New Correlation Vocoder

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Abstract
A correlation vocoder is described that uses delay lines composed of simple all-pass RC or RLC sections. The synthesizer of the correlation vocoder is represented as a time-variable filter whose response is controlled by the imagechannel signalsimage derived in the analyzer. It is shown that the all-pass delay line, having a phase delay that decreases with increasing frequency, gives the vocoder a spectral resolution better adapted to the properties of speech than does a delay line with constant delay. In particular, when RLC sections are used, the spectral resolution approximates that of a spectrum-channel vocoder. Results of a quality test are given. An imageopinion scaleimage was used that rates the subjective quality between zero (bad) and four (excellent). An experimental RLC vocoder received a rating of 1.35 for male speech as compared to 1.55 for a typical spectrum-channel vocoder and 1.75 for ordinary telephone speech.

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