| 摘要 |
PROBLEM TO BE SOLVED: To improve the sound quality of signals with a limited transmissible and reproducible peak frequency. SOLUTION: A high-pass filter 15 extracts a high frequency signal Si of an input audio signal. The absolute value signal Sl of a signal Sj obtained, by differentiating the high frequency signal Si is detected by a level detector 17b. A signal Sd is formed to instantaneously respond to the level increase of the signal Sl and to be gradually lowered, in response to its level decrease. The level of a high frequency random noise is controlled by this signal Sd. The ratio of the signal Sd and a signal Sm for which level control is performed to the absolute value of the signal Si is detected by a divider 19. Through this ratio, the characteristics of a variable filter are controlled. The frequency characteristics of the high frequency random noise are controlled by the variable filter, its level is controlled by the signal Sd, and as a result, an out-of-band high frequency random noise having a level and frequency characteristics corresponding to the high frequency signal Si is generated and is added to the input audio signal. |
