Clamping apparatus for a video signal
BACKGROUND OF THE INVENTION 
     The present invention relates to a clamping circuit for a video signal, and more particularly to a clamping circuit which sets the pedestal level of a digital video signal produced by an analog-to-digital converter, in response to an input analog video signal. 
     When an analog video signal is converted into a digital video signal (PCM video signal), it is necessary to set the pedestal level of the converted digital video signal at a predetermined level. In a conventional clamping circuit, an ordinary keyed clamping circuit is arranged in front of an analog-to-digital (A/D) converter to set the pedestal level of the analog signal which is then subject to A/D conversion. 
     Known A/D converters are sufficiently accurate to permit a relative error in a quantizing step to be disregarded. However, such A/D converters still have a drift error throughout their quantization range. Further, there are individual characteristic variation in the marketed A/D converters. 
     Conventional clamping devices cannot compensate for drift errors and for individual parameter variations in given A/D conerters. Therefore, pedestal level changes or errors after the A/D conversion are unavoidable. Further, these changes produce other changes in the set-up level in the case where a plurality of digital video signals are processed in a production switcher or recorded in a video tape recorder, and, then, the processed or recorded digital video signal is restored into an analog video signal by a digital-to-analog (D/A) converter after blank insertion process. The net result is that these changes are apt to cause delicate changes in the brightness on a picture monitor since the pedestal level is the reference level (totally black picture) for the video signal. 
     SUMMARY OF THE INVENTION 
     Accordingly it is an object of the present invention to provide a clamping circuit which accurately sets the pedestal level in a digital video signal at a predetermined level and which compensates for A/D converter induced drifts and errors. 
     According to the present invention, there is provided a clamping circuit for a video signal, comprising: a differential amplifier for receiving an input video signal through one input terminal thereof; an A/D converter for converting the output of the differential amplifier into a digital video signal; means for separating a synchronizing signal from the input video signal and producing a position pulse at a phase corresponding to the back porch of the synchronizing signal; a comparator for generating a differential signal representative of the difference between a pedestal level extracted from the digital video signal in accordance with the position pulse and a predetermined reference level; and means for feeding back the differential signal of the comparator to the other input terminal of the differential amplifier; whereby the pedestal level of the digital video signal is accurately set to a predetermined reference level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art clamping circuit for a video signal. 
     FIG. 2 is a block diagram of an embodiment according to the present invention. 
    
    
     DESCRIPTION OF THE PRIOR ART 
     Referring to FIG. 1, the known clamping circuit, illustrated therein, includes an input analog video signal at an input terminal 1 supplied through a buffer amplifier 2 to a keyed clamping circuit 3 in which a pedestal level is set at a predetermined value in analog manner. The clamped video signal from the clamping circuit 3 is converted into a PCM video signal by an A/D converter 6 and, then, transmitted to an output terminal 7. In order to produce a clamp pulse, a synchronizing signal separator 4 separates a horizontal synchronizing signal and a clamp pulse generator 5 generates the clamp pulse at a phase corresponding to a back porch of the horizontal synchronizing signal. However, as described above, drifts and similar effects occur in the A/D converter 6, and, thus, it is impossible to exactly fix the pedestal value at a predetermined level in the output digital video signal. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2 which shows an embodiment of the present invention, an input analog signal delivered from an input terminal 1 is supplied through a buffer amplifier 2 to one input terminal of a differential amplifier 8. A video signal from the differential amplifier 8 is converted into a digital (PCM) video signal in an A/D converter 6 and is output of an output terminal 7. A synchronizing signal separator 4 and a clamp pulse generator 5&#39; cooperate to generate a pulse at the back porch of the horizontal synchronizing signal which serves as a sampling pulse. 
     In the embodiment, this sampling pulse is supplied to a data selector or switching circuit 9, a first sample/hold circuit 10, and an inverting circuit 11. In the inverting circuit 11, the sampling pulse is inverted and, then, supplied to a second sample/hold circuit 12. 
     The PCM video signal of the A/D converter 6 is supplied to the data switching circuit 9, which also receives a predetermined reference level signal RL. The reference level signal RL is a digital signal having a value equal to the standard pedestal level of an ideally quantized signal. The data switching circuit 9 passes the PCM video signal in response to the sample pulse, which occurs at an instant corresponding to the back porch, and, of the horizontal synchronization signal, passes the reference level signal RL of all other times. A D/A converter 13 converts the output data delivered from the switching circuit 9 into an analog signal. This converted analog signal is equal to the pedestal level of the PCM video signal at terminal 7 or the reference level signal RL, depending on the state of the output of clamp pulse generator 5&#39;. The digital signal is supplied to the first and second sample/hold circuits 10 and 12. 
     The first sample/hold circuit 10 samples and holds the level corresponding to the pedestal level in the PCM video signal while the second sample/hold circuit 12 retains the level corresponding to the reference level RL. A comparator 14 compares the level from the first sample/hold circuit 10 and the level from the second sample/hold circuit 12. The output of the comparator is the difference between the pedestal level of the PCM video signal and the reference level RL. This output is independent of errors associated with the D/A converter 13 because the error is contained in both levels held in the first and second sample/hold circuits 10 and 12, and is cancelled by the comparator 14. 
     The comparator output, i.e., the difference between the pedestal level in the PCM video signal and the reference level RL is fed back to the other input terminal of the differential amplifier 8 so as to minimize the comparison result. As a result, the pedestal level in the PCM video signal is substantially fixed at the predetermined reference level. 
     As described above, the clamping circuit according to the invention, which directly compares the pedestal level in the PCM video signal after the quantization with the predetermined reference level of the pedestal, can compensate for any drift error which may occur in the A/D converter. Consequently, the present invention is very effective in stabilizing the pedestal level in a digital video signal.