quantization method which minimizes the expected spectral deviation bound is then derived and compared to the above mentioned schemes.

Thesis titles.