US 4,680,633 AGranted

Circuit and method for producing accurate dc restored video waveform, horizontal sync pulses, and vertical sync pulses

AssigneeThird Domain, Inc.

InventorsRichard C. Gerdes, Peter R. Vokac

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Abstract

A composite video signal is input to a DC restore circuit and a sync separator circuit, the output of which is connected to a sync decoder circuit. The DC restore circuit includes a negative sensing peak sample and hold circuit that generates a first reference voltage equal to the sync tip voltage. A first differential amplifier substracts the composite video signal from the reference voltage and produces an inverted, level shifted replica, the sync tip voltage of which is zero. A second differential amplifier having an output clamping circuit, in conjunction with a comparator, produces a composite sync voltage that is used to strobe a sample and hold circuit to produce a second reference voltage. The replica is inverted by a second differential amplifier that subtracts the replica from the second reference voltage to produce the DC restored video signal, a "back porch" portion of which is accurately referenced to zero volts.

Claims17

Dependent claims

A method of operating a video circuit comprising the steps of:

(a) applying a first signal, which is a composite video signal, to a negative input of a first differential amplifier and to an input of a negative sensing peak sense and hold circuit, to produce a first DC reference voltage that is equal to the negative sync tip voltage of the first signal;
(b) applying the first DC reference voltage to the positive input of the first differential amplifier to cause the first differential amplifier to produce a second signal that is an inverted replica of the first signal and has its level shifted so that the voltage of a sync tip portion of the second signal is equal to a ground reference voltage;
(c) applying the second signal to a negative input of a second differential amplifier;
(d) sensing the voltage of the second signal during a back porch portion thereof and producing a second DC reference voltage representative of the voltage of the back porch portion of the second signal;
(e) applying the second DC reference voltage to the positive input of the second differential amplifier and a fraction of the second DC voltage to the positive input of a comparator;
(f) coupling the output of the second differential amplifier through a diode to the negative input of the comparator to cause the comparator to produce a composite sync signal containing a series of narrow negative going pulses during a pre-equalization portion thereof followed by a plurality of narrow serration pulses during a vertical sync pulse interval portion thereof;
(g) applying the composite sync signal to a first monostable multivibrator and to an input of a first latch;
(h) causing an output of the first monostable multivibrator to produce pulses which are wider than the narrow negative-going pulses during the pre-equalization period of the composite sync signal but narrower than the interval between the serration pulses;
and
(i) clocking the first latch with a trailing edge of the output pulses produced by the first monostable multivibrator in order to produce a jitter-free vertical sync signal that undergoes a jitter-free transition in response to a transition between the pre-equalization portion and the vertical sync pulse interval portion of the composite sync signal.

7 more elements

A video circuit comprising:

(a) a first differential amplifier;
(b) a negative-sensing peak sense and hold circuit;
(c) means for applying a first signal, which is a composite video signal, to a negative input of the first differential amplifier and an input of the negative-sensing peak sense and hold circuit to cause the negative-sensing peak sense and hold circuit to produce a first DC reference voltage that is equal to the negative sync tip voltage of the first signal;
(d) means for applying the first DC reference voltage to the positive input of the first differential amplifier to cause the first differential amplifier to produce a second signal that is an inverted replica of the first signal and has its level shifted so that the voltage of a sync tip portion of the second signal is equal to a ground reference voltage;
(e) a second differential amplifier;
(f) means for applying the second signal to a negative input of a second differential amplifier;
(g) means for sensing the voltage of the second signal during a back porch portion thereof and producing a second DC reference voltage representative of the voltage of the back porch portion of the second signal;
(h) a comparator;
(i) means for applying the second DC reference voltage to the positive input of the second differential amplifier and a fraction of the second DC voltage to the positive input of a comparator;
(j) means for coupling the output of the second differential amplifier through a diode to the negative input of the comparator to cause the comparator to produce a composite sync signal containing a series of narrow negative going pulses during a pre-equalization portion thereof followed by a plurality of narrow serration pulses during a vertical sync pulse interval portion thereof;
(k) a first monostable multivibrator and a first latch;
(l) means for applying the composite sync signal to a first monostable multivibrator and causing an output of the first monostable multivibrator to produce pulses which are wider than the narrow negative-going pulses during the pre-equalization period of the composite sync signal but narrower than the interval between the serration pulses;
and
(m) means for clocking the first latch with a trailing edge of the output pulses produced by the first monostable multivibrator in order to produce a jitter-free vertical sync signal that undergoes a jitter-free transition in response to a transition between the pre-equalization portion and the vertical sync pulse interval portion of the composite sync signal.

11 more elements

Dependencies are inferred from the claim text.