Limitations of bandwidth compression hearing aids applied to the voiced portion of speech

Numerous speech processing techniques have been applied to assist hearing-impaired subjects with extreme high-frequency hearing losses who can be helped only to a limited degree with conventional hearing aids. The results of providing this class of deaf subjects with a speech encoding hearing aid, which is able to reproduce intelligible speech for their particular needs, have generally been disappointing. There are at least four problems related to bandwidth compression applied to the voiced portion of speech: (1) the problem of pitch extraction in real time; (2) pitch extraction under realistic listening conditions, i.e. when competing speech and noise sources are present; (3) an insufficient data base for successful compression of voiced speech; and (4) the introduction of undesirable spectral energies in the bandwidth-compressed signal, due to the compression process itself. Experiments seem to indicate that voiced speech segments bandwidth limited to f = 1000 Hz, even at a loss of higher formant frequencies, is in most instances superior in intelligibility compared to bandwidth-compressed voiced speech segments of the same bandwidth, even if pitch can be extracted with no error. With the added complexity of real-time pitch extraction which has to function in actual listening conditions, it is doubtful that a speech encoding hearing aid, based on bandwidth compression on the voiced portion of speech, could be successfully implemented. However, if bandwidth compression is applied to the unvoiced portions of speech only, the above limitations can be overcome (1).