DC Restoration Circuit for Television Receiver

The present invention relates to DC restoration circuits used in television receivers and, more particularly, to a DC restoration circuit employing current-mode feedback techniques to restore the DC level of a received television signal.

The video component of a transmitted television signal is characterized by a DC level establishing or defining the average background brightness of a scanned scene. In order for the reproduced picture at the receiver to have the proper background brightness, i.e. the proper level with respect to black, the transmitted DC component must be retained or otherwise restored in the receiver. In television receivers employing direct coupling techniques throughout the entire video section, the DC component of the composite video signal is inherently retained in the receiver and no special circuits are necessary to insure the reproduction of a picture having a proper average background brightness level. However, for purposes of simplicity and economy, as well as to prevent the accumulation of system errors, most current television receivers have video sections incorporating RC coupling circuits. As a result of the DC blocking action of the coupling capacitors in these circuits, the background brightness determining component of the video signal undergoes rather radical level shifts and must therefore be suitably restored in the receiver to enable reproduction of an acceptable picture.

Typically, the DC component of the video signal in an AC coupled video section is restored to establish a suitable average background brightness level in response to a manually operable brightness control which is used in association with a clamping capacitor to set the picture tube grid bias voltage. More specifically, the brightness control is used to establish an adjustable reference potential, a predetermined portion of the horizontal blanking interval of the video signal, e.g. the back porch interval, being clamped to the reference potential by the clamp capacitor to establish a desired black level. Thus, in response to adjusting the brightness control, the viewer effectively controls the black level, and thereby the average background brightness, of the reproduced picture by restoring the DC component of the video signal to a proper level. In this regard, while the back porch, as well as the front porch, of the horizontal blanking interval does not precisely correspond to black level as defined by FCC standards, the difference therebetween is considered to have only a negligible effect on DC restoration circuits of the foregoing type.

An embodiment of a DC restoration circuit employing the above principles is illustrated in an article entitled "A Versatile Low-Level Luminance IC for TV" appearing on pages 169-175 of the August, 1978 issue of IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3. In the illustrated circuit, the unblanked luminance component of the video signal is coupled to one input of a comparator while the second input to the comparator consists of a DC reference voltage derived from a manually adjustable brightness control. A back porch gate pulse enables the comparator for directing current flow into or out of a clamp capacitor which sets the DC voltage levels in a feedback loop for adjusting the black level of the luminance signal until a condition of equality between the DC reference voltage and the level of the back porch is established. While this circuit is adequate in certain applications, its operation is characterized by a rather limited dynamic range rendering the circuit unsuitable for use with different types of television receivers. The limited dynamic range characterizing the circuit is primarily attributable to the fact that the feedback loop is used to couple voltage signals for controlling system operation.

It is therefore a primary object of the present invention to provide a new and improved DC restoration circuit capable of widespread application and exhibiting a rather large dynamic range. As explained in detail hereinafter, these objects are achieved through the provision of a novel bi-directional gated back porch clamp circuit utilizing current-mode feedback techniques for controlling the black level of the luminance component of a composite video signal.

The sole FIGURE accompanying the specification illustrates, in block diagram form, a preferred embodiment of the DC restoration circuit of the present invention.

Referring to the FIGURE, an input video voltage signal, such as would be developed at the output of the video detector of a conventional monochrome or color television receiver, is developed at the input terminal 8 of a DC restoration circuit indicated generally by reference numeral 10. As discussed previously, in an AC coupled television receiver, the DC level of the video voltage signal, which determines the average background brightness of the televised scene, undergoes some rather radical level shifts as the video signal is propogated through the signal processing circuits of the receiver. If steps are not taken to somehow compensate for these level shifts, the reproduced scene will have an average background brightness which, not only does not correspond to the televised scene, but also results in a picture which is most displeasing to the viewer. The DC restoration circuit 10 is designed for re-establishing a suitable DC level in the processed video voltage signal so as to maintain a desired average background brightness in the reproduced scene.

Returning to the FIGURE, the input video voltage signal developed at the input terminal 8, whose DC level may be either too high or too low as a result of the effect of previous AC coupled stages in the receiver, is coupled through a resistor 12 which converts the input video voltage to a corresponding current I.sub.0 having a DC current component. The current I.sub.0 is divided at a node 14 into a signal current I.sub.1 flowing in a conductor 16 and a DC current feedback signal I.sub.2 flowing in a conductor 18. It will thus be recognized that the DC component of the signal current I.sub.1 flowing in conductor 16 corresponds to the difference between the DC component of the video current signal I.sub.0 and the DC current feedback signal.

The conductor 16 carrying the signal current I.sub.1 forms an input to a current mirror 20 which develops a corresponding current I.sub.1 on a conductor 22 coupled through a resistor 24 to a source of positive potential +V. As a result, the node 26 formed between the resistor 24 and the conductor 22 is characterized by a voltage V.sub.1 which is given by the expression V.sub.1 =V-I.sub.1 R, where R represents the resistance of resistor 24. The voltage V.sub.1 developed at the node 26 constitutes the output video voltage of the DC restoration circuit 10 and is coupled to the following circuitry of the television receiver by an output conductor 28. As will be explained in further detail hereinafter, the DC level of the output video voltage V.sub.1 will be suitably adjusted relative to the DC level of the input video voltage developed at the input terminal through the operation of the DC restoration circuit 10.

The output video voltage V.sub.1 developed at the node 26 is also coupled to one input of a charge pump comparator circuit 30 connected in a feedback loop between the nodes 14 and 26. A second input to the charge pump comparator circuit 30 is a black level reference voltage derived from the slider 32 of a manually operable brightness control potentiometer 34 connected between a source of positive potential +V and ground potential. The charge pump comparator circuit 30, which is enabled for operation by a gate pulse coinciding with the back porch of the horizontal blanking interval, includes a bi-directional output conductor 36 connected to one plate of a clamping capacitor 38, the other plate of the capacitor 38 being connected to a source of ground potential. When enabled by a back porch gating pulse, the charge pump comparator circuit 30 is operable for charging the clamp capacitor 38 by establishing a flow of current in the output conductor 36 in the direction of arrow 40 in response to the black level reference voltage exceeding the DC component of the output video voltage V.sub.1. On the other hand, the enabled charge pump comparator 30 will establish a flow of current in the output conductor 36 in the direction of arrow 42 for discharging the clamp capacitor 38 in response to the DC component of the output video voltage V.sub.1 exceeding the black level reference voltage. In this regard, it will be appreciated that since the charge pump comparator 30 is enabled for operation only during the back porch of the horizontal blanking interval, the comparisons effected thereby will represent the relative values of the black level reference voltage and the back porch of the horizontal blanking interval of the output video voltage V.sub.1.

The ungrounded plate of the clamp capacitor 38 is connected to the base terminal of an npn transistor 44 which acts as a buffer to prevent rapid discharge of the clamp capacitor. The collector terminal of the transistor 44 is connected to a source of positive potential +V while the transistor's emitter terminal is connected to a resistor 46 developing an adjustment current signal directly related to the voltage developed across the clamp capacitor 38. The adjustment current signal developed by the resistor 46 is supplied to one terminal of a second current mirror 48, the current mirror 48 causing the DC current feedback signal flowing in conductor 18 to assume a corresponding or proportional value.

In general terms, the DC restoration circuit 10 is operable for adjusting the output video voltage V.sub.1 through corresponding adjustments in the DC current feedback signal. That is, the value of the DC current feedback signal, which is determined by the adjustment signal current which, in turn, is established by the relative values of the black level reference voltage and the DC component of the output video voltage during the back porch of the horizontal blanking interval, determines the magnitude of the signal current I.sub.1 flowing in the conductor 16. The signal current I.sub.1, in turn, determines the magnitude of the DC component of the output video voltage V.sub.1. A desired DC level for the output video voltage is established by controlling the adjustment signal current in response to a selected black level reference voltage.

More particularly, assume that the brightness control potentiometer 34 has been set by the viewer for providing a selected black level reference voltage. As will be seen hereinafter, the effect of the DC restoration circuit 10 is to equalize the values of the selected black level reference voltage and the back porch of the horizontal blanking interval of the output video voltage V.sub.1. Now, assume that the DC level of the input video voltage developed at terminal 8 decreases to a value below that at which the circuit 10 was previously equalized. As a result of this decreased DC level, the current I.sub.0 flowing through resistor 12 correspondingly decreases as does the signal current I.sub.1 flowing in conductor 16. The mirrored current I.sub.1 flowing in conductor 22 consequently also decreases in value causing the output video voltage V.sub.1 to correspondingly increase in value. Since the level of the back porch of the horizontal blanking interval of the output video voltage V.sub.1 now exceeds the black level reference voltage, the charge pump comparator circuit 30 establishes an output current in the direction of arrow 42 for discharging the clamp capacitor during the back porch interval. The decreased voltage across the capacitor 38 causes the transistor 44 to conduct less heavily reducing the adjustment signal current flowing through the resistor 46. The mirrored DC current feedback signal flowing in conductor 18 is therefore also reduced causing the DC component of the signal current I.sub.1 to proportionately increase. The mirrored current I.sub.1 consequently also increases reducing the output video voltage V.sub.1 to a point where equalization with the black level reference voltage is again achieved.

Next, assume that the DC level of the input video voltage increases to a value above that at which the circuit was equalized. In this case, the current I.sub.0 flowing through resistor 12 correspondingly increases causing the signal current I.sub.1 to likewise increase. The increased mirrored current I.sub.1 reduces the output video voltage V.sub.1 causing the charge pump comparator circuit 30 to establish an output current flow in the direction of arrow 40 for charging the clamp capacitor 38. The increased voltage across the capacitor 38 causes the transistor 44 to conduct more heavily increasing the adjustment signal current flowing through the resistor 46. The increased adjustment signal current flowing in conductor 18 reduces the DC component of the signal current I.sub.1 and thereby increases the DC component of the output video voltage V.sub.1 to a point where equalization with the black level reference voltage is again achieved.

It will thus be seen that the DC restoration circuit 10 advantageously utilizes current-mode feedback techniques for equalizing the values of the back porch of the horizontal blanking interval of an input video signal and an adjustable black level reference signal. Adjustments in the black level reference signal control the average background brightness of the reproduced scene which may be set in accordance with the preferences of the viewer. Also, quite advantageously, a relatively wide dynamic range is achieved since the input current flowing through resistor 12 may vary considerably.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.