Study of perceptual balance for designing comb filters for binaural dichotic presentation
Pandurangarao N. Kulkarni, Prem C. Pandey, Dakshayani S. Jangamashetti · 2010
Earlier studies on binaural dichotic presentation by spectral splitting of speech signal using a pair of complementary comb filters, for improving speech perception by persons with moderate bilateral sensorineural hearing loss, have shown mixed results: from no advantage to improvements in recognition scores corresponding to an SNR advantage of 2 - 9 dB. The filters used in these studies had different bandwidths and realizations. For an optimal performance of the scheme, the perceived loudness of different spectral components in the speech signal should be balanced, especially for components in transition bands which get presented to both the ears. For selecting magnitude responses of such filters, we have investigated the relationship between the signal amplitudes for binaural presentation of a tone to evoke the same loudness as that of monaural presentation. Listening tests were conducted, on eight normal-hearing subjects, for comparing the perceived loudness of monaural presentations to that of the binaural presentation with different combination of amplitudes for the tones presented to the left and right ears, at 250 Hz, 500 Hz, 1 kHz, and 2 kHz. The sum of the amplitudes of the left and right tones in binaural presentation being equal to that of the monaural tone resulted in monaural-binaural loudness match, indicating that the magnitude response of the comb filters used for dichotic presentation should be complementary on a linear scale. An analysis of the magnitude responses of the comb filters used in earlier studies showed large deviations from the perceptual balance requirement, and those with smaller deviations were more effective in improving speech perception. A pair of comb filters, based on auditory critical bandwidths and magnitude responses closely satisfying the requirement for perceptual balance, was designed as 512-coefficient linear phase FIR filters for sampling frequency of 10 kHz. Listening tests on six normal-hearing subjects showed improvements in the consonant recognition scores corresponding to an SNR advantage of approximately 12 dB. Tests using 11 subjects with moderate bilateral sensorineural hearing loss showed an improvement in the recognition score in the range 14 - 31 %. Thus the investigations showed that binaural dichotic presentation using comb filters designed for perceptual balance resulted in better speech perception.