Quality improvement of bandlimited speech by adding higher‐order harmonic components produced by nonlinear circuit

Abstract

In this paper, the system to improve the quality of the bandlimited speech (0.3–3.4 kHz) through a telephone channel by adding the higher order harmonic components (3.4–7.0 kHz) produced by the simple nonlinear circuit is described. The restoration of the bandlimited speech is made based on the fact that most of the voiced sounds have periodicity. At the receive side, the higher‐order harmonic components are produced from the transmitted bandlimited speech by the nonlinear circuit and are added to the bandlimited speech with shaping in tho frequency domain. This nonlinear circuit operates in the way that the periodicity of speech is preserved and the output level is directly proportional to the input level. This system mainly improves the naturalness of the bandlimited speech. The subjective evaluation test using the samples of the speech announced in Japanese proves that improvement of scale value 0.5 by the method of paired comparison with five quality categories is obtained. This system also will be applied to the quality improvement of bandlimited music.