Veterans Administration Journal of Rehabilitation Research and Development Vol. 24 No. 4 Pages 7-20 Digital hearing aids: A tutorial review* HARRY LEVYITT, PIh,D. GenterJi)r Research in Speech nnd Nearing Sciences, Crud~ruieSchool and Uni~~ersityCenter of tlze Cily Univec~ityof Mew Yo&, New YorA, NY 10036 Abstract-The basic concepts underlying digital signal hearing aids because of the constraints on chip size processing are reviewed briefly, followed by a short and power consumption. But all of the above- historical account of the development of digital hearing mentioned features have already been demonstrated aids. Key problem areas and opportunities are identified, in a master digital hearing aid (12,14) and it is likely and the various approaches used in attempting to develop that (with the development of more advanced signal- a practical digital hearing aid are discussed. processing chips) an increasing number of these attractive features will be incorporated in the wear- able digital hearing aids of the future. ADVANTAGES OF DIGITAL HEARING AIDS The many advantages offered by digital hearing aids can be subdivided into three broad groups: Digital hearing aids promise many advantages I) Signal-processing capabilities that are analo- over conventional heartng aids. These include: gous to, but superior to, those offered by conven- Programmability; tional analog hearing aids. Much greater precision in adjusting electroacous- 2) Signal-processing capabilities that are ~~nique tic parameters; to digital systems and which cannot be implemented Self-monitoring capabilities; in conventional analog hearing aids. Logical operations for self-testing and self-cali- 3) Methods of processing and controfllng signals "oation ; that change our way of thinking about how hearing Control of acoustic feedback (a serious practical aids should be designed, prescribed, and fitted. problem with high-gain hearing aids); The third type of advantage is the most subtle- The use of advanced signal-processing techniques and the most important. For example, a digital for noise reduction; hearing aid can be programmed not only to amplify, Automatic control of signal levels; and but also to generate, audio signals. As such, the Self-adaptive adjustment to changing acoustic en- instrument can be programmed to serve as an vironments. audiometer in order to facilitate the measurement Only a few of these features are likely to be of audiological characteristics relevant to the pre- included in the first generation of wearable digital scriptive fitting of hearing aids (14,151. Using this approach, it is possible to circumvent the very difficult problem of correcting for the frequency- "Preparation of this paper was supported by Grant No. GO0830251 dependent differences in sound level between the from the National Institute on Disability and Rehabilitation Research and by PHS Grant PO1 NS17764 from the National Institute of traditional audiometer headphone and the patient's Neurological and Communicative Disorders and Stroke. own hearing-aid receiver. The idea of using a hearing Journal of Rehabilitation Research and Development Vol. 24 No. 4 Fall 7987 aid as an audiometer was born out of the realization The middle section of the diagram shows the that the primary differences between the two devices sampled data. Note that the signal is no longer are those of software (i.e., the controlling program) continuous but consists of a series of discrete pulses. rather than fundamental differences in hardware. The height of each pulse is equal to the value of the Another important stimulus to our thinking has continuous waveform at the sampling insturzl. Note been the use of computer simulation. As noted that, at this stage of the conversion process, the elsewhere in this paper, the development of com- pulse heights are still specified in analog form; i.e., puter-simulation techniques to facilitate the design the value of each sample can be recorded at its and development of vocoders and other telephone- precise numerical level within the range of the oriented speech processing systems led to the re- signal's continuous variation. Only the sampling alization that computers could also be used to instant is defined discretely. simulate hearing aids, and that eventually digital The bottom section of the diagram shows the processing of audio signals would be possible in a sampled data signal in binary form. For the purposes hearing aid. Further, the first working digital hearing of this example, eight arbitrary levels of quantization aid was achieved using real-time computer simuta- are shown. Each of the analog samples shown in lion (12,14), thereby providing a glimpse of the many the middle section of the diagram has been approx- possible features that could be incorporated in the imated by a sample with a height equal to one of digital hearing aids of the future. these eight levels of quantization (The binary rep- The results currently being obtained using com- resentation of these eight quantization levels appears puter simulation techniques are having a profound on the ordinate.) The continuous waveform has also effect on our thinking with respect to which features been reproduced here in order to show how well should or should not be included in a modern hearing the binary samples approximate the original signal aid. Whereas, in the past, progress in the develop- (at the sampling instants). ment of more effective hearing aids was limited The process of converting an analog sarnple to primarily by what could be achieved technologically, binary form involves a sequence of binary "deci- the present situation, with the help of computer sions." The first decision is whether the signal is sirnufation and the realization of practical digital greater or less than zero. Consider the first sarnple hearing aids, is one in which progress is limited by way of illustration: this sample has a value of primarily by our own lack of understanding of what 0.9 volts and therefore the answer to the first is important in processing signals for hearing im- question is positive (i.e., sample value is greater pairment. than zero) and a I is assigned as the first digit of its binary representation. (The analog-to-digital con- version has been set to cover the range from - I to HOW DOES DIGITAL SIGNAL PROCESSING + I volts. This range has been chosen to exceed WORK? that of the signal being quantized.) After the first binary decision it is evident that the value of the Analog-to Digital Conversion waveform at the first sampling instant must lie The key element in a digital hearing aid is its use between 0 and + 1 volts. of sound signals that have been sampled discretely The next binary decision is whether this sample in time, so that they have become represented by a value lies above or below the mid-value (0.5 volts) series of data poiizts instead of by a continuously- of this range. The answer is again positive (i.e., varying value analogous to the waveform itself. sample value greater than 0.5 volts) and a 1 is once Figure 1 illustrates the process whereby an analog again assigned as the next digit in the binary rep- signal is converted to digital form. The uppermost resentation. It is now known that the waveform section of the diagram shows a continuous wave- must have a value between 0.5 and 1.0 volts at this form; in this example, voltage as a function of time sampling instant. The mid-value of that range is 0.75 is shown. The signal is periodic with a period equal volts and so the third binary decision is whether the to 0.8 msec. It is sampled at regular intervals of 0.1 sample value is greater or less than 0.75 volts. The msec. The value of the waveform at each sampling answer is again positive so once again a 1 is assigned instant is shown by a solid circle. as the next digit in the binary representation. It is LEVITT: Digital Hearing Aids: A Tutorial Review 5 10 SAMPLE NUMBER Figure 1. Converting a Signal frorn Analog to Digital Form. 'The uppermost section of the diagram shows the waveform of a continuous signal. The solid points show the value of the waveform at each sampling instant. The middle section of the diagram shows the sampled-data version of the analog signal. The lowest section of the diagram shows the sample values after 3-bit quantization. (The original waveform is also shown here in order to illustrate the accuracy of quantization.) now known that the waveform must lie between example is 0.25 volts (e.g., Sample #I lies between 0.75 and 1 .0 volts at this sampling instant. 0.75 and 1 volt, Sample #2 lies between 0 and - 0.25 The above process can be continued iteratively volts, etc.). The largest error that can occur is half until the binary representation reaches the desired a quantization step, which in this case is 0.125 volts. accuracy. (The accuracy of approximation doubles As can be seen from the diagram, all of the binary with each additional binary decision.) In the above samples are within 0.125 volts of the original wave- example, three binary decisions were used leading form. In general, an N-bit analog-to-digital converter to a binary representation consisting of three binary will have 2N quantization levels and the error of digits. The term binary digit is abbreviated as bit quantization will not exceed 112 of 1/2N,or 11(2N+1), and the binary representation derived above is said of the range of the analog-to-digital converter. In to have "an accuracy of 3 bits." the above example, N=3 and the range of quanti- Referring once again to the lowest section of zation was 2 volts; the number of quantization levels Figure 1, it can be seen that each binary sample lies was thus 8 (= 2') and the precision of quantization eitherjust above orjust below the original waveform.