Patent Publication Number: US-2009237558-A1

Title: Teletext receiving circuit

Description:
TECHNICAL FIELD 
     The present invention relates to a teletext receiving circuit for receiving a teletext signal superimposed on a vertical blanking interval in an analog TV composite signal. 
     BACKGROUND ART 
     A teletext signal is sometimes superimposed on a vertical blanking interval in an analog TV composite signal. Examples of a teletext receiving circuit for receiving such a teletext signal include a circuit illustrated in a block diagram in  FIG. 3 . This teletext receiving circuit includes an A/D converter  310 , a horizontal synchronizing signal detection/CRI window generation section  320 , a frequency characteristic enhancement circuit  330 , a CPU  340 , a slicing circuit  350 , a teletext data decoding circuit  360 , a teletext data storage section  370 , and an OSD (On Screen Display)  380 , and receives an analog TV composite signal S 400 . A teletext signal is superimposed on the vertical blanking interval in the analog TV composite signal S 400 . 
     The A/D converter  310  digitizes the analog TV composite signal S 400  and outputs the digitized signal to the frequency characteristic enhancement circuit  330 . 
     The horizontal synchronizing signal detection/CRI window generation section  320  detects a horizontal synchronization interval in the analog TV composite signal S 400 , and, based on the detection result, generates a CRI window indicating a clock run-in (CRI) period showing a reference waveform for the teletext signal. 
     The frequency characteristic enhancement circuit  330  is a circuit for compensating for the frequency band of a CRI signal which is a reference signal for the teletext signal, and includes a band-pass filter  331  (which is abbreviated as BPF in the figure), a multiplier  332 , and an adder  333 . 
     The band-pass filter  331  only extracts a frequency of the input teletext signal that contains the clock run-in signal. The multiplier  332  multiplies the output of the band-pass filter  331  by a multiplier (which will be described later) provided by the CPU  340 , and outputs the multiplication result to the adder  333 . The adder  333  adds the output of the A/D converter  310  and the output of the multiplier  332  together and outputs the result to the slicing circuit  350 . 
     The CPU  340  monitors the amplitude of the output (i.e., the frequency component of the CRI signal) of the band-pass filter  331  and provides the multiplier  332  with the appropriate multiplier in accordance with the monitoring results. In this way, the target amplitude of the CRI signal is maintained without relying on an externally input amplitude. That is, the characteristics of the frequency component of the CRI signal are enhanced. 
     The slicing circuit  350  includes a MIN/MAX circuit  351 , a teletext data extraction circuit  353 , and a slicing level calculation circuit  352 . The MIN/MAX circuit  351  monitors the amplitude value during the CRI period indicated by the horizontal synchronizing signal detection/CRI window generation section  320 , and outputs the minimum and maximum of the amplitude value to the slicing level calculation circuit  352 . From the minimum and maximum values obtained by the MIN/MAX circuit  351 , the slicing level calculation circuit  352  determines a threshold value for binarizing the teletext signal and identifying 0/1, and outputs the threshold value to the teletext data extraction circuit  353 . From the threshold value provided by the slicing level calculation circuit  352 , the teletext data extraction circuit  353  substitutes data of 0/1 for the teletext signal data output from the frequency characteristic enhancement circuit  330 , and outputs the data of 0/1 to the teletext data decoding circuit  360  in the subsequent stage. 
     The teletext data decoding circuit  360  decodes the output of the teletext data extraction circuit  353  in accordance with a format in the broadcast system. The teletext data storage section  370  stores therein the output of the teletext data decoding circuit  360 . 
     Under control of the CPU  340 , the OSD  380  displays the teletext data in the teletext data storage section  370  on an output destination (for example, on the screen of a TV). 
     The operation of the teletext receiving circuit described above, in particular, the operation of the frequency characteristic enhancement circuit  330 , will be described in the order of signal flow. 
       FIG. 4  shows an example of the waveform of the analog TV composite signal S 400  in the vertical blanking interval. Teletext signals are transmitted in accordance with systems which differ from area to area around the world, and these teletext signals can thus be identified by the broadcast systems, the transmission rates, the superimposition lines, etc. For example, for World System Teletext (which will be hereinafter referred to as “WST”) transmitted in Europe, the transmission rate is 6.9375 MHz/bit, and the superimposition lines are from 7 to 22. 
     The analog TV composite signal S 400  is an example of a signal in the lines on which WST is superimposed. The analog TV composite signal S 400  contains a horizontal synchronizing signal S 401 , which determines a vertical location, and a burst signal S 402 , which is related to color signal demodulation technique. Also, a clock run-in signal S 403 , a framing code S 404 , and a teletext data signal S 405  are the elements forming the teletext signal. 
     The clock run-in signal S 403  is a reference for the teletext signal, and is related to a reference level for determining a value of 0/1 for the teletext data. In WST, the clock run-in signal S 403  is “1010101010101010” when 0/1 digitized, and has a frequency component of approximately 3.5 MHz. The framing code S 404  indicates the type of the teletext signal, and is “11100100” in WST. The teletext data signal S 405  is teletext data, and, in WST, always 42-byte data. 
     As indicated by S 501  in  FIG. 5 , the analog TV composite signal S 400  has a band up to 6.3975 MHz, and when the analog TV composite signal S 400  passes through the frequency characteristic enhancement circuit  330 , a part in the vicinity of 3.5 MHz, which is the clock run-in signal S 403 , is amplified by the multiplier provided from the CPU  340 . Therefore, as indicated by S 502  in  FIG. 5 , for example, when the amplitude characteristics of the clock run-in signal S 403  are not sufficient due to effects of the transmission path or the like, the frequency characteristics can be compensated for by the frequency characteristic enhancement circuit  330  as indicated by S 503 . 
     The MIN/MAX circuit  351  detects the minimum value  4 A (see  FIG. 4 ) and the maximum value  4 B (see  FIG. 4 ) of the amplitude during the CRI period indicated by the CRI window generated by the horizontal synchronizing signal detection/CRI window generation section  320 , and provides the detected values to the CPU  340 . From the minimum and maximum values  4 A and  4 B of the amplitude, the slicing level calculation circuit  352  determines the midpoint level  4 C (see  FIG. 4 ) as the slicing level for the 0/1 digitization. And the framing code S 404  and the teletext data signal S 405  are decoded using the midpoint level  4 C as the reference level. 
     As described above, the frequency characteristic enhancement circuit  330  amplifies the amplitude of the CRI signal band to increase the operation accuracy of the slicing circuit  350 , thereby contributing to the teletext signal receiving capability. 
     Other examples of a teletext receiving circuit include a circuit in which input data is binarized and decoded at a plurality of slicing levels to generate a plurality of pieces of decoded data, and then decoded data containing no errors is selected from those pieces of decoded data (see Patent Document 1, for example). 
     Patent Document 1: Japanese Laid-Open Publication No. 2004-40350 
     DISCLOSURE OF THE INVENTION 
     Problem that the Invention Intends to Solve 
     However, in the above-described frequency characteristic enhancement circuit, when the input (i.e., the analog TV composite signal) is minute, it is not possible to make the amplitude of the teletext data satisfy the set slicing level just by amplifying the frequency band of the clock run-in signal, and thus the possibility that the teletext data would be erroneously extracted increases. This means that, in that case, the teletext data also needs amplitude amplification. 
     In view of the problem described above, the present invention has been made, and it is therefore an object of the present invention to provide a necessary signal amplitude to a signal which is input to a slicing circuit, even when an analog TV composite signal having a minute amplitude is input. 
     Means for Solving the Problem 
     In order to achieve the object, an inventive teletext receiving circuit for receiving a teletext signal superimposed on a vertical blanking interval in an analog TV composite signal includes: a frequency characteristic enhancement circuit for amplifying a frequency band of the teletext signal when the frequency band has an amplitude lower than a predetermined level; and a slicing circuit for binarizing an output of the frequency characteristic enhancement circuit and outputting binarized data. 
     EFFECTS OF THE INVENTION 
     According to the present invention, even when an analog TV composite signal having a minute amplitude is input, a necessary signal amplitude is provided to a signal which is input to a slicing circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating the configuration of a teletext receiving circuit  100  according to a first embodiment. 
         FIG. 2  is a block diagram illustrating the configuration of a teletext receiving circuit  200  according to a second embodiment. 
         FIG. 3  is a block diagram illustrating an example of a conventional teletext receiving circuit. 
         FIG. 4  shows an example of the waveform of an analog TV composite signal in a vertical blanking interval. 
         FIG. 5  is an explanatory view of a teletext signal band. 
     
    
    
     EXPLANATION OF THE REFERENCE CHARACTERS 
     
         
         
           
               100  Teletext receiving circuit 
               110  A/D converter 
               120  Horizontal synchronizing signal detection/CRI window generation section 
               130  Frequency characteristic enhancement circuit 
               131  Band-pass filter 
               132  Selection circuit 
               133  Multiplier 
               134  Adder 
               140  Slicing circuit 
               141  First MIN/MAX circuit 
               142  Second MIN/MAX circuit 
               143  Slicing level calculation circuit 
               144  Teletext data extraction circuit 
               145  Determination circuit 
               150  Teletext data decoding circuit 
               160  Teletext data storage section 
               170  OSD 
               180  CPU 
               200  Teletext receiving circuit 
               210  Low-pass filter 
               220  Weak electric field detection circuit 
             S 101  Analog TV composite signal 
             S 102  Digital input signal 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the embodiments of the present invention will be described with reference to the accompanying drawings. In the descriptions of the following embodiments, components having the same functions as those already described are identified by the same reference numerals, and the descriptions thereof will be omitted. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating the configuration of a teletext receiving circuit  100  according to a first embodiment of the present invention. As shown in  FIG. 1 , the teletext receiving circuit  100  includes an A/D converter  110 , a horizontal synchronizing signal detection/CRI window generation section  120 , a frequency characteristic enhancement circuit  130 , a slicing circuit  140 , a teletext data decoding circuit  150 , a teletext data storage section  160 , an OSD (On Screen Display)  170 , and a CPU  180 . 
     The A/D converter  110  receives an analog TV composite signal S 101  and digitizes the input signal to output the digitized signal to the frequency characteristic enhancement circuit  130 . The analog TV composite signal S 101  is a signal in which a teletext signal is superimposed on the vertical blanking interval. 
     The horizontal synchronizing signal detection/CRI window generation section  120  detects a horizontal synchronization interval in the analog TV composite signal S 101 , and, based on the detection results, generates a CRI window indicating a clock run-in (which will be hereinafter referred to as “CRI”) period showing a reference waveform for the teletext signal. 
     The frequency characteristic enhancement circuit  130  compensates for the frequency band of a CRI signal which is a reference signal for the teletext signal. The frequency characteristic enhancement circuit  130  includes a band-pass filter  131  (abbreviated as BPF in the figure), a selection circuit  132 , a multiplier  133 , and an adder  134 . 
     The band-pass filter  131  only extracts a frequency of the input teletext signal that contains the CRI signal (which is the reference signal for the teletext signal). For example, in a case in which the analog TV composite signal S 101  is World System Teletext (which will be hereinafter referred to as “WST”) transmitted in Europe, the band-pass filter  131  is a digital band-pass filter that has a center frequency of approximately 3.5 MHz, which is the CRI signal frequency. In WST, the transmission rate is 6.9375 MHz/bit, and the superimposition lines are from 7 to 22. 
     The selection circuit  132  receives the output data (which will be called a digital input signal S 102 ) of the A/D converter  110  and the output data of the band-pass filter  131 , and selects and outputs one of these input data in accordance with a determination result obtained by a determination circuit  145  (which will be described later) in the slicing circuit  140 . 
     The multiplier  133  multiplies the output of the selection circuit  132  by a multiplier (which will be described later) returned form the CPU  180 . 
     The adder  134  adds the multiplication result obtained by the multiplier  133  and the digital input signal S 102  together, and outputs the result to the slicing circuit  140  in the subsequent stage. 
     The slicing circuit  140  binarizes the output of the frequency characteristic enhancement circuit  130  and outputs the binarized data. The slicing circuit  140  includes a first MIN/MAX circuit  141 , a second MIN/MAX circuit  142 , a slicing level calculation circuit  143 , a teletext data extraction circuit  144 , and the determination circuit  145 . 
     The first MIN/MAX circuit  141  monitors the amplitude value of the output of the frequency characteristic enhancement circuit  130  during the CRI period, and detects and outputs the minimum and maximum of the amplitude value to the slicing level calculation circuit  143  and to the determination circuit  145 . 
     The second MIN/MAX circuit  142  detects the maximum and minimum of the output of the frequency characteristic enhancement circuit  130  during the period of time between the current horizontal synchronization interval and the next horizontal synchronization interval, and outputs the results to the determination circuit  145 . 
     The slicing level calculation circuit  143  determines a threshold value for identification of 0/1 of the teletext signal in accordance with the output of the first MIN/MAX circuit  141 , and outputs the threshold value to the teletext data extraction circuit  144 . 
     Based on the threshold value determined by the slicing level calculation circuit  143 , the teletext data extraction circuit  144  substitutes data of 0/1 for the teletext signal data output from the frequency characteristic enhancement circuit  130 , and outputs the data of 0/1 to the teletext data decoding circuit  150  in the subsequent stage. 
     The determination circuit  145  controls the selection of the output data by the selection circuit  132  in accordance with the maximum and minimum values output from the first MIN/MAX circuit  141  and the difference between these maximum and minimum values and in accordance with the maximum and minimum values output from the second MIN/MAX circuit  142  and the difference between these maximum and minimum values. 
     Specifically, when these four values, i.e., the maximum and minimum values output from the first MIN/MAX circuit  141  and the maximum and minimum values output from the second MIN/MAX circuit  142 , fall within a predetermined value range, the determination circuit  145  determines that the analog TV composite signal S 101  is a minute signal, and makes the selection circuit  132  select and output the digital input signal S 102 . 
     When the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is greater than a predetermined value, and the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is also greater than a predetermined value, the determination circuit  145  determines that a normal operation (i.e., an operation performed when the analog TV composite signal S 101  is a normal signal) is performed, and makes the selection circuit  132  select and output the output data of the band-pass filter  131 . 
     When the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is greater than a predetermined value while the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is smaller than a predetermined value, the determination circuit  145  determines that the analog TV composite signal S 101  is in an abnormal condition, and makes the selection circuit  132  select and output the digital input signal S 102  so as to amplify the band of the entire input signal. 
     The teletext data decoding circuit  150  decodes the output of the teletext data extraction circuit  144  in accordance with a format in the broadcast system. 
     The teletext data storage section  160  stores therein the output of the teletext data decoding circuit  150 . 
     Under control of the CPU  180 , the OSD  170  displays the teletext data in the teletext data storage section  160  on an output destination (for example, on the screen of a TV). 
     The CPU  180  monitors the amplitude of the CRI signal frequency band in the output of the band-pass filter  131 , and provides the multiplier  133  with an appropriate multiplier in accordance with the monitoring results so as to achieve the target signal amplitude. 
     (Operation of the Teletext Receiving Circuit  100 ) 
     When a signal having a minute amplitude is input to the teletext receiving circuit  100 , the amplitude of the CRI signal is also small. This causes the maximum and minimum values output from the first MIN/MAX circuit  141  to be smaller than usual, and thus the difference between these maximum and minimum values also becomes smaller. Furthermore, since the amplitude of the signal other than the CRI signal portion is also considered small, the maximum and minimum values output from the second MIN/MAX circuit  142  are also smaller than usual, and the difference between these maximum and minimum values thus becomes smaller. Therefore, the determination circuit  145  makes the selection circuit  132  select and output the digital input signal S 102 . On the other hand, the CPU  180  monitors the amplitude of the CRI signal frequency band in the output of the band-pass filter  131 , and provides the multiplier  133  with an appropriate multiplier in accordance with the monitoring results so as to achieve the target signal amplitude. 
     Consequently, the digital input signal S 102  itself is input to the multiplier  133 , so that not only the CRI signal band, but also the band of the entire input signal is amplified. And the adder  134  adds the multiplication result obtained by the multiplier  133  and the digital input signal S 102  together, and outputs the result to the slicing circuit  140 . As a result, a necessary sufficient signal amplitude is provided to the slicing circuit  140 . 
     At this time, the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is greater than a predetermined value, and the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is also greater than a predetermined value. In this case, the determination circuit  145  determines that a normal operation is performed, and makes the selection circuit  132  select and output the output data of the band-pass filter  131 . 
     However, even in the case in which the determination circuit  145  determines that a normal operation is performed, if the input to the teletext receiving circuit  100  continues to have a minute amplitude, only the frequency band of the CRI signal is amplified, which may cause the signal amplitude of the teletext data to fail to achieve the signal amplitude necessary for the slicing circuit  140 . In that case, it is considered that the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is greater than a predetermined value, while the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is smaller than a predetermined value. 
     Therefore, the determination circuit  145  determines that the input signal is in an abnormal condition, and makes the selection circuit  132  select and output the output of the A/D converter  110  so as to amplify the band of the entire input signal. As a result, a necessary sufficient signal amplitude is provided again to the slicing circuit  140 . 
     As described above, according to this embodiment, the detection results obtained by the first and second MIN/MAX circuits  141  and  142  are used in the determination circuit  145  to make determinations about the input signal and thereby identify each case. This allows the selection circuit  132  to select the signal band to be amplified. For example, when a minute signal is input, the entire band of the digital input signal S 102  is amplified. 
     Hence, even when a minute signal is input, a necessary signal amplitude is provided to the signal that is input to the slicing circuit  140 , thereby enabling the slicing level calculation and the teletext data extraction characteristics in the later stages to be enhanced. 
     It should be noted that the center frequency (3.5 MHz) of the band-pass filter  131  is just an example, and a reference signal frequency that each teletext signal has can be the center frequency of the filter. 
     Second Embodiment 
       FIG. 2  is a block diagram illustrating the configuration of a teletext receiving circuit  200  according to a second embodiment of the present invention. As shown in  FIG. 2 , the teletext receiving circuit  200  is obtained by adding a low-pass filter  210  (which is abbreviated as LPF in the figure) and a weak electric field detection circuit  220  to the teletext receiving circuit  100 . As a result of the addition of theses members, the functions of the selection circuit  132  and of the determination circuit  145  are changed as will be described later. 
     The low-pass filter  210  is added to a frequency characteristic enhancement circuit  130 . The low-pass filter  210  is a digital low-lass filter for removing only part of an input teletext signal whose frequency is higher than a CRI signal frequency, and a digital input signal S 102  is input to the low-pass filter  210 . 
     In the teletext receiving circuit  200 , a selection circuit  132  receives the digital input signal S 102 , the output data of a band-pass filter  131 , and the output data of the low-pass filter  210 , and selects and outputs one of these three output data in accordance with a determination result obtained by a determination circuit  145 . 
     The weak electric field detection circuit  220  detects whether or not an analog TV composite signal S 101  has a weak electric field, and outputs the detection result to the determination circuit  145 . To be specific, the weak electric field detection circuit  220  detects whether or not the digital input signal S 102  is a signal having a small amplitude and a high noise ratio (i.e., a noise condition signal). 
     In the teletext receiving circuit  200 , when the weak electric field detection circuit  220  determines that the digital input signal S 102  is not a noise condition signal, the determination circuit  145  makes the selection circuit  132  select either the digital input signal S 102  or the output data of the band-pass filter  131  in accordance with the same determination criteria as in the first embodiment. On the other hand, when the weak electric field detection circuit  220  determines that the digital input signal S 102  is a noise condition signal, the difference between maximum and minimum values detected by a first MIN/MAX circuit  141  is equal to or smaller than a predetermined value, and the difference between maximum and minimum values detected by a second MIN/MAX circuit  142  is equal to or smaller than a predetermined value, then the determination circuit  145  makes the selection circuit  132  select and output the output data of the low-pass filter  210 . 
     (Operation of the Teletext Receiving Circuit  200 ) 
     The operation of the teletext receiving circuit  200  will be described by taking a case in which the analog TV composite signal S 101  is WST by way of example. 
     In a case in which a teletext signal is WST, if a signal having a weak electric field, that is, a signal having a small amplitude and a high noise ratio, is input to the teletext receiving circuit  200 , the analog TV composite signal S 101  is considered to have a minute amplitude and contain a signal whose frequency is higher than the transmission rate, i.e., 6.9375 MHz. Consequently, the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is small, and the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is also small. At the same time, the weak electric field detection circuit  220  detects the weak electric field condition. 
     Hence, the determination circuit  145  determines that the input signal has a weak electric field, and makes the selection circuit  132  select the output data of the low-pass filter  210  so as to remove the noise component whose frequency is higher than the transmission rate from the digital input signal S 102 . 
     On the other hand, the CPU  180  monitors the amplitude of the CRI signal frequency band in the output of the band-pass filter  131 , and provides a multiplier  133  with an appropriate multiplier in accordance with the monitoring results so as to achieve a signal amplitude necessary for a slicing circuit  140 . Then, the multiplier  133  multiplies the output data of the low-pass filter  210  by the provided multiplier, and outputs the result to an adder  134 . The adder  134  adds the multiplication result obtained by the multiplier  133  and the digital input signal S 102  together, and outputs the result to the slicing circuit  140 . As a result, the slicing circuit  140  is provided with the signal from which the high-frequency component noise, specific to the weak electric field condition, has been removed in advance and which has a necessary sufficient signal amplitude. 
     At this time, the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is greater than a predetermined value, and the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is also greater than a predetermined value. In this case, the determination circuit  145  determines that a normal operation is performed, and makes the selection circuit  132  select and output the output data of the band-pass filter  131 . 
     However, even in the case in which the determination circuit  145  determines that a normal operation is performed, if the input to the teletext receiving circuit  100  continues to have the weak electric field, only the frequency band of the CRI signal is amplified by the multiplier  133 , which may cause the signal amplitude of the teletext data to fail to achieve the signal amplitude necessary for the slicing circuit  140 . In that case, it is considered that the difference between the maximum and minimum values output from the first MIN/MAX circuit  141  is greater than a predetermined value, while the difference between the maximum and minimum values output from the second MIN/MAX circuit  142  is smaller than a predetermined value. 
     Therefore, the determination circuit  145  determines that the input signal is in an abnormal condition, and makes the selection circuit  132  select and output the output of the A/D converter  110  so as to amplify the band of the entire input signal. As a result, a necessary sufficient signal amplitude is provided again to the slicing circuit  140 . 
     As described above, according to this embodiment, the detection results obtained by the first and second MIN/MAX circuits  141  and  142  and obtained by the weak electric field detection circuit  220  are used in the determination circuit  145  to make determinations about the input signal and thereby identify each case. This allows the selection circuit  132  to select the signal band to be amplified. For example, when the analog TV composite signal S 101  is a noise condition signal, the entire band of the digital input signal S 102 , from which noise component has been removed, is amplified. 
     Accordingly, even in the case of a weak-electric-field input, it is possible to stably provide a necessary signal amplitude to the slicing circuit  140 , thereby enabling the slicing level calculation and the teletext data extraction characteristics in the later stages to be enhanced. 
     It should be noted that the cut-off frequency (which is the transmission rate, i.e., 6.9375 MHz, in the above-described example) of the low-pass filter  210  is just an example, and may be set by a signal band that each teletext signal has. 
     Moreover, the WST signals described as the teletext signal inputs in the first and second embodiments are also examples, and the teletext signal inputs may be any teletext signals around the world. 
     INDUSTRIAL APPLICABILITY 
     In the teletext receiving circuits according to the present invention, even when an analog TV composite signal having a minute amplitude is input, a necessary signal amplitude is provided to a signal that is input to a slicing circuit. Thus, the inventive teletext receiving circuits are applicable to teletext receiving circuits for receiving a teletext signal superimposed on a vertical blanking interval in an analog TV composite signal, and the like.