A Note on Pitch-Synchronous Processing of Speech

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Abstract
Voiced portions of speech wave forms are quasi-periodic in nature. This property arises from repeated excitation of the slowly changing vocal cavities by glottal puffs. Knowledge of this periodicity affords a means for reducing the channel capacity necessary to transmit a speech signal. One proposal [H. W. Dudley, U. S. Patent No. 2,115,803, May 3, 1938] to this end involves eliminating N − 1 of every N pitch period before transmission, where N is chosen small enough so that variations important to the ear-cortex in the intraperiod wave form are preserved. Thus, provided consonants are similarly chopped, the channel capacity in bits/second can be reduced by 1/N. At the receiver, missing parts are restored by interpolating them in a predetermined way, for example, by simply repeating each of the received periods N − 1 times. Circuits for operating on monotone speech demonstrating this principle have been constructed. Experiments with N = 2 to 5 indicate that minor distortion, which appears as submultiples of the pitch harmonics, is thereby introduced. Consonants are little affected by the chopping. A similar scheme which involves eliminating portions of the wave form in each period was also investigated. Less satisfactory results were obtained here, since the intraperiod wave form carries the phonemic information. A magnetic tape demonstration illustrating these points will be given.

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