Abstract:
A histogram generator creates a histogram to detect the characteristics of video signals. The histogram generator allows for a reduction in problems of instability caused by noise that may exist in other histogram generators. A width is allocated to thresholds used in creating the histogram, and signal values are counted after weighting those falling near a threshold.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of histogram generators, and particularly to the detection of distribution of levels of input data such as input video signals. 
     BACKGROUND OF THE INVENTION 
     In general, histograms are created in a signal processing circuit for television signals to extract the characteristics of video signals. Applied technology for creating histograms has been introduced in the past. One example of a device employing such technology is disclosed in Japanese Laid-open Patent No. S58-63989. 
     FIG. 17 shows an example of a histogram which divides sampled input signals into more than one group based on the size of sample value. In FIG. 17, sampled signals are divided into four groups. In order to group the sample values, thresholds a 1 , a 2 , and a 3  are set, and four intervals, S 1 , S 2 , S 3  and S 4  are then formed in accordance with these thresholds. Continuous input video signals are sampled and sample values allocated to one of the intervals S 1 , S 2 , S 3 , and S 4 . A histogram is created based on the intervals S 1 , S 2 , S 3 , and S 4 , and the counted number of samples (frequency) allocated to each intervals. 
     In a histogram generator such as the one of the aforementioned patents, however, the created histogram may be unstable. A sample value of the input signal that lies in small neighborhood of the threshold al may be judged by the histogram generator as belonging to the interval S 1  at one moment while judged as belonging to the interval S 2  at another moment, due to factors like noise. 
     SUMMARY OF THE INVENTION 
     A histogram generating apparatus according to the present invention finds a frequency of an input video signal level by dividing a maximum range of the input signal level into several intervals using several threshold values as boundaries. The histogram generating apparatus comprises a weighting circuit for weighting samples occurring near a threshold value and a counting circuit for counting the output signals of the weighting circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a histogram generator in accordance with a first exemplary embodiment of the present invention. 
     FIG. 2 is a block diagram of a histogram generator in accordance with a second exemplary embodiment of the present invention. 
     FIG. 3 is a block diagram of a histogram generator in accordance with a third exemplary embodiment of the present invention. 
     FIG. 4 is a block diagram of a histogram generator in accordance with a fourth exemplary embodiment of the present invention. 
     FIG. 5 is a block diagram of a histogram generator in accordance with a fifth exemplary embodiment of the present invention. 
     FIG. 6 is a block diagram of a histogram generator in accordance with a sixth exemplary embodiment of the present invention. 
     FIG. 7 is a block diagram of a histogram generator in accordance with a seventh exemplary embodiment of the present invention. 
     FIG. 8 is a block diagram of a histogram generator in accordance with a eighth exemplary embodiment of the present invention. 
     FIG. 9 is a block diagram of a histogram generator in accordance with a ninth exemplary embodiment of the present invention. 
     FIG. 10 is a block diagram of a histogram generator in accordance with a tenth exemplary embodiment of the present invention. 
     FIGS. 11A-E shows an order of creating a weighting function used for generating a histogram in the exemplary embodiments of the present invention. 
     FIGS. 12A-E shows an order of creating a weighting function used for generating a histogram in the exemplary embodiments of the present invention. 
     FIGS. 13A-E shows an order of creating a weighting function used for generating a histogram in the exemplary embodiments of the present invention. 
     FIGS. 14A-D shows an order of creating a weighting function used for generating a histogram in the exemplary embodiment of the present invention. 
     FIGS. 15A-C shows an order of creating a weighting function used for generating a histogram in the exemplary embodiment of the present invention. 
     FIG. 16 shows a weighting function for generating a histogram in the exemplary embodiment of the present invention. 
     FIG. 17 is a histogram created using the prior aft. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Assume that a lower threshold value (a first threshold) is denoted A and a upper threshold value (a second threshold) is denoted B to form an interval between A and B. In the conventional histogram generator (see FIG. 16) a histogram is obtained by classifying input video signals according to whether an input video signal S belongs to an interval or not, where the interval is defined by the threshold values A and B. According to the present invention, on the other hand, a histogram generating apparatus sets a threshold width W at both sides of the interval lying between threshold values A and B. Thus allowing both threshold values A and B to have a slope associated with them within a margin of ±W as drawn with a solid line in FIG. 16; whereby a weighting function is produced. The weighting function weighs the data, which are then counted to produce a histogram. 
     In other words, when a level of the input video signal S is near the center of a weighting function associated with A or B, the weighted data is unchanged, and when the level of the input video signal S is near the threshold value A or B, the weighted data takes a value proportional to the distance from the center. 
     In FIG. 16, steps drawn with a solid line within the ranges of ±W as the apparatus handles digital data. When analog data is handled, or the number of bits is increased in digital data, the steps (FIG. 16) closely approximate a line. In the lower neighborhood of the threshold A, the corresponding interval covering (A−W) to A has the same width as that of the upper neighborhood interval covering A to (A+W). Therefore, if the neighborhood of the threshold B is similarly arranged, the frequency can be calculated by using the weighting function, and the counted frequencies integrated to produce the desired histogram. 
     In the following description, one of a plurality of intervals of the histogram is described. This interval has a first threshold value A as well as a second threshold value B (A&lt;B) associated with it. 
     First Exemplary Embodiment 
     FIG. 1 shows a block diagram of a histogram generator for creating a histogram of video signals in a first exemplary embodiment of the present invention. 
     The histogram generator of the first exemplary embodiment comprises a first weighting circuit  100 , a second weighting circuit  200 , minimum detector circuit (MIN circuit)  11 , and a counter  12 . The histogram generator for video signals as configured in FIG. 1 receives input video signal S and outputs sample number (frequency) F of the input video signal S. The input video signal S is, for example, a luminance signal. 
     The configuration and operation of the first weighting circuit  100  is explained first. 
     The first subtractor  1  subtracts a threshold width W which is set externally and is provided to allocate width associated with to the first threshold A which is also set externally. The subtracted value (A−W) is outputted to the second subtractor  2 . 
     The second subtractor  2  subtracts the output of the first subtractor  1  from an input video signal S, and outputs the subtracted value (S−(A−W)) to the first zero limiter  3 . 
     The first zero limiter  3  outputs i) 0 if the output of the second subtractor  2  is zero or a negative value, or ii) the output value of the second subtractor  2  if the output of the second subtractor  2  is a positive value. In other words, the first zero limiter  3  outputs 0 or (S−(A−W)) in response to the input video signal S. 
     According to the above operation the weighting function associated with the threshold A that results is shown in FIG.  11 A. In FIGS. 11A-E, the level of the input video signal S is plotted along the abscissa, and the magnitude of weighted data determined by a weighting function is plotted along the ordinate F. In FIGS. 11A-E, the weighting function in a neighborhood of the threshold is shown by a slanted line but it is a step shaped if digital data is handled. 
     The first limiter  5  receives the output of the first zero limiter  3  and a limit level L which is set externally, and outputs i) the output of the first zero limiter  3  if the output of the first zero limiter  3  is smaller than the limit level L, or ii) the limit level L if the output of the first zero limiter  3  is greater than or equal to the limit level L. In other words, the first limiter  5  outputs 0, (S−(A−W)), or L in response to the input video signal S. In the present exemplary embodiment L is equal to 2W. 
     According to the above operation the first weighting circuit  100  outputs the weighted data values near the threshold A limited by the limit level L as shown in FIG.  11 B. In FIGS. 11A-E, the limit level L is shown along the ordinate. 
     The configuration and operation of the second weighting circuit  200  is explained next. 
     The first adder  6  adds a second threshold B which is set externally and the threshold width W, and outputs the added value (B+W) to the third subtractor  7 . 
     The third subtractor  7  subtracts the input video signal S from the output of the first adder  6 , and outputs the subtracted value (B+W−S) to the second zero limiter  8 . 
     The second zero limiter  8  outputs i) the output value (B+W−S) of the third subtractor  7  if the output (B+W−S) of the third subtractor  7  is a positive value, or ii) 0 if the output value (B+W−S) of the third subtractor  7  is 0 or a negative value. 
     According to the above operation weighting function associated with data near the threshold B is as shown in FIG.  11 C. 
     The second limiter  10  receives the output of the second zero limiter  8  and the limit level L, and outputs i) the output of the second zero limiter  8  if the output of the second zero limiter  8  is smaller than the limit level L, or ii) the limit level L if the output of the second zero limiter  8  is greater than or equal to the limit level L. 
     According to the above operation the second weighting circuit  200  outputs the weighted data values near the threshold B limited by the limit level L as shown in FIG.  11 D. 
     The minimum detector circuit (MIN circuit)  11  receives the output of the first limiter  5  and the output of the second limiter  10 , and outputs the smaller. 
     According to the operation described above a trapezoidal weighting function is realized as shown in a solid line in FIG.  11 E. 
     The counter  12  cumulatively adds the output signal of the minimum detector circuit  11 . 
     According to the above operation for a one frame or a one field video signal a histogram of video signals using a trapezoidal weighting function, shown in FIG. 11E, is generated. 
     A histogram generator of the present invention counts data with weighting function that increases within the threshold width near the first threshold, decreases within the threshold width near the second threshold, by allocating width near the second thresholds, and is constant near the center between the thresholds. 
     Accordingly, the first exemplary embodiment enables the creation of stable histogram by allocating width to thresholds and utilizing weighting function near the thresholds. The histogram, in turn, is used for extracting the characteristics of video signals. 
     Second Exemplary Embodiment 
     FIG. 2 shows a configuration of a histogram generator for video signals in a second exemplary embodiment of the present invention. The configurations and operations of the components in FIG. 2 with the same reference numeral as in the first exemplary embodiment (FIG. 1) are the same as in the first exemplary embodiment, and thus their explanation is omitted. 
     The configuration and operation of the first weighting circuit  101  is explained first. 
     The configurations and operations of the first subtractor  1  and the second subtractor  2  are the same as in the first exemplary embodiment, and thus their explanation is omitted. 
     The third limiter  15  receives the output value (S−(A−W)) of the second subtractor  2  and a limit level L which is set externally, and outputs i) the output of the second subtractor  2  if the output of the second subtractor  2  is smaller than the limit level L, or ii) the limit level L if the output of the second subtractor  2  is greater than or equal to the limit level L. 
     According to the above operation the output value of the third limiter  15  is shown in FIG.  12 A. 
     The third zero limiter  13  outputs i) 0 if the output of the third limiter  15  is zero or a negative value, or ii) the output of the third limiter  15  if the output of the third limiter  15  is a positive value. 
     According to the above operation the first weighting circuit  101  outputs the weighted data values near the threshold A and limited by the limit level L as shown in FIG.  12 B. 
     The configuration and operation of the second weighting circuit  201  is explained next. 
     The configurations and operations of the first adder  6  and the third subtractor  7  are the same as in the first exemplary embodiment, and thus their explanation is omitted. 
     The fourth limiter  20  receives the output value (B+W−S) of the third subtractor  7  and the limit level L, and outputs i) the output of the third subtractor  7  if the output of the third subtractor  7  is smaller than the limit level L, or ii) the limit level L if the output of the third subtractor  7  is greater than or equal, to, the limit level L. In the present exemplary embodiment L is equal to 2W. 
     According to the above operation the output value of the fourth limiter  20  is shown in FIG.  12 C. 
     The fourth zero limiter  18  outputs i) 0 if the output of the fourth limiter  20  is zero or a negative value, or ii) the output of the fourth limiter  20  if the output of the fourth limiter  20  is a positive value. 
     According to the above operation the second weighting circuit  201  outputs the weighted data values as shown in FIG.  12 D. 
     The minimum detector circuit  11  receives the output of the first weighting circuit  101  and the second weighting circuit  201 , and outputs the smaller. 
     The counter  12  cumulatively adds the output signal of the minimum detector circuit  11 . 
     According to the above operation a histogram based on value weighted by a trapezoid weighting function, shown in FIG. 12E, is generated. 
     Third Exemplary Embodiment 
     FIG. 3 shows a block diagram of a histogram generator for creating a histogram of video signals in a third exemplary embodiment of the present invention. 
     The configuration and operation of the first weighting circuit  102  is explained first. 
     The second adder  22  adds a first threshold A and a threshold width W. The added value (A+W) is outputted to the fourth subtractor  24 . 
     The fourth subtractor  24  subtracts an input video signal S from the output of the second adder  22 , and outputs the subtracted value (A+W−S) to the first zero limiter  3 . 
     The first zero limiter  3  functions as in the first exemplary embodiment, and outputs the calculated value to the first limiter  5 . The first limiter  5  functions as in the first exemplary embodiment, and outputs the calculated value as shown in FIG. 13A to the seventh subtractor  27 . 
     According to the above operation the output of the first weighting circuit  102  is as in FIG.  13 B. 
     The configuration and operation of the second weighting circuit  202  is explained next. 
     The fifth subtractor  25  subtracts the threshold width W from the second threshold B, and outputs the subtracted value (B−W) to the sixth subtractor  26 . 
     The sixth subtractor  26  subtracts the output of the fifth subtractor  25  from the input video signal S, and outputs the subtracted value (S−(B−W)) to the second zero limiter  8 . 
     The second zero limiter  8  functions as in the first exemplary embodiment, and outputs the calculated results to the second limiter  10 . The second limiter  10  functions as in the first exemplary embodiment, and outputs the calculated value as shown in FIG. 13C to the eighth subtractor  28 . 
     The eighth subtractor  28  subtracts the output of the second limiter  10  from the limit level L. In the present exemplary embodiment L is equal to 2W. 
     According to the above operation the output of the second weighting circuit  202  is shown in FIG.  13 D. 
     The minimum detector circuit  11  receives the output of the first weighting circuit  102  and the second weighting circuit  202 , and outputs the smaller. The counter  12  cumulatively adds the output signal of the minimum detector circuit  11 . 
     According to the above operation a histogram based on value weighted by a trapezoidal weighting function, shown in FIG. 13E, is generated. 
     Fourth Exemplary Embodiment 
     FIG. 4 shows a block diagram of a histogram generator for creating a histogram of video signals in a fourth exemplary embodiment of the present invention. The configurations and operations of the components in FIG. 4 with the same reference numerals as those in the second and third exemplary embodiments (FIG.  2  and FIG. 3) are the same as in the second and third exemplary embodiments, and thus their explanation is omitted. 
     The configuration and operation of the first weighting circuit  103  is explained first. 
     The configuration and operation of the second adder  22  and the fourth adder  24  are the same as in the 3rd exemplary embodiment (FIG.  3 ), and the configuration and operation of the third limiter  15  and the third zero limiter  13  are the same as in the second exemplary embodiment (FIG.  2 ). 
     The third limiter  15  received the output of the fourth subtractor  24  and a limit level L, and outputs the calculated value to the third zero limiter  13 . According to the above operation an output as shown in FIG. 13A is obtained. The seventh subtractor  27  subtracts the output of the third zero limiter  13  from the limit level L. 
     According to the above operation the output of the first weighting circuit  103  is shown in FIG.  13 B. 
     The configuration and operation of the second weighting circuit  203  is explained next. 
     The configuration and operation of the fifth subtractor  25  and sixth subtractor  26  are the same as in the third exemplary embodiments (FIG.  3 ), and the configuration and operation of the fourth limiter  20  and the fourth zero limiter  18  are the same as in the second exemplary embodiment (FIG.  2 ). 
     The fourth limiter  20  receives the output value (S−(B−W)) of the sixth subtractor  26  and outputs the calculated value to the fourth zero limiter  18 . According to the above operation the output value obtained is shown in FIG.  13 C. 
     The eighth subtractor  28  subtracts the output of the fourth zero limiter  18  from the limit level L. According to the above operation the output value of the second weighting circuit  203  obtained is shown in FIG.  13 D. In the present exemplary embodiment L is equal to 2W. 
     The minimum detector circuit  11  receives the output of the first weighting circuit  103  and the second weighting circuit  203 , and outputs the smaller. The counter  12  cumulatively adds the output signal of the minimum detector circuit  11 . 
     According to the above operation a histogram based on value weighted by a trapezoidal weighting function, shown in FIG. 13E, is generated. 
     In the above explanation four types of weighing circuits associated with the lower threshold value A and four types of weighing circuits associated with the upper threshold value B were described in the first to fourth exemplary embodiments in FIG. 1 to FIG.  4 . It is possible, however, to combine any one of the weighing circuits associated with the lower threshold value A in FIGS. 1 to  4 , for example circuit  100 , with any one of the weighing circuits associated with the upper threshold value B in FIGS. 1 to  4 , for example circuit  201 . 
     Fifth Exemplary Embodiment 
     FIG. 5 shows a configuration of a histogram generator for video signals in a fifth exemplary embodiment of the present invention. A difference with the first exemplary embodiment (FIG. 1) is that the first limiter  5  is not connected, and the output of the first zero limiter  3  is directly connected to the minimum detector  11 . The configurations and operations of the other components are the same as in the first exemplary embodiment, and thus their explanation is omitted. 
     The output produced by the operation of the first weighting circuit  104  is shown in FIG.  11 A. 
     The output produced by the operation of the second weighting circuit  200  is shown in FIG.  11 D. In the present exemplary embodiment L is equal to 2W. 
     The minimum detector circuit  11  receives the output of the first weighting circuit  104  and the second weighting circuit  200 , and outputs the smaller. The counter  12  cumulatively adds the output signal of the minimum detector circuit  11 . 
     According to the above operation a histogram based on value weighted by a trapezoidal weighting function, shown in FIG. 11E, is generated. 
     Sixth Exemplary Embodiment 
     FIG. 6 shows a configuration of a histogram generator for video signals in a sixth exemplary embodiment of the present invention. A difference with the fourth exemplary embodiment (FIG. 4) is that the third zero limiter  13  is not connected and the output of the third limiter  15  is directly connected to the subtractor  27 . The output of the third limiter  15  is shown in FIG.  14 A. The rest of the configurations and operations are the same as in the fourth exemplary embodiment, and thus their explanation is omitted. 
     The output of the third limiter  15  is shown in FIG.  14 A and the output produced by the operation of the first weighting circuit  105  is shown in FIG.  14 B. 
     The output produced by the operation of the second weighting circuit  203  is shown in FIG.  14 C. In the present exemplary embodiment L is equal to 2W. 
     According to the operation of a minimum detector  11  and counter  12 , a histogram based on value weighted by a trapezoid weighting function, shown in FIG. 14D, is generated. 
     It is possible, however, to combine any one of the weighing circuits associated with the lower threshold value A in the fifth and sixth exemplary embodiment with any one of the weighing circuits associated with the upper threshold value B in the first to the fourth exemplary embodiment. 
     Seventh Exemplary Embodiment 
     FIG. 7 shows a configuration of a histogram generator for video signals in a seventh exemplary embodiment of the present invention. A difference with the first exemplary embodiment (FIG. 1) is that the second limiter  10  is not connected and the output of the second zero limiter  8  is directly connected to the minimum detector  11 . The rest of the configurations and operations are the same as in the first exemplary embodiment, and thus their explanation is omitted. 
     The output produced by the first weighting circuit  100  is shown in FIG.  11 B and the output produced by the operation of the second weighting circuit  204  is shown in FIG.  11 C. In the present exemplary embodiment L is equal to 2W. 
     According to the operation of a minimum detector  11  and counter  12 , a histogram based on value weighted by a trapezoid weighting function, shown in FIG.  11 E, is generated. 
     Eighth Exemplary Embodiment 
     FIG. 8 shows a configuration of a histogram generator for video signals in a eighth exemplary embodiment of the present invention. A difference with the fourth exemplary embodiment (FIG. 4) is that the fourth zero limiter  18  is not connected and the output of the fourth limiter  20  is directly connected to the eighth subtractor  28 . The rest of the configurations and operations are the same as in the fourth exemplary embodiment, and thus their explanation is omitted. 
     The output produced by the first weighting circuit  103  is shown in FIG.  15 A and the output produced by the operation of the second weighting circuit  205  is shown in FIG.  15 B. In the present exemplary embodiment L is equal to 2W. 
     According to the operation of a minimum detector  11  and counter  12 , a histogram based on values weighted by a trapezoid weighting function, shown in FIG. 15C, is realized. 
     It is possible, however, to combine any one of the weighing circuits associated with the upper threshold value B in the seventh and eighth exemplary embodiment with any one of the weighing circuits associated with the lower threshold value A in the first to the fourth exemplary embodiment. 
     Ninth Exemplary Embodiment 
     FIG. 9 shows a configuration of a histogram generator for video signals in a ninth exemplary embodiment of the present invention. A difference with in the first weighting circuit  106  the third exemplary embodiment (FIG. 3) is that a first divider  4  is connected between the first zero limiter  3  and the first limiter  5 . A difference with in the second weighting circuit  206  the first exemplary embodiment (FIG. 1) is that a second divider  9  is connected between the second zero limiter  8  and the second limiter  10 . 
     The configuration and operation of the first weighting circuit  106  is explained first. 
     The first divider  4  divides the output of the first zero limiter  3 , (A+W−S) or 0 by a slope setting value G which is set externally; and outputs the divided value. In other words, the first divider  4  outputs (A+W−S)/G or 0 in response to the input video signal S. 
     The first limiter  5  receives the output of the first divider  4  and the limit level L, and outputs the result to the seventh subtractor  27 . Here the value of the limit level L is different from that of the first to the eighth exemplary embodiments. 
     The slope setting value defines a weighting function near the threshold value A so that the weighted data is 0 if the value of the input data is (A−W), and the weighted data is L if the value of the input data is (A+W). 
     The configurations and operations of the second weighting circuit  206  are essentially the same as those described above, and thus their explanation is omitted. In the second weighting circuit  206  the slope setting value G is a value determining a weighting function near the threshold value B so that the weighted data is L if the value of the input data is (B−W), and the weighted data is 0 if the value of the input data is (B+W). 
     The dividers which divide by the slope setting value (G may be connected in a different position in the circuit than the position shown in FIG. 9; e.g. in an appropriate position upstream of the minimum detector  11 . The symbol st in FIG. 16 denotes one step along the ordinate F and st=G*(L/(2*W)). Here (L/(2*W)) is a slope of the weighting function near the threshold. 
     A divider dividing by an externally set slope setting value G can be connected in the first to the eighth exemplary embodiments and to combination of the exemplary embodiments. 
     Tenth Exemplar Embodiment 
     FIG. 10 shows a configuration of a histogram generator for video signals in a tenth exemplary embodiment of the present invention. A difference with the ninth exemplary embodiment (FIG. 9) is that a third divider  40  is connected. The third divider  40  divides the output from the counter  12  by a limit level L. The rest of the configurations and operations of the present embodiment are the same as in the ninth exemplary embodiment, and thus their explanation is omitted. 
     The third divider  40  divides the output of the counter  12  by the limit level L. This process normalizes the output of the MIN circuit  12 . One aim of placing this third divider  40  downstream, instead of upstream, of the counter  12  is to minimize round-off errors generated by the division operation carried out by the third divider  40 . 
     The tenth exemplary embodiment as configured above enables to create the stepped weighting function, for generating a histogram, shown by a solid line in FIG.  16 . In this case, L indicated on the ordinate becomes 1. 
     According to the configuration and operation of the present invention, a histogram is generated using a trapezoidal weighting function as shown by a solid line in FIGS. 11E,  12 E,  13 E,  14 D,  15 C, and  16 . In this exemplary embodiment, L indicated on the ordinate F is 1. 
     Utilizing this exemplary embodiment, a histogram may be generated while suppressing the effects of errors during operations related to normalization. Furthermore, the magnitude of the output data may be reduced by normalization, enabling reduction of in size of downstream circuits after the histogram generator of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention may be employed in video equipment for extracting the characteristic of video signals. The present invention enables improvement in stability of histogram generators by allocating a width to thresholds for creating the histogram and by weighting the data near the thresholds. Accordingly, the problem of incorrect generation of a histogram due to concentration of samples in an interval near the threshold, caused by the effects of noise, which may occur with other histogram generators can be solved. 
     The histogram generator of the present invention refers in detail to a weighting circuit associated with a lower threshold value that weighs input signals by an increasing linear or step function in the neighborhood of the lower threshold of each intervals and to a weighting circuit associated with an upper threshold value that weighs input signal by a decreasing linear or step function in a neighborhood of the upper threshold of each interval. The function type used, however, is not limited to a linear or a step function but can be an arbitrary function. The threshold width extending on both sides of a threshold does not have to be symmetrical about the threshold as explained in the present invention. The exemplary embodiments specifically refer to hardware like configurations of the histogram generator of the present invention. As is apparent to one of ordinary skill in the art the exemplary embodiment discussed above may be implemented in hardware, software, or a combination thereof. The present invention can also be applied to equipment other than television sets, including a variety of display devices such as PC monitors. The preferred embodiments described herein are therefore illustrative and not restrictive. The scope of the invention being indicated by the appended claims and all modifications which come within the true spirit of the claims are intended to be embraced therein.