Electronic Black Matrix Circuitry

The present invention relates to electronic apparatus for improving the visual display on a T.V. screen by electronically creating a black matrix or mesh surrounding the individual luminescing dots. In the past, a black and white T.V. picture was a series of horizontal lines of light whose intensity varied along each line. Each horizontal line was separated by a thin, unilluminated area which appeared as a thin black line. Because such a T.V. picture in each field had a series of thin black horizontal lines, one above the other in the vertical direction, the eye of the viewer was forced to integrate the display in the vertical direction. However, because the light spots along the horizontal line ran together, the eye was not forced to integrate horizontally. By creating a series of vertical lines similar to the horizontal lines in a standard T.V. picture, the present invention forces the viewer's eye to integrate the image uniformly in both spatial directions. Such a picture, with the black mesh background, produces a sharper, clearer image and a more viewable display.

Not only does the periodic disruption of a horizontal scan cause a sharper image but the abrupt boundary definitions between light dots helps eliminate phosphor light blooming. In particular, when the video input is blacked out between each pixel of the horizontal display, a large dynamic range signal is less likely to cause phosphor light blooming. When the rate at which the blackout level is switched on and off approximates the video band width of the display system, then the potential for phosphor light blooming is decreased even further.

Previously, black mesh or matrix backgrounds were produced by placing a black grid on the face of the cathode ray tube. This procedure is expensive. The present invention, with the addition of a simple and low-cost, electronic circuit or circuit chip to the electronics of T.V., eliminates the need for a precision manufactured black matrix grid and the labor to install and align it. Although this circuit has its easiest application to black and white televisions which have a continuum phosphor on a screen, it is also adaptable to color T.V. By using an electronic black matrix, such as the present invention imposes, the precision in placing the phosphor dots can be reduced.

A major application of the present invention is in medical diagnostic equipment. This field commonly uses black and white T.V. monitors with a gray scale but demands great visual accuity--accuity not just to the human eye but to cameras which take photographs of the visual display on the T.V. screen. Photographic images from a T.V. screen with the black matrix background are more viewable to the eye and produce an image which is more easily and fully resolved. Such an image also can produce more accurate measurements of distances and sizes. Photographs from T.V. images without the black matrix tend to be striped which produce photographs that are relatively easily resolved perpendicular to the scan lines but which are more difficult to resolve along the scan lines.

A primary use for the present invention is to display the output from ultrasonic diagnostic equipment. Ultrasonic diagnostic equipment is able to make a very precise electronic record of the object scanned by the ultrasonic array. However, this very accurate information can be lost on a T.V. monitor which does not have excellent visual resolution in all directions. The present invention greatly improves the accuracy with which ultrasonic diagnostic equipment can view the internal areas of a patient. The invention makes a great improvement in the viewability of the display very inexpensively. To achieve similar improvement by modifying the ultrasonic diagnostic equipment, in particular the control circuitry or the transducer's array, would require a far greater cost that increases in complexity.

FIG. 1 shows a circuit diagram in accord with the present invention for electronically applying a black matrix to an analog T.V. or composite T.V. video signal;

FIG. 2 shows a circuit diagram in accord with the present invention for applying a black matrix to a digital T.V. or composite T.V. video signal; and

FIG. 3 shows the circuit diagram of FIGS. 1 and 2 in combination with ultrasonic scanning equipment.

In the standard T.V. image, there are about 480 horizontal scan lines separated by about 479 thin black lines of unirradiated phosphors between adjoining scan lines. These black lines produce the horizontal lines of the black matrix. The present invention adds to the standard T.V. a means for periodically blacking out the signal during each sweep of the electron beam. To produce a square matrix, it blacks out the electron beam about 640 times per sweep. By synchronizing the means for periodically blacking out the signal with the beginning of the sweep line, each pixel of light is directly below the pixels in preceding lines. In this manner, the blacked out points along each line form a series of vertical black lines causing the matrix to appear as a number of vertical black lines intersecting a number of horizontal black lines.

FIG. 1 illustrates a circuit for electronically producing a black matrix background. A T.V. or composite T.V. video signal is received at input 10. The signal is buffered by buffering means 12, such as an amplifier which has a very low output impedance. The output impedance of the buffering amplifier should be substantially lower than resistive element 14. A typical value of resistive element 14 would be 270 ohms.

The video signal is then subject to a periodic blacking out means. In FIG. 1, this means is a D-MOS switch which is controlled by clock 20. Clock 20 produces a symmetric square wave on the order of 12 megahertz. A square wave of 12 megahertz produces 640 pixels separated by 639 black cells, the length of the pixel and black cell being approximately equal. The D-MOS switch, in time with the clock pulse, switches a reference voltage which is sufficient to cause a black reference for full scale video onto video line 22. A typical reference voltage is about 7/10 of a volt. Thus, the data within each scan line is blacked out between each pixel of display information.

The video signal on line 22 is buffered by a second buffering means, such as amplifier 24, to match the impedance of the transmission line to a T.V. monitor. Normally a T.V. monitor has a 75 ohm video path. A typical value for resistive element 26 would be 75 ohms. Thus, the signal at the video output 28 is correctly buffered for a standard T.V.

When a composite T.V. signal is being chopped, it is necessary to prevent chopping of the composite portion of the synchronization system. To achieve this, a NAND gate 18 is used which prohibits the clock pulse signals from passing unless there is also a signal on the inhibit line 30. The inhibit signal blocks NAND gate 18 until the composite portion of the video has been transmitted on line 22, then allows the clock pulse to pass. This will destroy the symmetry of the clock pulses slightly because the inhibit signal will tend to decrease the size of the blacked out region slightly.

Further, the clock pulse generator 20 is synchronized with the sweep generator of the T.V. monitor. If the clock pulses fail to start in the same phase of the beginning of each sweep, the vertical black matrix lines will not be straight. Thus, there is a means which starts the clock 20 at a specific phase at the beginning of each sweep or, conversely, there may be a means on the T.V. sweep generator controlled by the clock so that the sweep can only start at a specific position on the clock pulse.

A second embodiment of the invention is shown in FIG. 2. This embodiment works with a digital video signal rather than the analog video signal of FIG. 1. The analog to digital converter produces a digital output on a set of lines--one for each binary digit. The signals on each line are a set of on/off signals whose duration is determined by the clock pulses of a clock circuit. The digital output passes through a gating circuit which cuts the duration of each digital signal in half. The gating circuit then allows the digital signal to pass for half a clock pulse and for the other half of the clock pulse replaces the signal with the digital signal to cause a black pixel. The digital-to-analog converter receives the digital signal for about half the normal duration, and a digital signal corresponding to a blacked out region for the other half of the duration. When the digital-to-analog converter converts these signals back into an analog signal, the signal appears as a series of analog values interspersed among a series of blacked out values.

The circuit of FIG. 2 again has a buffering amplifier 52 to buffer the video signal at input 50. Differential amplifier 52 is illustrated with a voltage divider for the input video signal and with a diode which prohibits the video signal from taking a negative value. Further, the amplifier is illustrated with a feedback loop to the inverted input. The buffered input is then fed to a six-bit analog-to-digital converter 54. The digital signal is a series of six high and low pulses appearing on lines 56 through 61, respectively, which taken together represent an analog input amplitude in a digital form. Lines 56 through 61 are connected to one input of AND gates 64 through 69, respectively. AND gates 64 through 69 have their outputs connected to a digital-to-analog converter 72 which converts the digital signals back to analog signals. Although it is possible to use either current-mode or voltage-mode digital-to-analog converters, voltage-mode converters have been found to be more desirable because they reduce interference from parasitics and other system time constants.

The other input of each of the AND gates 64 through 69 is connected to a chopping signal means. In this example, the chopping means is a clock generator 74 and an AND gate 76. AND gate 76 acts to transform the clock pulses into signals of only two amplitudes, either a high voltage amplitude or a low voltage amplitude which correspond in amplitude to the high and low voltages on output lines 56 through 61. When the output of AND gate 76 is high, then AND gates 64 through 69 will have a high output if the input on the respective one of inputs 56 through 61 is high and a low output if the respective input is low. When the output of AND gate 76 is low, the output of AND gates 64 through 69 are all low. When the outputs from AND gates 64 through 69 are all low, the digital-to-analog converter 72 will produce a signal indicative of a black region. When the output from AND gate 76 is high, AND gates 64 through 69 will pass or low pulses which digital-to-analog converter 72 transforms into an analog brightness amplitude. A pair of switches 78 may be used to connect to ground the third input to AND gates 68 and 69. This removes the least two significant digits of the six-bit digital brightness signal. This reduces by one-quarter the number of shades of gray which are available in the final output video signal.

The output of the digital-to-analog converter 72 is again fed through a buffering amplifier 80 which matches the output impedance to the input impedance of the T.V. monitor--typically 75 ohms.

It will be noted then that if the clock pulse generator runs at 12 megahertz, there will be 640 pixels of data separated by 639 blacked out pixels in each scan. If the analog-to-digital converter also converts the analog input signal into 640 digital representations per scan, then no digital information will be lost. However, if more than 640 conversions of analog-to-digital data are made per scan, then some of the data will be lost. To overcome the problem of matching the clock frequency to the rate at which the analog-to-digital converter converts analog signals into digital signals, AND gate 76 can be controlled directly by the analog-to-digital converter 54. Conversely, clock 74 can control analog-to-digital converter 54. It is desirable to have 400 to 700 blacked out pixels per line in order to produce a roughly square black matrix although a larger or smaller number of black pixels may be desirable for some uses. Accordingly, the analog-to-digital converter should be able to make 400-700 conversions per scan line.

FIG. 3 illustrates the electronic black matrix insertion circuit in an ultrasonic diagnostic system. Further details of such an ultrasonic diagnostic system are illustrated in such patents as U.S. Pat. Nos. 3,911,730, 3,881,466, and 3,919,683. In FIG. 3, there is a control panel 100 which selects the exact mode of operation of the diagnostic equipment. This control panel controls the pulser/receiver module 102. Pulser/receiver module 102 causes one or a group of transducers, typically on the order of four to eight in the transducer array 104, typically 64 transducers although the "array" 104 could be a single transducer, to be pulsed such that they produce an ultrasonic sound wave. The echo from the sound wave impinging upon transducer array 104 causes electrical signals which are sent back to the pulser/receiver unit and on to a video processing unit 106. The signals from each group are processed by the video processor 106 into one scan line of video data.

Pulser/receiver unit 102 then causes a different group of transducer elements to be pulsed to produce the information necessary for video processor 106 to produce a second scan line of video data. The output of video processor, a composite video signal, is fed to the input of circuit 108 which could be the circuit of either FIG. 1 or FIG. 2. The output of circuit 108 then goes to the input of a T.V. monitor 110 which provides the visual display of the patient or object scanned by the transducer array 104.

The above circuits are exemplary of the present invention. The specific embodiments as shown above are not intended to limit the present invention. Rather, the present invention includes the above specific embodiment as well as all of the equivalents thereto encompassed within the claims as follows.