Enhanced capacity display monitor

The character displaying capacity of a monitor is enhanced by operating the pixel clock at a frequency at which beam narrowing occurs so that adjacent pixels are substantially distinct from each other. The operating clock frequency exceeds the bandwidth of the video channel and is at a frequency higher than the clock frequency at which pixel smearing occurs.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
The invention relates to computer monitors particularly with respect to 
enhancing the character display capacity thereof. 
2. DESCRIPTION OF THE PRIOR ART 
Present day computer monitors display alphanumeric text by utilizing 
adjacently disposed character matrices of pixels (picture elements) for 
displaying the characters. An alphanumeric character is displayed in a 
pixel matrix by selectively illuminating the pixels of the matrix. 
Intercharacter spacing is effected by reserving one or more rows and 
columns of pixels at the edges of the matrix for the space. The number of 
pixels that can be displayed on a line is limited by the bandwidth of the 
video driving circuitry and thus the number of characters that can be 
displayed in a character row of the monitor is similarly limited by the 
video bandwidth. In order to maximize the number of characters on a line, 
the frequency of the pixel clock is set for operation at the high 
frequency band edge of the video amplifier. 
If the frequency of the pixel clock is increased somewhat so that operation 
occurs slightly beyond the high frequency end of the video amplifier 
bandwidth in order to increase the monitor capacity, a phenomemon denoted 
as pixel or dot smearing occurs. Since the slew rate of the video 
amplifier is inadequate to rapidly turn on and off at this increased 
frequency, the video amplifier does not completely turn off the video gun 
at the end of a pixel before the start of the next pixel. An adequate 
black level between pixels is not achieved resulting in inadequate 
contrast between the dots. Adjacent pixels tend to merge into one another. 
Because of this phenomemon, elements of the characters run into each other 
and adjacent characters run into each other severely degrading the 
legibility of the display. 
When utilizing a system that operates just beyond the band edge of the 
video amplifier, additional characters may be displayed on a line by 
reducing the width of the character matrix and utilizing a smaller 
intercharacter spacing. This approach, coupled with the pixel smearing 
effect further exacerbates display illegibility. 
A prior art approach to increasing the number of characters on the line is 
to increase the bandwidth of the video channel and hence increase the slew 
rate. The increased slew rate permits the beam to be turned on and off at 
a high rate thus generating an increased number of pixels on the line with 
adequate black level therebetween. Increased bandwidth, however, 
significantly increases the cost of the monitor. 
SUMMARY OF THE INVENTION 
The disadvantages of the prior art are obviated by increasing the frequency 
of the pixel clock such that operation occurs sufficiently beyond the high 
frequency band end of the video driving circuitry such that beam narrowing 
occurs to counteract the dot smearing effect and to utilize the 
significant increase in pixels on the line resulting from the increase in 
clock frequency to provide adequate intercharacter spacing. The operating 
point on the video driving circuitry frequency bandwidth curve is the 
point where beam narrowing results in adjacent pixels that are 
substantially distinct from each other.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a computer monitor display system environment in which 
the present invention may be utilized is illustrated. The apparatus of 
FIG. 1 includes a conventional computer monitor 10 having a display screen 
11. In a conventional cathode ray tube (CRT) monitor, the beam is 
deflected in an X direction (X-deflection) and in a Y direction 
(Y-deflection). Conventional deflection circuitry is included in the 
monitor 10 for sweeping the beam in the X and Y directions in accordance 
with a conventional raster scan. The intensity of the beam is controlled 
by a video amplifier 12 and the brightness level of the display is 
controlled by conventional brightness circuitry 13. The monitor 10 
includes conventional horizontal and vertical sync circuits responsive to 
composite sync signals on a line 14. It is further appreciated that the 
video amplifier 12 and the brightness circuitry 13 are conventionally 
included within the monitor 10 but are illustrated as separate components 
for convenience. 
A character font memory 15 stores a plurality of pixel maps or pages for 
the repertoire of characters to be selectively displayed on the screen 11. 
Each page contains a matrix array of bits with binary ones stored at the 
pixel locations to be illuminated in accordance with the shape of the 
character to be displayed and binary zeros at the remaining pixel 
locations in the matrix including the intercharacter spacing. The memory 
15 is read by control circuitry (not shown) to selectively provide, in 
parallel on a bus 16, the data bits from selected character pages. 
The character data bits on the bus 16 are applied in parallel to a 
parallel-to-serial converter 17. The converter 17 is responsive to a pixel 
clock 18 which clocks the converter 17 at the pixel clock frequency. The 
converter 17 may be a parallel-in serial-out shift register to convert the 
parallel character font data on the bus 16 into bit serial format at the 
pixel video rate for application to a digital video driver 19. The digital 
video driver 19 serially provides the digital video signals to the video 
amplifier 12 in accordance with the serial character bits provided by the 
converter 17. The digital video driver 19 also provides, to the deflection 
circuitry of the monitor 10, the composite sync signals on the line 14 in 
synchronism with the digital video. The signals are provided by the 
digital video driver 19 to the monitor 10 to display the pixels across the 
screen 11 in the appropriate positions required in the raster. FIG. 1a 
illustrates the horizontal sync signal (HSYNC) with the video pulses timed 
to occur between horizontal sync pulses for generating a line of the 
raster. 
Referring to FIG. 2, the high end of the frequency response curve of the 
video amplifier 12 is illustrated. In the prior art, the monitor 10 is 
operated at a point 30 which is at the high frequency band edge of the 
curve. At the point 30, the video amplifier 12 has an adequate slew rate 
to rapidly turn the beam on, increase to full intensity, and rapidly turn 
the beam off within a pixel cell of the system. FIG. 3a illustrates the 
luminosity versus X-distance depicting this prior pixel generation 
process. Four bits are illustrated with an adjacent one and zero as well 
as adjacent ones. It is observed that a pixel 31 generated in response to 
a binary one, terminates at the edge of the pixel cell. The next occuring 
bit, binary zero, provides a good black level at the bit cell. It is 
further appreciated that with respect to pixels 32 and 33, the video 
amplifier 12 has adequate bandwidth to provide a good black level 
therebetween. Because of the rapid turn on and turn off of the beam, 
proper contrast is provided between pixels and hence between characters 
resulting in a display with good legibility. 
A monitor that exhibits the performance described is commercially 
procurable from the Unisys Corporation in the product line of work 
stations provided thereby. The pixel clock utilized is 19.98 MHz. with a 9 
by 12 pixel character matrix (9 pixels wide by 12 pixels high) with 80 
characters to the line. Thus, when operating at the point 30 (FIG. 2) the 
monitor 10 provides 720 pixels to the line with good resolution and 
legibility. 
In order to provide mroe information on the display screen 11, it is 
desirable to increase the number of characters displayed on a line from 80 
characters to, for example, 132 characters. Attempting to display 132 
characters per line requires a change in the character matrix size. For 
example, a 6 by 10 character matrix (6 pixels wide by 10 high) may be 
attempted. A 6 by 10 character matrix requires a slight increase in the 
frequency of the pixel clock 18. A 6 by 10 character matrix requires 792 
pixels per line necessitating a pixel clock of approximately 22 MHz. This 
operating point is illustrated as point 40 on FIG. 2. Point 40 is slightly 
outside of the specification frequency band of the video amplifier 12. The 
point 40 is approximately 5 to 10 DB down from operation within 
specification limits at the point 30. 
Operation at the point 40 results in an unacceptably illegible display. 
FIG. 3b illustrates the bit smearing phenomemon discussed above that 
contributes to the degredation in legibility. Referring to FIG. 3b, a 
binary one is commanded for display in a pixel cell 41 and a binary zero 
is commanded for display in a pixel cell 42. The video amplifier 12 does 
not possess adequate bandwidth or slew rate at the operating point 40 
(FIG. 2) to sharply turn the beam on and off within the confines of the 
cell 41. Thus the beam smears into the cell 42 which should be displaying 
a black level. The beam smearing phenomemon is further illustrated with 
respect to pixel cells 43 and 44 wherein binary ones are commanded. It is 
observed that because of the inadequate video bandwidth and slew rate, the 
binary one pixels commanded for the cells 43 and 44 smear into one another 
without adequate black level contrast therebetween. 
Because of the beam smearing phenomenon, pixels and characters run into one 
another seriously degrading the legibility of the display. Additionally a 
6 by 10 character matrix only permits a one pixel spacing between 
characters further exacerbating the degredation in legibility. A prior art 
solution to the problem would be to increase the bandwidth of the video 
amplifier 12 but this would significantly increase the cost thereof. It is 
observed by comparing FIG. 3b to FIG. 3a that the pixel cell width is 
smaller in FIG. 3b than in FIG. 3a thus permitting the additional pixels 
to be displayed on a line as discussed above with respect to operating at 
the point 40 (FIG. 2). 
A significant enhancement in legibility is effected in accordance with the 
present invention by operating at a point 50 on the video amplifier 
frequency response curve of FIG. 2. The point 50 is aproximately 10 to 15 
DB down from operation at the point 30 and is significantly outside of the 
specification bandwidth of the video amplifier 12. The point 50 is 
effected by selecting a frequency for the pixel clock 18 that results in 
operation beyond the pixel smearing point 40 of FIG. 2 and where a beam 
narrowing affect occurs so that adjacent pixels are substantially distinct 
from each other and legibility improves to a satisfactory level. 
For the system described above, a pixel clock frequency of 26.4384 MHz. is 
utilized for operation at the point 50 providing 924 pixels to the line. 
At this operating point, a character matrix of 7 by 10 pixels may be 
utilized (7 pixels wide by 10 pixels high), providing 132 characters to 
the line. The optimum point of operation is where the luminosity between 
adjacent binary one pixels decreases to the black level. FIG. 3c 
illustrates pixel generation at the operating point 50. Four adjacent 
pixels are depicted illustrating that pixel luminosity attains the black 
level at the pixel cell boundary and that adjacent binary one pixels 
attain the black level therebetween. It is noted that the pixel luminosity 
amplitude attained during operation at point 50 is less than the 
luminosity amplitude during operation at point 30 as illustrated in FIG. 
3a. It is furthermore noted that operation at point 50 generates a 
narrower pixel cell than operation at point 30. This results in the 
significant increase in the number of pixels on the raster line. 
The beam narrowing effect, which counteracts the beam smearing effect, 
occurs because the video amplifier 12 does not have the bandwidth at point 
50 and hence the slew rate to respond to the applied frequency. The 
amplifier 12 therefore prematurely turns off and at a lower amplitude than 
during normal operation. The beam narrowing effect that occurs at the 
operating point 50 permits the video amplifier 12 to be turned off to a 
dark level and turned on again for the adjacent pixel providing enhanced 
interpixel contrast. There is, however, a brightness loss which can 
readily be compensated by adjustment to the brightness circuitry 13. 
By significantly increasing the number of pixels available on the line 
while counteracting the pixel smearing effect by the beam narrowing 
effect, provides additional pixels for intercharacter spacing which 
significantly improves the legibility of the display. 
Thus it is appreciated that by operation at the point 50, adequate 
intercharacter space is available without pixel smearing thereby providing 
a display with commercially acceptable legibility, with a vastly enhanced 
character display capacity and without an increase in video channel 
bandwidth that would otherwise have been expected for the level of 
performance achieved. The 7 by 10 character matrix utilizes two pixels of 
intercharacter spacing greatly enhancing the image readability. 
Referring to FIG. 4 details of the beam narrowing affect are illustrated. 
The normal response of the monitor 10 is illustrated by solid line curve 
51. The curve 51 is the pixel generation response when operating at the 
point 30. The dotted line curves comprised of portions 52 and 53 represent 
the beam narrowing affect caused by exceeding the monitor bandwidth at the 
point 50. At this operating point, the slew rate of the video amplifier 12 
is such that the pixel intensity prematurely turns off without reaching 
the full peak of the normal operating curve 51 and returns to the black 
level before the next pixel is displayed. There is a diminution in 
brightness which is compensated by the brightness circuitry 13. 
Although the operating point 50 of FIG. 2 may be empirically obtained, as 
described above the, the relationship between the optimum operating point 
50 on the frequency response curve of the video amplifier 12 and the slew 
rate of the video amplifier 12 is readily derivable by the routineer in 
the art utilizing well known principles of electrical engineering. FIG. 4 
illustrates the optimum response where the interpixel space between the 
curves 52 and 53 just attains the black level. 
In the beam narrowing operation illustrated in FIG. 4, the video amplifier 
12 does not have sufficient time to fully saturate before turning off the 
beam prior to the generation of the next pixel. In other words, when 
operating at the point 50 a less than specified pixel size results because 
the turn on time of the video amplifier is slower due to the bandwidth 
limitations. This results in smaller, finer pixels that permit the 
significantly increased number of pixels per raster line when operating in 
accordance with the present invention. 
While the invention has been described in its preferred embodiment, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that changes may be made within the 
purview of the appended claims without departing from the true scope and 
spirit of the invention in its broader aspects.