Generation of wideband speech from bandlimited speech by use of an autocorrelation function

Abstract

A high quality of speech sound is requested in speech communication circuits. This requirement, however, is not satisfied in telephone and mobile communication circuits. This paper proposes a new method to generate a lower speech signal from bandlimited speech (300–3400 Hz). A short-time autocorrelation function (ACF) is calculated on a speech waveform where the amplitude is compressed with a square root. A period is determined from the ACF. The period of the ACF is employed as an output signal. This signal is low-pass filtered and added to the original bandlimited signal. On the other hand, the higher band signal is synthesized by a pitch excitation formant synthesizer with fixed formant frequencies. The synthesized signal is filtered with BPF (3.4–7 kHz) and added to the original. Those processes are repeated with every pitch period. Short sentences uttered by two males and two females are assessed with a preference test. Synthesized speech is compared with bandlimited natural speech where the lowest frequency is varied between 0 and 300 Hz. The hearing test reveals that the proposed method provides a speech signal that is equivalent to the bandlimited signal where the limit of low frequency is 100–200 Hz.