Digital Speech: Coding for Low Bit Rate Communication Systems, 2nd Edition

6

Pitch Estimation and Voiced – Unvoiced Classification of Speech

6.1 Introduction

Low bit-rate speech coders, traditionally called vocoders, rely heavily on extracting the correct speech parameters from a given speech segment. The three main speech features are the spectral envelope, the pitch and the voiced–unvoiced classification. The spectral envelope is usually extracted by a standard autocorrelation method which results in a linear predictive (LP) parameters representation. However extracting the correct pitch and voicing classification is not as straightforward and may require a combination of methods.

When measuring the pitch, it is assumed that the voiced signals are formed by passing quasi-periodic excitation signals through the LPC filter. The duration between the pulses in the excitation signal is called the pitch period T₀ or fundamental frequency f₀. Correct estimation of the pitch is essential for good quality speech-coding. Incorrect estimation of the pitch period can seriously degrade the quality of synthesized speech. Pitch determination algorithms (PDAs) have been studied in both the time and frequency domains, and a comparison is discussed in [1, 2]. Traditionally, autocorrelation-based methods [3] and their variants [4, 5] have been intensively investigated and widely applied to various speech coders [6–11]. Frequency domain approaches [12–14], on the other hand, have become popular recently due to the growing interest in sinusoidal speech coders, such as the ...