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| Automatic guide-channel mapping method of a VCR |
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| Signal recording system using memory for audio signal |
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| Technique for automatically centering a lightguide fiber in a coating |
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| Still-image video signal processing circuit |
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| Memory address and display control apparatus for high definition television |
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| Run/standby control with switched mode power supply |
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Contrast control wherein reference pulse detection occurs every other line period and wherein clamping occurs in remaining line periods
| Details |
Inventors: Umemura, Shunji; Nakazono, Hiroyuki;
Assignee: Sony Corporation (Tokyo, JP)
Primary Examiner: Groody; James J.
Assistant Examiner: Miller; John W.
Attorney, Agent or Firm: Toto; Peter C., Miller; Jerry A.
The present invention provides a contrast control circuit capable of controlling the contrast even if the back porch is comparatively short. The contrast control circuit comprises monostable multivibrators (24, 26) which provides a background pulse and a reference pulse alternately, respectively, a switch (SW1) which clamps a video signal while the background pulse is HIGH so that the potential of the pedestal level is zero on an (N+1)th horizontal scanning line, and a switch (SW2) samples the leading edge of a reference pulse inserted in the video signal while the reference pulse is HIGH on an nth horizontal scanning line. |
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DETAILED DESCRIPTION Accordingly, it is an object of the present invention to provide a contrast control circuit capable of controlling contrast even if the duration of the back porch is less than 1.
6 .
mu.
sec.
A contrast control circuit in a first aspect of the present invention comprises a voltage detecting means for detecting the reference voltage of a reference pulse inserted in a horizontal back porch once every N (N is an integer not smaller than two.
) horizontal scanning cycles, and a control means for generating a contrast control signal on the basis of the reference voltage detected by the voltage detecting means and a specified voltage.
The reference voltage detecting means comprises, for example, a monostable multivibrator (26), a D flip-flop (28) and a switch (SW2) as shown in FIG.
1.
The control means comprises, for example, an operational amplifier OP1 shown in FIG.
1.
A contrast control circuit in a second aspect of the present invention comprises a clamping means for clamping a video signal so that the potential of the pedestal level is held zero once every N (N is an integer not smaller than two.
) horizontal scanning cycles, and a voltage detecting means for detecting the reference voltage inserted in a horizontal back porch once every N horizontal scanning cycles in a horizontal scanning cycle other than a horizontal scanning cycle in which the clamping means clamps a video signal so that the potential of the pedestal level is held zero, and a control means for generating a contrast control signal on the basis of the reference voltage detected by the voltage detecting means and a specified voltage.
The clamping means comprises, for example, a monostable multivibrator (24), a D flip-flop (28) and a switch (SW1) as shown in FIG.
1.
The voltage detecting means comprises, for example, a monostable multivibrator (26), a D flip-flop (28) and a switch (SW2) as shown in FIG.
1.
The control means comprises, for example, an operational amplifier (OP1) as shown in FIG.
1.
Desirably, N=2
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