referring first to fig1 , a cochlear implant system 10 constructed in accordance with this invention includes an external component 12 and an internal component 14 . the external component includes a speech processor 12 a and is associated with a microphone 16 for sensing ambient sounds and generating corresponding electrical signals . these signals are sent to the speech processor 12 a which processes the signals and generates corresponding encoded signals . the encoded signals are provided to a transmitter ( including a transmit coil 20 ) for transmission to the internal component 14 . the internal component 14 ( which may also be referred to as an implantable stimulator ) receives the power and data via a receive coil 22 . the rf power signal is stored by a power supply 24 ( see fig3 ) which provides power for the internal component 14 . the data signals control the operation of the internal component 14 so as to generate the required stimulation pulses which are applied to the auditory nerve of the patient via an electrode array 28 . the structure of the external speech processor 12 a is shown in more detail in fig2 . first , the audio signals received from microphone 16 are fed to a signal processor 30 . this signal processor 30 maps the audio signals into excitation signals in accordance with one or more mapping algorithms stored in a map memory 31 . these excitation signals are encoded by a digital data encoder 34 . the encoder data is combined with an rf signal in the data and power transmitter 36 , and passed to the transmit coil 20 via a tuneable tank circuit 38 . in accordance with the present invention , encoded telemetry data is received back from the internal component 14 via coil 20 , and is decoded by telemetry decoder 52 . the decoder telemetry data is passed to the tuning adjuster and power controller 40 , which uses the telemetry data to generate a tuning adjustment signal . the tuneable tank circuit 38 adjusts the tuning of the transmit coil 20 according to the tuning adjustment signal as described in more detail below . this can be achieved , for example , by using an electrically controlled variable capacitor in conjunction with a series tuning capacitor , or by any of various similar means known to the art . power to the whole system 10 is provided by a power supply 50 which typically includes a battery . referring now to fig3 , the internal component 14 includes a housing ( not shown ) which is hermetically sealed . the component 14 also includes a receiver tank circuit 32 having the receive coil 22 and a capacitor 66 . signals received through this tank circuit are fed to a power supply 24 generating an output voltage vdd . the power supply is represented in fig3 by a diode 68 charging a capacitor 70 . the power supply 24 uses the energy of the received rf signals to charge up the capacitor 70 . the rf signals are also fed to a data decoder 60 . the data decoder 60 derives from the rf signal the digital excitation signals generated by the data encoder 34 and generates corresponding stimulation control signals . these signals are fed to a programmable current source 62 and a switching control circuit 64 . these two circuits cooperate in response to the signals from data decoder 60 to apply the cochlear stimulation signals to predetermined electrodes of electrode array 28 in a known manner which is beyond the scope of this invention . implant 14 further includes a compliance monitor 66 which generates an output that is fed to a telemetry encoder 80 as discussed more fully below ; and a power supply monitor 82 which is used to monitor the voltage vdd generated by power supply 24 and which provides a voltage condition signal to telemetry encoder 80 . the compliance monitor 66 and power supply monitor 82 each sense certain specific functions of the internal component and transmit them to the telemetry encoder 80 . the telemetry encoder 80 then transmits this information to the telemetry decoder 52 . the data is decoded and used to adjust the power transmit between the coils , if necessary . an exemplary mode of operation indicating the voltage monitoring made is now described in conjunction with fig4 a , b and c and 5 a and 5 b . at predetermined intervals , for example , every 100 ms , or alternatively after every stimulation pulse , the telemetry encoder 80 generates a first pulse f . ( step 100 ). this pulse may have a duration of about 1 ms . this pulse f indicates to the external speech processor 12 a that the implantable stimulator 14 is sending data . next , the power supply monitor 82 compares the power supply output voltage vdd to a threshold value vt and sends the result to the telemetry encoder 80 . more specifically , starting with step 102 , the power supply monitor 82 first determines if vdd & gt ; vt . if it is , then in step 104 , a parameter pw ( pulse width ) is set to a predetermined value a , of for example , 2 ms by the telemetry encoder 80 . if in step 102 vdd is not larger than vt then in step 106 a check is performed to determine if vdd is approximately equal to vt . if it is , then in step 108 parameter pw is set to zero . if it is not then , vdd must be smaller than vt and in step 110 the parameter pw is set to a predetermined value b of , for example , 1 ms . next , in step 112 a pulse d is generated having a pulse width a or b , or no pulse is generated , depending on the outcome of the decisions 102 and 106 . the pulse d ( if present ) is generated a period t after pulse f . t may be about 1 ms . the results of this step are seen in fig4 a , 4 b , 4 c . for fig4 a it has been determined that vdd & gt ; vt , and hence pulse d with a pulse width a is sent about 1 ms after pulse f . in fig4 b , vdd has been found to be about equal to vt and hence no pulse d is present . in fig4 c , vdd is found to be smaller that vt and hence pulse d having a pulse width b is sent about 1 ms after period f , pulse width b being generally shorter than pulse width a . for example , pulse width a may be 2 ms and pulse width b may be about 1 ms . pulse f and , if present , pulse d are then sent to the tank circuit 32 . as a result , a corresponding signal appears on the transmit coil 20 , which is then decoded by the telemetry decoder 52 . the operation of the telemetry decoder 52 is now described in conjunction with fig5 b . starting with step 120 , a pulse f is first detected which indicates that the power supply monitor 82 is sending information about the status of the power supply 24 . next in step 122 a check is made to determine if a pulse d is present following pulse f . if this pulse is not detected , then in step 130 the previous operations are continued with no change . if in step 122 , a pulse d is detected then in step 124 a determination is made as to whether this pulse d has a pulse width a or a pulse width b . a telemetry pulse d having a relatively long pulse width , in a range corresponding to the pulse width a ( for example if pulse d exceeds 1 . 5 ms ), indicates that the implant supply voltage is high ( i . e . vdd & gt ; vt ). in step 126 , the tuning adjuster and power controller 40 therefore adjusts the tunable tank circuit 38 to reduce the power transferred to the implant . a preferred method to accomplish this effect is to reduce the resonance frequency of the tank circuit . if the telemetry pulse is less than 1 . 5 ms , ( indicating a pulse width b and that the power supply vdd & lt ; vt ) then in step 128 the tuning adjuster and power controller 40 adjusts the tunable tank circuit 38 to increase the transferred power . the tunable tank circuit 38 is adjusted by the tuning adjuster and power controller 40 via means of a tuning capacitor ( not shown ) which is preferably a voltage dependent capacitor . it should be appreciated that the tunable tank circuit 38 could also be tuned by other known means as would be understood by one skilled in the art . similarly , the above mentioned operation may be performed in respect of the compliance monitor signal , as described in more detail below . briefly , referring to fig3 , under the control of commands from data decoder 60 , the programmable current source 62 generates current pulses which are applied to the electrodes by switching control circuit 64 . fig6 depicts two typical stimulation current waveforms 70 and 73 which may be requested by the signal processor 30 . it can be seen that each - waveform is biphasic , consisting of two current pulses of equal amplitude and opposite polarity . thus , lower amplitude biphasic current waveform 70 consists of positive and negative pulses 71 and 72 respectively , and higher amplitude current waveform 73 consists of positive and negative pulses 74 and 75 . next , fig7 depicts the corresponding current waveforms that must be generated by the programmable current source 62 to produce the desired stimulation current waveforms 70 and 73 . that is , the programmable current source 62 must generate two lower amplitude square waves 76 and 77 to generate stimulus pulses 71 and 72 respectively , and two larger amplitude square waves 78 and 79 to generate the stimulus pulses 74 and 75 . pulses 77 and 79 are reversed by the switching control circuit 64 . however , if the current pulses 78 and 79 exceed the capability of the power supply 24 , an out of compliance condition occurs . this problem is resolved in the present invention as follows . referring to fig8 the voltage waveform 80 represents the voltage vn at the output of the programmable current source 62 . it can be seen from the shape of the voltage waveform 80 that the load contains a capacitive component . the level vc marks the minimum voltage across the programmable current source 62 at which compliance with the desired current waveforms of fig7 can be maintained . the voltage vca is a little higher than vc as shown and is selected to provide a safety margin . as seen in fig8 , pulse 83 required to generate pulses 78 and 74 of fig7 and 6 respectively , starts off at a level above vca but decreases linearly toward a minimum value ( p ) which is substantially below level vc and therefore is not attainable . when this pulse reaches vca ( at point 85 ), the compliance monitor 66 generates a compliance monitor signal indicating an out of compliance condition . the signal is encoded by the telemetry encoder 80 and transmitted to the external processor . the signal may be the same signal as when vdd drops below vt as discussed above , or it may be a different signal , as would be appreciated by one skilled in the art . in response , the tuning adjuster and power controller commands the tunable tank circuit 38 to increase the voltage transmitted to the internal section . the adjustment of the link tuning or rf power generated can be performed for every instance of a compliance monitor signal being received from the implant and may be maintained at a high level for a predetermined time , after which the rf power can be dropped to a previous level . alternatively , the frequency of the compliance monitor signal may be monitored by the tuning adjuster and power controller 40 . the link tuning or rf power generated could then be adjusted to maintain a desired ratio of compliance monitor signals to stimulation signals . for example , the link tuning or rf power generated could be adjusted to keep the ratio of compliance monitor signals to stimulation pulses to a desired target of for example 5 %, i . e . for this purpose , the tuning adjuster and power controller 40 includes a counter which counts every instance of non - compliance . after a predetermined number of stimulation pulses , for example a thousand , the counter is checked to determine the number of non - compliant instances . if the counter shows a number over the desired target ( i . e . 50 for a 5 % target ) then the tuning adjuster and power controller 40 adjusts the tank circuit 38 to increase its power level . on the other hand for a number of non - compliant instances below the target , the power level is increased . of course , this determination could also be made within the implant by the compliance monitor itself , as would be evident to one skilled in the art . obviously numerous modifications can be made to the invention without departing from its scope as defined in the appended claims .
in a cochlear implant system , the implantable stimulator includes a monitor which monitors parameters associated with the stimulation signals and / or the power stored in an energy storage element which stores energy transmitted from the processor . this parameter or parameters is / are analyzed and one or more feedback signals are generated and transmitted back to the processor . the processor uses the feedback signal to insure that power is transmitted to the stimulator optimally and that the stimulation signals are compliant .