Abstract:
A method and apparatus are provided for detecting network impairments through tilt-normalized measurement data, the method including: collecting data for a network signal; computing a best-fit tilt for the collected data; performing tilt-normalization of the collected data responsive to the computed best-fit tilt; and determining whether the tilt-normalized data crosses a threshold, and if so, pattern matching the tilt-normalized data to detect a network impairment; and the apparatus including: an input unit for collecting data from a network signal; a tilt unit connected to the input unit for computing a best-fit tilt for the collected data and performing tilt-normalization of the collected data responsive to the computed best-fit tilt; and a pattern matching unit connected to the tilt unit for determining whether the tilt-normalized data crosses a threshold, and if so, pattern matching the tilt-normalized data to detect at least one network impairment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 12/241,365, filed on Sep. 30, 2008 and entitled “CATV Digital Receiver Intermodulation Susceptibility Tester”, which, in turn, claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 60/976,823, filed on Oct. 2, 2007, the disclosures of which are incorporated by reference herein in their entireties for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present disclosure relates to network analysis. More particularly, the present disclosure relates to normalizing measurement data and compensating for tilt to facilitate detection of network impairments. 
         [0003]    Limit checking over multiple channels at the same time is typically only effective if the tilt of the data is at or near zero. This is a problem because users have to make limits too wide in order to account for tilt at a given location in the network. Once limits are expanded, certain error conditions such as roll-off, suck-out, and standing waves are difficult to detect. What is needed is a process for identifying these error conditions by performing tilt-normalized limit checks. 
       SUMMARY OF THE INVENTION 
       [0004]    These and other issues are addressed by a method and apparatus for detection of network impairments through tilt-normalized measurement data. Exemplary embodiments are provided. 
         [0005]    An exemplary method of the present disclosure includes collecting data for a network signal, computing a best-fit tilt for the collected data, performing tilt-normalization of the collected data responsive to the computed best-fit tilt, and determining whether the tilt-normalized data crosses a threshold, and if so, pattern matching the tilt-normalized data to detect at least one network impairment. 
         [0006]    An exemplary apparatus of the present disclosure includes apparatus for detecting network impairments through tilt-normalized measurement data, the apparatus comprising: an input unit for collecting data from a network signal, a tilt unit in signal communication with the input unit for computing a best-fit tilt for the collected data and performing tilt-normalization of the collected data responsive to the computed best-fit tilt, and a pattern matching unit in signal communication with the tilt unit for determining whether the tilt-normalized data crosses a threshold, and if so, pattern matching the tilt-normalized data to detect at least one network impairment. 
         [0007]    The present disclosure will be further understood from the following description of exemplary embodiments, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present disclosure will be described in greater detail with reference to the accompanying drawings, which represent exemplary embodiments thereof, in which: 
           [0009]      FIG. 1  is a schematic diagram of an intermodulation testing device in accordance with an exemplary embodiment of the present disclosure; 
           [0010]      FIG. 2  is a functional diagram of a controller for the testing device of  FIG. 1  in accordance with an exemplary embodiment of the present disclosure; 
           [0011]      FIG. 3  is a functional diagram of a controller with tilt compensation for the testing device of  FIG. 1  in accordance with an exemplary embodiment of the present disclosure; 
           [0012]      FIG. 4  is a graphical diagram of measured powers of analog channels in accordance with an exemplary embodiment of the present disclosure; 
           [0013]      FIG. 5  is a flow diagram for a method of intermodulation susceptibility testing device in accordance with an exemplary embodiment of the present disclosure; 
           [0014]      FIG. 6  is a flow diagram of a method for detection of network impairments through tilt-normalized measurement data in accordance with an exemplary embodiment of the present disclosure; 
           [0015]      FIG. 7  is a graphical diagram of measured level versus frequency for home certification results in accordance with an exemplary embodiment of the present disclosure; 
           [0016]      FIG. 8  is another graphical diagram of measured level versus frequency for home certification results in accordance with an exemplary embodiment of the present disclosure; 
           [0017]      FIG. 9  is yet another graphical diagram of measured level versus frequency for home certification results in accordance with an exemplary embodiment of the present disclosure; 
           [0018]      FIG. 10  is an additional graphical diagram of measured level versus frequency for home certification results in accordance with an exemplary embodiment of the present disclosure; 
           [0019]      FIG. 11  is a graphical diagram of a measurement menu in accordance with an exemplary embodiment of the present disclosure; 
           [0020]      FIG. 12  is a bar graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0021]      FIG. 13  is another bar graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0022]      FIG. 14  is still another bar graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0023]      FIG. 15  is yet another bar graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0024]      FIG. 16  is a filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0025]      FIG. 17  is another filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0026]      FIG. 18  is a bar graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0027]      FIG. 19  is a filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0028]      FIG. 20  is a bar graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0029]      FIG. 21  is a filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0030]      FIG. 22  is an additional filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; 
           [0031]      FIG. 23  is another filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure; and 
           [0032]      FIG. 24  is yet another filled graph diagram of measured level versus frequency for drop check results in accordance with an exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    The present disclosure provides a method and apparatus for detection of network impairments through tilt-normalized measurement data. Intermodulation (IM) distortion is generated within a digital receiver, such as a cable television (CATV) digital receiver, when adverse signal conditions are present. Such adverse signal conditions may include too much power relative to the desired signal at frequencies above and/or below the frequency band containing the desired signal, for example. 
         [0034]    As shown in  FIG. 1 , an intermodulation testing device is indicated generally by the reference numeral  100 . The tester  100  includes a controller  110 , a radio frequency (RF) tuner  120  connected to the controller, a detector  130  connected to both the controller and the tuner, and a display  140  connected to the controller. Here, the RF tuner  120  is capable of tuning to any channel being broadcast on the CATV network. The detector  130  may include one or more appropriate detectors for measuring power of either analog TV channels and/or digital channels. The controller  110  may include non-volatile memory for storing both an operating program and configuration data. The display  140  may be as simple as an indicator light or as elaborate as a touch screen for configuring the device, selecting channels, and reporting measurement progress and results. 
         [0035]    Turning to  FIG. 2 , a method executable by the controller  110  of  FIG. 1  is indicated generally by the reference numeral  200 . The method  200  includes a channel selection block  210 , which passes control to a digital channel measurement block  212 . The digital channel measurement block receives a channel plan  214 , which contains a description of the channels being transmitted on the cable, including frequency and modulation type. The digital channel measurement block  212  measures the power of a selected digital channel. For example, one method of measuring the digital channel power is known in the art as Digicheck. Another less accurate method is to measure the power at the center frequency and add a bandwidth compensation factor based on the ratio of digital channel bandwidth to measurement bandwidth. The digital channel measurement block  212  may perform the power measurement periodically in order to update the display with current results. 
         [0036]    Another channel measurement block  216  also receives the channel plan  214 , and measures the power of all or a subset of the channels being transmitted. The other channel measurement block  216  may measure all the channels, or just those that could substantially contribute to intermodulation distortion. Such other measured channels could be the video carriers of the analog TV channels, for example, since they normally have the highest power. The other channel measurement block  216  may perform only one measurement for each channel, measure all of them periodically, or select a small number with the highest power and measure them periodically in order to update the display with current results. 
         [0037]    An evaluation block  218  is connected to both the digital channel measurement block  212 , for receiving measured channel power and frequency, and to the other channel measurement block  216 , for receiving measured power versus frequency. The evaluation block  218  compares the power of the digital channel being tested to the power of the other channels and determines whether a device receiving the digital channel is susceptible to intermodulation distortions. 
         [0038]    Different types of evaluations are possible in the evaluation block  218 . In a first example, the evaluation block  218  may subtract the digital channel power from the highest of the other channels&#39; measured powers. If this result is over a threshold value or configurable limit  220 , the channel is indicated as susceptible. The degree of susceptibility may be indicated by the amount that the difference exceeds the threshold. In a second example, the evaluation block  218  may sum the measured powers of the other channels to get a total integrated power, and then subtract the digital channel power from this sum. 
         [0039]    As in the first example, the evaluation block compares the value to a threshold to evaluate the susceptibility to intermodulation distortions. In a third example, the evaluation block  218  may consider the capability of a tuner to reject off-frequency signals as a function of frequency or difference in frequency between the digital channel being received and the frequency of the other signal. Here, the evaluation block  218  sums the amount by which the power of any channel other than the digital channel being evaluated exceeds the device&#39;s rejection capability. The sum of these is defined herein as the “total overload power”. As in the first example, the evaluation block compares this value to a threshold to evaluate the susceptibility to intermodulation distortions. 
         [0040]    In an alternate embodiment, the method may measure the powers of only those channels for which sums and differences of harmonic frequencies of two or three channels falls within the frequency band of the first digital channel. Here, composite second order (CSO) distortion is the sum or difference of two signals or their harmonics, and composite triple beat (CTB) distortion is the sum and/or difference of three signals or their harmonics. 
         [0041]    The controller  110  or tester  100  of  FIG. 1  may further evaluate the susceptibility of a single digital channel selected by the user. The tester may also scan all digital channels and report the susceptibility of each. The tester may report which channel is most susceptible and the particular susceptibility of only that channel. The tester may display or otherwise output a susceptibility report or result  224 , which may be a pass/fail indicator and/or the degree of susceptibility, displayed either numerically or graphically. The tester may evaluate susceptibility compared to a reference specification from a device specification database  222 , which database may provide overload power versus frequency. The specification may be configurable. The tester  100  may further contain a database of the susceptibility characteristics of various receiver devices. A user of the tester could select a device from the database, and the tester would report its susceptibility. 
         [0042]    The susceptibility result  224  of this exemplary embodiment may be valid for devices connected at or near the same location as the tester. The signal may have a different tilt at other locations. 
         [0043]    Turning now to  FIG. 3 , another method executable by the controller  110  of  FIG. 1  is indicated generally by the reference  300 . The method  300  is similar to the method  200 ; so duplicate description shall be omitted. The method  300  includes extra functional blocks to perform tilt compensation, such as to measure susceptibility at other locations distant from the location of the tester  100 . 
         [0044]    The method  300  includes a channel selection block  310 , which passes control to a digital channel measurement block  312 . The digital channel measurement block receives a channel plan  314 . The digital channel measurement block  312  measures the power of a selected digital channel. Another channel measurement block  316  also receives the channel plan  314 , and measures the power of at least some of the other channels being transmitted. 
         [0045]    A tilt calculator  326  is connected to the other channel measurement block  316 , and provides a tilt reference  330 . A tilt compensator  328  is connected to each of the digital channel measurement block  312 , the other channel measurement block  316 , and the tilt calculator  326  for receiving a current tilt value. 
         [0046]    An evaluation block  318  is connected to both the digital channel measurement block  312  and the tilt compensator  328  for receiving compensated power of other channels. The evaluation block  318  may receive a limit specification or threshold  320  and/or information from a device specification database  322 . The evaluation block  318  compares the power and/or compensated power of the digital channel being tested to the compensated powers of the other channels, determines whether a device receiving the digital channel is susceptible to intermodulation distortions, and outputs a corresponding result  324 . 
         [0047]    Preferred embodiments of the test device  100  of  FIG. 1  may use a relative power ratio between the analog video channels and a digital channel in order to predict the likelihood that other devices receiving that digital channel will be impaired by internal intermodulations. Such other devices may be of different types, each having unique specifications stored in the device specification database  322 , for example. A tester  100  may further generate a tilt line for use as the reference power so that the same device will give the same results at different points in the network that have different tilts. 
         [0048]    As shown in  FIG. 4 , a plot of measured powers of analog TV channels is generally indicated by the reference numeral  400 . The plot  400  includes measured analog signal powers  412 ,  414 ,  416 ,  418 ,  420 ,  422 ,  424 ,  426 ,  428 ,  430 ,  432 ,  434 ,  436  and  438 , each at a different carrier frequency. Here, a first frequency span S 1  includes the measured powers  412 ,  414 ,  416  and  418 . A second frequency span S 2  includes the measured powers  418 ,  420 ,  422 ,  424 ,  426  and  428 ; and a third frequency span S 3  includes the measured powers  428 ,  430 ,  432 ,  434  and  436 . The tilt calculator  326  of  FIG. 3  may use these measured powers to compute a slope of a tilt line. A tilt line is a straight line intersecting the measured levels of two of the highest channels such that all other channels have less than or equal to the power at that frequency on the tilt line. 
         [0049]    In the exemplary plot  400 , tilt lines L 1 , L 2  and L 3  are present. If two or more possible tilt lines are found, as here, the one covering the widest frequency span is used. From the three possible tilt lines, L 2  is selected over L 1  and L 3  because it has the widest frequency span S 2 . If two or more tilt lines are found having equal frequency spans, the one with less tilt is used. The tilt calculator  326  outputs the slope of the tilt line, but need not calculate nor output the y-intercept. 
         [0050]    A user may assume that the signal has no tilt at the point that subscriber equipment is connected. Alternatively, the user may use the device to measure the actual tilt at the subscriber location. If the actual tilt is used, the device may store it as a reference tilt. If known, the reference tilt value may also be entered into the device without performing a tilt measurement. 
         [0051]    When tilt compensation is used, the device first measures the tilt. It uses the difference between the current tilt and the reference tilt to adjust the measured powers of all channels. The amount of adjustment is given by the equation: 
         [0000]      tilt Comp ( freq )=( ref Tilt−currentTilt)*( freq−digFreq )  (Eqn. 1) 
         [0052]    In Equation 1, currentTilt is the measured tilt at the current location, refTilt is the tilt at the location of the subscriber&#39;s receiver, freq is the frequency of the channel being adjusted, and digFreq is the frequency of the digital channel for which susceptibility is being evaluated. The adjustment is added to the measured value before passing it on to the evaluation block  318 . 
         [0053]    Turning to  FIG. 5 , a method for assessing susceptibility of a CATV receiver to intermodulation distortion is indicated generally by the reference numeral  500 . The method  500  includes a start block  510 , which passes control to a function block  512 . The function block  512  selects a first digital channel from a plurality of channels in a CATV signal, and passes control to a function block  514 . The function block  514  determines a first power measurement of the first digital channel at the CATV receiver input, and passes control to a function block  516 . The function block  516  determines a total power measurement from one or more of the other channels in the CATV signal at the CATV receiver input, and passes control to a function block  518 . The function block  518 , in turn, determines the susceptibility of the first digital channel to intermodulation distortion by comparing the first and total power measurements with known intermodulation distortion characteristics of the CATV receiver. 
         [0054]    Optionally, the method  500  may further include a function block  520 , which receives control from the function block  518  and determines a level-versus-frequency signal tilt at the CATV receiver input. In addition, the method  500  may further include a function block  522 , which receives control from the function block  520  and uses the level-versus-frequency signal tilt measured at the CATV receiver input to compensate for the differing signal tilt when performing the signal power measurements at a location in the network other than at the original CATV receiver input. The function block  522  may then pass control to an end block  524 . 
         [0055]    Turning now to  FIG. 6 , a method for detection of network impairments through tilt-normalized measurement data is indicated generally by the reference numeral  600 . The method  600  includes an input block  610  to collect a set of measurement points. The block  610  passes control to a function block  612 , which computes a best-fit tilt and passes control to a function block  614 . The block  614 , in turn, performs tilt normalization and passes control to a function block  616 . 
         [0056]    The function block  616  performs tilt-normalized limit checking, and passes control to a decision block  618 , which determines whether tilt-normalized limit checks are passed. If the checks are not passed, control passes to a function block  620 , which performs pattern matching and passes control to a function block  622 . If, on the other hand, the checks are passed, the decision block  618  passes control directly to the function block  622 . The function block  622 , in turn, queries or displays the results. 
         [0057]    In operation of the method  600 , the input block  610  collects a set of data points including level, frequency, and type. Here, the type may be analog or digital, where the digital type may include Quadrature Amplitude Modulation (QAM), which is a method for encoding digital data in an analog signal in which each combination of phase and amplitude represents one of a plurality of multi-bit patterns. 
         [0058]    The block  614  processes the data points by performing a tilt normalization of the data. For example, the tilt on a set of data may be calculated by computing the tilt between maximum points. The tilt computation with the greatest span is chosen to be the tilt of the data set. If there are multiple tilts with the same span, the minimum tilt is chosen. 
         [0059]    An alternate process for computing the tilt of the entire data set is to compute a best fit line to the data using the following equation: 
         [0000]        f ( x   i )= a   0   g   0 ( x   i )+ a   1   g   1 ( x   i )+ a   2   g   2 ( x   i )+ a   3   g   3 ( x   i )+ a   4   g   4 ( x   i )+ e   i   (Eqn. 2) 
         [0000]    where the functions are defined as follows: 
         [0000]    
       
         
           
             
               
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         [0060]    The linear regression will compute all constants (a 0 , a 1 , a 2 , a 3 , a 4 ) such that the sum of the squares of the error 
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         [0000]    is minimized. Constants (a 0 , a 1 , a 2 , a 3 , a 4 ) each directly correspond to useful quantities when performing normalization in a subsequent step. a 0  corresponds to the measured slope of the data. The computed tilt of the measured data is equal to the slope of the data times the frequency span of the data points included in the regression. Constants a 1 , a 2 , a 3 , a 4  correspond to the computed offset that will be subtracted from the measured value of the data point so as to center the data about the origin. For example, a 1  is subtracted from all analog video channel measurements. 
         [0061]    An alternate expansion to the above method uses a weighted data set. This allows for assigning low weights to outliers that might result in decreasing the reliability of the linear regression. In this case the process uses the equation: 
         [0000]        w   i   f ( x   i )= w   i ( a   0   g   0 ( x   i )+ a   1   g   1 ( x   i )+ a   2   g   2 ( x   i )+ a   3   g   3 ( x   i )+ a   4   g   4 ( x   i )+ e   i )  (Eqn. 3) 
         [0000]    with the goal of minimizing the weighted error: 
         [0000]    
       
         
           
             
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         [0000]    Multiple passes through the data may be performed to tune the weights of individual points. The algorithm here tunes the weights until the calculated slope, constants, and weights are stable, with the goal of eliminating a small number of outliers. 
         [0062]    Thus, the function block  614  can normalize the data points using the above calculated slope and constants. This results in flattening of the data so that there is zero tilt and the data points are centered about the origin. Normalization is accomplished by solving Equation (2) for the error, e i . A plot of the data points (e i , x i ) will be the set of normalized data. 
         [0000]        e   i   =f ( x   i )−( a   0   g   0 ( x   i )+ a   1   g   1 ( x   i )+ a   2   g   2 ( x   i )+ a   3   g   3 ( x   i )+ a   4   g   4 ( x   i ))  (Eqn. 4) 
         [0063]    Next the data is analyzed. There are three forms of analysis available at this point: the tilt normalization limit checks of blocks  616  and  618 , the pattern matching of block  620 , and error distribution analysis, which may be performed in conjunction with block  622 . 
         [0064]    The tilt normalization limit checks of blocks  616  and  618  look for data points that reside outside of a delta or threshold from the computed linear regression. Limit checking may be computed for each type of point. For example, digital channels might be allowed to have a delta of 3 dBmV from the best-fit line, but analog channels might only be allowed to have a delta of 1 dBmV. 
         [0065]    The pattern matching checks of block  620  may be performed if the tilt-normalized limit checks fail. Here, the method may perform an additional step of attempting to match the data set to a known problem pattern. The data points may be compared against a set of patterns and/or functions indicative of various network error conditions. Each pattern check may return a percent likelihood of a match to the pattern by performing a Euclidian distance to a known function or performing an appropriate non-linear regression technique. If the threshold is above a predetermined value, a conclusion may be drawn that the error condition is likely present. The following patterns have been identified and may be used to identify various network conditions: 1) High Frequency Roll-Off: In addition to identifying the roll-off condition, the frequency where it starts may also be identified; 2) Suck-Out: This may be able to identify the frequency where levels are decreased over a set of frequencies; and/or 3) Standing Wave: This may be able to identify that the data contains one or more standing waves. If a wave is present, the most significant wave may be analyzed to calculate the distance to the most significant impedance mismatch. 
         [0066]    An error value distribution analysis may be performed in conjunction with the function block  622 . Here, the normalized error values for each frequency may be plotted and analyzed. Various curves may indicate success or error conditions. This is an alternate approach to identify other network error conditions. 
         [0067]    Once the data has been analyzed, a useful graph can be displayed. The contents of the display may include: a) Tilt of the best fit line; b) Results of normalized limit checks; c) Results of pattern matching, such as pass/fail or error conditions identified; d) The original and normalized data points; and/or e) Points that fail tilt-normalized limit checks can be highlighted. 
         [0068]    As shown in  FIG. 7 , an exemplary plot of the results is indicated generally by the reference numeral  700 . The plot  700  shows measured level in dBmv on the vertical axis, versus frequency in MHz on the horizontal axis. A marker  710  is placed on channel  72 , which is here a Quadrature Amplitude Modulation (QAM) channel at 513 MHz. A Channel indicator  712  shows that channel  72  is selected. A Measured Level indicator  714  shows a measured level of −13.6 dBmV. A View pull-down menu  720  includes selections for Auto Reference, 1 dB/Div, 2 dB/Div, 5 dB/Div, 10 dB/Div, Video Summary, DOCSIS Summary, DOCSIS Details, Registration, VoIPCheck, and DOCSIS Status. The active selection  722  on the View menu is the 10 dB/Div choice. 
         [0069]    A Limits pull-down menu  730  includes selections for Off, Cable Modem, Ground Block, Tap, TV, Custom  1 , Custom  2 , Custom  3 , and Proof. The active selection  732  on the Limits menu is the TV choice. A Settings pull-down menu  740  includes selections for Normalized, Tilt Compensation, Tilt Line, and Limit Lines. 
         [0070]    The measured level of channel  72  is −13.6 dBmv, which is above a −15 dBmv minimum digital level limit. Thus, from this preliminary plot, it looks as though this test might pass. However, the data should be checked from a different perspective. 
         [0071]    Turning to  FIG. 8 , another exemplary plot of the results is indicated generally by the reference numeral  800 . The plot  800  is similar to the plot  700 , so duplicate description may be omitted. Here, an Adjusted Level indicator  816  shows an adjusted level of −5.1 dB. The active selection  822  on the Settings menu is the Normalized choice, and an active selection  824  on the View menu is the 2 dB/Div choice. Thus, this is a view of normalized data. The plot  800  shows the analog level brought down, and the different types of digital channels brought up to reference zero on the scale. A perfect system with zero tilt would have a straight line at 0 for the full frequency range, regardless of the differences in level between ideal analog and digital signals. The adjusted level is the difference between the current point and an ideal normalized value. Here, the adjusted level of −5.1 dB indicates a failure of the test. Various other settings can be applied to make the results easier to view or understand. 
         [0072]    Turning now to  FIG. 9 , yet another exemplary plot of home certification results is indicated generally by the reference numeral  900 . The plot  900  is similar to the plot  800 , so duplicate description may be omitted. Here, additional selections from the Settings menu are active, including an active Tilt Line choice  924 , and an active Limit Lines choice  926 . A tilt line  952  shows the computed tilt of the system after normalization. In the subtitle bar, the measured tilt for this data is shown to be −0.25 dB per 100 MHz. Upper and lower limit lines  950  and  954 , respectively, graphically show the application of a new user-defined limit. Limit lines allow the user to see how the acceptable limit values vary with frequency to account for system tilt. Here, channel  72  violated the lower limit line  954 , so the test failed. 
         [0073]    As shown in  FIG. 10 , an additional exemplary plot of the results is indicated generally by the reference numeral  1000 . The plot  1000  is similar to the plot  900 , so duplicate description may be omitted. Here, an additional selection from the Settings menu is active, namely a Tilt Compensation choice  1028 . Thus, this display setting allows the user to view the results without the tilt. The measured tilt is still reported in the subtitle bar, but here, the graph accounts for it. A perfect set of data, regardless of system tilt or modulation related level differences, would be represented as a horizontal line at 0 spanning the whole frequency. Thus, this view makes it even easier to identify potential network impairments. 
         [0074]    Turning to  FIG. 11 , a measurement menu with animation options is indicated generally by the reference numeral  1100 . The menu  1100  includes pull-up menu tabs for basic  1110 , service  1120 , spectrum  1130 , and sweep  1140 . The basic pull-up menu includes selections for hum mode  1112  to view hum levels, DQI mode  1114  to measure signal quality, and drop check mode  1116  to measure signal deviation for all channels. 
         [0075]    In operation, the drop check mode can be used with or without animation. When animation is enabled, the user is walked through two animation screens that show what is being done to a full scan to get to the final results of the drop check mode. There are also two separate paths one can take through the animation sequence. The first screen is a full scan graph of the channels included in the regression. The second screen is a view of the channels after they have been either tilt or type compensated. When the view is tilt compensated, all channels may be adjusted up or down based on the measured tilt of the system. When the view is type compensated, all channels may be adjusted up or down based on their type and level relative to the left most channel. For example, if the channels between 50-500 MHz are all analog and the channels higher than 500 MHz are QAM 256, the digital channels may be adjusted upward so that they are at a similar level to the analog channels. 
         [0076]    In addition to selecting the second screen of the animation, the user is also provided with an option to decide whether to view the data as a bar graph, or to view the data as a filled graph. Each graph displays the same data, but renders certain aspects easier to identify. The default is to use a filled graph for the second screen of the animation and drop check mode. 
         [0077]    Once the user has walked through the two animation screens, they reach the drop check mode storyboard. The storyboard provides a complete view of the type and tilt compensated data, and displays the deviation of the channel to the computed best fit line. The final mode screen allows for displaying results from limit checks, and also allows the user to enable or disable animation and select the screen to be used as the second screen of the animation. In an alternate embodiment, the storyboard graph might only show type or tilt compensated data without such a user configuration option. 
         [0078]    Turning now to  FIG. 12 , a drop check storyboard graph is indicated generally by the reference numeral  1200 . The graph  1200  includes a bar plot  1210  showing measured level in dBmv on the vertical axis versus frequency in MHz on the horizontal axis for all channels, a selected channel indicator  1212 , a selected channel data level indicator  1214 , a level data indicator  1216 , and a selected channel frequency indicator  1218 . There are also pull-up tabs for file  1240 , view  1220 , limits  1230 , and next channel  1250 . Here, the channel indicator  1212  shows TV Channel  002 , the frequency indicator  1218  shows 55.250 MHz, and the level indicator  1214  shows a measured level of 10.1 dBmV. 
         [0079]    In operation, after the user selects the drop check mode, the mode will be started and the first screen displayed will be the level view plot  1200 , including a full scan of the levels to be included in the regression, without any limit checking data displayed. 
         [0080]    As shown in  FIG. 13 , another drop check storyboard graph is indicated generally by the reference numeral  1300 . The graph  1300  is similar to the graph  1200  of  FIG. 12 , so duplicate description may be omitted. 
         [0081]    Here, the channel indicator  1312  shows QAM256 Channel  072 , the frequency indicator  1318  shows 513 MHz, and the level indicator  1314  shows a measured level of 13.4 dBmV. Moreover, the view pull-up menu  1320  has been activated to display selections for Auto Reference, 1 dB/div, 2 dB/div, 5 dB/div, 10 dB/div, Single Channel, Full Scan, Tilt, and Pause. 
         [0082]    In operation, the view menu options are available to the user at the first and all following screens. After the user selects next channel tab  1350 , the mode transitions to either a view of the tilt compensated or type compensated data. In this embodiment, the user has the option to select any item on the last screen, with tilt compensation being the default. 
         [0083]    Turning to  FIG. 14 , another drop check storyboard bar graph for a tilt compensated view is indicated generally by the reference numeral  1400 . The graph  1400  is similar to the graph  1200  of  FIG. 12 , so duplicate description may be omitted. 
         [0084]    Here, the channel indicator  1412  shows TV Channel  002 , the tilt compensated level  1414  is 9.9 dB, the view mode indicator  1416  indicates Tilt Compensation view, the selected frequency indicator  1418  shows 55.250 MHz, and the level indicator  1408  shows 10.0 dBmV at the selected frequency. A Back tab  1480  permits transitions to the previous view displayed. 
         [0085]    In operation, after the user selects the Next tab  1450 , the mode will transition to either a view of the tilt compensated or type compensated data. This decision is a configurable item on the last screen, with tilt compensation being the preferred default. 
         [0086]    Turning now to  FIG. 15 , a drop check storyboard bar graph for a type compensated view is indicated generally by the reference numeral  1500 . The graphical view  1500  is similar to the graphical view  1400 , so duplicate description may be omitted. The type compensated view  1500  includes a channel indicator  1512  showing TV Channel  002 , a type compensation level indicator  1514  showing a level of 10.1 dB, a Type Compensation indicator  1516 , a selected frequency indicator  1518  showing 55.250 MHz, and a selected frequency level indicator  1508  showing a level of 10.1 dBmV at the selected frequency. 
         [0087]    In operation, while tilt compensation will raise or lower all channels so that the displayed tilt in the system is zero, type compensation will raise or lower all channels so that they are all equal regardless of type. Thus, there may still be a noticeable tilt in the data set when viewed with type compensation. 
         [0088]    As shown in  FIG. 16 , a drop check storyboard filled graph for a tilt compensated view is indicated generally by the reference numeral  1600 . The graph  1600  is similar to the graph  1400  of  FIG. 14 , so duplicate description may be omitted. 
         [0089]    Here, the channel indicator  1612  shows TV Channel  002 , the tilt compensated level  1614  is 10.1 dB, the view mode indicator  1616  indicates Tilt Compensation view, the selected frequency indicator  1618  shows 55.250 MHz, and the level indicator  1608  at the selected frequency shows 10.2 dBmV. 
         [0090]    Turning to  FIG. 17 , a drop check storyboard filled graph for a type compensated view is indicated generally by the reference numeral  1700 . The graph  1700  is similar to the graph  1500  of  FIG. 15 , so duplicate description may be omitted. The type compensated view  1700  includes a channel indicator  1712  showing TV Channel  002 , a type compensation level indicator  1714  showing a level of 10.1 dB, a Type Compensation view indicator  1716 , a selected frequency indicator  1718  showing a frequency of 55.250 MHz, and a selected frequency level indicator  1708  showing a level of 10.1 dBmV at the selected frequency. 
         [0091]    In operation, after the Next button  1750  is selected, the display will change to the final mode to view the deviation for each channel. There are two screen options printed here for viewing the data, one based on a bar type graph, and the other based on a filled type graph. 
         [0092]    Turning now to  FIG. 18 , a drop check bar graph with limits off is indicated generally by the reference numeral  1800 . The graphical view  1800  is similar to the view  1200  of  FIG. 12 , so duplicate description may be omitted. The view  1800  includes a selected channel indicator  1812  showing TV Channel  002 , a selected channel frequency indicator  1818  showing a selected frequency of 55.250 MHz, a selected channel level indicator  1808  showing a level of 10.2 dBmV at the selected frequency, a deviation indicator  1806  showing a deviation of −0.8 dB, and a headroom indicator  1804  showing headroom of 2.2 dB. There are also pull-up tabs for file  1840 , view  1820 , limits  1830 , and Settings  1860 . 
         [0093]    As shown in  FIG. 19 , a drop check filled graph with limits off is indicated generally by the reference numeral  1900 . The graphical view  1900  is similar to the view  1800  of  FIG. 18 , so duplicate description may be omitted. The view  1900  includes a selected channel indicator  1912  showing TV Channel  002 , a selected channel frequency indicator  1918  showing a selected frequency of 55.250 MHz, a selected channel level indicator  1908  showing a level of 10.1 dBmV at the selected frequency, a deviation indicator  1906  showing a deviation of −0.9 dB, and a headroom indicator  1904  showing headroom of 2.1 dB. 
         [0094]    Turning to  FIG. 20 , a drop check bar graph view with limits on is indicated generally by the reference numeral  2000 . The bar graph view  2000  with limits on is similar to the bar graph view  1800  of  FIG. 18  with limits off, so duplicate description may be omitted. The view  2000  includes a selected channel indicator  2012  showing TV Channel  002 , lower and upper limit lines  2013  and  2015 , respectively, a selected channel frequency indicator  2018  showing a selected frequency of 55.250 MHz, a selected channel level indicator  2008  showing a level of 10.1 dBmV at the selected frequency, a deviation indicator  2006  showing a deviation of −0.9 dB, and a headroom indicator  2004  showing headroom of 2.1 dB. In addition, the view  2000  includes a test failure indicator  2016 , which indicates a failure because at least one of the frequency levels falls below the lower limit  2013  or above the upper limit  2015 . 
         [0095]    Turning now to  FIG. 21 , a drop check filled graph with limits on is indicated generally by the reference numeral  2100 . The bar graph view  2100  is similar to the bar graph view  2000  of  FIG. 20 , so duplicate description may be omitted. The view  2100  includes filled graph data  2110  in place of the bar graph data  2010  of  FIG. 20 . 
         [0096]    In operation, the views  2100  and  2000  of  FIGS. 21 and 20 , which both have limits on or active, may be compared with the views  1900  and  1800  of  FIGS. 19 and 18 , which both have limits off or inactive. Limits are checked on the Tilt, Level, and Deviation values displayed on the screen. The Headroom value displayed is equal to the Maximum Deviation limit value minus the absolute value of the channel&#39;s deviation. Values involved in the calculation may be rounded to dB tenths before taking the difference. In addition, selectable options for the File, Limits, and Settings pull-up menus are described with respect to  FIGS. 22 through 24 . 
         [0097]    As shown in  FIG. 22 , a drop check view with activated File menu is indicated generally by the reference numeral  2200 . The view  2200  is similar to the view  2100  of  FIG. 21 , so duplicate description may be omitted. Here, the activated file menu  2240  includes selections for Save or Open. 
         [0098]    Turning to  FIG. 23 , a drop check view with activated Limits menu is indicated generally by the reference numeral  2300 . The view  2300  is similar to the view  2200  of  FIG. 22 , so duplicate description may be omitted. Here, the activated limits menu  2330  includes selections for Off, Cable Modem, Ground Block, Tap, TV, Custom 1 , Custom 2 , Custom 3 , Proof, or Edit. 
         [0099]    Turning now to  FIG. 24 , a drop check view with activated Settings menu is indicated generally by the reference numeral  2400 . The view  2400  is similar to the view  2300  of  FIG. 23 , so duplicate description may be omitted. Here, the activated settings menu  2460  includes selections for Animation On, Type First, Tilt First, Bar Graph, or Channel Plans. 
         [0100]    In operation, the settings menu allows enabling or disabling the animation sequence. If animation is disabled, the mode view will open directly to the final screens, such as described above. If animation is enabled, the choice to set the second screen to be either Type or Tilt Compensation as the view is also available. If the Bar Graph option is enabled, the preferred default setting is that the mode view will display data using a bar graph for all screens. Otherwise, the mode view will use a filled graph. 
         [0101]    The above and alternate embodiments provide a method and apparatus for detection of network impairments through tilt-normalized measurement data. Alternate embodiments may include determining the susceptibility of a digital receiver to intermodulation (IM) distortion. The IM distortion results when the total power received across all digital and analog signal frequencies exceeds by a critical amount the strength of the selected digital signal being demodulated. Embodiments may also use a level-versus-frequency signal tilt compensation feature, which enables a tester  100  to evaluate a receiver connected at a different location in the network from that of the tester. 
         [0102]    Although illustrative embodiments have been described herein with reference to the accompanying drawings, it is to be understood that the present disclosure is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by those of ordinary skill in the pertinent art without departing from the scope or spirit of the present disclosure. All such changes and modifications are intended to be included within the scope of the present disclosure as set forth in the appended claims.