tailieunhanh - Cochlear Implants: Fundamentals and Application - part 6

Như đã nói ở trên, formants nồng độ năng lượng tần số quan trọng cho intelligibility. Vocoder formant là một tiến bộ trong các hệ thống truyền thông như tốc độ dữ liệu ít hơn nhiều so với khi truyền một tín hiệu phát biểu chưa qua chế biến. Bài phát biểu lần đầu tiên được phân tích với một máy dò lên tiếng | Acoustic Models of Cochlear Implant Speech-Processing Strategies 397 quency is a product of the outputs from the filters. The optimal number of filters for channel vocoder and fixed filter strategies is discussed below. Formant Vocoders As discussed above formants are concentrations of frequency energy of importance for intelligibility. The formant vocoder was an advance in communication systems as the data rate was much less than when transmitting an unprocessed speech signal. Speech was first analyzed with a voiced unvoiced detector and a pitch detector measured the frequency of the glottal openings. Information about the frequencies and amplitudes of the formants was obtained from a bank of bandpass filters and envelope detectors. Formant vocoders were first developed by Lawrence 1953 Parametric Artificial Talker PAT and Fant and Martony 1962 Orator Verbis Electris OVE II . These were adequate for the transmission of vowels but for some consonants the resonances poles and antiresonances zeros needed to be specified. This formant vocoder required less bandwidth than the channel vocoder but was not used in communication systems because of the complexity of the circuitry. However it has been very useful for studies on speech perception Ainsworth 1976 . The design of a formant synthesizer is illustrated in Figure . The formant vocoder became the basis for the formant speech-processing strategies used with multiple-electrode stimulation discussed below and in Chapter 8. F1 A1 F2 A2 F0 SW F3 A3 F4 A4 FIGURE . Block diagram of a parallel-coupled formant synthesizer Reprinted from Ainsworth 1976 with permission. . 398 7. Speech Sound Processing Acoustic Representation of Electrical Stimulation An acoustic model to evaluate multiple-electrode speech-processing strategies was developed by Blamey et al 1984a b . The model used a pseudo-random white noise generator with the output fed through seven separate band-pass filters with center frequencies corresponding to .

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