Abstract:
A clock generation apparatus generates a synchronous clock based on an input analog signal. The average of maximum and minimum values of a digital signal in a predetermined period is used as a threshold. Rise and fall times which are times when the threshold and an approximated line of two values of the digital signal crosses are detected. The time intervals between the adjacent rise and fall times are obtained during a predetermined period. The minimum value of the time intervals is used as the input rate. The synchronous clock is output on the basis of the input rate and the rise and fall times. The synchronous clock and a comparison signal which is obtained by comparing the threshold and the digital signal are supplied to a latch circuit, thereby outputting a synchronous signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a clock generation apparatus and, more particularly, to technology of correctly acquiring VBI data from television signals or the like on which the VBI data are superimposed. 
   BACKGROUND OF THE INVENTION 
     FIG. 13  is a block diagram illustrating a semiconductor integrated circuit which constitutes a prior art clock generation apparatus for generating a synchronous clock for a signal which is input at a fixed rate. 
   As shown in this figure, the semiconductor integrated circuit  1300  constituting the prior art clock generation apparatus comprises an analog input terminal  1301 , a threshold input terminal  1302 , a synchronous signal output terminal  1303 , a synchronous clock output terminal  1304 , a comparator circuit  1305 , a clock supply circuit  1306 , a counter circuit  1307 , a decoder circuit  1308 , an edge detection circuit  1309  and a D-type flipflop  1310 . 
   The comparator circuit  1305  is a circuit for comparing an analog signal S 1301  with the level of a threshold S 1302  and outputting the result of the comparison as a binarized signal, i.e., a comparison signal S 1305 . The comparator circuit  1305  operates using a clock supplied by an oscillator circuit in the clock supply circuit  1306  as the reference clock. 
   The clock supply circuit  1306  is realized by a crystal oscillator circuit using a crystal or the like. The frequency of the clock which is output by the clock supply circuit is an integral multiple of the rate at which the analog signal S 1301  is input. In addition, the edge detection circuit  1309  is a circuit for detecting the edge of the comparison signal S 1305  which is output by the comparator circuit  1305 . The signal which has been subjected to the edge detection is supplied to the counter circuit  1307 . 
   The counter circuit  1307  operates using the clock signal S 1306  from the clock supply circuit  1306  as the reference clock. The count value output by the counter circuit  1307  is supplied to the decoder circuit  1308 . The counter circuit  1307  operates using the output from the edge detection circuit  1309  and the output from the decoder circuit  1308  as the clear signals. 
   Hereinafter, the operation of the digital PLL device will be described. 
   Initially, the analog signal S 1301  and the threshold S 1302  are input to the comparator circuit  1305  via the analog input terminal  1301  and the threshold input terminal  1302 , respectively. 
   The comparator circuit  1305  makes the comparison to see whether the level of the analog signal S 1301  is larger or smaller than the threshold S 1302 , and outputs the result of the comparison. 
   A binarized comparison signal S 1305  output by the comparator circuit  1305  is input to the edge detection circuit  1309 , and the edge of the comparison signal S 1305  is detected herein. The signal whose edge was detected is supplied to the counter circuit  1307  and clears the counter. 
   Owing to the series of operations of edge detection and counter clear, the count value of the counter circuit  1307  and the edge, i.e., phase of the comparison signal S 1305 , match. 
   The count value of the counter circuit  1307  is usually composed of plural bits. Therefore, the decoder circuit  1308  executes decoding so as to output a sample clock signal S 1304  and strobe the analog input signal S 1301  in an appropriate phase. The D-type flipflop  1310  stably latches the comparison signal S 1305  with supply of a sample clock signal S 1304  which is output by the decoder circuit  1308 . 
   As described above, in the semiconductor integrated circuit  1300  constituting the prior art clock generation apparatus, a clear signal S 1308  for clearing the count value of the counter circuit  1307  decides the frequency division ratio of the counter circuit  1307 . The D-type flipflop  1310  stably latches the comparison signal S 1305  with the sample clock signal S 1304  which is output by the decoder circuit  1308 . Therefore, the semiconductor integrated circuit  1300  outputs a synchronous signal and a synchronous clock, which are stable toward the variations in outside environments, such as the variations in temperature or supply voltage and variations with time. 
   However, in the semiconductor integrated circuit constituting the prior art clock generation apparatus, the frequency of an oscillated clock S 1306  which is supplied by the clock supply circuit  1306  is an integral multiple of the input rate of the analog signal S 1301 , and the variations in the count value of the counter circuit  1307  directly result in the resolution showing the phase of the input analog signal S 1301 . Therefore, the error in the phase which occurs in the counter circuit  1307  results in the phase error in the case where the signal is captured by the D-type flipflop  1310 . In order to solve this problem, the only way is to increase the frequency of the clock supply circuit  1306  to improve the performance. Further, when this is implemented and an extremely high frequency is selected as the frequency of the clock supply circuit  1306 , unnecessary radiation is generated from the semiconductor integrated circuit. 
   Further, in the semiconductor integrated circuit constituting the prior art clock generation apparatus, in the case where the oscillated clock s 1306  is not an integral multiple of the input rate of the analog signal S 1301 , when the input signal S 1301  is kept in a certain condition during a period longer than the cycle of the oscillated clock S 1306  (for example, when the high level continues), the phase error for the input signal finally exceeds the tolerance and the input signal is erroneously recognized. This results in limiting the frequency of the oscillated clock which is used in the semiconductor integrated circuit comprising the prior art clock generation apparatus. Accordingly, when the input signal has plural kinds of input rates, plural oscillator circuits which correspond to respective input rates are required. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a clock generation apparatus that generates a synchronous clock on the basis of an input analog signal. 
   A clock generation apparatus according to a 1st aspect of the present invention comprises: A/D conversion means for converting an input analog signal into a digital signal; arithmetic means for generating a threshold which is used as a reference when the digital signal is binarized to generate a binarized signal and a synchronous clock for sampling the binarized clock, on the basis of the digital signal; binarization means for comparing the digital signal with the threshold generated by the arithmetic means, and outputting a result of the comparison as a binary signal; and latch means for latching the binary signal with the synchronous clock and outputting a synchronous signal, and the arithmetic means comprise threshold detection means for detecting a maximum value and a minimum value of the digital signal in a predetermined period, and outputting an average of the maximum value and the minimum value as the threshold; rise time detection means for detecting a rise time as a time of intersection of the threshold and a line connecting two values of the digital signal, one of which is lower and the other of which is higher than the threshold, when the digital signal changes from the lower value to the higher value; fall time detection means for detecting a fall time as a time of intersection of the threshold and a line connecting two values of the digital signal, one of which is higher and the other of which is lower than the threshold, when the digital signal changes from the higher value to the lower value; input rate detection means for obtaining time intervals between the adjacent rise and the fall times during a predetermined period, and outputting a minimum value of the time intervals as an input rate of the analog signal; and synchronous clock output means for obtaining a half timing of the input rate after an edge of the input analog signal is detected on the basis of the input rate and the rise and fall times and outputting a first one of the synchronous clock at that timing, and obtaining a timing of the input rate after the first synchronous clock is output and outputting a second or later one of the synchronous clock at that timing. Therefore, the input rate is detected from the digital signal which is obtained by subjecting the input analog signal to the A/D conversion, and the synchronous clock is generated on the basis of the input rate. Thereby, when the binarized signal is to be latched, the phase error between the synchronous clock and the binarized signal can be kept within one clock of the synchronous clock. Further, even when the input analog signal has plural kinds of input rates, the plural clock supply circuits are not required. 
   A clock generation apparatus according to a 2nd aspect of the present invention comprises: A/D conversion means for converting an input analog signal into a digital signal; arithmetic means for generating a threshold which is used as a reference when the digital signal is binarized to generate a binarized signal and a synchronous clock for sampling the binarized clock, on the basis of the digital signal; binarization means for comparing the digital signal with the threshold generated by the arithmetic means, and outputting a result of the comparison as a binary signal; and latch means for latching the binary signal with the synchronous clock and outputting a synchronous signal, and the arithmetic means comprise threshold detection means for detecting integrals of the digital signal in a predetermined period, and outputting an average of the integrals as the threshold; rise time detection means for detecting a rise time as a time of intersection of the threshold and a line connecting two values of the digital signal, one of which is lower and the other of which is higher than the threshold, when the digital signal changes from the lower value to the higher value; fall time detection means for detecting a fall time as a time of intersection of the threshold and a line connecting two values of the digital signal, one of which is higher and the other of which is lower than the threshold, when the digital signal changes from the higher value to the lower value; input rate detection means for obtaining time intervals between the adjacent rise and the fall times during a predetermined period, and outputting a minimum value of the time intervals as an input rate of the analog signal; and synchronous clock output means for obtaining a half timing of the input rate after an edge of the input analog signal is detected on the basis of the input rate and the rise and fall times and outputting a first one of the synchronous clock at that timing, and obtaining a timing of the input rate after the first synchronous clock is output and outputting a second or later one of the synchronous clock at that timing. Therefore, in addition to the effects of the 1st aspect, the clock generation apparatus is hard to be affected by noises or the like in detecting the threshold, thereby detecting the more accurate threshold. 
   According to 3rd and 4th aspects of the present invention, the clock generation apparatus of the 1st and 2nd aspects, respectively, comprises an oversampling digital filter for interpolating the adjacent digital signals. Therefore, arbitrary frequency characteristics are given to the digital signal, whereby unnecessary signals, such as noises, can be removed. Further, the oversampling increases the number of sample data, whereby the temporal resolution of the digital signal can be increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating a clock generation apparatus according to a first embodiment of the present invention. 
       FIG. 2  is a block diagram illustrating an arithmetic circuit in the clock generation apparatus of the first embodiment. 
       FIG. 3  is a flowchart showing the operation of the arithmetic circuit of the first embodiment. 
       FIG. 4  is a flowchart showing a threshold detection method according to the first embodiment. 
       FIG. 5  is a flowchart showing a rise time detection method according to the first embodiment. 
       FIG. 6  is a timing chart for explaining the rise time detection method of the first embodiment. 
       FIG. 7  is a flowchart showing a fall time detection method according to the first embodiment. 
       FIG. 8  is a flowchart showing an input rate detection method according to the first embodiment. 
       FIG. 9  is a flowchart showing a synchronous clock output method according to the first embodiment. 
       FIG. 10  is a block diagram illustrating an arithmetic circuit in a clock generation apparatus according to a second embodiment of the present invention. 
       FIG. 11  is a flowchart showing a threshold detection method according to the second embodiment. 
       FIG. 12  is a block diagram illustrating a clock generation apparatus according to a third embodiment of the present invention. 
       FIG. 13  is a block diagram illustrating a semiconductor integrated circuit comprising a prior art clock generation apparatus. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described. 
   [Embodiment 1] 
     FIG. 1  is a block diagram illustrating a clock generation apparatus according to a first embodiment of the present invention. 
   The clock generation apparatus  100  according to the first embodiment comprises an analog signal input terminal  101 , a clock signal input terminal  102 , a synchronous signal output terminal  103 , a synchronous clock output terminal  104 , an A/D converter  105 , an arithmetic circuit  106 , a comparator circuit  107  and a latch circuit  108 . The clock generation apparatus  100  receives an analog signal S 101  to which VBI data are superimposed in the blanking interval and a clock signal S 102 , and outputs a synchronous signal S 103  and a synchronous clock S 104 . 
   The A/D converter  105  samples the analog signal S 101  in accordance with the timing of the clock signal S 102 , thereby outputting a digital signal S 109  as a digital discrete value. 
   The arithmetic circuit  106  receives the digital signal S 109  and the clock signal S 102 , and outputs a threshold S 106   a  as a reference value for binarizing the digital signal S 109  and a synchronous clock S 106   b  for latching a binarized signal S 110 . The details will be described later. 
   The comparator circuit  107  receives the digitally converted signal S 109  and the threshold S 106   a , operates in synchronization with the clock signal S 102 , and makes a comparison to see whether the digital signal S 109  is larger or smaller than the threshold S 106   a . Then, the comparator circuit  107  outputs “1” when the digital signal S 109  is larger than the value of the threshold S 106   a , and outputs “0” when the digital signal S 109  is smaller than the threshold S 106   a , as the binarized signal S 110 . 
   The latch circuit  108  receives the binarized signal S 110  which is output by the comparator circuit  107  as a D input and the synchronous clock S 106   b  as a clock input, and outputs the synchronous signal S 103 . 
     FIG. 2  is a block diagram illustrating the arithmetic circuit in the clock generation apparatus according to the first embodiment. 
   The arithmetic circuit  106  of the first embodiment comprises a threshold detection block  200  for detecting the threshold S 106   a , a rise detection block  201  for detecting a rise time as a time of intersection of the threshold S 106   a  and an approximated line of two values of the digital signal S 109  when the digital signal S 109  exceeds the threshold S 106   a  and, a fall detection block  202  for detecting a fall time as a time of intersection of the threshold S 106   a  and an approximated line of two values of the digital signal S 109  when the digital signal S 109  is lower than the threshold S 106   a , an input rate detection block  203  for detecting the rate of the digital signal S 109  using the rise time and the fall time, and a synchronous clock output block  204  for outputting the synchronous clock S 106   b.    
   Hereinafter, the operation of the clock generation apparatus according to the first embodiment is described. 
   The analog signal S 101  is input to the A/D converter  105  via the analog signal input terminal  101 . The clock signal S 102  is input to the A/D converter  105 , the arithmetic circuit  106  and the comparator circuit  107  via the clock signal input terminal  102 . 
   The A/D converter  105  samples the analog signal S 101  in accordance with the timing of the clock signal S 102 , thereby outputting the digital signal S 109  as the digital discrete value to the arithmetic circuit  106  and the comparator circuit  107 . 
   The arithmetic circuit  106  receives the digital signal S 109  and the clock signal S 102 , and outputs the threshold S 106   a  as the reference value for binarizing the digital signal S 109  and the synchronous clock S 106   b  for latching the binarized signal S 110  in the latch circuit  108 . The details will be described later. 
   The comparator circuit  107  receives the digital signal S 109  and the threshold S 106   a , and makes the comparison to see whether the digital signal S 109  is larger or smaller than the threshold S 106   a  in synchronization with the clock signal S 102 . The comparator circuit  107  outputs “1” when the digital signal S 109  is larger than the threshold S 106   a , and outputs “0” when the digital signal S 109  is smaller than the threshold S 106   a , as the binarized signal S 110 . 
   The latch circuit  108  receives the binarized signal S 110  as the D input and the synchronous clock S 106   b  as the clock input, and outputs the synchronous signal S 103  as the synchronous signal output. 
   Hereinafter, the operation of the arithmetic circuit  106  outputting the threshold S 106   a  and the synchronous clock S 106   b  is described with reference to FIG.  3 . 
     FIG. 3  is a flowchart showing the operation of the arithmetic circuit of the first embodiment. 
   Initially, the threshold detection block  200  detects the threshold S 106   a  (Step  300 ), and outputs the threshold S 106   a  to the rise detection block  201 , the fall detection block  202  and the comparator circuit  107  (Step  301 ). Here, the method for detecting the threshold will be described later. 
   Next, the rise detection block  201  detects the rise time Rise(j) (Step  302 ), and outputs the rise time Rise(j) to the input rate detection block  203  and the synchronous clock output block  204  (Step  303 ). Similarly, the fall detection block  202  detects the fall time Fall(j), and outputs the fall time Fall(j) to the input rate detection block  203  and the synchronous clock output block  204 . Here, “j” is the argument and shows the order in which the respective times have been detected. The rise time detection method and the fall time detection method will be described later. 
   Then, the input rate detection block  203  detects a rate (Rate) of the digital signal S 109  on the basis of the rise time Rise(j) and the fall time Fall(j) (Step  304 ), and outputs the rate (Rate) to the synchronous clock output block  204  (Step  305 ). The method for detecting the input rate will be described later. 
   Then, the synchronous clock output block  204  outputs the synchronous clock S 106   b  (Step  306 ). The method for outputting the synchronous clock will be described later. 
   Hereinafter, the operations in the respective blocks of the arithmetic circuit  106  are described in detail with reference to  FIGS. 4  to  9 . 
   Initially, a threshold detection method in the threshold detection block  200  is described with reference to the flowchart of FIG.  4 . 
     FIG. 4  is a flowchart showing the threshold detection method according to the first embodiment. 
   Initially, the digital signal S 109  as the output of the A/D converter  105  is accepted at an arbitrary time as 0th data A 0 , and the data A 0  is set as the initial value (Step  400 ). The data A 0  is input to internal registers Amax and Amin in the arithmetic circuit  106 , respectively, and “1” is input to the internal pointer i as well as a repeat count N is given (Step  401 ). The repeat count N shows the number of the digital signals S 109 . The larger the number, the more the accuracy of the threshold S 106   a  is increased. 
   Then, the digital signals S 109  are accepted in the order pointed by the internal pointer i, and the comparison is made to see whether the accepted data A i  is larger or smaller than the data in the internal register Amax (Step  402 ). When the data A i  is larger than the data in the internal register Amax, the data in the internal register Amax is replaced with the data A i  (Step  403 ). On the other hand, when data  Ai  is smaller than the data in the internal register Amax, a comparison is made to see whether the data A i  is larger or smaller than the data in the internal register Amin (Step  404 ). When the data Ai is smaller than the data in the internal register Amin, the data in the internal register Amin is replaced with the data A i  (Step  405 ). 
   Thereafter, the internal pointer i is incremented (Step  406 ) and whether or not the value of the internal pointer i is equal to the repeat count N is checked (Step  407 ). When the value of the internal pointer i is not equal to the repeat count N, i.e., the value of the internal pointer i is smaller than the repeat count N, the processing proceeds to Step  402  so as to accept the next digital signal S 109 . When the value of the internal pointer i is equal to the repeat count N, the average value of the data in the internal register Amax and the data in the internal register Amin is output as the threshold S 106   a  (Step  408 ). 
   A rise time detection method in the rise detection block  201  is described with reference to the flowchart of FIG.  5 . 
     FIG. 5  is a flowchart showing the rise time detection method according to the first embodiment. 
   Initially, “2” is input to the internal pointer i, and further, the internal pointer j is cleared. Then, the digital signal S 109  which is accepted at an arbitrary time is set as data A 0 , the digital signal S 109  which is accepted subsequent to the data A 0  is set as data A 1 , and the input of the repeat count M is accepted (Step  500 ). 
   Further, the digital signal S 109  is accepted (Step  501 ) When Step  501  is executed for the first time at this time, the digital signal S 109  which is accepted here is data A 2 , because “2” has already been set in the internal pointer i. 
   Then, it is determined whether the value of data A i−1  is smaller than the threshold S 106   a  and the value of data A i  is larger than the threshold S 106   a  (Step  502 ). When the value of the data Ai−1 is larger than the threshold S 106   a  or the value of the data Ai is smaller than the threshold S 106   a , the internal pointer i is incremented and then Step  501  is executed (Step  503 ). When the value of the data Ai−1 is smaller than the threshold S 106   a  and the value of the data Ai is larger than the threshold S 106   a , the analog signal S 101  intersects the threshold S 106   a  and the rise time occurs. Therefore, the j-th rise time Rise(j) is output (Step  504 ). The details of this arithmetic will be described later. 
   Then, the internal pointer j is incremented (Step  505 ) and whether or not M pieces of the rise time Rise(j) have been detected is monitored (Step  506 ). When M pieces of the rise time have been detected, the processing is completed. Otherwise, the processing returns to Step  501  and the above-mentioned processing is repeated until M pieces of the rise time are detected. 
   Here, the details of the method of obtaining the j-th rise time Rise(j) are given with reference to FIG.  6 . 
     FIG. 6  is a timing chart for explaining the rise time detection method of the first embodiment. 
   At time T i−1 , the data A i−1 , of the digital signal S 109  as the output of the A/D converter  105  is accepted, and the value of the data is lower than the threshold S 106   a . At time T i , the data A i  of the digital signal S 109  is accepted. This data A i  is higher than the threshold S 106   a . Therefore, the analog signal S 101  intersects the threshold S 106   a  between time T i−1  and time T i . 
   The dot-dash line  600  shows a straight line which is obtained by linear approximation with the two points of the data. Making the approximation that a time when the analog signal S 101  intersects the threshold S 106   a  is a time when the dot-dash line  600  intersects the threshold S 106   a , that time is a time when a period x i−1 , has elapsed from time T −1 . That is, the dot-dash line  600  is an intersection time approximation line and x i−1 , is a rise intersection time correction time. 
   Assuming that the origin of the time axis of the dot-dash line  600  is a time when the data Ai−1 is input, the parameter of the time axis is x, and the amplitude axis of the input signal is y, their relationship is given by the following equation:
 
 y=A   i−1 +( A   i   −A   i−1 ) x  
 
   The time X i-1  when the dot-dash line  600  intersects the threshold S 106   a  is given by the following equation assuming that the threshold is THR,
 
 THR=A   i−1 +( A   i   −A   i−1 ) x   i−1  
 
   Therefore, when this linear equation is solved to obtain x i−1,  
 
 x   i−1 =( THR−A   i−1 )/(A i −A i−1 ) 
 
This x i−1  corresponds to the decimal part of the rise time Rise(j). Therefore, the rise time Rise(j) obtained in Step  504  is given by the following expression.
 
Rise( j )= i−x   i−1  
 
   A fall time detection method in the fall detection block  202  is described with reference to FIG.  7 . 
     FIG. 7  is a flowchart showing the fall time detection method according to the first embodiment. 
   In  FIG. 7 , the same reference numerals as those in  FIG. 5  correspond to the same processes in FIG.  5 . 
   In the fall time detection method as shown in the flowchart of  FIG. 7 , whether or not the analog signal became lower than the threshold is detected in Step  702 . Therefore, the determination is made by a criterion opposed to that in Step  502 . The fall time detection method is different from Steps  502  and  504  of the rise time detection method in that it is determined whether the value of the data A i−1  is larger than the threshold S 106   a  and the value of the data A i  is smaller than the threshold S 106   a , and that an arithmetic result in Step  704  is output as Fall (j), respectively. 
   An input rate detection method in the input rate detection block  203  is described with reference to the flowchart of FIG.  8 . 
   The input rate of the input analog signal S 101  can be detected on the basis of the rise time and fall time which are detected by the above-mentioned processing. That is, when the period from a rise time Rise(j) to a fall time Fall(j) occurring subsequently is obtained, this period is always a multiple of the input rate. Therefore, difference values between plural rise times and fall times are obtained, and the minimum value of the difference values is used as the input rate. 
     FIG. 8  is a flowchart showing the input rate detection method according to the first embodiment. 
   Initially, an initial value is input to the internal register Rate which holds the input rate of the input signal, the initial value “1” is input to the internal pointer j, and the repeat count M is set (Step  800 ). In this case, M means the number of data of a plurality of the rise times Rise and the fall times Fall obtained in Step  302 . 
   Next, the time interval between the rise time Rise(j) and the fall time Fall(j) is obtained, and the result is retained in the internal register Temp (Step  801 ). The internal register Temp is a saving register to which the difference between the rise time Rise(j) and the fall time Fall(j) corresponding to the argument provided for convenience of arithmetic is saved. 
   Then, the value of the internal register Rate is compared with the value of the internal register Temp (Step  802 ). When the value of the internal register Temp is smaller than the value of the internal register Rate, the value of the internal register Temp is input to the internal register Rate to make a replacement of the value (Step  803 ). When the value of the internal register Temp is larger than the value of the internal register Rate, the internal pointer j is incremented and Step  801  is executed again (Step  804 ). 
   Then, in Step  805 , when the value of the internal pointer j is equal to the repeat count M, i.e., when the prescribed number of times of processing has been completed, the processing is completed. The value of the internal register Rate at this time is used as the input rate. When the prescribed number of times of processing has not been completed, Step  804  is executed. 
   A method for outputting the synchronous clock in the synchronous clock output block  204  is described with reference to the flowchart of FIG.  9 . 
     FIG. 9  is a flowchart showing a synchronous clock output method according to the first embodiment. 
   Initially, the threshold S 106   a  detected in Step  300  and the input rate (Rate) detected in Step  305  are accepted, and further, the internal pointer i is cleared (Step  900 ). 
   Next, the digital signals S 109  are accepted in the order in which the internal pointer i points the signals. The product of the difference between the accepted data A i  and the threshold S 106   a  and the difference between the previously accepted data A i−1  and the threshold S 106   a  is obtained (Step  902 ). When the product is 0 or more, the internal pointer i is incremented (Step  903 ). When the product is less than 0, x i  and the edge time Edge(i) as a time when the data intersects the threshold S 106   a  are obtained on the same principles in Steps  504  and  704  (Step  904 ). The obtained Edge(i) is composed of an integer part i and a decimal part X i . In addition, Rate/2 is composed of an integer part r and a decimal part r i . The Edge(i) and Rate/2 are added and consequently an integer part Sam and a decimal part x s  are obtained (Step  905 ). Owing to the arithmetic in this Step, the first synchronous clock S 106   b  is generated at a half timing of the input rate (Rate), i.e., at the middle of one rate of the input signal, after the edge of the signal comes. 
   Next, it is monitored that the Sam-th data Asam for generating the synchronous clock S 106   b  is input (Step  906 ) When it is detected that the data Asam has been input, the synchronous clock S 106   b  is generated once (Step  907 ). 
   Then, the arithmetic for a timing value of the second or subsequent synchronous clock is performed (Step  908 ). The timing for generating the synchronous clock in the middle of the input rate, i.e., Sam+x s  has been already obtained in Step  905  when the first synchronous clock is generated. Therefore, in this Step, only the value of the internal register Rate is added to Sam+Xs, whereby the second or subsequent clock can be generated in the middle of the input rate. Thereafter, the above-mentioned processing is repeated, thereby outputting the synchronous clock S 106   b.    
   In the clock generation apparatus according to the first embodiment, the input rate is detected from the digital signal which is obtained by subjecting the input analog signal to the A/D conversion, and the synchronous clock is generated on the basis of the input rate. Therefore, when the binarized signal is to be latched, the phase error between the synchronous clock and the binarized signal can be within one clock of the synchronous clock. Accordingly, the VBI data can be correctly acquired from analog television signals on which the VBI data are superimposed in the blanking interval. 
   In addition, the clock generation apparatus of the first embodiment, is usually realized by a semiconductor integrated circuit while, in this case, in order to improve performance of the clock generation apparatus, it is not required to increase the frequency of the supplied clock. Therefore, the unnecessary radiation generated from the semiconductor integrated circuit is not increased. Furthermore, even when the input analog signal has the plural kinds of input rates, the plural clock supply circuits are not required. 
   [Embodiment 2] 
     FIG. 10  is a block diagram illustrating an arithmetic circuit of a clock generation apparatus according to a second embodiment of the present invention. 
   In the clock generation apparatus according to the second embodiment, the arithmetic circuit  106  in the clock generation apparatus of the first embodiment as shown in  FIG. 1  is replaced with an arithmetic circuit as shown in FIG.  10 . Other construction is the same as that in the clock generation apparatus according to the first embodiment. 
   As shown in  FIG. 10 , the arithmetic circuit  106  comprises a threshold detection block  1000  for detecting a threshold S 106   a , a rise detection block  201  for detecting a rise time as a time of intersection of the threshold S 106   a  and an approximated line of two values of the digital signal S 109  when the digital signal S 109  exceeds the threshold S 106   a , a fall detection block  202  for detecting a fall time as a time of intersection of the threshold S 106   a  and an approximated line of two values of the digital signal S 109  when the digital signal S 109  gets lower than the threshold S 106   a , an input rate detection block  203  for detecting a rate of the digital signal S 109  using the rise time and the fall time, and a synchronous clock output block  204  for outputting a synchronous clock S 106   b.    
   Hereinafter, the operation is described. 
   Here, the operations of the rise detection block  201 , the fall detection block  202 , the input rate detection block  203  and the synchronous clock output block  204  are the same as those in the first embodiment. Therefore, their descriptions are not given here. Hereinafter, the description is given of the operation of the threshold detection block  1000  detecting the threshold S 106   a , with reference to FIG.  11 . 
     FIG. 11  is a flowchart showing a threshold detection method according to the second embodiment. 
   Initially, an internal register Acc and an internal pointer i in the arithmetic circuit  106  are cleared, respectively, and a repeat count N is accepted (Step  1100 ). Here, the repeat count N shows the number of the digital signals S 109 . The larger the number, the more the accuracy of the threshold S 106   a  is increased. 
   Next, the digital signal S 109  is accepted as well as it is monitored whether the value of the internal pointer i is larger than the repeat count N (Step  1101 ). When the value of the internal pointer i is smaller than the repeat count N, data Ai which has been accepted in Step  1101  is successively added to the internal register Acc, and further the internal pointer i is incremented (Step  1102 ). Accordingly, the integral of data A i  of (N+1) digital signals S 109  (i.e., data A i  of the digital signal S 109  where i=0, i.e., 0-th data, to data A i  of the digital signal S 109  where i=N, i.e., N-th data) is stored in the internal register Acc. In addition, when the value of the internal pointer i is larger than the repeat count N, the value of the internal register Acc is divided by the number of the integrated data, i.e., N+1. Then, the obtained value is output as the threshold S 106   a  (Step  1103 ). 
   In the clock generation apparatus according to the second embodiment, the average of the integrals of the digital signals is used as the threshold. Therefore, the effects of the clock generation apparatus according to the first embodiment are obtained, and further effects that the detection of the threshold resists influences by the noises or the like and a more accurate threshold is detected are obtained. 
   [Embodiment 3] 
     FIG. 12  is a block diagram illustrating a clock generation apparatus according to a third embodiment of the present invention. In this figure, the same reference numerals as those in  FIG. 1  denote the same or corresponding parts, and their descriptions are not given here. 
   The clock generation apparatus according to the third embodiment comprises an oversampling digital filter  1201  in the subsequent stage of the A/D converter  105  of the clock generation apparatus according to the first embodiment. 
   The oversampling digital filter  1201  gives an arbitrary frequency characteristic to the input signal as well as performs oversampling and outputs the oversampled signal to the comparator circuit  107 . 
   In the clock generation apparatus according to the third embodiment, the oversampling digital filter  1201  gives an arbitrary frequency characteristic to the digital signal, whereby unnecessary signals, such as noises, can be removed. Further, the oversampling digital filter performs the oversampling, whereby the number of sample data is increased and the temporal resolution of the digital signal can be increased. 
   The clock generation apparatus of the third embodiment comprises the oversampling digital filter in the subsequent stage of the A/D converter in the clock generation apparatus of the first embodiment. However, the oversampling digital filter can be provided in the subsequent stage of the A/D converter in the clock generation apparatus of the second embodiment. 
   In the first to third embodiments, the clock generation apparatus according to the present invention are described taking cases where television signals to which VBI data are superimposed in the blanking interval are input as examples. However, the signals are not restricted to the television signal, and playback signals of CD (Compact Disk) or MD (mini disk) or the like can be input.