Study on Perceptibility and Measuring Method of Nonlinear Distortion

Abstract

The conventional measuring method of nonlinear distortion using one or two tones of a fixed level does not always provide an objective value of distortion which agrees with the subjective sound quality of speech or music signal passing through the apparatus or the system to be measured. To find a better agreement, the distortion produced in the actual performance of the system should be obtained, and the distortion components appearing in various frequency bands should be extracted separately. A measuring method that meets these requirements is proposed in this paper. Fig. 1 shows the principle. As a test signal, a wide-band signal whose spectrum is similar to that of speech or music is used. The test signal is applied to the system to be measured after removing the narrow band components with band-elimination filter. Distortion components that appear in the eliminated band are extracted with an appropriate band-pass filter having the same frequency as that of the band-elimination filter. In this way the distortion component appearing in a specified frequency band is extracted, and the whole spectrum of distortion is also obtained by using pairs of filters of various central frequencies. Perceptibility of nonlinear distortion for three kinds of typical program sound was obtained as a function of frequency of distortion components using a distortion circuit having an input-output characteristics as shown in Fig. 6. The results are shown in Fig. 11, from which it is clearly seen that the distortion appearing in the high frequency band is much noticeable. Results of this measurement and a few additional experiments were discussed from the viewpoint of masking, and it was shown that the perceptibility of distortion components in a specified frequency band is mainly determined by the signal level in the same and the neibouring frequency bands, and that the just perceptible level of distortion is almost constant when the level is given relative to the signal level of the same frequency band as far as a fixed program signal is concerned.