Abstract:
Methods and apparatus for audio signal clip detection are disclosed. The clip detectors may receive audio signals, from which peak reference signals, indicative of the highest voltage of the received audio signals, may be derived. The received audio signals may also be differentiated and phase-lagged to produce differentiated audio signals which may, in turn, be rectified to produce rectified differentiator signals. The rectified differentiator signals and the peak reference signals may be compared to thereby produce clip detect signals indicative of whether the received audio signals are clipped. The clip detect signals may then be used to indicate whether the received audio signal are clipped.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to systems, software, processes, and/or apparatus for detecting audio signal clipping. Accordingly, the general objects of the invention are to provide novel systems, software, methods, and/or apparatus such character. 
     2. Description of the Related Art 
     It is well known that audio signal sources have a finite output voltage level capability. Maximum output capability is typically determined by the supply voltage available to the audio components in the signal path. Audio signal clipping results when the output signal level exceeds the available supply voltage. Audio signal clipping is generally not desired because, when clipping occurs, the audio signal is no longer a linear representation of the original unclipped signal; this increases total harmonic distortion and reduces the quality of the audio signal. It is, therefore, generally preferred that the audio signals remain unclipped through the entire signal path to a listening device. 
     Such considerations are particularly important in automotive audio systems where a typical signal path may include one or more signal sources, preamplifiers, and power amplifiers that are separated from one another. If any one or more of the signal sources, preamplifiers, and power amplifiers exceed their capability, a clipped signal will result. Thus, various level-setting methods and apparatus have been employed to achieve maximum capability at each stage of the signal path and to minimize or eliminate clipping of the system as a whole. 
     If the maximum unclipped output voltage of an audio source is known, creating a fixed reference voltage for comparison to the known maximum output level would be a suitable method for clip detection and/or prevention. Current solutions of this nature involve monitoring the output of the amplifier to determine if a clipped signal is generated. Once a clipped signal is detected, the preamplified signal is adjusted to limit the amount of clipping. In, Botti et al. U.S. Pat. No. 5,068,620, the amplifier input voltage acts as a reference and is compared to the amplifier output. When the input voltage increases above the output voltage a detect signal is activated. In Buck et al. U.S. Pat. No. 5,430,409, the amplifier output voltage, minus fixed gain, is compared to the amplifier input voltage. A detect signal is activated when the input voltage exceeds the reference voltage. This method also incorporates an adjustable dc offset plus reference voltage which programs the total clipping distortion permitted before the detect signal is activated. 
     However, in many instances, the maximum unclipped output signal level from an audio source is unknown. In such instances, audio component input clip detection is desired. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies the above-stated needs and overcomes the above-stated and other deficiencies of the related art by providing methods, software, systems and apparatus for audio signal clip detection. 
     One aspect of the present invention is directed to an audio signal clip detector with an audio input for receiving the audio signal. The clip detector may have a peak hold, electrically associated with the audio input, for generating a peak reference signal indicative of the highest voltage of the audio signal received by the audio input. The clip detector may also have a differentiator, electrically associated with the audio input, for differentiating and phase-lagging the audio signal received by the audio input to thereby produce a differentiated audio signal. The differentiator may also have a rectifier for rectifying the differentiated audio signal to thereby produce a rectified differentiator signal. A comparator may be electrically associated with the peak hold and the differentiator, and may compare the rectified differentiator signal and the peak reference signal to thereby produce a clip detect signal indicative of whether the audio signal received by the audio input is clipped. The clip detector may also have an indicator, responsive to the clip detect signal, for indicating whether the audio signal received at the audio input is clipped. 
     Some of the preferred embodiments of the invention may implement the peak hold, differentiator and comparator as analog circuitry. While some apparatus embodiments of the invention may generate positive rectified signals in the peak hold and in the differentiator, other embodiments may generate negative rectified signals in the peak hold and in the differentiator. 
     Some analog circuitry embodiments of the invention may employ a diode within the peak hold circuit and four diodes in the differentiator circuit. Other embodiments of the invention may employ at least one buffer between the input and the peak hold circuit, the buffer having a feedback loop with a diode; these embodiments may employ as few as two diodes in the differentiator circuit. 
     Those of ordinary skill will readily appreciate that the inventive input clip detectors disclosed herein may reside within an audio component that serves another function (such as a receiver, CD player, equalizer, preamplifier, amplifier, etc.) Alternatively, inventive detectors may be in the form of a stand-alone device including, but not limited to, devices with a handheld form-factor. Such devices may include a housing that encloses the audio input, the peak hold, the differentiator, the comparator, and the indicator such that the audio input and the indicator are user-accessible. Providing electrical power by any known means (batteries, a remote power supply, with solar capabilities, etc) to any of the disclosed detectors is within the skill of an ordinary artisan. 
     The invention can also take the form of a method of detecting audio signal clipping in which a received audio signal may be used to generate a peak reference signal indicative of the highest voltage of the received audio signal. The received audio signal may also be differentiated and phase-lagged to thereby produce a differentiated audio signal and the differentiated audio signal may be rectified to produce a rectified differentiator signal. The peak reference signal may be compared with the rectified differentiator signal to thereby produce a clip detect signal indicative of whether the received audio signal is clipped. Finally, the method may include the step of indicating, responsive to the clip detect signal, whether the received audio signal is clipped. 
     Naturally, the above-described methods of the invention are particularly well adapted for use with the above-described apparatus of the invention. Similarly, the apparatus of the invention are well suited to perform the inventive methods described above. 
     Numerous other advantages and features of the present invention will become apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiments, from the claims and from the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred embodiments of the present invention will be described below with reference to the accompanying drawings where like numerals represent like steps and/or structures and wherein: 
         FIG. 1  is a functional block diagram of an audio signal clip detector in accordance with one preferred embodiment of the present invention; 
         FIG. 2  is a schematic diagram of the audio signal clip detector in accordance with a first preferred analog circuitry embodiment of the present invention; 
         FIG. 3  shows a simplified model of a portion of the differentiator circuit in accordance with the preferred embodiment of  FIG. 2 ; 
         FIGS. 4A and 4B  depict transient pre-charging of a reference voltage occurring within the preferred peak hold circuit of  FIG. 2 ; 
         FIGS. 5A and 5B  depict various signals occurring within the preferred audio signal clip detector of  FIG. 2  when the received audio signal is not clipped; 
         FIGS. 6A and 6B  depict various signals occurring within the preferred audio signal clip detector of  FIG. 2  when the received audio signal is clipped during 20% of the audio signal cycle; 
         FIGS. 7A and 7B  depict various signals occurring within the preferred audio signal input clip detector of  FIG. 2  when the received signal is clipped during 57% of the audio signal cycle; 
         FIG. 8  is a partial Fourier transform plot of a non-clipped 40 Hz audio signal as detected using the preferred embodiment of  FIG. 2 ; 
         FIG. 9  is a partial Fourier transform plot of a 40 Hz audio signal that is clipped during 20% of the audio signal cycle; 
         FIG. 10  is a partial Fourier transform plot of a 40 Hz audio signal that is clipped during 57% of the audio signal cycle; 
         FIG. 11  is a schematic diagram of the audio signal clip detector in accordance with a second preferred analog circuitry embodiment of the present invention; and 
         FIG. 12  is a schematic diagram of the audio signal clip detector in accordance with a third preferred analog circuitry embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a functional block diagram of an audio signal clip detector  20  illustrating one possible preferred embodiment of the invention. As shown, detector  20  may include an input  22  electrically associated with a peak hold  26  and a differentiator  28  and with optional buffers  24   a  and  24   b . Also as shown, detector  20  may also include a comparator/processor  30  electrically associated with peak hold  26 , differentiator  28  and a clip detect indicator  32 . 
     An audio signal may be received by audio input  22  and may be generated from an external source/component such as a radio, a CD player, an MP3 player, an equalizer, a preamplifier, a power amplifier, a signal generator, or other audio source known in the art (not shown). The input signal may be either balanced or unbalanced and may be applied to peak hold  26 , either directly or through optional buffer  24   a . The input signal is also applied to differentiator  28 , either directly or through optional buffer  24   b . The output of peak hold  26  may be applied to one input of comparator  30  and the output of differentiator  28  may be applied to the opposite input of comparator  30 . A clip detect signal output from comparator  30  may be set at voltage high or voltage low, depending on the polarity configuration of the clip detect indicator  32  and on whether the audio input signal is clipped or non-clipped. 
       FIG. 2  is a schematic diagram of an analog circuitry implementation of the audio signal clip detector  20  of  FIG. 1 . As shown, an audio input signal may be applied to the non-inverting inputs of the analog operational amplifiers U 1 A and U 1 B preferably configured as unity gain buffers. Buffers U 1 A and U 1 B act as high impedance buffers to prevent loading effects from upstream circuit stages. Buffers U 1 A and U 1 B also electrically isolate peak hold circuitry  26  from differentiator circuitry  28 ; as such, almost any conventional buffer having a high impedance input and a low impedance output may be used. 
     The acceptable audio input signal voltage levels can be specified at any value as long as the resulting voltages do not exceed the rated operating voltages of the various components in the circuit as shown. Audio input signals also should not exceed the supply voltages of the operational amplifiers and comparator shown. However, those of ordinary skill will recognize that exceeding the supply voltages of the operational amplifiers and comparator can be prevented by dividing down the input signal voltage (with conventional circuits and/or methods) prior to the inputs of buffers U 1 A and U 1 B to a value that is within acceptable voltage levels. 
     In the circuit shown in  FIG. 2 , the buffered audio signal at the output of U 1 A may be applied to analog peak hold circuit  26  where it is preferably positively rectified by diode D 1 . Resistors R 1  and R 2  form a voltage divider that, in conjunction with transistor Q 1 , pre-charges capacitor C 2  with no signal present. The pre-charge voltage value at capacitor C 2  is determined by supply voltage+Vsupply, and resistors R 1  and R 2 . The pre-charge of capacitor C 2  helps prevents false clip detect at the initial signal transition when time t=0, where t=0 is defined as the moment before the initial transition of the audio input signal. 
     In an alternative embodiment, op amps U 1 A and U 1 B may be configured to apply gain to the input signal being detected so that any minimum audio input signal can be clip detected. The gain ratios of U 1 A and U 1 B should be at least substantially equal and the resulting amplitude of the audio signal at VS 1  must be large enough to compensate for the forward-bias-diode voltage drops of diode D 1  and transistor Q 1  to maintain ability to charge capacitor C 2 . 
     In the preferred analog peak hold circuit  26 , transistor Q 1 , configured as an emitter follower, provides practical current drive capability to charge capacitor C 2 . The charging current for capacitor C 2  is preferably reasonably unimpeded because charge current limiting at the initial charge of capacitor C 2  could place the peak hold reference signal Vref at an incorrect low value and this condition could cause the clip detect to falsely trigger. This is especially true at system start up, when capacitor C 2  is initially charged at the first transition of the audio input signal. Transistor Q 1  also serves as a high impendence buffer, preventing discharge of capacitor C 2  through resistor R 2 . 
     With joint reference now to  FIGS. 2 ,  3 ,  4 A and  4 B,  FIG. 4A  shows the initial charge current of capacitor C 2  and supply voltage+Vsupply at time t=0.  FIG. 4B  shows the voltage level rise at peak hold reference signal Vref in relation to differentiated audio input signal Vtrig at time t=0. 
     The voltage signal at capacitor C 2  (peak hold reference signal Vref), applied to the positive input of U 2  is approximately: 
     two forward bias diode voltage drops (Vf and Vbe) lower than Vs 1 . 
     Where: 
     Vpk=Voltage Peak at VS 1 ; 
     Vf=Forward Voltage of D 1 ; and 
     Vbe=Vbe Voltage of Q 1 . 
     In some applications it might be possible for noise artifacts (for example, in the form of AC line or switching noise) to overcharge capacitor C 2 . To ensure that peak hold reference signal Vref is not higher than intended due to such noise in the system, resistor R 3  acts as a discharge path for capacitor C 2 . A wide variety of conventional peak hold options could be substituted for the particular configuration shown in  FIG. 2 , with possible minor modifications being within the skill of ordinary artisans. Care should be taken, however, to ensure that peak hold circuit voltage drops are balanced with differentiator circuit voltage drops, as is the case with the various embodiments disclosed herein. 
     With primary reference, again, to  FIG. 2 , the buffered audio signal VS 2  at the output of op amp U 1 B is applied to capacitor C. Since, in this preferred embodiment, U 1 A and U 1 B are unity gain buffers, the voltage values at VS 1  and VS 2  (buffered audio signals) are assumed equal. 
     Focusing on differentiator  28 , resistors R, R 4 , R 5  (Rt) and capacitor C (in conjunction) form a differentiator, a phase shifter and a voltage divider. The steady state signal Vdiff (the differentiated audio input signal) will be less than or equal to the signal VS 2  depending on the frequency of the input signal.  FIG. 3  shows this voltage division for the differentiator. In particular, it is noted that the peak voltage at Vdiff is approximately equal to the peak voltage at VS 2  for audio input signals at frequencies much greater than 1/(2πRC). By contrast, the peak voltage at Vdiff is less than the peak value at VS 2  for audio input signals at frequencies much less than 1/(2πRC). This is due to the reactance of capacitor C, where the impedance of capacitor C is given by Xc=1/(2πfC) and where f is the frequency signal VS 2 . A wide variety of conventional differentiator options could be substituted for the particular configuration shown in  FIG. 2 , with possible minor modifications being within the skill of ordinary artisans in light of this disclosure. Care should be taken, however, to ensure that differentiator circuit voltage drops are balanced with peak hold circuit voltage drops and that suitable phase-lagging is present, as is the case with the various embodiments disclosed herein. 
     Operation of preferred clip detector  20  will now be discussed with reference to the signal traces and Fourier transform plots of  FIG. 5A  through  FIG. 10 . Turning first to  FIG. 5A  there is shown therein signals Vdiff, VS 1 , and Vc when presented with an non-clipped sine wave at a frequency where proper clip detection can occur (see discussion below). Vc is the voltage across the capacitor C. It can be seen that the peak value of differentiated audio signal Vdiff is below that of VS 1 . The value of Vdiff is the difference in voltage between VS 2  and Vc where Vdiff=VS 2 −Vc. (See  FIG. 3 ). It can be seen in  FIG. 5B  that when the absolute value of Vdiff is below the absolute value of VS 1 , Vtrig is below Vref and the output of U 2  (the clip detect signal) is a voltage high. Those of skill in the art viewing  FIGS. 5A ,  6 A and  7 A will recognize that Vref is about 1.4 volts lower than an implied upstream voltage limit of about 5 volts due to two forward-bias-diode voltage-drops in the peak hold signal path. 
     Turning now, primarily to  FIGS. 6A and 6B , it can be seen that the Vdiff peak voltage will exceed VS 1  when a clipped sine wave is received by audio signal input  22 . A clipped audio input signal which causes Vdiff to exceed VS 1  is limited to a calculated frequency range and clipping percentage. The detectable frequency range and clipping percentage is determined by the tuning of the values of resistors R, R 4 , R 5  (Rt) and capacitor C.
 
Clipping percentage can be found by  C  %=( Tc/t )*100.
 
     Where: 
     Tc=total time duration of clipping within a single cycle of the signal; and 
     T=time duration of a single cycle of the signal. 
       FIG. 6A  shows the values of Vdiff, VS 1 , and Vc when presented with a clipped sine wave. It can be seen that the peak value of Vdiff is higher than that of VS 1 . It can be seen in  FIG. 6B  that, when presented with a clipped signal, the peak value at Vtrig is higher than Vref at the onset of clipping. When the peak value of Vtrig surpasses Vref, the output of U 2  (the clip detect signal) goes to a voltage low and remains there until/unless this condition no longer exists. This is known as the clip detect pulse. 
     For preferred clip detection conditions to occur, the minimum percentage of clipping should be at least about 20% of the total period of the audio input at the targeted frequency (here, a sine wave). The lower the clipping percentage of the total period, the narrower the detectable clipped frequency range becomes. The higher the clipping percentage, the broader the detectable clipped frequency range becomes. The Vdiff peak voltage is preferably at least 100 mV above VS 1  for reliable input clip detection to occur. As noted above, Vdiff can be calculated as VS 2 −Vc, where Vc is the voltage drop across the capacitor. To obtain at least 100 mV above VS 1  at Vdiff, the phase lag (a negative phase shift) of the signal voltage Vc across capacitor C should be between about 88 degrees and about 90 degrees. This corresponds to a phase lag (a negative phase shift) in signal Vdiff of between about 2 degrees and about 0 degrees. This phase lag will also correspond to a negative voltage drop in signal Vc of at least 0.1 volts (restated, −0.1V or more negative) and an equivalent positive voltage spike in signal Vdiff. Surprisingly, Vdiff phase lags of more than about 2 degrees result in little to no voltage spike in Vdiff and the magnitude of the Vdiff voltage spike increases for phase lags approaching the theoretical limit of 0 degrees. The impedance Xc of capacitor C in relation to the resistances of resistors R, R 4 , and R 5  will determine the voltage drop across C and the corresponding voltage rise in Vdiff (where Xc is the impedance of capacitor C at any frequency and Xc=1/(2πfC)). 
     It has been found empirically that the preferred clip detector operating frequency range (at the preferred minimum 20% clipping) is between about 3.5(Rt/Xc) and about 9(Rt/Xc) (where 3.5(Rt/Xc) represents the lowest frequency of the range and 9(Rt/Xc) represents the highest frequency of the range). Where Rt is the total resistance for positive going signals of R and R 5  in parallel expressed as (R*R 5 )/(R+R 5 ). When considering negative going signals, Rt is the total resistance of R and R 4  in parallel expressed as (R*R 4 )/(R+R 4 ). The preferred frequency range in which clipping may be detected may be determined as follows:
 
 f   LOW =1/(2π* X   CL   *C );
 
 f   HIGH =1/(2π* X   CH   *C );
 
 X   CL  for  f   LOW   =Rt/ 3.5; and
 
 X   CH  for  f   HIGH   =Rt/ 9, where  f   LOW  is the lowest frequency and  f   HIGH  is the highest frequency.
 
     Many audio amplifier applications typically fall into low frequency and or high frequency applications (e.g., bi-amplified systems). In such applications, it is preferred to design the clip detection frequency range around the nominal range of frequencies required for an amplifier application. Setting the clip detect range can be done by first selecting the target frequencies. 
     In a low frequency application, the lowest practical frequency is 20 Hz. Based on this application, f LOW  can be set to 20 Hz. A reasonable standard value capacitor for low frequency clip detection is 10 uF.
 
At 20 Hz,  X   CL =1/(2           *20 Hz         10 uF)=796 Ohms;
 
 Rt=X   CL *3.5=796 Ohms*3.5=2.8K Ohms;

     When applying:
 
 f   LOW =1/(2 π*X   CL   *C )=20 Hz;
 
     For solving at f HIGH:  
 
 X   CH   =Rt/ 9=2.8K/9==311 ohms;
 
     When applying:
 
 f   HIGH =1/(2π* X   CH   *C )=51 Hz.
 
     In low frequency applications where 20 Hz is not the desired f LOW , the frequency can be shifted by adjusting Rt. Assuming 50 Hz is the desired f LOW  
 
At 50 Hz,  X   CL =1/(2           *50 Hz         10 uF)=318 Ohms;
 
 Rt= 318*3.5=1.1K Ohms;
 
 f   LOW =50 Hz;
 
 f   HIGH =130 Hz.

     In a high frequency application, 1 kHz, is an appropriate nominal target frequency for clip detection. 1 kHz could be preferred as the center frequency of the clip detect range. To solve for Rt use X CN *6.25 (where X CN  is the capacitor&#39;s impedance at the nominal frequency and 6.25 is the median ratio between 3.5 and 9). A reasonable standard value capacitor for high frequency clip detection is 0.47 uF.
 
At 1 kHz  X   CN =1/(2           *1000 Hz         47 uF)=339 ohms;
 
 Rt= 339 Ohms*6.25=2.11 k;

     When solving for f Low :
 
 X   CL =2.11 k/3.5=603;
 
 f   LOW =1/(2 π*X   CL   *C )=561 Hz.
 
     When solving for f HIGH :
 
 X   CH =2.11 k/9=234;
 
 f   HIGH =1/(2 π*X   CH   *C )=1.44 kHz.
 
     It will be appreciated that diodes D 4  and D 5  also rectify the audio signal and compensate for forward voltage losses of diode D 1  and transistor Q 1  Vbe and seen at output of peak hold  26 . This compensation maintains a substantially equal voltage to the non-inverting and inverting inputs of U 2  when a signal is received. As a result of this configuration, the rectified differentiator audio signal at the negative input of U 2  is Vtrig. Vtrig peak value is Vtrig=Vdiff−Vf 4 −Vf 5 , where Vf 4  equals the forward peak voltage across D 4  and Vf 5  equals the forward peak voltage across D 5 . 
     It will further be appreciated that diodes D 3  and D 2 , and resistor R 4  create a balanced AC load for the output of the differentiator. The load balance prevents voltage offset at Vdiff that would otherwise cause a false clip detect. 
     To a point, the peak voltage at Vdiff is proportional to the percentage of time period clipping. Because of this relationship, an increase in clipping percentage will increase the applicable frequency range of clip detection. As the percentage of clipping increases, the amplitude of the 3 th , 5 th , and 7 th  harmonics of the fundamental frequency also increase. This is true at the moment the angular voltage rise of the audio signal transitions to DC, or clipping. Xc is lower at the higher harmonic frequencies due to the inversely proportional relationship of Xc versus frequency. When harmonic amplitudes increase at the moment of clipping, Xc is reduced at those frequencies, and the resulting voltage seen across resistor R (labeled Vdiff) is increased.  FIG. 8  shows the Fourier transform response of an unclipped 40 Hz sine wave.  FIG. 9  shows the Fourier transform response of a 40 Hz sine wave exhibiting 20% clipping.  FIG. 10  shows the Fourier transform response of a 40 Hz sine wave exhibiting 57% clipping. It can be seen that the harmonic amplitudes are greater at the 3 rd , 5 th , and 7 th  harmonics as the clipping percentage increases. 
     Increased Vdiff peak value due to clipping over 20% can be seen in  FIG. 6A  and  FIG. 6B . The clipping percentage in  FIG. 7A  and  FIG. 7B  is approximately 57%. It can be seen that the increased clipping has also increased the clip detect error signals duty cycle. This is due to the increase in duration of time that Vdiff peak voltage is above VS 1  peak voltage. 
       FIG. 11  is a schematic diagram of an audio signal clip detector  20 ′ in accordance with a second preferred embodiment of the present invention. In this Figure, the feedback of op amp U 1 A ( 24   a ′) includes diode D 1 . This effectively eliminates need to use diodes D 3  and D 4  in the differentiator  28 ′ as compared with the preferred embodiment(s) of  FIGS. 2-10 . Otherwise, this circuit operates in at least generally the same manner as the prior discussed embodiments with the differences readily apparent to those of skill in the art. 
       FIG. 12  is a schematic diagram of an audio signal clip detector  20 ″ in accordance with a third preferred embodiment of the present invention. Unlike the above-discussed preferred embodiments of the invention using positive rectification to achieve clip detection, the preferred embodiment of  FIG. 12  uses negative rectified signals to achieve clip detection. Otherwise, this circuit operates in at least generally the same manner as the prior discussed embodiments with the differences readily apparent to those of skill in the art. 
     It will be appreciated that the present invention permits detection of clipped audio signals as a function of that audio signal itself. Thus, the clip detection described herein is not reliant on a predicted maximum unclipped signal at any one or more stages along the signal path. Indeed, the detector described herein has the ability to detect clipped and non-clipped audio signals when the voltage limitations of upstream audio sources is unknown. However, those of ordinary skill will appreciate, in light of the inventive disclosure, that the invention is even capable of identifying implied/deduced voltage limitations of upstream audio sources. For example, in the embodiment of  FIGS. 2-10  it is readily apparent that the audio input signal shown is voltage limited to about 5 volts (somewhere upstream of detector  20 ) even though the supply voltage for the preferred detector is about 17 volts. 
     The detection pulse produced when clipping is detected can be used to perform any one or more of several functions in an audio system. A given audio application can help determine how best to use the clip detect pulse. Some of the possible uses include, to produce a visual clip indicator, to produce an audible clip indicator, to trigger signal-compression or distortion reduction/prevention, and/or to record clip events, etc. 
     Clip detect indicator  32  is preferably one or more conventional LED&#39;s but may be any one of the many conventional types of visual indicators (LED&#39;s, liquid crystal displays, CRT&#39;s, etc.), audible indicators (piezo tweeters, speakers, etc), vibratory indicators, and/or digital recording means. 
     Comparator/Processor  30  may be implemented as any one of the many known forms of comparators but it also may be implemented in any configuration that functions to receive two or more values/signals and to produce a predictable result based on those values/signals. For example, comparator  30  may be implemented as ratio circuitry which yields a clip detect alert when the ratio of the two input signals is less than (or greater than) one. Other equivalent implementations are well within the skill in the art and the term comparator is intended to literally encompass those as well. 
     Those of ordinary skill will readily appreciate that various aspects of the present invention (including, but not limited to the disclosed peak holds, differentiators, comparators/processors) may be implemented in hardware, software and/or firmware; such implementations are intended to be encompassed by the literal terms of the appended claims and the claims expressly intended to be interpreted as such. 
     For simplicity, certain preferred embodiments have been discussed/explained using sine waves for the received audio input signal(s) and it has been indicated that the invention may be used with known methods of “ringing out” an audio system. However, it will be appreciated that the invention is not so limited. For example, and as discussed above, the frequency characteristics of the preferred differentiator circuitry exhibit a natural tendency to reject frequencies outside of a desired target range. Therefore, the invention is capable of clip detection at frequencies within that range even if the received audio signal contains other frequencies. Although some limitations may apply and some audio signals may perform better than others, this characteristic enables inventive clip detection to use conventional wide range music as the audio input signal. 
     While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to encompass the various modifications and equivalent arrangements included within the spirit and scope of the appended claims. With respect to the above description, for example, it is to be realized that the optimum dimensional relationships for the parts of the invention, including variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the appended claims. Therefore, the foregoing is considered to be an illustrative, not exhaustive, description of the principles of the present invention. 
     Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties, which the present invention desires to obtain. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. 
     Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. 
     Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations. 
     For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, and derivatives thereof shall relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting.