Video contrast enhancement device

Abstract: The video contour correction system for a video display unit consists of an auxiliary deflection coil having coil axis parallel with the axis of the horizontal deflection coil of the cathode-ray tube, a differentiating circuit for detecting the primary differential waveform of a video signal and an am­ plification circuit for amplifying the output of the differentiat­ ing circuit. The output current of the amplification circuit is supplied to the auxiliary deflection coil so that the magnetic field produced by said auxiliary deflection coil is super­ posed on the horizontal deflection magnetic field produced by said horizontal deflection coil, thereby improving the bor­ der contrast of the picture.

Description: The present invention relates to a video contrast enhancement (also referred to as contour correction) device for a video display unit such as television. When, in a system which presents a picture on a cathode-ray tube, the acute rising and falling edges of a video signal are rounded as shown in Fig. 1(A), the border contrast of the picture produced by the video signal is dimmed, resulting in a fuzzy picture. Conventionally, to overcome this problem, the frequency characteristics of the video circuit were changed so that the rising and falling edges of the video signal have preshoots and overshoots, as shown in Fig. 1(B). Thus, correction was applied to the video signal waveform itself so as to improve the apparent contrast at the picture border. This method had a disadvantage that it broadened the picture width, or that when the electron beams were intensified at the preshoots and overshoots, the electron beam focus was dimmed. US-A-4185301 and GB-A-2064911 both disclose an improved technique of contrast enhancement, or contour correction, in which the horizontal scan speed on a video screen is modulated in accordance with a video signal, and particularly in acordance with the differential of the video signal. However, in both systems the technique is applied continuously and this can produce distorted image display, for example where an auxiliary video signal is superimposed on the incoming video signal for superimposition of image display portions. such as characters or symbols, on the displayed image. This problem is solved by the arrangement disclosed in Valvo Technische Information 820111 entitled "Improvement of picture sharpness in television receivers". This document discloses another system for video contrast enhancement of TV pictures, and mentions the need to switch off the scan speed modulation during a screen fade-in, during a video text mode, and during mixed operation. In the latter, mixed mode, the modulation circuit is said to be blanked by the video text blanking pulse. In all of the disclosed circuits, modulation for the TV picture is performed using the picture luminance (Y) signal. According to this prior art, therefore, there is known a video display unit including a video contrast enhancement device, in which video display unit the scanning operation in a display tube is controlled using a colour video signal to display a colour picture on the tube screen, and means is provided for varying the speed of scanning in accordance with the video signal by supplying to an auxiliary horizontal beam deflection means of the tube an auxiliary drive signal derived by differentiating in a differentiating circuit a signal derived from said video, the unit including: means for superimposing upon said video signal a further signal representing a display image portion, such as characters or symbols, to be superimposed on said picture; and contrast enhancement inhibiting means responsive to an inhibiting signal synchronised with said further signal to selectively inhibit the varying of said scanning speed by said scanning speed varying means in accordance with the video signal for said display image portion. The aim of the present invention is to provide a video contrast enhancement device in such a display unit using, instead of the luminance (Y) signal, the separate primary colour signals. Although the afore-mentioned GB-A-2064911 discloses a circuit in which a switch is provided for selecting between modes employing the luminance signal, and the primary colour signals, respectively, there is no inhibition of contour correction for a superimposed display image portion, and the primary colour signals, with superimposed primary colour text signals, are mixed using a summing network of resistors. The present invention uses the primary colour signals in a different way, and is characterised in that there is provided an OR circuit coupled to receive at three separate inputs thereof three respective primary colour signals of the colour video signal representing the picture and to deliver to said differentiating circuit an output signal which is kept at high level while at least one of said primary colour signals exceeds a certain level. A preferred embodiment of the invention will hereinafter be described by way of example with reference to Figures 10 and 11 of the accompanying drawings. In the drawings : Fig. 1 is a view illustrating a video signal correction method employed in the conventional art. Fig. 2 is a block construction diagram in which a video contour correction system is applied to the video display circuit. Fig. 3 is a view explaining the operation of the circuit in Fig. 2. Fig. 4 is a construction diagram of the differentiating circuit used in the circuit in Fig. 2. Fig. 5 is a characteristic view of the retardation element used in said differentiating circuit. Fig. 6 is an equivalent circuit of the differentiating circuit in Fig. 4. Fig. 7 shows a current amplification circuit in the final stage of the differentiated signal amplification circuit having a signal clamping circuit. Fig. 8 is a diagram of the video correction terminating circuit provided in the video contour correction system of Fig 2. Fig. 9 is an operation explanatory view for the video correction terminating circuit of Fig 8. Fig. 10 is a block diagram in which in accordance with the invention the video contour correction system is applied to the video display circuit in a color video signal receiver. Fig. 11 is an operation explanatory view for the example in Fig. 10. In the system of Fig 2, the detection circuit 1 separates the input carrier wave into a video signal and horizontal and vertical synchronizing signals and outputs them. The video signal is amplified in the video signal amplification circuit 2 and input into the cathode-ray tube 9 for controlling the intensity of electron beams. Meanwhile. the horizontal and vertical synchronizing signals are fed into the synchronous deflection circuit 3 which outputs horizontal and vertical deflecting currents according to said synchronizing signals. The horizontal and vertical deflecting currents are supplied to the deflection coil unit 4 for producing horizontal and vertical deflection magnetic fields which sweep the electron beams in the cathode-ray tube 9. To this ordinary video display circuitry without video contour correction means is added the video contour correction system 8, which is composed of a differentiating circuit 6, an amplification circuit 7, an auxiliary horizontal deflection coil 5 and a video correction terminating circuit 10, as shown in Fig. 2. The differentiating circuit 6 differentiates a video signal output from the detection circuit 1. The differentiated signal output from the differentiating circuit 6 is then fed to the amplification circuit for amplification, and supplied to the auxilliary deflection coil 5 which is arranged coaxially (in parallel) with the horizontal deflection coil contained in the deflection coil unit 4. In the construction described above, when the detection circuit 1 outputs the video signal shown in Fig. 3(A), the differentiating circuit 6 differentiates the waveform of the signal to produce a differentiated signal shown in Fig. 3(B). The differentiated signal is amplified in the amplification circuit 7 and suppiied to the auxilliary horizontal deflection coil 5. Meanwhile, the synchronous deflection circuit 3 is triggered by a horizontal synchronizing signal (not shown) added immediately before the video signal so that it supplies the horizontal deflection coil in the deflection coil unit 4 with a horizontal deflecting current which varies linearly with respect to time. Fig. 3(C) shows the variation with time of the horizontal deflection magnetic field by the horizontal deflecting current or the horizontal deflection coil. Accordingly, the total horizontal deflection magnetic field applied to the cathode-ray tube 9 is the sum of the magnetic field produced by the horizontal deflection coil in the deflection coil unit 4 and the magnetic field produced by the auxilliary deflection coil 5. The time course change of the total magnetic field is shown in Fig. 3(D), from which it is obvious that the horizontal deflecting speed of the video signal in the rising portion is relatively fast at the first half and relatively slow at the latter half. Correspondingly, as obvious from the variations with time of the electron beam intensity shown in Fig. 3(E), the rate of change of the intensity is slow at the first half but very fast at the latter half of the rising portion of the video signal. In this way, the edge portion of the picture corresponding to the rising portion of the video signal has a strong contrast. The falling portion of the video signal has a similar characteristic, except that the attenuation speed of the electron beam intensity is fast at the first half and slow at the latter half of the falling portion of the video signal, thereby intensifying the border contrast of the picture. The differentiating circuit 6 used in the above example will be explained in the following. As shown in Fig. 4, the differentiating circuit 6 consists of a signal retardation element 62 comprising a delay line, and a resistor 61. An end of the retardation element 62 is grounded. A signal to be differentiated is inputted through the resistor 61 to the other end of the element 62. Output is taken out from the contact between the resistor 61 and the retardation element 62. With its end grounded, the retardation element forms a resonate circuit having multiple resonance points at the frequency (2n+1)f₀ with λ₀/4 frequency peculiar to the element as f₀, as shown in Fig. 5. In Fig. 5, the horizontal axis indicates the relative frequency to f₀ or f/f₀, and the vertical axis indicates the complex impedance. Within the range 0<f/ f₀<1, for example, the complex impedance is reactive. Under such a condition, the circuit shown in Fig. 4 is replaced by an equivalent circuit shown in Fig. 6 which is apparently a differentiating circuit. Therefore, the circuit in Fig. 4functions as a differentiating circuit at all times, if adequate value is selected for f0 of the retardation element according to the repetition cycle of the video signal. It is possible, of course, to use an ordinary concentrated constant circuit element instead of the distributed constant circuit element such as the retardation element for constructing the differentiating circuit 6 in Fig. 2. The circuit composed of the ordinary concentrated constant circuit element involves increased number of circuit elements and requires complicated design consideration in determining the resonance frequency and Q value. According to the above video picture contour correction method, when the rounding of the video signal is not so severe, and therefore when the differentiated signal in Fig. 3(B) becomes excessively large, the picture can have excessively intensified luminance at the border, deteriorating the picture quality. Moreover, large current may be generated in the amplification circuit 7, giving damage to the output transistor. To prevent such an excess picture correction, a clamping circuit consisting of resistors and diodes may be added to the circuit for amplifying differentiated signals. The clamping circuit will be explained below. The final stage current amplification circuit of the amplifier 7 has such a construction as shown in Fig. 7 so that positive and negative differentiated signals in excess of a certain level are clamped. Referring to Fig. 7, C1 and C2 are coupling capacitors R1, R2, R3, R4 and R5 are bias resistors, Di1, and Di2 are diodes, Ti and T2 are transistors, R6 and R7 are d.c. biasing feedback resistors for transistors, R8 and R9 are a.c. biasing feedback resistors for transistors, C3 and C4 are emitter bias capacitors for transistors, R10 is a damper of the auxilliary deflection coil 5, and C5 is a direct current blocking capacitor. Accordingly, differentiated signal output from the preamplifier, if positive, is inputted through the coupling capacitor C2 into the base of the transistor Tr2. The clamping circuit composed of the resistance of resistor R2 and the diode Di2 arranged in series is connected between the base and the ground. Therefore, if the resistance of resistor R2 is smaller than the output impedance of the preamplifier, the portion of the differentiated signal exceeding a certain level is clamped. During this period, the output of the transistor Tr2 is maintained constant. Thus, the excessive current flow to the auxilliary deflection coil 5 is blocked. The negative portion of the differentiated signal controls the transistor Trl via the coupling capacitor C1. In this case as well, the clamping circuit composed of a resistance R1 and a diode Di1 clamps the portion of the negative differentiated signal smaller than a certain level, blocking the excessive current flow to the auxilliary deflection coil 5. As mentioned above, if a clamping circuit is incorporated in the amplification circuit 7, it clamps the portion of the differentiated signal in excess of a certain level when excessively large differentiated signal is transmitted from the differentiating circuit 6 due to the severe rounding in the video signal. This prevents excessive current flow to the auxilliary deflection coil 5 so that overload operation of the differentiated signal amplifier 7 as well as excessive correction of the picture are prevented. If characters or symbols prepared on the receiver side are to be displayed overlapping the video picture contour, it is necessary to input signals for these characters or symbols into the luminance signal input terminal of the cathode-ray tube so that the signals are superposed on the video signal. The characters or symbols thus displayed at the picture contour will tend to be distorted in that they may have abnormal luminance and therefore be difficult to distinguish if the luminance correction is applied over the entire picture contour according to the video contour correction method described above. This disadvantage is eliminated by providing a video correction terminating circuit to terminate the correction operation for border contrast of the picture for the entire or any optional portion of the video contour. The video correction terminating circuit will be explained in the following. The output of the video correction terminating circuit 10 shown in Fig. 8 is connected between the preamplifier 7a and the final stage current amplification circuit 7b of the amplifier 7 so that luminance correction of the video contour can be interrupted at least partially. The terminal 11 in Fig. 2 is the additional video signal input terminal to which signals for characters or symbols are inputted. Referring to Fig. 8, R10. R30 and R60 are resistors for controlling base current, C10 and C20 are speed-up capacitors, R20, R40 and R50 are waveform shaping resistors, R70 is a resistor to cause potential difference between the drain and the source of the field-effect transistor T30, T10 and T20 are transistors, and T30 is a field-effect transistor. If it is necessary to partially interrupt the luminance correction operation, for example, in the portion of the picture contour where the signals for characters or symbols inputted through the additional video signal input terminal 11 are superimposed on the video signal, a square pulse video correction terminating signal shown in Fig. 9(B) must be inputted to the input terminal A of the video correction terminating circuit shown in Fig. 8 according to a correspondingly constructed program loaded in a microcomputer. The reference numerals 7a and 7b in Fig. 8 denote the preamplifier and the final stage current amplifier respectively of the amplifier 7 for differentiated signals shown in Fig. 2. The output terminal B of the preamplifier 7a outputs a signal as shown in Fig. 9(A), which is proportional to the differential waveform of the video signal shown in Fig. 3(B). The video correction terminating signal inputted from the terminal A is reversed by the input step transistor Tr10 and outputed from the output terminal a as such a signal as shown in Fig. 9(C). Then, the signal is amplified and reversed in the amplifying transistor Tr20 and taken out at the output terminal b as a gate signal proportional to the input signal, as shown in Fig. 9(D). The gate signal is inputted into the gate terminal of the field-effect transistor Tr30 which is a switching element. Owing to this gate signal, the impedance between the drain and the source of the field-effect transistor Tr30 is maintained virtually zero only while the video correction termination signal is at high level, as shown in Fig. 9(E). The video correcting differentiated signal outputted, during this period, from the preamplifier 7a of the differentiated signal amplifier 7 is shortcircuited so that the luminance correction operation for the picture contour is partially interrupted. Thus, correction operation for luminance of the entire or partial video contour can be terminated as desired. When characters or symbols are to be displayed, superimposed on the border of the picture, the luminance correction is interrupted only at the superposing section so that the characters or symbols are easily recognized on the screen. An example of the present invention will be described in reference to Fig. 10 and Fig. 11(A), (B) and (C). According to this example, the video contour correction system is incorporated in the video display circuit of a multi-color video signal receiver. Referring to Fig. 10, the color signal output circuits 100, 100a and 100b divide a colour video signal containing a synchronizing signal into three primary color (red, green and blue) signals or generate the three primary color signals. The three primary color signals thus divided are amplified in the respective amplification circuits 200, 200a and 200b and inputted into the cathode-ray tube 9 to control the intensity of the electron beam corresponding to each of the three primary colors. Meanwhile, the synchronous deflection circuit 3 generates horizontal or vertical deflection current according to the synchronizing signal and supplies he current to the deflection coil unit 4 consisting of horizontal and vertical deflection coils. To this ordinary color video display circuit is added a color video contour correction system 8 of this invention, which consists of an auxilliary horizontal deflection coil 5, an OR circuit 60, a differentiating circuit 6 and an amplification circuit 7, as shown in Fig. 10. Three primary color signals are inputted from the color signal output circuits 100, 100a and 100b into the OR circuit 60. The differentiating circuit 6 differentiates the output signal from the OR circuit 60. The signal thus differentiated is current-amplified in the amplification circuit 7 and then supplied to the auxilliary horizontal deflection coil 5 which is arranged coaxially with the horizontal deflection coil in the parallel coil unit 4. In this construction, if at least one of the color signal output circuits 100, 100a and 100b outputs a color signal including rounded rising edges and falling edges as shown in Fig. 11(A), the output from the OR circuit 60 is kept at a high level while the color signal exceeds a certain level, and the OR circuit 60 transmits a square wave signal shown in Fig. 11(B). This signal is differentiated by the differentiating circuit 6. If the circuit constant has been appropriately selected for differentiation, the differential waveform with slightly rounded rising and falling edges as shown in Fig. 11(C) is produced as a differential output. Explanation for the subsequent operations which are identical with those in the example shown in Fig. 2 is omitted. In Fig. 10, components identical with those in Fig. 2 are allotted with the same numerals as in Fig. 2, their explanations being omitted. The video correction terminating circuit, and optionally also the signal clamping circuit described above with reference to Figs 7 and 8 are incorporated in the multi-color video contour correction system shown in Fig. 10. As obvious from the foregoing explanation, according to the present invention, horizontal sweep on the cathode-ray tube is modified by adding preshoots and overshoots so that the border contrast of the picture is improved without modification of the video signal itself. This can eliminate such a complicated process as adjustment of the video signal circuit as well as a broadened picture, dimmed electron beam focus or other defects accompanied with the conventional art. Moreover, the differentiating circuit involved in the present invention can be composed only of the delay line and resistors, making the entire circuit construction of the system very simple.

Claims: A video display unit including a video contrast enhancement device, in which video display unit the scanning operation in a display tube (9) is controlled using a colour video signal (Fig. 3(A)) to display a colour picture on the tube screen, and means (5, 6, 7) is provided for varying the speed of scanning in accordance with the video signal by supplying to an auxiliary horizontal beam deflection means (5) of the tube an auxiliary drive signal (Fig. 3(B)) derived by differentiating in a differentiating circuit (6) a signal derived from said video signal, the unit including: means (11) for superimposing upon said video signal a further signal representing a display image portion, such as characters or symbols, to be superimposed on said picture; and contrast enhancement inhibiting means (10) responsive to an inhibiting signal (Fig. 9(B)) synchronised with said further signal to selectively inhibit the varying of said scanning speed by said scanning speed varying means (5) in accordance with the video signal for said display image portion, characterised in that there is provided an OR circuit (60) coupled to receive at three separate inputs thereof three respective primary colour (RGB) (Fig. 11(A)) of said colour video signal representing said picture and to deliver to said differentiating circuit an output signal (Fig 11B) which is kept at high level while at least one of said primary colour signals exceed a certain level. A video contrast enhancement device according to claim 1 characterised in that said contrast enhancement inhibiting means (10) is arranged to interrupt the supply of said auxiliary drive signal to said auxiliary horizontal beam deflection means (5). A video contrast enhancement device according to claim 1 or claim 2, characterised in that said scanning speed varying means (5, 6, 7) also includes an amplifying circuit (7) for amplifying the output of said differentiating circuit, and in that said amplifying circuit (7) is adapted to clamp its output to a predetermined level when the output of said differentiating circuit exceeds a given threshold thereby to avoid over-enhancement of border contrast.

Inventors: SHIMA HIDEAKI, SHIMA, HIDEAKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1985
Main classification: H04N5
IPC codes: H04N5