THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

232 10. Physiology of Hearing and Psychoacousticsa receiver (i.e., a mini-loudspeaker) that converts the signals into amplified sound.Current hearing aids evolved from a long history of continuously improving technology,from the bulky electrical hearing aids of the 1930s (which were poweredby rather large multiple batteries) to the smallest models inserted into the ear canal.The electronic part of a hearing aid can be based on different technologies: analog,programmable, and digital. Each of these technologies has advantages anddisadvantages, but the choice rests on the optimum benefit to the user.In the analog models, variable resistors perform the trimming adjustments, andany changes of resistor value will change the properties of the circuit accordingly,thus providing a change in the frequency response, usually prescribed by a qualifiedaudiologist, to meet the needs of the user. In principle, the same applies toa programmable hearing aid. In this case the total variable resistor componentconsists of a ladder of resistors, each attached to a digital switching element. Settingthe individual switches either “on” or “off,” thereby bypassing some of theresistors in the ladder changes the total resistor value. These switches or gates arethe fundamental building blocks of all digital circuits. CMOS are used to producegates that function as long as there is electrical power. But in order to avoid losingthe switch settings (and hence the adjustment), a common solution is to constructa secondary set of switches, using a technology that enables the switch settingsto be retained even when there is no battery attached. This technology known asE2PROM (programmable read only memory) provides the nonvolatility necessaryto retain the settings.By their very nature digital adjustments are discreet, i.e., the resistor can onlytake on a finite number of values. Although there is theoretically no limit onhow fine the resolution can be, the penalty is a larger circuit using more power.The resolution is therefore set to be sufficiently adequate for the parameter to beadjusted, typically in the region of 0.5–5 dB. Programmable hearing aids mayalso use digital switching to select functions (ON/OFF, M/T). The M/T functionrepresents a choice between the microphone (M) and telephone (T) mode. In thetelephone mode, induction signals are received from the telephone receiver ratherthan the acoustic signal, so as to provide better coupling and eliminate feedback.A digital volume control may be used to work in steps, typically 1 dB.In digital hearing aids, an analog-to-digital converter reads the analog voltagesignal from the microphone and converts it to a digital signal. The digital signalrepresents the original sound as a series of number, which then can be manipulatedmathematically by the digital signal processor (DSP). Once digitized, the signal ismore robust and is no longer subject to electronic noise and distortion. To obtainthe highest possible signal quality the input signal is sampled at a very high rate(500 kHz–1 M Hz) before the digitization. The sampling rate at the input affects thesignal quality, so as the sampling rate increases, so does the quality of the signal.Another important parameter for sound quality is the resolution, which determinesthe initial precision of the signal and the precision that later can be achieved duringthe mathematical manipulations that constitute signal processing. After the sampling,the converted signal is then ready for manipulation by a specially designeddigital signal processor (DSP). The DSP is programmed by an audiologist on a