Cepstrum-Based Harmonics-to-Noise Ratio Measurement in Voiced Speech

The estimation of the harmonics-to-noise ratio (HNR) in voiced speech provides an indication of the ratio between the periodic to aperiodic components of the signal. Time-domain methods for HNR estimation are problematic because of the difficulty of estimating the period markers for (pathological) voiced speech. Frequency-domain methods encounter the problem of estimating the noise level at harmonic locations. Cepstral techniques have been introduced to supply noise estimates at all frequency locations in the spectrum. A detailed description of cepstral processing is provided in order to motivate its use as a HNR estimator. The action of cepstral low-pass liftering and subsequent Fourier transformation is shown to be analogous to the action of a moving average filter. Based on this description, short-comings of two existing cepstral-based HNRs are illustrated and a new approach is introduced and shown to provide accurate HNR measurements for synthesised glottal and voiced speech waveforms.