US Patent 6,577,739
United States Patent and Trademark Office
University of Iowa Research Foundation (Iowa City, IA, US)
Apparatus and methods for audio compression and frequency shifting retain the spectral shape of an audio input signal while compressing and shifting its frequency. The fast Fourier transform of the input signal is generated, to allow processing in the frequency domain. The input audio signal is divided into small time segments, and each is subjected to frequency analysis. Frequency processing includes compression and optional frequency shifting. The inverse fast Fourier transform function is performed on the compressed and frequency shifted spectrum, to compose an output audio signal, equal in duration to the original signal. The output signal is then provided to the listener with appropriate amplification to insure audible speech across the usable frequency range.
Hurtig, Richard Ray and Turner, Christopher William Apparatus and methods for proportional audio compression and frequency shifting. US Patent 6,577,739 filed September 16, 1998 and issued June 10, 2003. https://ir.uiowa.edu/patents/329