Digital Hearing Aids Frequency

• In the past decade digital HA research and development has resulted in a number of improvements in clarity and S/N ratio (in certain environments). Most of current ASP (automatic signal processing) devices are still limited however by the number of frequency bands; thus are generally only effective in the presence of low frequency competition. All but Ensoniq have limited their approach to a few distinct processing channels . We should note when considering masking by noise, it is well accepted that the frequency range of the human auditory system is divided into 24 discreet channels, or critical bands (for each ear) [20] and each cochlear neuron responds to a narrow range of frequency stimuli [21]. It would seem appropriate then to at least separate the frequency range into like divisions. Philips has recently developed the Digital Compact Cassette (DCC), which utilizes 32-band digital processing for perceptual coding of audio signals [27]. This technology could be very easily adopted to make 32-band HA's. However, DCC will not be accepted as a new recording format, since recordable optical discs will be released soon to the consumer market.

• We still lack sufficient understanding as to the nature of many hearing impairment problems and their relationship to one another [2,5,29].Generally hearing problems are divided into: conductive, cochlear, eighth nerve and central nervous system disorders [2]. However the number of the distinctive disorders may reach easily into the hundreds [2], and each may require a specific signal processing algorithm [2,24]. Further research is still needed to explore the usefulness of compression systems [23]. Information theory can be applied to calculate inherent channel capacity for the ear [28]. On the basis of this theory analysis of a hearing impaired communication channel could be performed and the most appropriate information coding obtained.

• Digital Signal Processing workstations (for example NeXT computer) should be used to perform further psychoacoustic tests in order to learn more about the human auditory system. Also a Digital Master HA can be simulated on these types of computers and used to design and check different DSP strategies to be used in HA's.

• The complexity of processing which is needed to address many hearing disorders requires highly sophisticated signal processing. DSP offers substantial improvements over analog techniques [30] along with unmatched flexibility and precision to adopt the processing to individual requirements of each patient [2]. Also the paired-comparison judgment technique may be used more effectively with this technology for precise HA fitting [22].

• DSP should complement rather then substitute for signal processing which is performed in the auditory system (in other words it should be transparent when not needed). This will allow the best signal processor so far- the human brain - to extract information most efficiently [18].

• The following DSP techniques could be used in future HA's : arbitrary filtering and frequency shaping, arbitrary gain (as function of frequency and signal amplitude), frequency shifting, feedback control, noise reduction (various techniques), peak clipping or limiting etc [24,30]. Also multichannel, parallel processing can be done with DSP improving speed and sophistication of sound processing. "Smart HA's" with adaptive algorithms and performing logical operations can be build around DSP technology to further improve HA's capabilities.

  In our opinion DSP is still an underexplored technology in the area of HA's, but this may change in the near future with anticipated benefits to the hearing impaired.

  HA's however sophisticated never would be a panacea for hearing impairment. Hearing impairment reduces information channel capacity (from outside world to auditory system) and this can't be restored with a hearing aid. HA's can only help to better utilize the remaining information channel capacity.