The feasibility of using the formant analysis-synthesis approach to replace the voicing sources of esophageal speech was explored. Using inverse-filtered signals extracted from normal speakers provided the voicing sources. Pitch extraction was tested with various pitch extraction methods, and then a computationally simple, band-limited auto-correlation method was chosen. To accomplish stable and practical speech enhancement, the input signal was divided into low-and highfrequency channels, then only the low-frequency channel was processed by the formant analysissynthesis method. A special purpose DSP-hardware unit was designed to perform the proposed analysis-synthesis process in real-time. Subjective evaluation tests (rating scale method) have been made with seven well-trained esophageal speakers and three speech therapists. Results of the subjective test showed that the synthesized speech was significantly improved, especially in cases of

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