this invention finds use in many applications where the electronics is essentially the same but the external appearance of the device may vary . fig1 illustrates a desk telephone including base 10 , keypad 11 , display 13 and handset 14 . as illustrated in fig1 , the telephone has speaker phone capability including speaker 15 and microphone 16 . the cordless telephone illustrated in fig2 is similar except that base 20 and handset 21 are coupled by radio frequency signals , instead of a cord , through antennas 23 and 24 . power for handset 21 is supplied by internal batteries ( not shown ) charged through terminals 26 and 27 in base 20 when the handset rests in cradle 29 . fig3 illustrates a conference phone or speaker phone such as found in business offices . telephone 30 includes microphone 31 and speaker 32 in a sculptured case . telephone 30 may include several microphones , such as microphones 34 and 35 to improve voice reception or to provide several inputs for echo rejection or noise rejection , as disclosed in u . s . pat . no . 5 , 138 , 651 ( sudo ). fig4 illustrates what is known as a hands free kit for providing audio coupling to a cellular telephone , illustrated in fig5 . hands free kits come in a variety of implementations but generally include powered speaker 36 attached to plug 37 , which fits an accessory outlet or a cigarette lighter socket in a vehicle . a hands free kit also includes cable 38 terminating in plug 39 . plug 39 fits the headset socket on a cellular telephone , such as socket 41 ( fig5 ) in cellular telephone 42 . some kits use rf signals , like a cordless phone , to couple to a telephone . a hands free kit also typically includes a volume control and some control switches , e . g . for going “ off hook ” to answer a call . a hands free kit also typically includes a visor microphone ( not shown ) that plugs into the kit . audio processing circuitry constructed in accordance with the invention can be included in a hands free kit or in a cellular telephone . the various forms of telephone can all benefit from the invention . fig6 is a block diagram of the major components of a cellular telephone . typically , the blocks correspond to integrated circuits implementing the indicated function . microphone 51 , speaker 52 , and keypad , 53 are coupled to signal processing circuit 54 . circuit 54 performs a plurality of functions and is known by several names in the art , differing by manufacturer . for example , infineon calls circuit 54 a “ single chip baseband ic .” qualcomm calls circuit 54 a “ mobile station modem .” the circuits from different manufacturers obviously differ in detail but , in general , the indicated functions are included . a cellular telephone includes both audio frequency and radio frequency circuits . duplexer 55 couples antenna 56 to receive processor 57 . duplexer 55 couples antenna 56 to power amplifier 58 and isolates receive processor 57 from the power amplifier during transmission . transmit processor 59 modulates a radio frequency signal with an audio signal from circuit 54 . in non - cellular applications , such as speakerphones , there are no radio frequency circuits and signal processor 54 may be simplified somewhat . problems of echo cancellation and noise remain and are handled in audio processor 60 . it is audio processor 60 that is modified to include the invention . how that modification takes place is more easily understood by considering the echo canceling and noise reduction portions of an audio processor in more detail . fig7 is a detailed block diagram of a noise reduction and echo canceling circuit ; e . g . see chapter 6 of digital signal processing in telecommunications by shenoi , prentice - hall , 1995 , with the addition of four vad circuits as disclosed in u . s . pat . 6 , 754 , 337 ( domer et al .). the following describes signal flow through the transmit channel , from microphone input 62 to line output 64 . the receive channel , from line input 66 to speaker output 68 , works in the same way , except that the gain of a particular stage may be different from the gain of a corresponding stage in the transmit channel . a new voice signal entering microphone input 62 may or may not be accompanied by a signal from speaker output 68 . the signals from input 62 are digitized in a / d converter 71 and coupled to summation network 72 . there is , as yet , no signal from echo canceling circuit 73 and the data proceeds to non - linear processing circuit 74 , which includes a noise reduction circuit , a residual echo cancelling circuit , and a center clipper connected between the input and output thereof . the output from non - linear processing circuit 74 is coupled to summation circuit 76 , where comfort noise 75 is optionally added to the signal . the signal is then converted back to analog form by d / a converter 77 , amplified in amplifier 78 , and coupled to line output 64 . data from the four vad circuits is supplied to control 80 , which uses the data for allocating sub - bands , echo elimination , and other functions . circuit 73 reduces acoustic echo and circuit 81 reduces line echo . the operation of these last two circuits is known per se in the art ; e . g . as described in the above - identified text . the path from speaker output 68 to microphone input 62 is simulated in echo canceler 73 by operation on signal x from the receive channel . in the simple case where there is no echo , signal x is attenuated and no signal is subtracted from signal y in circuit 72 . in the more complex case where there is echo , echo canceler 73 simulates the attenuation , frequency response , and phase ( delay ) of the path for subtracting the echo from the signal at microphone input 62 . the external path can be simulated with any linear filter , such as an fir filter known in the art . the basic operation of an echo canceler constructed in accordance with the invention is illustrated in fig8 . linear filter 84 operates on signal x from the receive channel to produce an estimate of the echo that is subtracted from input signal y in summation circuit 72 . in accordance with the invention , the estimate is adjusted for the range of clipping in circuit 86 , which adjusts the estimate to remain within the dynamic range of the microphone input . the range signal is obtained from clipping detector 89 , which is also coupled to input signal y . as described in greater detail in conjunction with fig9 , range is sensed by observing flat spots in the speech ( echo plus noise ) input . successive maximum or minimum values of the signal indicates clipping . the number of successive values is not critical . feedback circuit 88 compares the output from summation circuit 72 with the estimate to change coefficients as necessary for imitating the echo path . circuit 86 adjusts amplitude and circuit 88 adjusts frequency response and phase for the simulated echo path through filter 84 . fig9 is a block diagram of clipping detector 90 , constructed in accordance with a preferred embodiment of the invention . clipping detector 90 provides signals representing clipping range , clipping depth , and a binary signal indicating that clipping is occurring . the maximum value and minimum value are tracked over the full length of an input sample by detectors 91 and 92 , respectively , and combined to make the range signal in logic 93 . these values are inaccurate until saturation ( clipping ) occurs . clipping is detected as repeated values of the input at the current maximum or minimum levels . the range output signal gives the maximum and minimum values used for clipping the echo estimate from filter 84 . the clip signal is a system signal to indicate that clipping has occurred . the depth signal , the length of the clipping or the number of successive maxima or minima , is available to the audio processing circuit for further gain adjustment , if desired . analog clipping may generate values that do not repeat because of filtering and the resolution of the a / d converter . the “≧” and “≦” symbols in comparators 94 and 95 represent a comparison of higher order bits . in other words , the output is “ true ” or a logic one when the difference between one input and the other is less than 1 bit or less than 2 bits of resolution . a repeated maximum or minimum value is signaled as a clip . for example , in binary notation , a maximum of 11101101 and a maximum of 11101110 match if the two lower order ( right hand ) bits are ignored . thus , the second data is considered a repeated maximum , indicating clipping . the number of bits one chooses to ignore is arbitrary . ignoring the 0 and 1 bits has been found satisfactory . ignoring more ( higher order ) bits can lead to false positives , which are more undesirable than false negatives . the repeats are usually either maxima or minima . in fact , it is very unlikely for a maximum to be followed by a minimum , or vice - versa . also of interest is the depth of the clipping , defined as the number of successive maximums or minimums . the number gives some idea of how much the microphone signal is being clipped . each maximum , or minimum , increments counter 96 through or - gate 97 . the count is accumulated during a sample period determined by clock 101 , d - flip - flop 102 , inverter 103 , and and gate 104 . a logic one output from and gate 104 resets counter 96 . the output from and gate 104 is inverted by inverter 106 to provide a binary indication of clipping . the range data is used to modify the echo estimate using range limiting circuit in circuit 86 . in the event of a large amount of clipping , the estimate is locked to prevent the adaptation loop from diverging . other portions of the echo canceling circuitry ( not shown ) can remain in operation , e . g . attenuation and center - clipping . the invention thus provides a clipping detector for improving acoustic echo canceling in a telephone and , in particular , for improving the stability of an acoustic echo canceling circuit using a linear filter . the invention enables an echo cancelling circuit to operate even when the input has been clipped within the range allowed by the echo cancelling algorithm . having thus described the invention , it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention . for example , circuits 72 and 76 ( fig7 ) are called “ summation ” circuits with the understanding that a simple arithmetic process is being carried out , which can be either digital or analog , whether the process entails actually subtracting one signal from another signal or inverting ( changing the sign of ) one signal and then adding it to another signal . stated another way , “ summation ” is defined herein as generic to addition and subtraction . the depth of clipping is estimated by counting the number of repeated maximums or minimums . the count value gives a heuristic estimate of how much of the signals is being clipped . interpolation of the clipped values would give a more accurate estimate , but this would require more processing ( band - limited interpolation filter over the missing samples ). d - flip - flop 102 acts as a single stage counter , requiring that there be two maxima or minima for the output of inverter 106 to go true . a counter of greater capacity could be substituted , requiring more than two successive maxima or minima for an indication of clipping . while echo canceling can continue in a system constructed in accordance with the invention , one can use the available signals to provide alternative modes of operation . for example , the , depth signal can be used to shut off echo canceling if the clipping exceeds some threshold . in other words , just because an echo canceler constructed in accordance with the invention can continue to operate does not mean that it has to continue . similarly , the clip signal can cause an preset change in gain in one or more stages of the audio processing circuit . this preset change in gain , combined with the adjustment caused by the range signal , increases the dynamic range of the circuit .