Television – Camera – system and detail – Combined image signal generator and general image signal...
Reexamination Certificate
1997-12-11
2001-02-20
Garber, Wendy (Department: 2612)
Television
Camera, system and detail
Combined image signal generator and general image signal...
C348S243000, C348S312000
Reexamination Certificate
active
06191816
ABSTRACT:
The invention relates to an interface circuit having an input intended to receive a pseudo-periodical input signal, each pseudo-period of which comprises at least a first and a second consecutive time interval, said input signal having a reference level during the first time interval and a video level during the second time interval, and an output intended to supply a signal having a level which is representative of the difference between the reference level and the video level assumed by the input signal in the course of one and the same pseudo-period, said interface circuit comprising:
a first and a second sampling branch each constituted by series-arranged trackand-hold circuits, each having an input and an output, each branch having an input and an output constituted by the input of its first tack-and-hold circuit and the output of its last track-and-hold circuit, respectively, the first branch comprising one track-and-hold circuit more than the second, the input of each branch being intended to receive the input signal from the interface circuit,
a subtracter having a first input connected to the output of the first branch, a second input connected to the output of the second branch and an output constituting the output of the interface circuit,
a DC level regulation loop comprising a first capacitance arranged between the input of one of the branches and the input of the interface circuit, and a first comparator having a first input connected to the output of one of the sampling branches, a second input intended to receive a regulation signal having a predetermined value and an output intended to control the conduction of a current through the first capacitance.
The interface circuits which are currently used for realizing the function described in the opening paragraph comprise two control loops, the first regulating the DC component of the input signal during the first time interval, i.e. when the input signal has the reference level, the second regulating the DC component of the input signal during the second time interval, i.e. when the input signal has the video level. The signal at the output of the first branch, whose value is representative of the reference level, is compared with a first predetermined value and adjusts, during the first time interval, the voltage at the terminals of a capacitance arranged between the input of the first branch and the input of the interface circuit. The signal at the output of the second branch, whose value is representative of the video level, is compared with a second predetermined value and adjusts, during the second time interval, the voltage at the terminals of a capacitance arranged between the input of the second branch and the input of the interface circuit. These two regulation loops necessarily have similar feedback times because both of them must act in the course of each pseudo-period. The correction realized by the first control loop may thus modify the value of the voltage at the terminals of the capacitance of the second control loop, and conversely. In high-frequency applications, in which the pseudo-period of the input signal of the interface circuit is very short, the feedback time of each of the two loops must be very small. The interaction described above may then cause oscillations of the DC components of the signals at the inputs of the sampling branches, which oscillations are detrimental to the operation of the interface circuit.
It is an object of the invention to remedy this drawback by providing an interface circuit in which the inputs of the two sampling branches receive one and the same signal whose DC component is controlled by means of a single regulation loop.
To this end, an interface circuit according to the invention is characterized in that the inputs of the first and second branches are interconnected, forming a junction point connected via the first capacitance to the input of the interface circuit, and in that the interface circuit also comprises control means arranged between the sampling branches and the subtracter, which means are activated when the levels of the signals at the outputs of the first and second branches correspond to the reference levels, said control means allowing adjustment of the values of the signals at the inputs of the subtracter in order that said values are equal.
In such an interface circuit, the control loop and the control means do not act directly on the same signals, which enables to limit the risks for oscillations to occur. Moreover, if the feedback time of the control loop is necessarily short, because a regulation of the DC component of the input signal of the sampling branches must be carried out in each pseudo-period of the input signal of the interface circuit, the feedback time of the control means may be longer because these means only act in the case where the video level of the input signal of the interface corresponds to the reference level. The difference between the feedback times of the two loops all the more limits the risk of interaction between these loops.
An embodiment of the invention provides an interface circuit as described above, which is characterized in that the control means comprise:
a second comparator having a first and a second input connected to the outputs of the first and second sampling branches, respectively, and an output connected via a second capacitance to a fixed voltage terminal, and
an offset module having a signal input connected to the output of one of the sampling branches, a signal output connected to one of the inputs of the subtracter, the other input of which is connected to the output of the other sampling branch, the offset module also having a control input connected to the output of the second comparator and being intended to generate, between its signal input and its signal output, a potential difference whose value depends on the value of the signal received at the control input.
In such an interface circuit, a difference between the values of the DC components of the output signals of the sampling branches is detected by means of the second comparator and immediately corrected thanks to the offset module which may be realized in various forms.
In an embodiment of the invention, which is particularly advantageous because of its simplicity, an interface circuit as described above is characterized in that the offset module comprises:
a resistor arranged between the signal input and the signal output,
a third comparator having an output, a first input intended to receive a control signal having a predetermined value and a second input constituting the control input of the offset module, and
a variable current source having an output connected to one of the terminals of the resistor and being intended to supply a control current whose direction and value are controlled by the state of the output of the third comparator.
The correction of the difference between the values of the DC components is thus easily realized by generating a voltage drop between the output of one of the sampling branches and the corresponding input of the subtracter.
An interface circuit as described above may be used for receiving and reshaping all types of pseudo-periodical signals having a reference level and a video level in each pseudo-period and from which useful information, consisting of the difference between these levels, must be obtained. The invention thus also relates to a video camera comprising:
a device for detecting and converting light into analog electronic signals of a pseudo-periodical kind,
an input stage having an input intended to receive said analog electronic signals, and an output intended to supply an analog video signal,
an amplifier device having an output and an input intended to receive the video signal, and
a processing unit intended to process the signal at the output of the amplifier device,
characterized in that the input stage comprises an interface circuit as described above.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments describ
Darthenay Frederic
Morisson Richard
Garber Wendy
Piotrowski Tony E.
U.S. Philips Corporation
White Mitchell
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