Acoustic parameters measured by a formant‐estimating speech processor for a multiple‐channel cochlear implant

In order to assess the limitations imposed on a cochlear implant system by a wearable speech processor, the parameters extracted from a set of 11 vowels and 24 consonants were examined. An estimate of the fundamental frequency EF0 was derived from the zero crossings of the low‐pass filtered envelope of the waveform. Estimates of the first and second formant frequencies EF1 and EF2 were derived from the zero crossings of the waveform, which was filtered in the ranges 300–1000 and 800–4000 Hz. Estimates of the formant amplitudes EA1 and EA2 were derived by peak detectors operating on the outputs of the same filters. For vowels, these parameters corresponded well to the first and second formants and gave sufficient information to identify each vowel. For consonants, the relative levels and onset times of EA1 and EA2 and the EF0 values gave cues to voicing. The variation in time of EA1, EA2, EF1, and EF2 gave cues to the manner of articulation. Cues to the place of articulation were given by EF1 and EF2. When pink noise was added, the parameters were gradually degraded as the signal‐to‐noise ratio decreased. Consonants were affected more than vowels, and EF2 was affected more than EF1. Results for three good patients using a speech processor that coded EF0 as an electric pulse rate, EF1 and EF2 as electrode positions, and EA1 and EA2 as electric current levels confirmed that the parameters were useful for recognition of vowels and consonants. Average scores were 76% for recognition of 11 vowels and 71% for 12 consonants in the hearing‐alone condition. The error rates were 4% for voicing, 12% for manner, and 25% for place.