Multimedia Signal Processing Laboratory

P. Kabal

Paper Abstracts 1998


A. K. Khandani and P. Kabal

"Optimization of a Lattice-Based Constellation for Signaling Over a Partial Response Channel", IEEE Trans. Commun., vol. 46, no. 7, pp. 854-856, July 1998.

This letter discusses the problem of optimizing a lattice-based signal constellation for signaling over a partial response channel. The objective is to minimize the probability of error which is determined by the combined effects of the additive Gaussian noise and channel memory.

Conference papers

N. Sheikholeslami and P. Kabal

"A Family of Nyquist Filters Based on Generalized Raised-Cosine Spectra", Proc. Biennial Symp. Commun. (Kingston, Ont.), pp. 131-135, June 1998.

Data transmission over bandlimited channels requires pulse shaping to eliminate or control Inter-Symbol Interference (ISI). A widely used filter for this purpose is the raised-cosine filter which satisfies Nyquist's first criterion. We design a phase compensator so that the square-root raised-cosine filter also satisfies Nyquist's first criterion. Such a technique is particularly useful to accommodate two different structures for the receiver, one with a filter matched to the transmitting filter and one without any matched filter. In the case of a raised-cosine spectrum with full excess bandwidth, we show that the phase compensator corresponds to a pure time delay.

We also extend the raised-cosine spectra to a more general family of Nyquist filters which their compensated square-root spectra satisfy Nyquist's criterion. In practice, it is important that the transmitting and receiving filters be well approximated with short impulse responses. From this point of view, the raised-cosine spectrum is not necessarily the best choice for Nyquist filter design. A new family of Nyquist pulses are designed such that they have faster asymptotic decay compared to the raised-cosine filter impulse response.

C. Papacostantinou and P. Kabal

"Improved Pitch Modeling for Low Bit-Rate Speech Coders", Proc. Biennial Symp. Commun. (Kingston, Ont.), pp. 3-7, June 1998.

The poor modeling of the pitch pulse waveforms during voiced speech contributes to the degradation of CELP coded speech. This becomes more vivid with the reduction of the number of pulses and other constraints imposed on the fixed codebook excitation. We have developed a Pitch Pulse Averaging (PPA) algorithm to enhance the periodicity of such segments, where during steady state voicing the pitch pulse waveforms in the excitation signal evolve slowly in time. The PPA algorithm extracts a number of such pitch pulse waveforms from the past excitation, aligns them, and then averages them to produce a new pitch pulse waveform with reduced quantization noise. We have simulated and tested our algorithm on a floating point C-simulation of the G.729 8 kbps CS-ACELP coder. The results we present verify that the algorithm has generally improved the periodicity of voiced segments by reducing the average of the weighted mean-squared error.

Other publications

K. El-Maleh and P. Kabal

"Frame-Level Noise Classification in Mobile Environments", Document TD 15-E (WP3/12), ITU-T Study Group 12, Question 17 ("Noise Aspects in Evolving Networks"), Nov. 1998.

Background environmental noises degrade the performance of speech-processing systems (e.g. speech coding, speech recognition). By modifying the processing according to the type of back-ground noise, the performance can be enhanced. This requires noise classification. In this work, four pattern-recognition frameworks have been used to design noise classification algorithms. Classification is done on a frame-by-frame basis (e.g. once every 20 ms). Five commonly encountered noises in mobile telephony (i.e. car, street, babble, factory, and bus) have been considered in our study. Our experimental results show that the Line Spectral Frequencies (LSFs), derived from the linear prediction coefficients, are robust features in distinguishing different classes of noises.

Paper titles.