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Spectrum Flattening in Vocoders

IP.com Disclosure Number: IPCOM000074337D
Original Publication Date: 1971-Apr-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Buron, R: AUTHOR

Abstract

In a conventional base-band vocoder synthesizer, the speech quality is improved by the so called 'spectrum flattening" feature. This is performed by using on each of the excitation channels a premodulation network made of a band-pass filter BPFi followed by a clipper CLi, L=1, 2..., n with generally n approx.= 15. The clipping operation generates odd harmonics which could fall within another channel bandwidth, and, therefore, could distort the output signal provided by the summing in stage Sigma. These harmonics must be removed. In conventional synthesizers the removal is obtained through use of a second band-pass filter on each channel (post-modulation network).

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Spectrum Flattening in Vocoders

In a conventional base-band vocoder synthesizer, the speech quality is improved by the so called 'spectrum flattening" feature. This is performed by using on each of the excitation channels a premodulation network made of a band-pass filter BPFi followed by a clipper CLi, L=1, 2..., n with generally n approx.= 15.

The clipping operation generates odd harmonics which could fall within another channel bandwidth, and, therefore, could distort the output signal provided by the summing in stage Sigma. These harmonics must be removed. In conventional synthesizers the removal is obtained through use of a second band-pass filter on each channel (post-modulation network).

In order to facilitate a digital implementation of the synthesizer, it is of high interest to remove the post-modulation filters. A solution is provided through use of a frequency shifting operation. A carrier frequency Fo is used to modulate the excitation signal in stage M1 before driving the premodulation filters. The frequency bandwidth of each BPFi is chosen such that the lower sideband generated by modulator M1 is removed. In other words the excitation frequency spectrum is shifted towards the upper frequency. Therefore, the odd harmonics generated by the clippers are also shifted the same way. The frequency Fo is such that these shifted harmonics fall outside the bandwidth of the vocoder channels and, therefore, cannot interfere with a useful signal provided by any ...