a digital hearing aid according to the present invention is used to measure several aspects of the hearing aid system . the transfer function of the feedback path , consisting of the amplifier , receiver , feedback acoustics , and microphone in series , can be measured in the ear using the hearing aid microphone . the transfer function of the series combination of the amplifier and receiver , and the maximum output levels and distortion for the amplifier receiver combination , can be measured as well using an external microphone . once the receiver characteristics are known , the hearing aid microphone transfer function can also be determined . these measurements are possible because the hearing aid contains a digital signal processing ( dsp ) integrated circuit that provides a programmable computer inside the hearing aid . fig2 is a flow diagram showing a first integrated hearing aid performance measurement and initialization method 200 . the hearing aid might be very similar to that shown in fig1 a , but the hearing aid processor is a programmable dsp . the initial processing steps 204 - 208 shown in fig2 are preferably run prior to the run time operation 214 of the hearing aid , as an initialization process . this initialization may be performed once , or each time the hearing aid is turned on . in fig2 the initialization process is entirely integrated within the hearing aid . the code used to perform measurements and set initialization parameters is resident in hearing aid memory 202 , and all processing is performed within the hearing aid . in step 204 , the measurement program is loaded from hearing aid memory 202 . preferably , this program is stored in some form of rewritable memory , so the program can be updated if desired . step 206 performs measurements of hearing aid performance , for example transfer functions of various combinations of hearing aid elements . fig4 - 6 illustrate various measurement configurations which could be employed . in step 208 , processing parameters are computed from these measurements . in step 212 , the hearing aid run time program is loaded into operating memory , and in step 214 , run time hearing aid processing begins . note that the two step procedure of loading the initialization code followed by loading the run time code is most appropriate when the hearing aid has a limited amount of program memory ; if enough memory is available , the initialization and run time code can be combined into a single program . fig3 is a flow diagram showing a second , partially integrated , hearing aid performance measurement and initialization method . in this embodiment , the hearing aid interacts with a host computer 302 to download program code and process the measurements . in step 304 the measurement program is downloaded from host computer 302 . the hearing aid dsp runs the program and acquires data in step 306 , and may also partially process the data . the raw or partially processed data is then sent up to host computer 302 , which computes the processing parameters . step 308 stores the processing parameters for use by the run time processing program . the run time code is then downloaded to the hearing aid in step 312 . real time processing begins in step 314 . the method shown in fig3 might be used when there is simply not enough storage memory in the hearing aid for both the measurement program and the run time program . thus , each must be separately loaded into the hearing aid when needed . in general , the initialization is performed only once in the scenario shown in fig3 since an external computer is required to load the two programs . however , if better initialization or run time programs became available , the process of fig3 could be repeated . fig4 - 6 show configurations for performing measurement steps 206 and 306 of fig2 and 3 . fig4 is a block diagram showing a first measurement configuration . the characteristics of the feedback path , which includes the amplifier 404 , receiver 406 , and microphone 410 along with the acoustic and mechanical feedback 408 , can be measured by exciting the system with a probe signal 402 and recording the response 412 at the hearing aid microphone 410 . the impulse response of the feedback path can be obtained , for example , by using a periodic maximal - length sequence as the probe and accumulating the corresponding periods of the microphone response . the circular correlation of the microphone response with one period of the excitation will then give the impulse response of the feedback path . system identification techniques can then be used to produce an all - zero , all - pole , or pole - zero model of the feedback path from the impulse response . an alternative would be to excite the system with a white noise probe sequence and adapt a set of filter coefficients to produce the model of the feedback path . fig5 is a block diagram showing a second configuration for performing measurement steps 206 and 306 of fig2 and 3 . the characteristics of receiver 406 can be determined using the configuration of fig5 . as in the case of the fig4 configuration , a test signal is generated by block 402 which passes through amplifier 404 and receiver 406 . however , in this configuration , a calibrated microphone 508 is electrically connected to the hearing aid input via pre - amp 510 , thus bypassing the feedback path and the hearing aid microphone . the signal is recorded at 512 . a maximal - length sequence can be used as the excitation , and the impulse response of receiver 406 determined from the output of the calibrated external microphone connected to the audio input of the hearing aid . or , as in the case of the feedback path estimation shown in fig4 a white noise excitation can be used and a set of filter coefficients adapted to produce a model of the receiver response . if only the magnitude frequency response is desired , the system can be excited with a sine - wave sweep and the response recorded at the hearing aid input , or individual tones can be used with the magnitude at each frequency determined at the hearing aid input . distortion can be estimated by increasing the level of the sinusoids and measuring the power at the frequencies of the harmonics , or a noise signal can be used to measure the coherence between the excitation signal and the signal recorded at the hearing aid input . the minimum digital signal level that drives the amplifier or receiver into saturation can be determined by increasing the level of a sinusoid excitation and observing the maximum output signal level . the maximum receiver output signal level for the d / a converter input at full scale can be determined by generating a sinusoid at full scale and measuring the output signal power . fig6 is a block diagram showing a third configuration for performing the measurement steps 206 and 306 of fig2 and 3 . the system shown in fig6 can be used to estimate the characteristics of microphone 410 once the receiver impulse response or transfer function has been measured . receiver 406 is connected directly to microphone 410 via a short acoustic tube 608 , so the feedback path is bypassed but amplifier 404 , receiver 406 , and microphone 410 are included . since the amplifier - receiver characteristics are already known , only the microphone remains to be measured . a maximal - length sequence , white noise , or a sinusoidal sweep or a set of tones can be used to measure the overall system response , after which the known receiver characteristics are divided out to produce the estimate of the microphone response . there are several ways in which the measurements can be used to adjust the processing parameters . in a feedback cancellation system , for example , the impulse response of the feedback path can be used as the input to a system identification procedure to produce a nonvarying all - pole , all - zero , or pole - zero filter to model the feedback path or to provide the starting filter coefficients for an adaptive all - zero or pole - zero model . receiver measurements made in an acoustic coupler can be used on the production line to check that the receiver sensitivity , maximum output signal level , frequency response , and distortion are within specifications . differences in frequency response could then be used , for example , to adjust equalization used to compensate for irregularities in the receiver frequency response . an external microphone connected to a probe tube inserted in the ear canal along with the hearing aid earmold could be used to provide equalization for the receiver as it will be used in situ . the microphone response measurements can be used to provide equalization for the microphone in a manner similar to that used for the receiver . the receiver and microphone response and calibration curves can also be used to adjust compression parameters for each hearing aid so that the amplified input signal never exceeds the maximum digital level that can be handled by the dac without distortion . while the exemplary preferred embodiments of the present invention are described herein with particularity , those skilled in the art will appreciate various changes , additions , and applications other than those specifically mentioned , which are within the spirit of this invention .
a digital hearing aid according to the present invention is capable of measuring its own performance . the hearing aid includes a test signal generator for feeding a test signal into the hearing aid amplifier . the response to the test signal is acquired at a specific point in the hearing aid , depending upon what aspect of performance is to be measured . various elements of the hearing aid and / or the hearing aid feedback may be bypassed . the hearing aid further includes the capability of initializing hearing aid parameters based upon the performance measurements . the measurement and initialization capability may be entirely integral to the hearing aid , or an external processor may be used to download the measurement program and the run time program , and assist in computing the parameters .