A method and apparatus for enhancing modulation of certain speech sounds, such as trill sounds, are provided for radios which utilize digital vocoders. A digitized speech stream is sampled and the sampling is adjusted to determine, detect and enhance trill nulls in the digitized voice stream by one or more of: frame shifting the digitized speech input stream prior to vocoding, time expanding a digitized speech steam prior to vocoding, time compressing a digitized speech output stream after vocoding, and/or modulation enhancement and filtering of the a digitized speech output stream after vocoding.