1. Field of the Invention
The present invention relates to apparatus and method for speech signal processing for improving the intelligibility of a speech signal in a hearing aid or a public address system.
2. Description of the Prior Art
A speech signal making apparatus for processing a speech easier to perceive for the hard of hearing has been hitherto studied, and an example was disclosed by R. W. Guelke in "Consonant burst enhancement: A possible means to improve intelligibility for the hard of hearing," Journal of Rehabilitation Research and Development, Vol. 24, No. 4, fall 1987, pages 217-220.
In such a conventional apparatus for speech signal processing, first the input signal is entered into a gap detector, an envelope follower and a zero crossing detector. Next the gap detector, envelope follower, dlfferentiator, and zero crossing detector detect the burst of a stop consonant. Then a one-shot multivibrator produces pulses in a specific interval corresponding to the burst to an amplifier. Finally, the amplifier amplifies the input signal for the interval length of pulses produced by the one-shop multivibrator at a specific amplification factor.
In such a conventional constitution, it is difficult to detect the burst of a stop consonant, and it is particularly hard if noise is superposed. Further, only the stop consonant can be enhanced, and many other consonants cannot be emphasized. Yet, since the amplifying interval and amplification factor are constant, It is not possible to follow up changes.
Also hitherto, an apparatus and method for speech signal processing for making speech easier to perceive for the hard of hearing have been studied, and the present inventors previously disclosed an example in "Apparatus and method for speech signal processing." U.S. application Ser. No. 748,190, filed Aug. 20, 1991, now U.S. Pat. No. 5,278,910.
In such a speech signal processing apparatus, first the level measuring means measures the level of the input signal, and the coefficient calculating means finds the value, on the basis of the output of the level measuring means, which becomes a large value when the level of the input signal at a specific time is smaller than the levels before and after in time, and becomes a small value when larger than the levels before and after in time, then the output of the input signal delay means for delaying the input signal for compensating for the delay of processing and the output of the coefficient calculating means are multiplied by first multiplying means and produced.
In such a constitution, as the coefficient calculating means determines the value for suppressing the change of the level of the input signal on the basis of the level of the input signal determined by the level measuring means, a large memory capacity is required, and the hardware load increases and the processing delay is prolonged at the same time, and the response speed of the value for suppressing the level changes is delayed, and consonants may not be enhanced sufficiently. Furthermore, if the output of the coefficient calculating means is directly used, not only are the consonants is enhanced, but also the vowels are suppressed, whereby a natural sounding speech is not obtained.