Enhancement of electrolaryngeal speech by spectral subtraction, spectral compensation, and introduction of jitter and shimmer
Prem C. Pandey, S. Khadar Basha · 2010
An electrolarynx, a verbal communication aid used by laryngectomy patients, is a vibrator held against the neck tissue to provide excitation to the vocal tract, as a substitute to that provided by the glottal vibrations. Although the user can set the vibration level and pitch, a dynamic control of level, voicing, and pitch during speech production is not feasible. In addition to this basic limitation, the electrolaryngeal speech suffers from (i) presence of background noise caused by leakage of acoustic energy from the vibrator and vibrator-tissue interface, (ii) low-frequency spectral deficiency, and (iii) unnatural quality due to constant pitch and level. Background noise decreases the intelligibility, while the other two factors affect the speech quality. Present study involved investigations for improving the intelligibility and quality of electrolaryngeal speech. Pitch-synchronous application of generalized spectral subtraction was used for reducing the background noise. In order to track the variation in the spectrum of the leakage noise due to changes in vibrator orientation and pressure during speech production, a dynamic estimation of noise was carried out from a set of past frames. The estimated noise spectrum was subtracted from that of the noisy speech and the resulting magnitude spectrum was combined with the original phase spectrum. The speech signal was resynthesized using overlap-add method, with two-pitch period analysis frames and one period overlap. Estimation of phase spectrum by minimum-phase assumption and the assumption of phase continuity did not improve the speech quality. An introduction of jitter and shimmer in the speech signal, using LPC based analysis-synthesis, was investigated for improving its naturalness. The excitation for synthesis was an impulse train with the frequency equal to that of the vibrator, with random frequency and amplitude modulations for providing the jitter and the shimmer, respectively. An FIR filtering of the excitation was used to match the long-term average spectral envelope of the processed electrolaryngeal speech to that of the normal speech. A peak-to-peak jitter of up to 6 % increased the naturalness, while introduction of shimmer decreased the quality.