XY display transition intensifier

Abstract: An XY display transition intensifier increases the intensity of an electron beam of a CRT device when the deflection voltages are changing, indicating a transition between dots on the CRT display. The absolute values of the rate of change of the deflection voltages are determined and summed. The resulting output voltage is applied to the intensity grid of the CRT device to increase the intensity of the electron beam.

Description: Background of the Invention The present relates to cathode ray tube (CRT) displays, and more particularly to an XY display transition intensifier for a CRT display which equalizes the brightness between dots and the transition between the dots, the dots and transitions providing useful information. In XY CRT displays, such a vectorscope display, the displayed image is generated by applying signal voltages to the deflection plates of a CRT. When the voltages are momentarily constant the electron beam, which excites the screen phosphor of the CRT to produce the image, is stationary and a bright dot appears. Traditionally this is the only portion of the display image that is providing useful information. However the transitions between dots also are useful for relative timing information. Unfortunately the transitions are too dim to be easily seen when the dots are at normal brightness. Therefore what is desired is a means for equalizing the brightness, or reducing the difference in brightness, between the dots and the transitions between dots. US-A-4,198,661 discloses a cathode ray tube apparatus where the deflection current through the deflection coils is sensed and differentiated. The absolute values of each of the voltages corresponding to the derivative of the horizontal and vertical deflection currents are summed and compared to a signal derived from a brightness control to selectively blank the cathode ray tube. US-A-3,714,502 discloses a circuit for controlling the intensity of scanning in an electromagnetic deflection type cathode ray tube in which the voltage across the deflection coil, which corresponds to the time derivative of the deflection current, is sensed. An absolute value amplifier converts positive or negative going time derivative values to a single polarity signal and this signal is used as the basis of an intensity control signal, to obtain constant brightness regardless of the scanning velocity. Summary of the Invention According to the present invention there is provided a vector display transition intensifier for providing an intensity control voltage to a display such that a transition between two points on the display in response to a step change in a deflection voltage is intensified, comprising differentiating means responsive to a step change of either polarity in said deflection voltage for providing two time derivative values of said deflection voltage, said two values having waveforms of mutually inverse polarity, and switching means receiving said time derivative values for routing one of said time derivative values from said differentiating means to an output so as to produce an intensity control voltage of the same polarity regardless of the direction of the step change of said deflection voltage. When said deflection voltage comprises a pair of orthogonal deflection voltage components, conveniently said differentiating means produces respective first and second time derivative values of the pair of orthogonal deflection voltage components, said switching means switches said first and second time derivative values to respective first and second outputs and means are provided for combining the first and second outputs to provide the intensity control voltage. Accordingly, the present invention discloses a display transition intensifier which equalizes the brightness, or reduces the difference in brightness, between the dots and transitions between dots of a displayed image. Since the brightness of the transitions is a function of the electron beam speed, the beam speed is compensated for by taking the absolute value of the time derivative of the deflection signals, summing these values and applying the summed signal to the CRT grid to increase the transition brightness. The objects, advantages and novel features of the present invention will be apparent from the following detailed description when read in conjunction with the appended claims and attached drawing. Brief Description of the Drawing The Figure is a schematic view of an XY display transition intensifier according to the present invention. Description of the Preferred Embodiment The brightness of the transitions between stationary dots on an XY CRT display is a function of the display duty cycle and the electron beam speed. The circuit shown in the Figure compensates for the electron beam speed by taking the absolute value of the time derivative of the X and Y deflection signals, V x and V y , summing the two time derivative signals, and applying the sum signal to the intensity grid of the CRT to increase the transition brightness. Due to CRT gamma the exact correction would be obtained by the following expression: V g = (V x ² + V y ²) 1/4 , the gamma exponent being two. However, it has been determined empirically that the exact correction is not necessary. Therefore, the circuit of the Figure employs the following function: V o = /V x / + /V y /. The circuits for the horizontal and vertical deflection signals are identical, so only one of them will be described in detail. A pair of transistors Q₁, Q₂ act as a differentiator 12. For positive input transitions the collector current in the left transistor Q₁ increases and in the right transistor Q₂ decreases. The reverse occurs for negative transitions. Differential pairs of transistors Q₃₁, Q₃₂ are driven by an RC differentiator 14 and act as a switch 16 for the outputs of the differentiator 12, with the collector of the left transistor Q₁ being connected to the emitter pair of the left differential pair Q₃₁ and the collector of the right transistor Q₂ being connected to the emitter pair of the right differential pair Q₃₂. The transistors of switch 16 and differentiator 12 are available as a single, inexpensive multiplier integrated circuit from several integrated circuit vendors, such as the Motorola MC1496. The switch 16 routes the increasing current of the differentiator 12 through a common load R L for either a positive or negative transition. Thus the output current on line 18 is a function of the absolute value of the input rate of change. The outputs from the X and Y transition deflection circuits over lines 18 are summed at node 20 and input to a Z-axis driver 22, the output of which is applied to the CRT intensity grid to increase the intensity during transitions. In operation so long as the deflection voltages V x , V y are constant, i.e., a dot is being displayed on the CRT display, an equal bias voltage is applied to both bases of each differential transistor pair Q₃₁, Q₃₂ and each transistor of each pair conducts a current of I/2, resulting in an output current equal to the I. When the input deflection voltage changes positively, the left transistor Q₁ of the differentiator 12 is biased more positively, causing the emitter to increase in voltage. The increase of voltage at one emitter is transferred via a coupling capacitor C c to the emitter of the other transistor Q₂ which effectively reduces the bias of that transistor. Thus the current increases through Q₁ and decreases through Q₂. The positive going input voltage at the RC differentiator 14 causes the bias at the left transistor of the differential pair Q₃₁ to increase and conduct so that the increased current from Q1 is switched through that transistor resulting in an output current equal to the I plus a current proportional to V, where V is the time derivative of the input deflection voltage. Likewise when the input voltage decreases, the current through Q₂ increases and the right transistor of the differential pair Q₃₂ has a decreased bias which cuts off that transistor and turns on the left transistor so that the increased current from Q₂ is switched to the output. Therefore, regardless of the direction of the change of the input voltage, the output voltage represents an absolute value representative of that rate of change. The output currents from the two deflection circuits are input to an operational amplifier 24 at the inverting input, the non-inverting input being held at a fixed potential. The resulting output is an increased positive voltage from the Z-axis driver 22 when the deflection voltages are changing, i.e., during transitions, which results in increased intensity on the CRT display. Thus the present invention provides an XY display transition intensifier which increases the intensity of the electron beam during changes in the deflection voltages regardless of the direction of the change by summing the absolute values of the rate of change and outputting the resultant to the intensity grid of a CRT display.

Claims: A vector display transition intensifier for providing an intensity control voltage to a display such that a transition between two points on the display in response to a step change in a deflection voltage is intensified, comprising differentiating means (12) responsive to a step change of either polarity in said deflection voltage for providing two time derivative values of said deflection voltage, said two values having waveforms of mutually inverse polarity, and switching means (14, 16) receiving said time derivative values for routing one of said time derivative values from said differentiating means (12) to an output (18) so as to produce an intensity control voltage of the same polarity regardless of the direction of the step change of said deflection voltage. A vector display transition intensifier according to claim 1 wherein said switching means receives said deflection voltage for controlling the routing of said time derivative value. A vector display transition intensifier according to claim 1 or claim 2 wherein said differentiating means (12) comprises a differentiator having two inputs and two outputs, the deflection voltage being applied to one input, the other input held at a fixed potential, a time derivative of one polarity corresponding to an increase in deflection voltage appearing at one output, and a time derivative of said one polarity corresponding to a decrease in deflection voltage appearing at the second output. A vector display transition intensifier according to claim 3 wherein said switching means comprises a pair of switches (16), each coupled to a respective output of the differentiator, the pair of switches having a common output as said output (18) ; and a switch differentiator (14) to drive the pair of switches, the switch differentiator having the deflection voltage coupled as an input and being coupled to the pair of switches to provide said intensity control voltage at said output. A vector display transition intensifier according to any preceding claim wherein said intensity control voltage is applied to an intensity grid of a display device. A device comprising a first and a second vector display transition intensifier, each according to any one of claims 1-5, wherein said deflection voltage for said first and second vector display intensifiers is provided by first and second, orthogonal deflection voltage components, and further comprising means for adding the signals from said outputs (18) of said first and second vector display intensifiers to provide the intensity control voltage.

Inventors: BAKER DANIEL G, BAKER, DANIEL G.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1987
Main classification: G09G1
IPC codes: G09G1