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RELATED U.S. PATENT APPLICATIONS
U.S. Patent applications directly or indirectly related to the subject
application are as follows:
Ser. No. 836,842, filed Sept. 26, 1977, by C. L. Seitz et al, and entitled,
"A Video Synthesizer for a Digital Video Display System Employing a
Plurality of Gray-Scale Levels"; and
Ser. No. 836,747, filed Sept. 26, 1977, by C. L. Seitz et al, and entitled,
"Control Means to Provide Slow Scrolling, Positioning and Spacing in a
Digital Video Display System".
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a digital video display system and more
particularly to such a display system utilizing a plurality of gray-scale
levels so as to provide for a more natural display of character.
2. Description of the Prior Art
There are commercially available today many examples of so called "video
character displays" or "video terminals". These systems form
representations of characters on the face of a cathode ray tube where each
character is formed as a matrix of discrete points. Such a method of
representation of characters predates electronics and may be found in
embroidery, mosaics, and other disciplines employing a grid pattern. Such
characters displayed on a cathode ray tube are of inferior readability
unless the dot matrix allowed for each character is much larger than
5.times.7 or 7.times.9 which are the most commonly used matrices. Larger
matrices require the electron beam to transverse many more points on each
complete display frame, and thus require higher speed logic and cathode
ray tub deflection circuits.
Although such systems may be called "video terminals", they are without
exception incompatible with broadcast video standards and use a standard
television set or monitor in such a crude fashion that half of its spatial
and all of its gray-scale resolution is lost in forming an image.
The most important reason why designers have avoided the standard broadcast
type of video raster is that it is interlaced. By producing the complete
frame with two interlaced fields, broadcast video achieves twice the
vertical resolution than can be achieved at that scan rate without
interlace. However, dot-matrix characters appear to "flicker" when
interlaced fields are used, particularly if the great majority of dots
over any local region happen to fall within one or the other field.
Flicker is a problem when interlace is employed not only for dot-matrix
characters but also when there are very high contrast images. Thus, even
when a standard video monitor is employed as the display device for a
video terminal, it is used without interlace and is limited in its
vertical resolution. Because this resolution is inadequate to display a
large number of good quality dot-matrix characters, some systems must
employ video rasters which are incompatible with standard video monitors.
It is desirable to employ standard video monitors because of their economy
due to mass production.
Such dot-matrix raster scan video displays are disclosed for example in the
Cole U.S. Pat. No. 3,345,458. However, the concept of dot-matrix
character generation for video display goes back to 1948 as evidenced by
the Dirks U.S. Pat. No. 2,972,016.
It is, then an object of the present invention to provide a digital video
display system which provides more natural display of characters and other
information.
It is another object of the present invention to provide a digital video
display system having interlaced scan for higher resolution but which
display is "flicker" free.
It is still another object of the present invention to provide a video
digital display system that can employ standard commercial video monitors.
SUMMARY OF THE INVENTION
In order to achieve the above-described objects, the present invention
resides in a character generator with an associated video synthesizer to
convert digital codes into video images of characters to be displayed,
which images employ a plurality of different gray-scale levels or levels
of luminance to create the entire character image so as to more fully
utilize the resolution and gray-scale level of a standard video monitor.
The image code is a binary code of a sufficient number of bits to
represent the different number of levels of gray-scale or luminance values
employed. These codes are stored in a character generator storage which is
responsive to a character code which addresses the storage to retrieve the
image code which is then transferred to a video synthesizer. The video
synthesizer then generates the video analog signal.
A feature then of the present invention resides in a digital video display
system including a display monitor, a video synthesizer coupled to said
display monitor and a character generator which includes a character image
storage coupled to the video synthesizer which storage includes a
plurality of image codes representing different levels of gray-scales or
luminance to create the full image of a character to be displayed on the
display monitor.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the present
invention will become more readily apparent from a review of the following
specification when taken in conjunction with the drawings, wherein:
FIG. 1 is a representation of a dot-matrix character as employed in the
prior art;
FIG. 2 is a representation of a character image as employed in the present
invention;
FIGS. 3A and 3B are representations of the transformation of luminance
values as employed in the present invention;
FIG. 4 is a diagram of a system employing the present invention;
FIG. 5 is a diagram of the video synthesizer of the present invention;
FIG. 6 is a diagram of the display mode transform logic of the present
invention;
FIG. 7 is an example of character data mapping as employed in the present
invention;
FIG. 8 is a diagram of the digital-to-video convertor of the present
invention;
FIG. 9 is a diagram of the character generator of the present invention;
FIG. 10 is a diagram of a font storage element as employed in the present
invention;
FIG. 11 is a diagram of the control element logic of the present invention;
FIG. 12 is a representation of the "character wheel" of the present
invention; and
FIG. 13 is a schematic diagram of the timing system of the present
invention.
GENERAL DESCRIPTION OF THE SYSTEM
For video representation of an image, a signal must be provided which
produces on a video monitor an approximation of that image which is close
enough to what is perceived by the human eye. A video camera will convert
the image to a signal for the display of natural scenes. For synthetic
images, such as character or graphic displays, a synthetic video signal is
produced electronically according to information stored within or
delivered to the electronic system.
The usual analog video art requires that two discrete approximations be
made to present an approximation to a natural scene. The scene is composed
of a sequence of discrete frames and each frame is composed first of a
sequence of discrete scan lines. As the scan lines are traversed in
sequence, the time-varying video signal is a representation of the
luminance of the corresponding point in the scan. The justification for
the discrete frames is the unconscious reconstruction of motion by the
human observer and the justification for discrete scan lines is the
limited resolution of the human eye.
For the display of digitally synthesized video images such as character and
graphic displays, two additional discrete approximations are made. Each
scan line is composed of a sequence of discrete picture elements (pixels).
Each pixel has a luminance value which in the present invention is
approximated by one of a number of discrete values according to a sequence
of binary digits. The justification for the discrete approximation of the
pixel is the limited resolution of the eye and equates through the
sampling theorem to the bandwidth limit in an analog video signal. The
justification for the discrete approximation of the luminance value is the
limited ability of the eye to discriminate luminance when a sufficient
number of levels of approximation are employed.
According to these additional approximations, an image may be represented
as an array of values, either discrete or continuous. If these values are
extracted from digital storage and produced in the order and rate
corresponding to the adopted scanning order and rate, these values may be
converted to an analog video signal conforming to some adopted standard
and the image may be displayed using a standard video monitor.
In prior art video terminals, the character codes to be represented on the
display screen are stored in a memory in the order in which they are to be
displayed, each code representing a particular character. The code is
fetched from the memory in synchronism with the raster scan and causes a
character generator to generate the appropriate dot signals during that
raster scan as described in the above referenced Cole patent. An example
of the dot matrix type character formed in such a system is illustrated in
FIG. 1. If a standard video monitor is employed, as the display unit, then
the video terminal must contain the appropriate character store in a video
synthesizer outside of the display unit.
As distinct from the dot matrix type of character of the prior art, the
present invention is adapted to employ the full resolution and gray-scale
of the standard video monitor. The synthetic images representing the
characters to be displayed are stored outside of the standard video
monitor and activate a video synthesizer to provide the appropriate
signals to create an entire picture of the character during its display.
An artistic rendition of a character as displayed by the present invention
is illustrated in FIG. 2 for comparison with the dot matrix type character
of FIG. 1.
Before generally describing the system of the present invention, the
description will be provided of the nature of the character images and how
they are formed. As was explained above, the present invention utilizes
the full resolution and gray-scale of a standard video monitor. Thus, the
character image display employs all of the picture elements in the
particular character space. In the present invention, eight levels of
gray-scale are employed corresponding to eight different levels of
luminance which eight states can be accomodated by a three-bit number in
binary code. In addition, the eight different levels are directly
proportional to the luminance to be displayed rather than to the signal
voltage, which is nominally the 2.2 root of the luminance. Eight discrete
levels were experimently determined to be the least power-of-two number of
levels sufficient to represent characters without any apparent jaggedness.
More or less levels might be employed depending on the quality desired,
character scale, display device, or other variables.
FIGS. 3A and 3B illustrate the manner in which the eight levels of
luminance are employed in the present invention to represent a given
character, in this case "A". The process by which the table of numbers is
derived from the character image can be mechanized in many ways by optical
and electro-optical systems. The simplest method conceptually is to
digitize to eight levels the signal output of a video camera scanning the
desired character at the desired scale. Unlike the dot-matrix character,
which is "composed" as a table of binary values; the fully-formed
character is a stored image of a symbol composed without reference to the
underlying grid of picture elements. A fully-formed character font may
either be designed or any existing printing or typewriter font may be
employed. The individual characters are each captured and processed to
form representations such as that of FIG. 3B.
Optimum results can be achieved only with great difficulty by digitizing
the output of a video camera, because no camera has an ideal physical
aperture. If the spot size of the camera is too small compared to the scan
line spacing, the video image will contain higher spatial frequencies than
sampling considerations allow. Conversely, if the spot size of the camera
is too large, the effective aperture is non-isotropic depending on
scanning direction, and the video image will be at too low a resolution.
In the present invention, the procedure employed is one in which the image
of a character is captured at approximately ten times the resolution at
which it will appear on the video display, and this high resolution image
is processed digitally in order to simulate the effect of an ideal
scanning aperture. Because the aperture itself is a spatial filter, the
process of computing the luminance values to represent the character image
limits the spatial frequencies approximately to those limits required by
the sampling theorem. The proper luminance value for each picture element
is computed by weighing luminance contributions from an area centered on
the picture element. The weighing function is exactly like a physical
aperture. The weighted sum of the luminance contributions from the area
centered on the picture element whose value is to be determined is rounded
to eight (or some chosen number of) levels.
There are many choices of a weighing function, or aperture, that will
produce satisfactory results. A function that is unity within the picture
element area, d by d, and zero elsewhere, is the simplest weighing
function from the computational standpoint. Somewhat better results can be
achieved by using a triangular weighing function with a base of 2d, which
is equivalent to convolving the picture element aperture with itself.
(Other possibilities will occur to those skilled in the art.) The
displayed image shown in FIG. 3B subtends 7 scan lines with eight gray
levels, and the stored image was derived from a computation based on the
picture element aperture.
The images produced on a video monitor by the synthetic video signal
derived from such stored character images are technically
indistinguishable from those that would be produced by an ideal video
camera pointed at a page of text. Accordingly, the video medium is used as
it was intended to be used, and "flicker" due to excessive contrast edges
with the interlaced display is absent.
The CRT spot of the video monitor employed is a physical aperture whose
size and shape determine a spatial filter exactly like the abstract
apertures used to compute luminances. In practice, the spot is not exactly
uniform at all points in the raster, but the spot size can be adjusted
generally to minimize the scan line texture, and this adjustment produces
a suitable aperture for constructing the image from sampled data picture
elements. It is an economically important property of the fully-formed
symbol technique that the very small and consistent spot size sought for
dot-matrix characters is neither necessary nor desirable. A spot in which
about 75% of the energy emits from a region of diameter d is easily
achieved in commercial video monitors and close to ideal for the
application of the present invention.
The organization of a system employing the present invention is illustrated
in FIG. 4. As shown therein, stored information structures are fetched
from information storage 10 by character display processor 11 which
basically controls the communication between storage 10 and the display
system of the present invention. Character codes are transferred to
character buffer 12 for reasons that will be more thoroughtly described
below. Character codes then drive character generator 13 in which are
stored the character font signals. These character font signals are mixed
by video synthesizer 14 with other signals such as synchronization signals
to provide the composite video signals supplied to standard video monitor
15. Image display processor 16 and image buffer 17 are provided in
parallel with character display processor 11 and character buffer 12 when
it is desired to superimpose character text upon some other image. Thus,
the system of FIG. 4 accomodates modular inclusion of additional
facilities as dictated by any specific application.
Video monitor 15 is the transducer which converts a composite video signal
to an image that can be viewed. Because the composite video signal
conforms to some standard, other video devices and systems may be used in
conjunction with this signal such as video tape to store the signal, cable
television or a transmitter or television receiver to convey the image to
another location and various video devices to process the signal.
Video synthesizer 14 is a sampled data signal synthesizer which converts
and combines the digital image specification and synchronizing signals to
the single composite video signal. This hybrid digital-analog system
operates from a clock and system timing unit 18 which clock corresponds to
the picture element rate that in the present invention is 12.3 MHz. The
derivation of the timing signals are more thoroughly described below. The
cycle time of video synthesizer 14 which in the present invention is 81
nanoseconds, places a practical limit on the complexity of the functions
that can be performed even though relatively high-speed logic is employed.
However, the image combining operations in various modes and output
conversion are pipe-lined through a series of intermediate registers
containing partial results so that a very high data throughput is
maintained. Because the timing cycle is typically too small for the
sources of image point data, the data is typically provided with a number
of image points in parallel and parallel to serial conversion is performed
prior to interfacing with video synthesizer 14.
The character generator 13 contains the stored image of each character that
may be displayed. It is particularly effective to organize this storage so
that the image points across the scan lines are read in parallel, selected
by the character code and vertical position information, and converted to
serial form for input to video synthesizer 14. Either fixed or variable
horizontal pitch characters may be employed.
Character buffer 12 allows the lowest data rate from the processor and
consists of two or more interchangeable shift register type buffers in
which the display is refreshed from one buffer while the processor fills
another buffer with the next character line. Each shift register type
buffer contains character code and possibly some display mode information.
The refresh process generally requires multiple cycles through the buffer
corresponding to the multiple sequential scan lines for each character
line. The loading of the next buffer shift register should be complete by
the time the display of the character line is complete so that the control
of the respective shift register buffer may be switched.
Character display processor 11 is a micro-processor-based system whose
primary role is to move text strings from the stored information structure
of storage 10 to character buffer 12. In the implementation of the present
system, display processor 11 is a programmable device which, by its power
to compute and extract information from a complex information structure,
assists in the formating of the text display and provides character line
parameters such as the height and origin position of the character line, a
mask that is useful in scrolling text smoothly, mode information, blink
information and the control of the vertical synchronization.
As was indicated above, the digital video characters generated by the
character display processor 11, character buffer 12 and character
generator 13 may be combined in video synthesizer 14 with other digital
video images. The general combining rule allows formating of the
full-screen display either by text codes which key other video sources or
by luminance codes which cause character or symbol display to be overlayed
or superimposed.
Some of the characteristics of the system of FIG. 4 and variations therein
will now be discussed. Since the video medium must be refreshed at its
characteristic field and frame rate, and in raster order, the system must
produce the image repetitively and in raster order. The repetitions
required to refresh the display are to be at least partly independent so
that changes in the information structure will be reflected immediately in
the displayed image. One of the benefits of such a display system as
opposed to hard copy is the ability to change the display in accordance
with the users interaction.
If all of the display information is not extracted from the information
structure on each frame or field, the refresh function occurs from
character buffer 12, (or image buffer 17 as the case may be). In this
situation, it is not important whether information is provided from
display processor 11 in raster order as buffer 12 serves also to implement
a scan conversion function. This variation requires a large buffer,
nominally in which each buffer location corresponds to a character
location or to a picture element, which buffer can be modified
selectively.
If all of the display information is extracted from the information
structure on each frame or field, the refresh operation may be regarded as
occurring directly from the information structure. If the information
provided from the display processor 11 from the information structure is
necessarily uncorrelated from raster order, buffer 12 may be used to
perform a scan conversion process. If the information provided from the
display processor 11 from the information structure is even approximately
in raster order, buffer 12 need only be a small buffer whose size depends
on the most out-of-raster-order elements and speed matching
considerations.
Certain types of displays of characters and symbols have very wide
application and can be accomodated by raster order production, so long as
the character codes are provided from display processor 11 in the order in
which the character first appear in the raster. In this case, buffer 12
will store character codes in order to repeat the character sequence
across the multiple scan lines that subtend each character line. An
alternative is to fetch the text sequence from the information structure
repetitively.
Character and symbol display are of such wide applicability as to be
required for almost any system display. For example, a system for banking
applications might well include special facilities for displaying
facsimiles of financial documents, signatures, or the like; but would
certainly require character display for the control information related to
a transaction. Similarly, engineering applications may require stylized
drawings to be displayed, that will certainly require character display
for legends.
DETAILED DESCRIPTION OF THE SYSTEM
Video synthesizer 14 of FIG. 4 is illustrated in FIG. 5 and includes
display mode transform logic 20 which receives character data as well as
the character mode signal and cursor signal for combination with other
image data, which combined data is then transferred to digital-to-video
converter 21 which converts the data to the analog video signal in
combination with the composite sync and composite blanking signals in
order to produce the composite video signal. The monochrome composite
video signal produced thereby may be optionally combined at output summing
amplifier 22 with a chroma signal as will be more thoroughly explained
below. The synthesizer must operate entirely at the pixel clock rate with
new data appearing at each clock cycle. However, this data is irrelevant
during blanking periods. Data input appears one clock time early in order
to allow for pipelining.
Display mode transform logis 20 of FIG. 5 is illustrated in FIG. 6 and
includes read-only storage 23 that is addressed by the incoming character
data, optional image data, mode signals and cursor. Read-only storage 23
serves to perform the various functions by mapping the 3-bit character
data to the 4-bit output data in the various ways required. Thus,
characters may be displayed in different display modes according to user
preference or to highlight an area of particular interest. Text displays
also benefit from a cursor which can high-light a character in a text
string to indicate visually the location of the text editing operation.
Typical text modes are: white-on-black, black-on-white, half-bright,
white-on-gray, black-on-gray. For each of these modes it is necessary to
have a contrasting cursor.
In addition, the text may be made to be invisible in order to hide
classified information such as passwords. The text may also be made to
blink between invisible and visible to draw attention to a particular
item. This is accomplished by the receipt of blink rate and blink on
signals by AND gate 25 the output of which is supplied along with the
invisible signal to NOR gate 26. The output from NOR gate 26 is then
"ANDED" with respective character data bits supplied to read only storage
23. Specifically, the input network consisting of AND gate 25 and NOR gate
26 treats the various cases of invisible or blinking characters by forcing
the character data input to 000 when appropriate.
FIG. 7 is a table of some of the various transformations that may be
accomplished by reading out a portion of the contents of read only storage
23 of FIG. 6. In this table, 15 output levels (0-14) are used in order to
have a middle level of 7. The effect of the transformation is to scale the
character data by either -2, -1, +1, or +2 and to offset it. This is done
in order to perform the transformations readily and to use the same
character data to display light-on-dark as dark-on-light which requires
that the internal representation as read out of read-only storage 23 be
linear to the luminance required rather than its 2.2 root and the
digital-to-video converter thus must perform the gamma correction.
By use of read-only storage 23, the display mode transform logic of FIG. 6
provides for any transformation desired. It may also be used to mix
digital video signals from different sources. For example, in one mode,
characters can be overlayed in white on an image background. For each
luminance value of the image, the character value is linearly scaled to
the proportionate luminance between the image luminance and the brightest
value. Similarly, mode information can be used to key the image, or
particular images values can be used to key to text thus giving complete
and general control over the full screening formating.
The output signals of read-out storage 23 are then supplied to register 24
from which they are then clocked at the next pixel clock time to the
digital-to-video converter.
The digital-to-video converter 21 of FIG. 4 is illustrated in more detail
in FIG. 8. This converter is a digital-to-analog converter which must be
fast enough to operate at the pixel clock rate and also to have a
non-linear output in order to correct for the video gamma when the data
represents luminance rather than the output voltage. Furthermore, its
output must be free of switching transients which are inevitable in the
ladder network type digital-to-analog converter. These transients would
typically occur in the transition from values such as 0111 to 1000 in
which case the signals driving the resistive conversion network are
changing in different directions and at slightly different times.
The binary input signals received from the transform logic of FIG. 6 are
converted through a combinational mapping implemented in read only storage
27 to a unary code. This mapping converts four input signals to 15
different outputs in the following manner. The input 0000 produces an
output of all zeros. The input 0001 causes just the first of the fifteen
outputs of become 1. The input 0010 causes the first two of the fifteen
outputs to become 1, and so on until the input 1111 causes all fifteen
output to be 1. This code has the property that the transition between any
two input combinations can only cause a group of adjacent outputs to
change in the same direction. Thus, when these outputs are weighted and
summed through resistive network, after being reclocked, the analog output
changes monotonically from one value to another without switching
transients.
The resistance network is illustrated in FIG. 8 as being formed of the
plurality of resistors 29 which are of appropriately varying resistance
values. The 16 output levels produced thereby are spaced by 15 steps, and
the conductances of each of the fifteen weighting resistors 29 is
proportional to the size of that step. If all of the conductance were the
same, the output would be linear with the data input. However, any
positive monotonic non-linear function can be produced just as easily by
making each conductance proportional to the corresponding step size. Thus,
the resistance values may be calculated to achieve correction for whatever
value of gamma is expected.
As has been indicated above, the voltage representation of luminance in a
video signal is not linear. Instead, according to U.S. Standards,
luminance is approximately the 2.2 power of the voltage. Conversely, the
voltage is approximately 1/2.2 power (2.2 root) of the luminance. In FIG.
8, the values of the respected resistances 29 are calculated accordingly.
The values of the respective resistances are as follows:
Bit position 1--475 ohms
Bit position 2--953 ohms
Bit position 3--1180 ohms
Bit position 4--1330 ohms
Bit position 5--1500 ohms
Bit position 6--1620 ohms
Bit position 7--1740 ohms
Bit position 8--1870 ohms
Bit position 9--1960 ohms
Bit position 10--2050 ohms
Bit position 11--2150 ohms
Bit position 12--2210 ohms
Bit position 13--2260 ohms
Bit position 14--2370 ohms
Bit position 15--2430 ohms
These values were calculated with the grounded load resistance being 51.1
ohms. The composite sync resistance is 221 ohms and the composite blank
resistance is 1210 ohms. These values were calculated for a voltage output
ranging from a minimum from 0.8 volts to 2 volts. The 15 resistances
provided 15 intervals between 16 output signals ranging from 0 volts to 2
volts.
The advantages of having voltage output as the 2.2 root of luminance is
that it allows the transmitter to compensate for a non-linearity inherit
in the receiver monitor. Secondly, the root function, being a good
approximation of the logarithmic response of the eye, has the advantage
that noise of particular amplitude has approximately the same subjective
effect when superimposed on dark and light areas. If a linear relationship
existed between luminance and voltage, noise voltage would be more evident
in the dark display than in the light areas of the image. This technique
also greatly simplifies the computational transformation in regard to
scaling, rotation, and arbitrary positioning of synthetic figures.
Another advantage of this type of converter resistance values is that the
resistance values need only be as precise as the steps size dictates.
Typically 5% resistance tolerances will be more than adequate. The
switches likewise need not be particularly precise. In fact, the flip flop
outputs of register 28 will generally serve as voltage sources to drive
the weighting and summing networks. Accordingly, the converter is easily
made fast.
In general, the conversion techniques discussed above uses more components
than the ladder network type of converter. However, components of much
lower precision can be employed and the present conversion technique is
well suited for integrated circuit implementation. The composite sync and
blanking signals are weighted into the summing network in the same way as
the data. In addition, the presence of the blanking signal gates the data
input to 0 in order to reject any spurrious information on the data input
during blanking periods.
Character generator 13 of FIG. 4 is illustrated in more detail in FIG. 9.
This generator includes, for illustration, from 1 to 8 font storage
elements 30 where each storage element stores the image of 32 characters
for a total of 256 possible characters. The generator also includes
decoder 31 to select one font storage element according to the incoming
character code and control element 32 that will be more thoroughly
described below.
The various font storage elements 30 of FIG. 9 are shown in more detail in
FIG. 10. A digital video character is produced on character data bus 41 in
response to the receipt of the character code and vertical position data.
This information together forms the address to the respective read-only
storage elements 33. The image data for each character has been computed
by techniques explained above and this data stored in the three similar
read-only storage sections 33 by the usual mark process. It will be
understood that read-write storages may be employed with provision for
loading the contents from the character display processor of FIG. 4. When
a load signal is produced by AND gate 39 of FIG. 10, the data from
respective read-only storage elements 33 are transferred synchronously
into their corresponding shift registers 34 from where they are read out
in a serial digital video data sequence to the video synthesizer 14 of
FIG. 4 as was described above.
The appearance of a start signal to an enabled storage element also sets
the internal select flip flop 37 and the storage element will remain
selected until another start signal is received with enable logical zero.
The signal input to the parallel to serial conversion shift registers 34
is a logical zero and the selected storage element will simply output
zeros to the character data bus 41 after the prescribed number of picture
elements have been produced from the corresponding storage element. In
addition, the control may disable the output at times in order to overlay
an underline, crossout text, or cursor symbol.
For variable - pitch characters, it is most natural to store the character
width in the same element as the character image. For variable pitch
characters, optional read-only storage element 35 may be employed which
contains 32 four-bit words which are masked to contain the character width
as a prescribed number of picture elements. In this case, when an enabled
element is started, the width of the selected character is transferred
synchronously into a binary down counter 36 and the underflow signal from
this counter is gated onto a bus line which returns to the control to
indicate the completion of the current character.
Control element 32 of FIG. 9 is illustrated in more detail in FIG. 11. The
control element receives the parameters required to control such features
as vertical format slow scrolling, underlining, or crossout text. To this
end, initial position and count parameters are received respectively by
counting registers 42 and 43. Mask counter 44 receives the mask parameter.
These registers are driven by a systems clock. The function of these
parameters will be more thoroughly described below. In addition, the
control element is provided with a decoder read-only store 48 which
contains the data for underlining such as required. The last line and
field parameters from miscellaneous register 45 are used to control
vertical sync counter 46 that drives decoder read-only store 47 to supply
a vertical sync and vertical blinking signals.
Slow scrolling, positioning and spacing of the text within any scan line
pair will now be described. The stored image is placed within a larger
address space as illustrated graphically in FIG. 12. The addresses are
allowed to "wrap around", i.e., modulus 256. For this reason, the address
space is referred to as the "character wheel". Because of the interlaced
scan, the lowest-order binary digit of the address to the character wheel
is the signal called FIELD and this indicates whether the upper or lower
scan line in the address scan line pair is used for display.
The manner in which slow scrolling or smooth scrolling is achieved will now
be more specifically described in relation to FIGS. 11 and 12. The
location of the character in the character wheel of FIG. 12 is defined by
the three parameters supplied to the control element of FIG. 11. As
indicated above, these parameters are the initial position, the count and
the mask. These parameters are loaded into the respective registers of the
control element at the beginning of each character line. By adjusting
these parameters, the initial scan lines of the character line can be
deleted with the remaining scan lines automatically moving up with
additional scan lines being added at the bottom of the character space and
the character appears to move up. Similarly, the characters can be made to
appear to move or scroll downwardly.
The function of the mask parameter is to mask out all portions of the
character wheel except that portion being displayed which, in the case of
FIG. 12 is each scan line pairs 0-7. Thus, as the character is moved
upwardly in the character line, it will appear to disappear behind the
barrier. At the same time, in the previous character line, a different
character wheel employed therefor as defined by its own parameters will
give the appearance of the character moving up from behind the barrier.
Thus, the character can be made to appear to slowly move upward from
character line to character line in a very smooth fashion. Conversely, the
character can be made to appear to move downward in the same slow smooth
manner. This is accomplished because position register 42, count register
43, and mask register 44 are loaded at the beginning of each character
line with the actual location of the character line being determined by
character display processor 11 of FIG. 4 which fetches the respective
character codes and their corresponding parameters from storage 10 in a
continuous manner. Thus, there is no need to have another counter in the
system to count out vertical synchronization since it is the number of
times per field that the parameter registers are loaded that determines
how many character lines there are plus one more to do the vertical
synchronization.
In FIG. 11, mask register 44 need only contain 4-bits for the individual 15
scan lines of the 8 scan line pairs which make up the character frame.
There is no harm in the mask parameter being periodic modulus 16. When the
high order bit of the mask is zero, it allows the character to be visible
through the mask. When the high order bit is one, the character is
invisible as this causes an output disable signal to be generated as
indicated in FIG. 11.
Slow, smooth scrolling can be achieved without use of the mask register and
operation by manipulation only of the initial values of the position and
count registers at the first and last character lines in a scrolling
segment. However, it is possible in this case to display only a single or
very small number of scan lines in a contracting character line, and the
small number of scan lines may consume so short a period of time that the
processor may be unable to complete filling the next character buffer
before it is required for display. Accordingly, if smooth scrolling is to
be performed without use of the mask register, the character buffer must
have the capacity to store three or more character lines, and the
processor must be able to replenish this speed-matching buffer more
rapidly than display depletes the buffer.
As was indicated above, the two fundamental parameters which control the
format of the display are the initial position on the character wheel and
the count of the number of scan lines to be produced. For example,
starting at address zero with a count of eight scan lines will result in
the character lines being spaced vertically by their normal minimum
separation analogous to single spacing on a typewriter. Starting at
address 252 with a count of 16 display lines, lines 252 through line 11,
is similar to double-spaced typing. Display of lines 254 through line 9
with a count of 12 scan line pairs along the character wheel is similar to
one-and-one-half spacing on a typewriter.
The same functional mechanism is employed to provide a program-controlled
vertical synchronization. For example, 480 of 483 possible visible scan
lines for text display might be employed in a format in which 240 scan
line pairs are used to produce 30 lines of single spaced text with a count
parameter of eight. The vertical synchronization intervals will consist of
either 22 or 23 scan lines depending upon which field is completed. In
this vertical synchronization mode, the display is blanked (vertical
blanking) and the vertical synchronization signal characteristic of the
particular video standard is produced beginning either one or two half
scan lines later depending on the field completed.
This technique avoids the necessity of having a scan line counter and yet
allows program-controlled vertical synchronization to accomodate, with the
same instrument, the multiple scan standards employed.
In the operation of the character generator control, the control element
receives directly from the display processor, the initial value and count
parameters stored in the respective counting registers 42 and 43 and
supplies vertical position information to the respective font storage
elements 30 of FIG. 9. Control element 32 of that figure detects when the
character wheel address is within the active segment, nominally position
0-7 of the character wheel and initiates each character and advances the
character line buffer one position for the next character. The control | | |
