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The Intelligibility of Speech Transposed Downward in Frequency by One Octave

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A method is described for transposing speech down by one octave in real time. Speech is filtered so as to suppress all formant components except the second formant and then clipped; the resulting signal is highly intelligible. Transposition to lower frequencies to accomplished by dividing this clipped signal by two, using a simple digital divide-by-two technique. A modulator is used to reimpose the envelope of the filtered speech onto the constant amplitude transposed signal. Finally, a lowpass filter is used to prevent high-frequency distortion products from reaching the output. Scores on listening tests after training average 77%. The potential use of such a device in aiding the partially deaf is discussed.

JAES Volume 18 Issue 1 pp. 56-62; February 1970
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AES - Audio Engineering Society