1. Field of Invention
The present invention relates to delta-gun shadow mask type color cathode ray tubes (CRT) and more particularly to means for causing the electron beams from each of the delta-arranged guns to converge on a single shadow mask opening and, therefore, the phosphor triad on the display surface or screen of the CRT.
2. Description of the Prior Art
As is well known to those skilled in this art, the delta-gun, color cathode ray tube (CRT) differs most significantly from conventional CRT's having a single gun in that the delta-gun has three guns, each of which generate a distinct electron beam for exciting its corresponding phosphor dot of the red, green and blue dot triad matrix constituting the CRT display surface.
Each of the delta-arranged gun beams must land at precisely the same opening in the shadow mask at every point on the screen in order to produce a perfect rendition of a desired color and symbol shape. To achieve the desired rendition on the screen, the beams, substantially forward of where they leave the guns, are magnetically deflected, as a set, by excitation of a main deflection yoke comprising an electromagnet arrangement mounted around the neck of the CRT so as to control the angle at which the beams, as a set, approach the desired landing site on the display screen.
Registration of all three beams at the landing site cannot be accomplished through all deflection angles without some individual deflection of each beam in addition to the main deflection. The auxiliary deflection system is termed convergence deflection because of the function it performs in causing each beam to converge to a single point for all deflected angles.
Aside from the geometry errors (pin cushion effect) which necessitate a convergence system, there are errors due primarily to alignment tolerances associated with the deflection yoke and electron gun assemblies. These errors have the effect of distorting the ideal convergence function and consequently, each CRT assembly must be uniquely converged.
In most, if not all delta-gun CRT's, convergence deflection is accomplished by electrical current excitation of a small deflection yoke for each yoke is usually part of the electron gun assembly (See FIG. 1). Each of these yokes produce a magnetic field which is applied to the beam emanating from their respective gun in such a way that deflection of the beam is along a single axis, that is, the axis which extends radially from the center of the CRT, through the center of each of the three guns. These axes are called red radial, green radial and blue radial, for a typical CRT and are then, by the nature of the delta-gun, 120 angular degrees apart.
It is customary to align the blue radial with the vertical deflection of the main deflection yoke. Thus, as facing the front of the CRT, the blue gun converges along a vertical axis, red along an axis 120.degree. left and green along an axis 120.degree. to the right (240.degree. to the left).
The full function of convergence is not possible without a fourth degree of freedom of movement associated with one of the electron beams. This is generally applied to the blue gun along the lateral axis of the CRT at 90.degree. to the blue radial and is consequently termed blue lateral convergence.
As is the case with radial convergence, the blue lateral convergence can be partitioned into two components, a dynamic portion and a static, or constant portion. The cynamic portion of blue lateral correction (unlike that of the dynamic portion of the radial correction) can be, in general, performed by the design of the main deflection yoke. When this is the case, only an adjustable external static magnet is required to position the blue beam laterally to a reference location at the center of the screen. This magnetic adjustment is termed the static blue lateral adjustment.
In one prior display system which was developed using digital and analog hybrid circuitry, the screen was divided into 256 regions or cells and convergence for each gun was characterized by a digital word for each gun in each of the 256 locations. An analog circuit generated the major component of the convergence function and the digital word for each gun and location was converted to an analog equivalent to complete the convergence function.
Calibration of the hybrid system was accomplished by placing a symbol, having a color which consisted of combinations of two of the primary colors. The color of the symbol could be changed under operator control to give the three possible combinations of two guns (red blue, blue green, green red) and also the combination of three guns (white). In each cell, a potentiometer for each gun for each of the cells was manually adjusted by observation of the landing registration of the symbol color components, two guns at a time. The voltage at the wiper of the potentiometer was converted into a digital word and substituted for the digital word normally stored and accessed by the convergency circuitry for that cell. When the proper registration was observed, the digital values that produced it were recorded. All the convergence characteristics of each cell was generated in this way and the resulting digital data was stored in PROMS for use by the convergence circuitry in subsequent operation of the CRT.
Because this process of convergence calibration for each gun took place in each of 256 cells, calibration often took more than eight hours. To minimize time in calibration, a smoothing routine was created to linearly extropolate the data between two cells so that only every other cell required manual calibration. Although this resulted in a savings of time of nearly six hours, it also compromised the convergence quality and very often cell by cell data had to be taken at the edges of the screen.
It was reasoned that the linear smoothing model was incorrect and that a parabolic model was more correct. This resulted in needing to take only nine points of data but had the disadvantage of compromising the convergence quality even further. The subjective opinion of each viewer of what constituted adequate convergence led to many recalibrations and served as a major motivation behind developing a new convergence technique.
In addition to the calibration shortcomings, other disadvantages existed. Vertical bars were discernable in the raster near the edges where the quantized convergence distorted the beam placement as it crossed from one cell to the next. The same phonomena manifested itself as "jogs" in stroke generated lines as they passed from one cell to the next. Furthermore, the digital words stored in the PROMS were not easily modified for even the smallest of corrections.
Most other convergence methods developed are inappropriate or inadequate for the high quality resolution required of aircraft Electronic Flight Instrument System (EFIS) displays. Most schemes have their origin in the home television industry, where the convergence quality is lower than is necessary in EFIS and where the only mode of operation is a raster scan. EFIS has the requirement that the display operate both as a high speed raster device and as a stroke or vector monitor. The random nature of the vector mode of operation eliminates schemes where convergence is dependent on an anticipatory knowledge of where the beam is going. Such schemes are ideal in raster systems. These schemes allow smoothing of the digital component of the convergence through a "pre-access" of the next digital word. The digital smoothing can be improved through an increase in the number of cells on the screen but because of the high scan rates of EFIS raster, the limit of digital access times is reached before the solution is effective.
Analog convergence schemes exist and are commonly used. The general problem with these schemes are that they are complex in alignment. Most adjustments are interactive with each other and few can achieve the degree of convergence quality provided by the present invention. Most of the analog schemes divide the screen into regions or sectors and a correction term is generated and applied specifically for that region. This amounts to a piecewise approximation where new terms become active and are added in as the region is entered.