the Symbion/Richards implant [8]. The adequacy of this simulation of auditory frequency analysis is limited by the small number of electrodes which are used, ...
SIGNAL PROCESSING STRATEGIES IN CURRENT USE FOR COCHLEAR IMPLANTS.
Andrew Faulkner, D. Phil *
The provision of useful information about speech and other important acoustic events through the electrical stimulation of the cochlea is a complex task. Present methods are sufficiently good to be of real use, but still leave much to be desired. The design of systems for this purpose depends on four areas of knowledge, relating to the function of the normal auditory system, the receptive potential of the residual capacities of the profoundly impaired ear, the informational basis of speech communication, and an understanding of applicable signal processing technologies.
Although electrical stimulation of the auditory nerve produces auditory sensations, there are many important ways in which these sensations differ from those produced by acoustic stimulation of the normal ear. In particular, the information capacity of the electrically stimulated impaired ear is very substantially reduced. The dynamic range is extremely limited, often to only 10 to 15 dB. Frequency range is also very much restricted, especially in systems using extra-cochlear electrodes. Here many patients cannot detect stimulation at rates above a few hundred Hz because the levels of stimulation required cause the spread of current to other nerves which can give rise to pain or other undesirable effects. With both extra- and intra-cochlear electrodes, most patients have a very limited ability to distinguish different stimulating frequencies above 400 Hz, whereas lower frequencies are generally adequately distinguished. Similarly, the potential for providing an multi-channel electro-cochlear system to simulate auditory frequency analysis is very limited compared to the normal ear. In normal ears, frequency analysis over even the limited audio bandwidth used in telephony requires the equivalent of 19 auditory filter channels [1]. Even with multiple intra-cochlear electrode arrays, often only one or two channels can effectively be used simultaneously over this same frequency range [2].
There have been a number of different signal processing strategies proposed for electrocochlear stimulation. Here, these are grouped under three headings according to the approach taken to the problem of matching information to the impaired ear. These are a) single-channel compressed and equalized whole-speech systems, b) multiple-channel filter-bank systems which present compressed and equalized whole-speech, and c) speech pattern or feature extracting systems. These different approaches to signal processing will be discussed in relation to the speech pattern information which needs to be transmitted to the user to allow the perception of different classes of speech sounds. The range and function of speech pattern information is broadly outlined in table I. Those speech pattern elements which are not readily visible, that is, intensity, larynx frequency, and aperiodic excitation, are of particular importance in supplementing lipreading, and for almost all implant users, it is this which is of the greatest utility in speech communication [3]. These three elements cannot, however, by themselves support the understanding of speech in the absence of lipreading, which requires in addition the reception of spectral information.
The first and simplest approach is used, for example, in classic single-channel devices such as those developed by House in the USA [4] and by Hochmair and Hochmair-Desoyer in * Department of Phonetics and Linguistics, University College London. 3/1
Vienna [5], and is currently employed in the UCH/RNID implant [6]. Here, the main concern is to provide an electrical stimulus which is audible, and which is suited to the frequency and
intensity range over which the patient can comfortably hear sound. Although the temporal fine-structure of the speech signal, which is related to the speech spectral envelope, is preserved by these systems, users are not generally able to use this information in speech
perception. These systems are, therefore, capable only of transmitting information related to intensity, larynx frequency, and the presence of random excitation, and thus, they are principally of use in supplementing lipreading. A further limitation of these systems is that
these speech pattern elements are represented in a complex manner which makes considerable
demands on the temporal coding and analysis capacities of the ear, and users often have great difficulty in extracting larynx frequency and aperiodic elements from the whole-speech signal. Table L Important speech pattern elements. The speech contrasts which are underlined are those which are most strongly related to each element
The second, more complex, approach, is intended to simulate the processing which occurs in the peripheral auditory system, that is, frequency analysis and neural transaction. It is exemplified by a number of multi-channel devices, including the Project Ear system [7], and the Symbion/Richards implant [8]. The adequacy of this simulation of auditory frequency analysis is limited by the small number of electrodes which are used, and more importantly, by the spread of current between cochlear electrode sites. Nevertheless, in patients with better
residual auditory nerve function, and in quiet listening conditions, these systems can provide the full range of speech pattern elements, and many users are able, at least to some degree, to understand speech without the need to lipread.
The third type of design, exemplified by the UCL Microstim and VSP systems [9] and by Cochlear AG's Nucleus implant [10], is intended to simulate not only aspects of the peripheral auditory systems, but also to provide processing which simulates the pattern extraction processes used in the perception of speech. These systems are specifically adapted to speech, and are designed to simplify the signal so as to provide optimal transmission of essential speech information. Important aspects of speech information which are used include fundamental frequency, intensity patterns, and spectral patterns corresponding to the first and second formant frequencies, and to higher frequency spectral peaks. Where devices of this class extract and present the full range of speech pattern information, the results are generally comparable to those seen with filter-bank based whole-speech systems.
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There is still very substantial scope for improvements, particularly in multi-channel devices. The filter-bank based systems are likely to be considerably improved by new schemes for
rapidly sampled interleaved stimulation of multiple electrodes, which offers considerable improvements over present simultaneous stimulation schemes [11]. Developments of speech
pattern extracting devices are most likely to come from improved methods for the robust
extraction of important speech information [12,13], and from advances in our understanding
of the role of different elements in the perception of speech [14,15]. Hybrid devices, with switchable modes to suit the patient's range of abilities and the needs in relation to each
communicative environment are also likely. In this way, a speech processor could provide optimal auditory support for lipreading in face-to-face conversation, which would probably be of particular importance to the patient with poorer residual auditory nerve function, and offer alternative processing strategies optimized for speech perception in the absence of lipreading, or for sensitivity to a wide range of environmental sounds. References
[1] Moore, B. C. J., and Glasberg, B. R. (1983) "Suggested formulae for calculating auditoryfilter bandwidths and excitation patterns" J. Acoust. Soc. Am., 74, 750-753. [2] Dorman, M., Dankowski, K., McCandless, G. and Smith, L. (1989) "Consonant
recognition as a function of the number of channels of stimulation by patients who use the Symbion cochlear implant" Ear and Hearing, 10, 288-291.
[3] Working Group on Communication Aids for the Hearing-Impaired (1991) "Speech-
perception aids for hearing-impaired people: Current status and needed research" J. Acoust. Soc. Am., 90, 637-685.
[4] House, W. H., Berliner, K. L, and Gary, W. G. (1976) "Cochlear implants" Ann. Otol.
RhinoL Laryngol. Suppl. 27 85, 1-93.
[5] Hochmair, E. S., and Hochmair-Desoyer, I. J. (1985) "Aspects of sound processing iusing the Vienna intra-and extra-cochlear implants" In R. A. Schindler and M. M. Merzenich (Eds.)
Cochlear Implants, pp 291-304, New York, Raven Press. [6] Boyle, P. (1991) "The UCH/RNID cochlear implant", this meeting.
[7] Evans, E. F. (1991) "Multichannel cochlear implants: signal processing by the cochlea; physiological ideal; Project Ear multichannel cochlear implant....", this meeting.
[8] Gray, R., (1991) "The Ineraid cochlear implant", this meeting. [9] Walliker, J. R., (1991) "A versatile digital speech processor for hearing aids and cochlear
implants", this meeting.
[10] Wong, J., "The Nucleus cochlear implant", this meeting. [11] Wilson, B. S., Finley, C. C, Lawson, D. T. et al, (1991) "Better speech recognition with cochlear implants" Nature, 352, 236-238. [12] Walliker, J. R., and Howard, L (1990) "Real-time portable multi-layer perceptron voice fundamental-period extractor for hearing aids and cochlear implants". Speech Communication.
9, pp 63-71.
[13] Faulkner, A., Walliker, J. R., Howard, I. S., Ball, V., and Fourcin, A. (1991) "New developments in speech pattern element hearing aids for the profoundly deaf Paper presented at 2nd International Workshop on Hearing Impairment and Signal-Processing Hearing Aids., London, June 1991.
[14] Fourcin, A. J. (1990) "Prospects for speech pattern element aids". larvngologica (Stockh1 Suppl. 469, pp 257-267.
Acta Oto-
[15] Rosen, S. (1989) Temporal information in speech and its relevance for cochlear implants. In Cochlear Implant: Acquisitions and Controversies. B. Fraysse & N. Cochard (eds.) Cochlear AG, Basel, pp. 3-26.
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I
*
SCIENCE, EDUCATION AND
TECHNOLOGY DIVISION Colloquium on
cochlear implants in the uk
Organised by Professional Group
s9 (.blomedical engineering) on Thursday, 28 November 1991
Digest No: 1991/179
