US 3597533 A
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United States Patent 3,402,258 9/1968 Lerner 3,410,953 11/1968 Quinlan.... 3,479,453 1 l I969 Townsend Priority Gordon Hughes Manchester, W
Aug. 21, I968 Aug. 3, 1911 hm Computers Limited hudon, W
Aug. 26, 1967 Great Brink lnventor Assignee cnmc'rsn GENERATION 6 Claims, 2 Drawing Figs.
Refierences Cited UNITED STATES PATENTS l78l DlG. 3 l78/DIG. 3 l78/6.8
f PULSE 1AM:
osc-- FOREIGN PATENTS 546,462 7/1942 Great Britain 17 8/6 794,531 5/1958 Great Britain l78/DlG. 3
Primary Examiner- Robert L. Griffin Assistant ExaminerJoseph A. Orsino, J r. Anomey--l-lane & Baxley ABSTRACT: A system for high-speed scanning of graphical symbols in which a line is scanned by a first waveform at a first speed and upon the scan passing over the leading or trailing edge of a symbol, an output is developed and applied to a pulse generator. The pulse generator produces a substantially rectangular pulse whenever the scan passes a leading or trailing edge of a symbol portion, while the pulses are integrated by an appropriate amplifier. The integrated pulses form a second triangular-shaped wavefonn which is combined with the first scanning waveform such that the scan speed of the first waveform is temporarily reduced below normal scan speed and is temporarily increased above normal scan speed before reaching and remaining at a normal speed. The combined scanning waveform is then transmitted to a display device along with the first waveform and appropriate control signals such that the symbol scanned may be remotely displayed.
CHARACTER GENERATION BACKGROUND or THE INVENTION The scanning of character or symbol outlines to generate I electrical signals which are used to control a cathode-ray tube in such a way as to display a corresponding character or symbol outline is well known. In present devices, it is desirable to scan symbols as rapidly as possible for maximum efficiency. However, it will be apparent that an increase in the number of characters scanned per second requires a corresponding increase in the rate of scanning each character and, therefore, in the high frequency components of the resulting electrical signals. One of the problems .encountered in such systems is that electrical signals must be fed from the scanning system to the display system over some form of transmission channel. However, the bandwidths of the transmission channels, if the channels are of any substantial length, will not effectively permit the transmission of such high frequency signals and, will be in many cases the factor which limits the maximum scanning rate. Also, the bandwidths of video amplifiers employed to drive bright up circuits of the display device limit the practical rate of scan. Therefore, bandwidth reduction of electrical signals generated from the scanning of symbols has been employed to allow the effective transmission of such signals to a display device.
Various prior art systems have been developed in which bandwidth reduction techniques are employed for narrowing the band of transmitted frequencies in such characterscanning arrangements. For example, one such technique is shown in British Pat. No. 659,596, which shows a two-speed scanning method. In this patent, a line-scanning technique is employed such that the area of a television screen is scanned at a high rate and, upon the scan passing an image, the scan rate is reduced. As the scan line covers the image and encounters a blank area again, the scan speed is returned to a rapid rate. As noted in this patent, a considerable reduction of the transmitted frequency band representative of the image is obtained as a result of scanning the image at a slower rate. However, in the subject patent, a comparison between the intensity of blank" and image areas must be effected before the scan is switched to a slower rate, the switching operation also requiring external circuitry.
SUMMARY According to the present invention, a system for scanning graphic symbols is provided wherein a first scanning waveform is adapted to scan a line across a symbol at a first speed with the passage of such scan across a symbol detected by suitable means. First and second signals corresponding to the position of the scan across the leading and trailing edges of the symbol, respectively, are generated and applied to suitable circuit means for producing second waveforms. The second waveforms are combined with the first scanning waveform such that the scan speed of the first waveform is temporarily reduced below the normal speed and then temporarily increased above normal speed as a result of the scan passing each one of the symbol portion edges. Thus, the first and second pulses generated each effectively both reduce and increase the speed of the first scanning waveform.
In the present invention, the normal scanning speed along a line is slowed down as a result of the scan encountering an edge of a symbol, with the scan speed subsequently increased to a rate greater than the normal scanning speed. Both the slow and high scan rates are temporary with the high scan rate lasting until the normal scan rate is reached. At this point, the normal scan speed is continued until the second edge of the symbol portion is encountered. At this second edge, the scan rate is temporarily reduced and increased as previously described. Changes in the scan rate, as mentioned above, are
effected by adding or combining second waveforms with the main scanning waveform such that the symbols are rapidly scanned, yet efficiently transmitted to a remote display device. It should be noted that while the present invention provides a system for rapid scanning of graphic symbols, the outlines of such symbols are scanned at a substantially lower speed, thereby allowing effective transmission of electrical signals representative of the symbol outlines.
As a result of altering the main scan rate by combining a second waveform therewith, various complex switching circuits have been rendered unnecessary. Also, while it is known to determine scan rates by the discharge of a capacitor in a resistor-capacitor discharge circuit, the need to switch appropriate resistors, and the attendant problems of such switching circuits, is eliminated in the present invention.
BRIEF DESCRIPTION OF THE DRAWING the system of DESCRIPTION OF THE PREFERRED EMBODIMENTS Electrical signals representing symbols are generated by a conventional monoseope character generator tube 1 (FIG. 1 The screen of the tube carries secondary emission areas corresponding to the shapes of symbols, such as alphabetic characters. The monoseope electron beam is scanned in a line across the screen by the application of scanning waveforms to deflection electrodes 2 and 3. When the electron beam strikes a portion of a secondary emission area, an output voltage is developed across resistor 4. The monoseope tube is provided with a conventional deflection system for deflecting the beam in a direction perpendicular to that deflection produced by the electrodes 2 and 3. This further deflection system is not shown, since it is not necessary to an understanding of the invention.
The scanning waveform which is applied to the deflection electrode 2 is derived from an oscillator 5, operating at a frequency of 1 MHz., for example. The waveform may be of a conventional sawtooth form,.or it may be sinusoidal, the substantially linear part of the sine wave being effective for scanning the beam across the screen.
The output voltage across resistor 4 is applied to the input of a video amplifier 6,-having a limited bandwidth of 3.3 MHz. in the present example, and a pulse generating circuit 7. The passage of the monoscope beam across a portion of a symbol area produces a substantially rectangular pulse, as shown by waveform B (F IG. 2). The pulse generating circuit 7 produces a pair of pulses (waveform C) in response to the rectangular input (waveform B), the edges of the pair of pulses coinciding with the leading and trailing edges of the input pulse. This may be done, for example, by differentiating the input pulse and using the resulting pulses derived from the leading and trailing edges of the input pulse to trigger a monostable delay line pulse generator (not shown). The pair of pulses are of equal duration, 50 nsec. duration being suitable in the present example. The output of video amplifier 6 is shown in waveform E of FIG. 2. The bandwidth of video amplifier 6 is directly determined by the time necessary for the edges of waveform E to settle with this time increment (50 nsec.) being equal to the duration of the pulses of waveform C.
The pulse pair is fed to an amplifier circuit 8 which integrates each pulse to form a substantially triangular pulse of nsec. duration (waveform D). The output waveform from the amplifier 8 is applied to the deflection electrode 3. Waveform A illustrates the net deflection voltage applied to the monoseope tube, which is the resultant of combining the linear waveform =from oscillator 5 with the waveform D. The effect of the additionof waveform D is to reduce and then increase the rate of change of the scanning voltagefor each character edge which is scanned. Hence, the rate of scanning is first reduced and then increased as compared with the average scanning rate produced by the main scanning waveform from the oscillator 5. The pair of pulses (waveform C) are slightly delayed relative to the waveform B so as to allow the leading edge of this waveform to stabilize before the waveform D is applied to the electrode 3. As stated previously, while waveform B (FIG. 2) is a substantially rectangular pulse and the edges of the pulse will not be completely vertical, the pulse will have a finite rise time which, as is the case with conventional monoscope tubes, will depend on the size of the scanning spot and on the particular materials comprising the secondary emission areas of the tube 1. While the production of waveform D by pulse generator 7 and integrating amplifier 8 will require a finite time delay, pulse generator 7 will respond to the voltage across resistor 4 and may produce waveform B before the exact edge of a symbol portion is scanned as a result of the scanning spot having a finite size etc. Thus, by the time waveform D is applied to deflection plate 3, the main scanning waveform produced by oscillator 5 may be considered as crossing the actual" edge of the character portion. Similar considerations will apply to the trailing edge of waveform B.
The signals from the amplifier 8, the oscillator 5, and the video amplifier 6 are fed over a cable 9 to control a remote display cathode-ray tube 10. The scanning waveforms from the amplifier 8 and the oscillator 5 are fed, via current drivers 1 l and 12, respectively, to electromagnetic deflection coils l3 and 14, respectively. The response characteristics of the drivers 11 and 12, together with those of the coils l3 and 14 are such that the electron beam of the cathode-ray tube scans in synchronism with the beam of the monoscope 1. The output of the video amplifier 6 is fed to control grid 15 of the cathode-ray tube 10. The control grid is normally biased to such a level that the CRT beam is cut off and the signal from the video amplifier acts as a bright up" waveform to bring the control grid above cutoff. Hence, when the monoscope beam scans a portion of a character, a corresponding bright image is produced on the CRT screen. The amplitude and shape of the waveform D is such that the scan produced by the main scanning waveform is first substantially cancelled and then increased to twice the average speed of the main scanning waveform. It will be appreciated that the amplitude and/or shape of the wavefonn D may be such that the changes in the scan rate due to scanning an edge are less than that shown and that the changes may be nonlinear, for example, they may be exponential.
It has been assumed that the width of the portion of the character which has been scanned is such that the variations in scanning rate due to the trailing edge commence immediately after the variations due to the leading edge. if the width of the character portion is less than that of the example, the waveforms produced by successive edges will overlap to some extent This can result in a reduction of the amplitude of the net scanning voltage, if several portions of a character occur in one scan. This reduction may be compensated by increasing the amplitude of the main scanning waveform from the oscillator 5. Such an increase in the amplitude of the main scanning waveform will result in less of a slowing of the scanning speed as the addition of waveform D (FIG. 2) have a smaller effect in slowing down the scanning speed and a greater effect in increasing the scan speed. Therefore, the scanning speed will be restored to an original rate sooner, and thereby allowing smaller character widths to be scanned. If the width of the character portion is greater than that of the example, the
variations in scanning rate due to the leading and trailing edge of the character portion will be separated by an interval during which scanning is at the normal rate. Consequently, there will be a variation in brightness across the character portion displayed on the cathode-ray tube. In order to produce a uniform brightness a suitable compensating waveform is added to the si nal passing through the video amplifier.
he invention is applicable to forms of character scanning other than the monoscope. For example, the characters may be apertures in a mask and the scanning waveforms may be used to control the beam of a cathode-ray tube to produce a spot of light which scans across the mask. The light which passes through the apertures in the mask is picked up by an optical system and a photoelectric cell, the output from which corresponds to the output from the monoscope. Alternatively, the characters may be recorded as images on a photographic film or printed outlines on paper.
1. A system for scanning graphic symbols including means for generating a first scanning waveform which is adapted to scan a line across a symbol at a first speed, means for detecting the passage of the scan across a portion of the symbol, means for generating first and second pulses corresponding to the portion of the scan across the leading and trailing edges of said symbol portion, respectively, means responsive to said first and second pulses to generate second scanning waveforms and means to combine said first and second waveforms to cause the scanning speed along said line to be reduced temporarily to less than said first speed and subsequently increased temporarily to a speed greater than said first speed as a result of said scan passing each one of said edges of said symbol portion.
2. A system as claimed in claim 1 in which said means responsive to said first and second pulses includes an integrating amplifier operative to integrate said first and second pulses and thereby produce said second waveforms, each of said second waveforms having a substantially triangular waveshape.
3. A system as claimed in claim 2 in which said means to combine said first and second waveforms includes means to add the first half of each of said second waveforms to said first waveform to temporarily reduce the scanning speed below the speed of said first waveform and to add the second half of each of said second waveforms to said first waveform to temporarily increase the scanning speed above the scanning speed of said first waveform.
4. A system as claimed in claim 2 in which said means for detecting the passage of a scan across a portion of said symbol includes an electron tube having a first deflecting means to which said first waveform is applied anda second deflecting means, the output of said tube being to a pulse generator for producing said first and second pulses that are integrated by said amplifier to produce said second waveforms, and means to apply said second waveforms to said second deflecting means so that said first and second waveforms are combined.
5. A system as claimed in claim 4 in which the output of said electron tube is applied over a transmission means to a remote display device for displaying said symbol scanned by the combined first and second waveforms.
6. A system as claimed in claim 5 in which said display device includes a second electron tube having third and fourth deflecting means, said first and said combined waveforms applied over said transmission means to said third and fourth deflecting means, respectively, and means for applying a control signal over said transmission means to a control electrode of said second electron tube.