Some hearing-impaired persons are completely loosing their ear sensitivity in the middle- and high-frequency bands. Consequently, there is a need to transpose the audio signal downward on the frequency scale in order to maintain the speech signal intelligible. This task can be achieved with the use of DSP circuitry provided with some algorithms which can match some special demands of the partially functioning hearing sense. A series of tests was organized, and their results are discussed in this paper.
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