Abstract:
A method for preventing damage to a document by a document transport apparatus provides a member for receiving the document as a stack of sheets to be serial fed to a feeding station. At least two spaced-apart microphones are disposed at the feeding station and responsive to audio to produce signals representing audio energy received by each microphone respectively. The energy received from each microphone is compared to determine if it is ambient noise or if it indicates that two attached sheets are being fed or a single sheet is being damaged. The document transport apparatus is shut off to prevent damage to documents when it has been determined that two attached sheets are being fed or that a single sheet is being damaged.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to document handling systems and more particularly to methods and apparatus for sensing document handling problems during the feed and transport cycle. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various types of sensors have been employed for monitoring document handling in imaging apparatus such as scanners, copiers, printers, fax machines, and other equipment that obtains data from, or imprint images and text onto, paper or other sheet media. Automatic document feed systems have used a range of different types of mechanical, optical, and audio sensors for this purpose. 
         [0003]    Document feeding stations are particularly prone to problems caused by staples and other fasteners, poor document preparation or stacking, folds or wrinkles in the fed media sheet, different media weights and thicknesses, and other media-related problems, as well as problems with the media transport components themselves, caused by wear, dust and dirt, and other factors. These problems can be particularly acute with high-speed scanning systems or with scanners that handle financial and other business documents. Failure to detect a jam or other misfeed condition in time can damage the original document, cause loss of data, require special handling to correct the problem, and reduce equipment efficiency due to down time. 
         [0004]    With no moving parts, requiring no contact with the moving media, and because they are less susceptible to problems caused by dirt or other particulates, audio sensors have some advantages for use along the media transport path. Audio sensors can perform acceptably when used in place of electrical or optical sensors. However, with conventional solutions in deployment of audio sensors, many of the same problems in document feeding and handling persist. 
         [0005]    Thus, it can be seen that there is a need for improved apparatus and methods for detecting document feed problems and helping to prevent damage to documents due to misfeeds and jams. 
       SUMMARY OF THE INVENTION 
       [0006]    Embodiments of the present invention are directed to advancing the art of document feeding and handling in an imaging apparatus. An arrangement of audio sensors helps to detect jams and misfeeds at the document feeding station in a timely manner, enabling the media transport apparatus to stop before further damage to the document occurs and, optionally, signaling an error condition to an operator. 
         [0007]    According to an aspect of the present invention, there is provided a method for preventing damage to a document by a document transport apparatus, comprising 
         [0008]    a) providing a member for receiving the document as a stack of sheets to be serial fed to a feeding station; 
         [0009]    b) providing at least two spaced-apart microphones disposed at the feeding station and responsive to audio to produce signals representing audio energy received by each microphone respectively; 
         [0010]    c) comparing the energy received from each microphone to determine if it is ambient noise or if it indicates that two attached sheets are being fed or that a single sheet is being damaged; and 
         [0011]    d) shutting off the document transport apparatus to prevent damage to documents when it has been determined that two attached sheets are being fed or a single sheet is being damaged. 
         [0012]    These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings, in which: 
           [0014]      FIG. 1  is a side cross-sectional view of a document scanner that uses the document feed apparatus sensing according to an embodiment of the present invention; 
           [0015]      FIG. 2A  is a top view of the document feed apparatus and sensors according to an embodiment of the present invention; 
           [0016]      FIG. 2B  is a top view of the document feed apparatus and sensors showing characteristics of an audio signal having the same amplitude at each left and right microphone. 
           [0017]      FIG. 2C  is a top view of the document feed apparatus and sensors showing characteristics of an audio signal having different amplitudes at left and right microphones. 
           [0018]      FIG. 3A  is a graph showing cross-correlation with reference to the example of  FIG. 2B . 
           [0019]      FIG. 3B  is a graph showing cross-correlation with reference to the example of  FIG. 2C . 
           [0020]      FIG. 4  is a schematic block diagram that shows the audio signal path in the document feed sensing apparatus; 
           [0021]      FIG. 5  is a logic flow diagram that shows audio frame processing according to an embodiment of the present invention. 
           [0022]      FIG. 6  is a schematic block diagram showing an adaptive audio filter. 
           [0023]      FIG. 7  is a logic flow diagram that shows execution of jam detection and algorithm application. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    In the following detailed description of embodiments of the present invention, reference is made to the drawings in which the same reference numerals are assigned to identical elements in successive figures. It should be noted that these figures are provided to illustrate overall functions and relationships according to embodiments of the present invention and are not provided with intent to represent actual size or scale. 
         [0025]    Embodiments of the present invention address the problem of document feed monitoring and jam detection using an arrangement of audio sensors that cooperate to indicate that a problem has occurred. As noted previously in the background section, audio sensing has advantages over other types of sensing mechanisms, such as those that require contact with the moving document. There are, however, problems peculiar to audio sensing, such as the requirement to distinguish sounds indicating a jam or feed problem from ambient noise from outside the document handling system as well as from noise due to document handling system components themselves. Unlike conventional approaches that merely replace mechanical or optical sensors with audio equivalents, embodiments of the present invention take advantage of signal differences among multiple audio sensors and are thus capable of obtaining more accurate information about the nature and source of document feed problems than is available using conventional sensor solutions. As a result, a jam condition is more accurately detected and corrective action initiated to help prevent further damage to the document as it enters the media transport path. 
         [0026]    In the context of the present embodiment, the terms “microphone” and “audio transducer” are used interchangeably to describe a component that provides an analog or digital output signal according to an audio input signal. 
         [0027]    The description that follows references an exemplary document handling and transport system that is used in a digital scanner. It should be noted that the apparatus and methods of the present invention can also be used with document handling equipment for imaging apparatus and equipment of other types, such as those used for copiers, fax machines, printers, binding devices, and other systems. The document feed tray, or other member for receiving the document as a stack of serial-fed sheets, can include single-sheet feed, top feed, bottom feed, or other serial feed configurations. Particular arrangements of rollers, feed mechanisms, and other document handling components can vary significantly from those described below. 
         [0028]      FIG. 1  is a side cross-sectional view of one type of imaging apparatus, a document scanner  4 , showing portions of a document transport apparatus  48 . As shown in  FIG. 1 , documents  5  are fed into scanner  4  in serial form from an input tray  10  of scanner  4 . When documents  5  enter scanner  4 , urge rollers, feed rollers, and separation rollers of a document feed station, document feed apparatus  15 , urge the document  5  from input tray  10 , separating the documents  5  from one another, and direct the document  5  down a media transport path  30  and past scanning elements  40  and  45 . The newly scanned document then proceeds along transport path  30  until it is ejected onto an output stack  35 . As shown in  FIG. 1 , input tray  10  can include documents that have some amount of folds or wrinkles. An elevator  25  lifts input tray  10  into vertical position for serial document feeding. 
       Audio Sources and Sensing 
       [0029]    The action of various document transport apparatus components in operation, such as separation rollers of document feed apparatus  15  in separating the documents  5  from one another, generates sound that is characterized and used to sense normal operation or to indicate some type of document handling problem. In the case of a stapled or jamming document, for example, separation roller handling produces a wrinkling sound that is characteristically different from the sound that is produced when an unwrinkled, well-preserved document enters scanner  4 . Referring to the top view of a document feeding station  46  in  FIG. 2A , with travel direction T indicated from the front of the scanner  4  or other document feed apparatus  15 , the sounds of the document  5  are picked up by an arrangement of microphones or other audio transducers  26 ,  11 , and  6 , with corresponding microphone positions marked L (left), R (right) and M (middle), that are part of a document feed sensing apparatus  20 . Feed rollers  32  are shown as part of a feed roller apparatus  16  and schematically represent any of a number of roller types, including rollers for document urging and separation. The obtained audio signal  55  ( FIG. 1 ) is sent to be conditioned, digitized, and processed on a digital signal processing (DSP) platform  50  ( FIG. 1 ). DSP platform  50  can be any of a number of types of digital processors for processing the audio signals from audio transducers  26 ,  11 , and  6 . Suitable processors for the DSP function can include microprocessors or other dedicated processors, programmable logic devices, or a local or networked computer or computers that respond to programmed instructions to analyze the audio data and provide one or more suitable output signals, which signals can include data content or can be analog signals. 
         [0030]    The particular arrangement of audio transducers  6 ,  11  and  26  at L, R, and M positions is provided in order to help to detect and filter out ambient noise and to sense proper operation of document feeding. Ambient sound coming from the front of the scanner (at 0° incidence) will be detected equally at L and R positions and a cross correlation between the signals coming from the corresponding audio transducers  11  and  26  will yield a high value of cross-correlation coefficient around the 0 th  lag. The top view of  FIG. 2B  shows how sound emanating from a source S that is in front of the document feed sensing apparatus  20  strikes both L and R front microphones, audio transducers  26  and  11 , equally. This is similar to the sensed signal arrangement with ambient sound, such as sound from an office environment in which a scanner or other document handling apparatus is used, for example. That is, broadly-distributed ambient sound is indicated by generally equal audio signals at L and R microphones. 
         [0031]    The top view of  FIG. 2C  shows an alternate case, in which sound appears to emanate from one side, shown as the left side in this example, or the other. Here, the sound signal appears to emanate from source S at about 45 degrees from the front of document feed sensing apparatus  20 . This can be due to ambient sound from one side of the unit or to document handling sound along one side of the document feed path input, for example. 
       Cross-Correlation 
       [0032]    Embodiments of the present invention use cross-correlation to help determine whether or not detected noise is ambient noise and to help to differentiate ambient noise from document handling noise. Well known to those skilled in the signal processing arts, cross-correlation gives a measure of how similar two signals are, as a function of the time lag between them. In execution, cross-correlation sums the product of two signals where they overlap. For signals from microphones at L and R, cross-correlation processing tests by shifting one signal past the other to determine where the computed correlation is highest. 
         [0033]    The graph of  FIG. 3A  shows a cross-correlation curve  44  for two audio signal frames that are substantially identical, such as in the example of  FIG. 2B . Here, the peak of cross-correlation curve  44  shows 0 lag between the compared signals. This type of cross-correlation, with near-zero lag, is common for ambient signal detection. By comparison, the graph of  FIG. 3B  shows a negative cross-correlation curve  44  that corresponds to the signal sensing condition of  FIG. 2C . The absolute value of the cross-correlation peak remains high; only the lag (by some sample value) indicates the difference between signals. 
         [0034]    To improve comparison between L and R microphone channels, the cross-correlation coefficient is normalized by dividing it by an auto-correlation coefficient that relates a microphone&#39;s output to itself. This helps to reduce the microphone&#39;s effect on the data, permitting the underlying characteristics of the audio signal to be compared on a common scale. 
         [0035]    Where cross-correlation is below a predetermined correlation threshold, as described in subsequent procedures, the two L and R microphones are not capturing signals from the same audio source. This indicates, for example, a staple or other problem that is near one or the other microphone. Where cross-correlation exceeds the correlation threshold, on the other hand, the L and R microphones are assumed to be capturing the same noise. 
       Audio Detection and Processing Sequence 
       [0036]    Referring again to  FIG. 2A , where there is a staple along a trail edge or rear edge of the document  5 , the document  5  first gets pulled into the scanner  4  until the feed rollers  32 , used as separation rollers in the described embodiment, attempt to un-staple the document  5 . This document un-stapling happens very close to either microphone at L or R, depending on the position of the staple; hence the jamming sound is pronounced at one of the L or R positions, corresponding to the staple location, and weak at the other position. As described previously, the cross-correlation between the signals from the two L/R microphones in this case will have a lower peak value around the 0 th  lag, and thus the sound source for handling the document is identified. 
         [0037]    In the event that both jam sound and ambient sounds are present, further signal processing is helpful. In this case, adaptive filtering is performed, with the selected L/R microphone channel as primary signal and the other channel as reference signal, to help isolate and remove the ambient sound from the jam sound. The detailed description of this signal processing is given subsequently. 
         [0038]    Consistent with an embodiment of the present invention, the algorithm for jam detection is a frame based-processing technique that works on a frame of some number N of audio samples at a time from the L, R, or M audio transducers  26 ,  11  and  6 . Initially, the algorithm receives one frame from each L, R and M microphone and it selects one microphone at a particular instant for further processing, based on the highest energy detected in these sampled frames. Since ambient sound approaching the scanner  4  is always received first at L or R microphones, by the time the ambient sound reaches the middle M microphone, its energy is correspondingly reduced. Hence, when middle microphone M has the highest energy of all the frames and is then selected, it is sent on for further processing, to determine whether or not this indicates a jam condition. Because of its position within the scanner  4 , the middle microphone M is neither used to measure ambient sound nor involved in the determination of the sound source, as described previously. 
         [0039]    Advantageously, methods of the present invention are able to adjust transducer sensitivity as the medium advances from the input tray  10  and begins to move to positions along transport path  30 . For example, the algorithm also accommodates noise associated with entry of the trail edge of the document into document transport path  30  ( FIG. 1 ). During this time, as the document falls from a higher level of elevator  25  ( FIG. 1 ) to a lower level on the document transport path  30 , a sharp impulse occurs in the audio signal  55 . This impulse is similar to the sound made by a rear stapled document being separated at the trail edge. Microphones at L and R positions, because they are located toward the input tray  10  and are generally directed outward to detect ambient and input tray sound, do not readily detect this impulse, since the sound is generated behind these microphones relative to the media path. Instead, the middle microphone at M detects this sound. To help avoid false positives in the jam detection system due to this noise, the sensitivities to signals from the microphones at L, R, and M are varied as the document travels along the document transport path  30  ( FIG. 1 ). Change in microphone or signal sensitivity and how this is used is described in more detail subsequently. 
         [0040]    The audio signal path for each of audio transducers  6 ,  11 , and  26  is designed to condition the audio signal  55  and to provide a corresponding digital signal to DSP platform  50  for analysis. As shown in  FIG. 4 , audio transducer  6  (or other transducer  11  or  26 ) provides a signal to a signal conditioning circuit  60  that provides filtering to remove noise and can further condition the signal for subsequent processing. The conditioned analog signal is then sampled and digitized by an analog to digital A/D converter  65  at an appropriate rate to avoid aliasing of the highest frequency present in the signal. A/D converter  65  then provides this data to DSP platform  50 . DSP platform  50  provides an output signal that indicates jam detection. This signal can then be used to stop document feed apparatus  15  or to stop transport rollers or other components that otherwise conduct the document  5  along media transport path  30  and, optionally, to provide a warning, such as by energizing an indicator light, for example. 
         [0041]    Embodiments of the present invention differentiate between the sound made by a normal paper sheet entering the document scanner  4  and a wrinkling sound made by the paper in case of jam. In this system, it is useful to ignore or in some way to isolate background sounds of the scanner  4  or other document handling device from the wrinkling or jamming sounds. Normal background sounds come from different moving parts of the scanner, such as the transport motor, fan, clutch, front and rear lamp, and elevator. These background sounds are periodic and have low frequency components. On the other hand, the audio signal from a wrinkling document, is a short duration signal in the time domain, has frequency components spread over a wide range in the frequency domain. Therefore, computing the energy of the audio signal  55  in a band between two frequencies F 1  and F 2 , and assuming that the sound from the background and from a clean document  5  entering the scanner  4  has frequencies concentrated below F 1 , differentiation is made between normal scanning of documents and an event in which a document starts to jam. 
         [0042]    Another aspect of this system is to ignore ambient background sound typical of an office environment, such as sound from people talking, music playing, and other normal workplace sounds that can occur near the scanner. To avoid interference of these sounds with the working of the algorithm, multiple audio sensors are placed inside the scanner in such a way that the system can reliably detect jams occurring near the feeder area by discriminating expected sounds from those that indicate a jam. 
         [0043]      FIG. 5  is a logic flow diagram that shows how the N audio frames are processed according to an embodiment of the present invention. A selection step  100  selects the highest energy frame, whether from either microphone at positions L and R or from middle microphone M. Having the highest energy indicates the strongest audio signal at that particular audio transducer. Consistent with one embodiment of the present invention, selection step  100  uses the following formula for each microphone channel to obtain the largest absolute average: 
         [0000]    
       
         
           
             
               ( 
               
                 
                   ∑ 
                   1 
                   N 
                 
                  
                 
                   abs 
                    
                   
                     ( 
                     
                       mic 
                       data 
                     
                     ) 
                   
                 
               
               ) 
             
             / 
             N 
           
         
       
     
         [0000]    If middle microphone M is selected, a leading-edge jam (e.g., due to a staple in the front) or a misfeed is more likely, most readily detected due to positioning of the microphone at this point. If microphone M is selected, a jam detection process  110  is initiated. Jam detection process  110  works through a number of sub-steps in order to determine whether or not a jam is detected. It is useful to establish how far the document  5  has progressed from the input tray  10  and into the transport path  30 . This is determined by simply counting the number of audio frames obtained for this document. A decision step  120  compares the frame count against a threshold value for sensitivity switching, termed a Sensitivity Switch Point (SSP) in one embodiment of the present invention. A count above the SSP value indicates, for example, that the document has progressed further than the first few inches into the transport path  30 . If this is the case, an algorithm application step  130  with low sensitivity is executed, as shown in  FIG. 5 . If the document has not yet progressed this far, an alternate algorithm application step  140  with high sensitivity is executed. 
         [0044]    As used for this purpose, ‘SSP’ relates to the number of samples corresponding to the time taken by the smallest length document, that can be fed to the scanner, to enter the document transport from the elevator: 
         [0000]      SSP= Fs*L/S    
         [0000]    Where, Fs—Sampling frequency in Hz
       L—Length of smallest document specified for a particular scanner in inches   S—Speed of the media transport in inches/s.
 
This check is useful to help compensate for the impulse noise associated with the microphone position M, when the trail edge of the document enters the document transport.
       
 
         [0047]    Thus, in embodiments of the present invention, the sensitivities of microphones at L, R, and M are adjusted according to the position of the medium in the media transport path, with transitions timed appropriately. When the document is initially fed into the transport path  30 , with up to about 4-5 inches fed into the transport path  30 , middle microphone M has high sensitivity. As the media moves further along beyond that point, sensitivity changes so middle microphone M then has low sensitivity. With this adjustment, it is possible to improve the response of document feed sensing apparatus  20  to jams and misfeed problems at different points in the document feed cycle. Document feed sensing apparatus  20  can include a way for adjusting the sensitivity of one or more of the audio signals, such as the sensitivity of the middle audio signal, according to the length of the document. Sensitivity adjustment are performed by signal conditioning circuit  60 , for example, as shown in  FIG. 4 . 
         [0048]    Along the alternate execution path in the logic flow of  FIG. 5 , an audio frame from microphones at L or R might exhibit the highest energy. This high audio average signal can indicate ambient noise or some problem with a document fastened or stapled along its trailing edge. If the frame is from left microphone L, energy from right microphone R is compared with an energy based noise threshold (termed the Absolute_Ambient_thresh or Ambient_thresh according to one embodiment of the present invention) in a decision step  150 . In the case of ambient sound, energy from the right microphone R signal is higher than this Absolute_Ambient_thresh value, since ambient sound is arriving at both the microphones. A normalized cross correlation, with its value ranging between 0 and 1, is performed on the left microphone L signal and on the right microphone R signal. The maximum value (or peak) of the cross-correlation coefficient around +/−X samples of 0 th  lag is compared to a set threshold (termed corr_thresh in this example). Corr_thresh can be set to 0.5, indicating a 50% confidence level. A correlation coefficient at or above 0.5 indicates that the two input signals are similar or well correlated, as that terminology is used in the present disclosure. A correlation coefficient below this threshold value indicates input signals that are not well correlated. The value of X can be calculated by knowing the system parameters as: 
         [0000]    
       
      
       X=Fs*D/S  
      
     
         [0000]    Wherein, Fs—sampling frequency in Hz;
       D—Distance between left and right microphone in m;   S—Speed of sound in air m/s.
 
Hence, depending on the direction left or right from which the ambient sound is approaching the microphones, the peak of the cross-correlated signal shifts correspondingly left or right of the 0 th  lag, by X samples respectively.
       
 
         [0051]    Continuing with the processing shown in  FIG. 5 , the particular frame, whether L or R, is processed in subsequent steps. If a decision step  150  determines that the energy is less than the Ambient_thresh threshold value, the frame is sent for further processing, as indicated by a jam detection process  112 . Here again, entry into jam detection process  112  does not indicate that a jam exists; further processing is needed to determine this. Results are passed to an algorithm application step  140  with high sensitivity since the frame is either from L or R microphone. If decision step  150  determines that that the energy exceeds the threshold, the cross-correlation of frames for L and R microphones is performed in step  160 , as noted above. Another decision step  170  checks the cross-correlation coefficient against the corr_thresh value described previously. If this is in excess of this threshold, the audio energy is determined to be from ambient noise  180  and the corresponding audio frame is ignored. If less than this threshold, an adaptive filter step  190  is executed on the selected channel and execution passes to algorithm application step  140 . 
         [0052]    When there is a staple or other fastener on the left trailing edge of the document  5  and ambient sound is also present, the energy from right microphone exceeds the Absolute_Ambient_thresh value and the left microphone L signal does not correlate with the right microphone R signal. The peak value of cross correlation step  160  is below the corr_thresh value. In this example, both signals are given to the adaptive filter in step  190  with the left microphone L acting as primary signal and right microphone R acting as reference signal. 
         [0053]    Using the method of the present invention, not only can a jam condition be detected, but the location of the problem can also be identified in many cases. This capability permits document feed sensing apparatus  20  to both stop feeding the document  5  from the input tray  10  and report the location of the problem, whether along the leading or trailing edge and whether on the left or right side of the document  5 . Document feed sensing apparatus  20  generates an output signal that is used to energize a control panel indicator or provide an electronic message that indicates the likely problem source. 
         [0054]    The schematic block diagram of  FIG. 6  shows an adaptive filter  92 . The objective of the adaptive filter is to change (adapt) the coefficients of an FIR (Finite Impulse Response) filter, W  80 , so that the reference signal matches as closely as possible the response of the filter system, Y. The filtered signal from the adaptive filter is free from the ambient sound and is passed on for further processing by the algorithm. 
         [0055]    It is appreciated that the logic flow shown in  FIG. 5  can be executed in different ways and can be modified and adapted to suit the requirements and problems of a particular document handling system. General principles that have been found to be of particular value for jam detection when using the multiple audio transducer arrangement of  FIG. 2A  include the following:
       (i) Processing of the highest energy frame(s). Jam or misfeed noise is most likely to be detected in the highest energy frame or frames.   (ii) Cross-correlation to compare L and R microphone signals. Where these signals are very similar, cross-correlation increases and ambient noise is assumed, as noted earlier.   (iii) Additional data is collected. Because the process of  FIG. 5  is executed quickly, there is time to repeat processing with an updated set of N frames. This capability can help to reduce ambiguity in jam detection.       
 
         [0059]    The logic flow diagram of  FIG. 7  shows the execution used for jam detection steps  110  and  112  and algorithm application steps  130  and  140  according to an embodiment of the present invention. The selected audio frame first goes to a band pass filter  84  with cut-off frequencies from F 1  to F 2 . Cut-off frequency F 1  is selected such that the background sound from different moving parts of the scanner or other device and the sound associated with a document in good condition is fed into the scanner lie below this cut-off value. In one embodiment of the present invention, cut-off frequency F 1  is approximately 1000 Hz. F 2  is the upper frequency of a document that is jamming or wrinkling. Frequencies above F 2  are not related to document handling. Ambient noise is broadly distributed in terms of frequency and spans the full range of frequencies up to and beyond F 2 . 
         [0060]    A median filter  86  is used to help avoid any false jam detection, such as what might occur if a pre-wrinkled or folded document were fed to the scanner. By comparing the audio signals for a wrinkling document (true jam) and a pre-wrinkled document, an observation can be made that the audio signal for the wrinkled document has intermittent high peak values as against an actively wrinkling document that has continuous high values of amplitude as the paper starts to jam. Thus, applying a 1D median filter to the audio signal of the incoming document helps to reduce the effect of intermittent spikes and thus reduce the potential false jam detection. After passing the audio frame through filters  84  and  86 , energy of this filtered signal is calculated in an energy calculation step  88  and compared with a threshold in a threshold detection step  90  to determine if it indicates a jam or a “clean” document, that is, a document in good condition that fed properly into the scanner or other device. 
         [0061]    One problem that is addressed by embodiments of the present invention relates to differences in sound that are characteristic of the location of the trailing edge of the document  5  along the transport path  30 . A page separation mechanism or other device along the transport path  30  can be the cause of a sound impulse that is normal, but can be misinterpreted, as the document sheet moves past. To help reduce the effects of this type of normal sound variation, embodiments of the present invention adjust the sensitivity of detection logic to the audio signal  55  according to relative location along the media transport path. Position along the transport path is determined, for example, by maintaining a count of audio frames obtained for a document. 
         [0062]    The invention has been described in detail with particular reference to presently preferred embodiments, but it will be understood that variations and modifications can be effected that are within the scope of the invention. For example, adjustment of sensitivity to audio signals can be obtained in a number of ways, such as by attenuating the signal obtained by the corresponding microphone or by conditioning digital data from the received signal. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.
     4  Document scanner     5  Document     6  Audio transducer     10  Input tray     11  Audio transducer     15  Document feed apparatus     16  Feed roller apparatus     20  Document feed sensing apparatus     25  Elevator     26  Audio transducer     30  Transport path     32  Feed roller     35  Output stack     40  Scanning element     45  Scanning element     44  Correlation curve     46  Document feeding station     48  Document transport apparatus     50  DSP platform     55  Audio signal     60  Signal conditioning circuit     65  A/D converter     80  FIR filter     84  Band pass filter     86  Median filter     88  Energy calculation step     90  Threshold detection step     92  Adaptive filter     100  Selection step     110  Jam detection process     112  Jam detection process     120  Decision step     130  Algorithm application step     140  Algorithm application step     150  Decision step     160  Cross-correlation step     170  Decision step     180  Ambient noise     190  Adaptive filter step   S Source   T Travel direction   L Left   R Right   M Middle   W Filter   Y Filter System