Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
- This invention relates to a digital Yideo display
s~stem and more particularly to such a display system
having a ~ideo synthesizer providing a plurality of
- gray-scale le~els for a more natural display of charac-
ters.
Descri~tion o t_e Prior Art
There are commercially avallable today many examp-
les of so called "video character displays" or "video
terminals"~ These systems form representations of
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characters on the ~ace o~ a cathode ray tube where each
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character i5 ormed as a matrix of discrete points. Such
a method o~ representation of characters predates elect-
ronics and may he found in embroider~, mosaics, and other
disciplines employing a grid pattern. Such characters
displayed on a cathode ray tube are of inferior readabil-
ity unless the do~ matrix allowed for each character
is much larger than 5 x 7 or 7 x 9 which are the most
commonly used matrices. har~er matrices require the
electron beam to transverse many more points on each
complete display frame, and thus require higher speed
logic and cathode ray tube deflection circuits.
Although such systems may be called "video termin
als'~, they are without exception incompatible with
broadcast vid o standards and use a standard television
set or monitor in such a crude ~ashion that half of its
spatial and all of its gray-scale resolution is lost in
onming an image.
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The most impo~tant reason why designers have
a~oided the standard ~roadcast type o video raster i5
that`it is interlaced. B~ producing the complete frame
with two interlaced ~ield~, broadcast ~ideo achieves
twice the vertica]. resoLution than can be achieved at
I that scan rate without interlace. However, dot-matrix
charac~ers appear to "~licker" when interlaced fields
are used, particularl~ if the great majority o dots
o~er any local region happen to fall within on~ or the
other ~.ield. Flicker is a problem when interlace is
employed not only for dot-matrix charactels but also
when there are ~ery high contrast images. Thus, even
uhen a standard ~ideo mOnitor is employed as the display
de~ice for a ~ideo terminal, it is used without inter-
lace and is limited in its ~ertical resolution. Be-
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cause this resoIution is inadequate to display a large
number of good quality dot-matrix characters, some
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systems must employ ~ideo rasters which are incompatible
with ~tandard ~ideo monitors. It is desirable to employ
standard video monitors because o their economy due to
mass production.
Such dot-matrix raster scan video displays are
disclosed for example in U.S. Patent No. 3,345,458
(October 3t 1967 ~ D.A Cole et al~ ~owe~er, the
concept o~ dot-matrix character generation for video
display goes bac~ to 1948 as evidence by the U.S. Patent
2,972,016 t~ebruary 14, 1961 - Gerald Dirks).
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It is, then, an object of the present invention to
provide in at least olle embodiment a digital video display
system which provides more natural display of charact~rs and
other information.
It is ano~ler object o~ the present invention to
provide in at least one e ~ociiment a digital video disL~lay
syst:m having interlaced scan for higher resolution but
whicl display is "flickerl' free.
It is still another object of ~he present invention
to provide in at least one embodiment a video digital display
sys~em that can employ standard commercial video monit~rs.
Accordingly, the present invention provides a vid~o
synt~lesi2er for a digital video display system, said video
syn~lesizer comprising: a summing network to provide in
sequence a plurality of different voltage levels which levels
~ are selected from three or more such levels to designate
; different luminance values to be displayed, said su.~ning
network having a pluralit~ of different input leads, said
summlng network including a plurality of resistances
coupled between said respective input leads and a common
connection, said resistances having dif~erent resistance
values to provide to said co.~non connection different
- ~ voltage levels which increase proportionately to a root
of the luminance to be displayed; a storage device having a
plurality o~ output leads coupled to respective ones of
said summing network input leads, said storage device stoxing
different monadic operation codes to activate di~ferenl
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combinations of said input leads, and means coupled to said
- storage device to transmit a binary code to said stora~e
device which blnary code desi~nates a monadic operation code
to select the particular luminance to be displayed.
: An embodiment of the pr~QsQnt invention will no~ be
described, by way of example, \lith reference to the accompa-
nying drawings in which:-
Figure 1 is a representation of a ~o~-matri~ character
as em~loyed in the prior art;
Figure 2 is a representation of a character imaye as
emplo~ed in the present embodi.ment;
- - . .'~igures 3A and 3B are representaticns of the transfor-
mation of luminance values as employed in the present
~ . embodJ' ment;
-` E~igure 4 is a diagram of a system employiny the present
.`~. embodiment;
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Figure S is a diagram of the video synthesizer of the
~ present embodiment;
.. ~ Figure 6 is a diagram o the display mode transform
logic of the present embodiment;
Figure 7 is an example of character data mapping as
employed ln the present embodiment;
Figure ~ is a diagram of the digital-to-~ideo convertor
of the present embodiment;
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-: Figure 9 is a diagram o~ the character generator of
the present em~odiment;
Figure 10 is a diagxam o a front storage element
asemployed in the present embodimen~;
Figure 11 is a diagram of the control element logic
of the present embodiment;
Figure 12 is a representation of ~he "character
wheel" of the present embocliment; and
~ igure 13 is a schematic diagram of the timing
system of the present embodiment.
GENERAL DESCRIPTION O~ THE SYSTEM
~ or video repr sentation o~ an imaye, a signal must
- ~ be provided which producës on a video monitor an approx-
imation of that image which is close enough to what is
perceived by the human eye. A video camera will convert
- the image to à signal for the display of natural scenes
For synthetic images, suc~ as character or graphic dis-
- ~ plays, a synthetic video signal is produced electroni
cally 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 approxima-
tion 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
~iscrete scan lines is the limited resolution of the
human eye. ~
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For the displa~ o~ di~itally s~nthesized video
images such as charactex and graphic displays, two
additional discrete approximations are made. Each scan
line is composed of a saquence of discrete picture
elements (pixels). Each pix81 has a luminance value
which in the present invention is approximated by one
of a number o discrete values according to a sequence
of binary digits~ The justification for the discrete
- approximation of the pixeI 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 o~ the
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luminance ~alue is the limited ability of the eye to
discriminate luminance when a sufficient number of levels
of approximation are emplo~ed.
According to these additional approximations, an
image may be represented as an array of values, either
- discrete or continuous. If these values are extracted
~rom di~ital 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 ~ideo monitor.
In prior art video terminals, the chararter 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
ex~mple of the dot matrix type character formed in such
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; a system is illustrated in Figure 1. If a standard video
monitor is employed,as the displa~ unit, then the video
terminal must contain the appropriate charactex store in
a video synthesizer outside of the display unit.
As distinct from the dot matrix type o~ 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 rapresenting the
characters to be displayed are stored outside of the
standard video monitor and activate a video synthesi~er
~ to provide the appropriate signals to create an entire
- picture of the character durin~ its display. An artistic
rendition of a character as displayed by the present
invention is illustrated in Fi~ure 2 for comparison with
the dot matrix type character o~ Pigure 1.
Before generally describing the system of the
, ~ ; present embodiment, the descrip~ion will be provided of
the nature of the chaxacter images and how they are form-
ed. As was explained above, the present invention utiliz-
es 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 o~ gray-scale are
employed correspondlng to eight different levels o~
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
oltage, which is nominall~ the 2.2 root of the lumin-
i 30 ance. Eight discrete levels were experimently detexmin-
¦ ed to be the least power-of-two number of levels suffi-
cient to represent characters ~ithout any apparant
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jag~edness. More or less le~els might be employed de-
pending on the ~ualit~ desired, character scale r display
device, or other variables.
- Figures 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
'IA". The process by which the table of numbers is deriv-
ed from the character ima~e can,be mechanized in many
ways by optical and electro-optical systems. The
simplest method conceptually is to diyitize to eight
levels the signal output of a video camera scanning the
desired character at the de~ired scale. Unlike the dbt~
matrix character, which is "composed" ag a table of binary
values; the fully-foxmed character is a stored image of a
s~m~ol composed without re~erence to the underlying grid
~- of picture elemPnts. 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
` 20 of Figure 3B.
Optimum results can ~e achieved only with great
difficulty by digitizing the output of a video camera,
because no camera has an ideal physical aperture. I~ the
spot size of the camera i5 too small compared to the scan
line spacing, the video image will contain higher spatial
frequencies than sam~ling considerations allow. Convers-
ely, if the spot ~ize of the camera is too large, the
e~fecti~e aperture is non-isotropic depending on scanning
direction, and the video image will be at too low a
resolution~
In the present embodiment, the procedure employed
is one in which the image of a character is captured
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at approximatel~ ten times the resolution at which it~
` will appear on the video display, and this high resolution
image is pro~essed digitally in orde~",,to simulate the
e~ec~ o~ an ideal scanning aperture. Because the aper-
ture itself is a spatial ~ilter, the process of computing
' ~he 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 alement is computed by
weighing luminance contributions ~rom an area centered on
- the picture element. The weig~ y function is exactly
like a physical aperture. The weighted sum of the lumin-
ance contributions from the area centered on'the picture
element whose value is to be determined is rounded to
~ - ei~ht ~or some chosen number of) levels.
,' ~here are many choices of a weighing ~unction, or
'~ aperture, that will produce satisfactory results. A
function that is unity within the picture element area, -
I d by d, and ~ero el~ewhere, is the simplest weighing
~unction from the computational standpoint. Somewhat
better results can be achieved by using a triangular
'weighingfunction with a base of 2d, which is equivalent
to convolving the picture element aperture with itself.
; ~Other possibilities will occur to those skillad in the
ar~ . The dlsplayed image shown in Figure 3B subtends 7
scan lines with eight gray levels, and the stored image
was derived ~rom a computation based on the picture
element aperture~
Tha images produced on a video monitor by the
~ 30 s~nthetic video signal derived ~rom such stored character
- ima~es are technically indistinguishable ~rom those that
would be produced by an ideal video camera pointed at a
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page of text. ~cco~dingly, the video medium is used as
it was intended to ~e used, and "flickerl' due to excess-
i~e c~ntrast edges ~ith the interlaced display is absent.
The CRT spot o~ the video monitor employed is a
physical aperture who~e 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 point~ in the raster, bu~ ~he spot size can be
-` adjusted generally to minimize the scan line texture, and
this adjustment produces a suitablè aperture ~or construct-
in~ the im~ge from sampled data ~icture elements. It is
~n economically important property of the fully-formed
symbol technique that the very small and consistent spot
i size sought for dot-matxix characters is neither necess-
ary nor desirable. A spot in which about 75% of the - -
energ~ emits from a region of dlameter d is easily achiev~
ed in commercial video monitors and close to ideal for
` the application of the present invention.
The organization of a system employing the present
em~odiment is illustrated in Figure 4~ As shown therein,
stored information structures are fetched from informa-
tion storage 10 by character display processor 11 which
basically controls the communication between storage 10
and the display system of the present invention. Charac-
ter codes are transferred to character ~uffer 12 for
~easons that ~ill be more thoroughly described below.
Char~cter codes then drive character generator 13 in
- ~hich are stored the character font signals. These
character font signals are mixed by ~ideo synthesizer
3b 14 with other signals such as synchronization signals to
pro~ide the composite video signals supplied to standard
video monitor 15. Image display processor 16 and image
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- buffer 17 are ~rovided in pa~allel with character display
processor 11 and chaxact~r buffer 1~ when it is desired
~o superimpose character text upon some'other image.
Thus, the system of Fi~ure 4 accomodates modular inclu-
sion o~ additional ~acilities as dictated by any specific
application.
~ideo monitox 15 is the transducer which conver s
a composite video signal to an image that can be viewed.
Becau~e the composite video signal corlorms to some
standard, other ~ideo devices and systems may be used in
conjunction wlth this signal such as video tape to store
the signal, cable tele~ision or a transmitter or tele-
` ~ision receiver to conve~ the image to another location
; and various video devices to process the signal.
Video synthesizer 14 is a s~mpled data signal
s~nthesizer ~hich converts and combines the digital image
specification and synchronizin~ signals to the single
composite ~ideo signal. This hybrid digital-analog
; ' system operates ~rom a clock and system timing unit 18
which clock corresponds to the picture element rate that
in the present in~ention is 12.3 MHz. The derivation of
the timing signals are mo~e thoroughly described ~elow.
The cycle time o~ ~ideo synthesizer,l4 which in the
present invention is 81 nanoseconds r places a practical
limit on the complexity of the functions that can be
per~ormed even though relatively high-speed logic is
employed. Howe~er, the image combining operations in
various modes and output conversion are pipe-lined
through a series ofintermediate registers containing
- ~ 30 partial results so that a ~ery high data thxoughput is
mai-ntained. Because the timing cycle is typically too
small for the sources of image point data, the data is
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- ` typically provided ~ith a number of image points in
p~rallel and parallel to serial conversion is performed
pxior to interfacing ~ith ~ideo synthesizer 14.
The character generatox 13 contains the stored image
of each character that may be displayed4 It is particular
ly effecti~e to oxganize this storage so that the image
points across the scan lines ara read in parallel, select-
ed by the character code and vertical position information,
and converted to serial ~orm for ir1put to video synthesi-
zer 14. Either fixed or variable horizontal pitch charac-
ters may be employed.
Character buffer 12 al1Ows the lowest data rate
from the processox and consists of two or more inter-
changeable shit register t~pe buffers in which the dis-
pla~ is refreshed from one bufXer ~hil the processor
fills another buffer ~ith the next character line. ~ach
shift Legister t~pe buffer contains character code and
po~sibly some disp1ay mode information. ~he r9fresh
~rocess general1y re~u1res multiple cycles through the
bu~fer corresponding to the multiple sequenti~l scan lines
for each char~cter line. The loading of the next buf~:er
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 displa~ pxocessor ll is a micro-processor~
based system whose primary role is to move ~ext strin~s
from the stored in~ormation structure of storage lO to
character ~uffer L2, In the implementation of the
present system, display processor ll is a programmable
device which, by its power tG compute and axt.ract infor-
mation from a complex information structure, assists in
the formatiny of the text display and provides character
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line parametexs such as the hei~ht and origin position of
the character line, a mask that is useful in scrolling
text smoothly, mode infoxmation, blink information and
the control of the vertical synchronization.
As was indicated above, the digital video charact-
ers 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. Tha general combining rule allows ormating of
the full~screen display either ~y text codes which key
other ~ideo sources ox b~ luminance codes which cause
- character or symbol display to be overlayed or superim-
posed~
Some o~ the charactexistiGs of the system of Figure
4 and ~ariations 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
m~st produce the image repetitively and in raster order.
The repetitions required to refresh the display are to
~0 be at least partly independent so that changes in the
infoxmation structure will be reflected immediately in
the displayed image. One o the bene~its of such a dis~
play system as opposed to hard copy is the ability to
change the display in accordance with the users interact-
ion.
I~ all o~ the display information is not extracted
~rom 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 situa-
- , 30 tion, it is not important whether informatio~ is provided
XrQm display processor 11 in xaster order as buffer 12
ser~es also to implement a scan conversion function.
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This ~ariation re~uixes a large buffer, nominally in which
each buffer location corxesponds to a character location
or to a picture element, which bu~fer can be modified
selectively.
If all of the display infoxmation is extracted
from the inormation s~ructure on each frame or field,
the re~resh operation may be regarded as occurring direct-
ly from the information str~cture. If the information
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provided from the display processor 11 from the informa-
10 . tion structure is necessari~y 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 xaster order, buffer 12 need only be a
small buffer whose size depends on the most out-of-raster-
order elements:and speed ma~ching considerations~
Certain types~o displays o~ characters and symbols
have very wide application an~ can be accomodated by
. raster order production, so lon~ as the character codes
20 axe provided from display processor 11 in the order in
which the characters first appear in the raster. In this
case, buffer 12 ~lill store character codes in order to
repeat the character seguence 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
:. ~ applicabilit~ as to be required for almost any system
display. For example, a system for banking applications
might well include sp~cial facilities for displaying
facsimiles o~ ~inancial documents, signatures f or the
like; but would cextainly require character display ox
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the contxol information related to a transaction. Simi-
larly, engineering applications may require stylized
dra~ings to be displayed, that will certainly require
character display ~or legends.
DETAILED DESCRIPTION OF TE~E SYSTEM
Yideo synthesizer 14 of Figure 4 is illustrated in
Figure 5 and includes display mode transform logic 20
which receives charactex data a,s well as ~he character
mode signal and cursor signal ~or combination with othex
image data, which ombined data is then transferrPd to
digital-to~video con~erter 21 ~hich 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 signalO The monochrome composite
~ideo signal produc d thexeby may be optionally combined
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~; at output summing~ampliier 22 with a chroma signal as
will be more thoroughly explained below~ Thè synthesizer
~ust operate entirely at the pixel clock rate with new
data ~ppearing at each clock cycle. However, this data
is irrelevant during blanking periods. Data input appears
one clock time eaxly in order to allow ~ox pipelining.
Display mode transform logia 20 of Figure S is
illustrated in Figure 6 and includes read~only storage
23 that is addressed by the incorning character data,
optional irnage data, mode signals and cursor. Read-only
storage 23 ser~es to perform the various functions by
mapping tha ~-bit character data to the 4-bit output data
i in the various ways required. Thus, characters may be
; displayed in different display rnodes according to user
j 30 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
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- the location of the text editing operation. Typical text
modes are: white~on black r black-on-white, half-bright~
; white~on~gray, black~on-gray. For each of these modes it
is necessary to have a contrastin~ cursor.
In addition, the text may ~e made to be in~isible
in order to hide classifie~ information such as passwords.
- The text may al60 ~e made to blink betwae.n invisible and
visib}e to draw attention to a particular item. This is
accomplished by the receipt o~ blink ra~e and blink on
.signals by AND gate 25 the output of which is supplied
along with the in~isible signal to NOR gate 26. The out-
put ~rom NOR gate 26 is then "ANDED" with respective
character data bits supplied to read only storage 23.
Specifically, the input network consisting of A~D gate
25 and NOR gate 26 treats the various cases o~ invisible
; or blinking characters by orcing the character data in-
: put to 000 when approprlate.
~ ~igure 7 is a table o~ some of the various trans-
- ~ormations that ma~ be accomplished b~ reading out a
portion of the contents o~ read only storage 23 of Figure
6. In this table, ].5 output levels ~0-14~ are used in
order to have a middle le~el of 7. The ef~ect of the
- ~ trans~ormation is to sc~le the character data by either
~2, -1, +1, or ~2 and to offset ito This is done in
order to perform the trans~ormations readily and to use
th~ same character data to display llght-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 digit-
al to~video co~erter thus must perform the yamma correc-
tion O
By use o read~onl~ storage 23l the display mode
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trans~orm logic o~ Figure 6 pxovides for any transforma-
tion desired. It may also ~e used to mix digi~al video
si~nals from diferent sources. For example, in one mode,
characters can be o~erlayed in white on an image back-
ground. For each luminance valu~ of the image, the char-
acter value is linearly scaled to the proportionate lum-
inance 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 contxol o~er the ~ull
screening formatin~.
The output signals.of read~out storage 23 are then
supplied to register 24 from which they are then clocked
at the next pix~l clock time to the digital-to-video
con~erter. ~ ~
The digital~to~iaeo converter 21 of Figure 4 is
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illustrated in more detail in Figure 8. This converter
is a digital-to-analog converter which must be ~ast
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 Yolta~e. Furthermore, its output must be free of
switching transients which are inevitable in the ladder
network type digital-to-analog converter. These transi-
ents would typically occur in the transition from values
such as 0111 to lO00 in which case the signals driving
- the resistive conversion network are changing in di~ferent
dir~ctions and at slightly different times.
; The binary input signals recei~ed from the trans-
~orm logic o Figure 6 are converted through a combina-
tional mapping implemented in read only storage 27 to a
unary or monadic opera~ion code. This mapping con~erts
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four input signals to 15 dif~erent outputs in the follow-
ing manner7 The input 0000 produces an output of all
- zeros~ The input 0001 causes just the first o the fif~
teen outputs to become 1. The input 0010 causes the first
two o~ the fifteen outputs to ~ecome 1, and so on until
the input 1111 causes all fl~tePn output to be 1. This
code has the property that the transition between any two
inpu~ combinations can only cau'se a group o~ adjac~nt
outputs to change in ~he same direction, Thus, when
these outputs are weighted and summed through resistive
network, after being reclocked, the analog output changes
monotonically ~rom one ~alue to another without switching
transients.
-~ The resistance net~ork is illustrated in Figure 8
- as being ~ormed of the plurality of resistors 29 which
are of appropriately var~ing resistance values. The 16
output levels produced thereby are spaced ~y 15 steps,
; and the conductances of each of the fifteen weightingresistors 29 is proportional to the size of that step. If
all of the conductance ~ere the same, the output would be
linear with the data input. ~owever, any positi~e mono-
tonic non-linear function can be produced just as easily
by making each conductance proportional to the correspon-
ing step size. Thus, the resistance ~alues may be calcu-
lated to achiev coxr~ction ~or~whatever Yalue of gamma
- is expectedO As has been indicated above, the voltage
xepresentation of luminance in a video signal is not
linear. Instead, according to U.S~ Standards, luminance
is approximately the 2.2 power of the voltageO Conversely,
the voltage is approximately 1/2.2 power ~2.2 root~ o~
the luminance. In Figure 8, the values o~ the respected
resistances 29 are calculated accordingly. The values
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of the respective resistances are as ~ollowsO
. Bit position 1 - 4~5 ohms
Bit position 2 - 9S3 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
E3it pOSitiQn 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 I4 - 2370 ohms
~` : Bit position 15 ~ 2430 ohms
These values wer~ calculated with the g~ounded load
resistance being 51.1 ohms. The composite sync resistance
.~ is 221 ohms and the composite blank resistance is 1210
. 20 ohms. These values were calculated for a voltage output
ranging Erom a minimum from 0.8 volts to 2 volts. Ths
15 resistances provided 15 intervals between 16 output
signals ranging from 0 volts to 2 volts.
The ad~antages o~ ha~ing Yoltage output as the 2.2
root of luminance is that it allows the transmitter to
compensate for a non-linearlty inherit in the receiver
; monitorO Secondly, the root functionr being a good
approximation o~ the logarithmic r~sponse of tha eye, has
the advantage that noise o~ parkicular amplitude has
approximately the same subjective effect when superimpos~
l ed on dark and light areas. If a linear relationship
: . ' existed betwe~n luminance and ~oltage, noise voltage
~ 20 ~
pj~
:
5i3
; would be more e~ident in the dark displa~ than in the
light areas of the image. This technigue also gxeatly
simplifies the computational transformation in regard to
scaling, rotation, and arbitrary positioning o~ synthetic
~igu~es.
Another ad~antage o~ this type of con~erter resist-
ance values i5 that the resis~ance ~alues need only be as
precise as the steps size dlctates. Typically 5~ resist~
ance 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
~oltage sour~es to drive the weighting and summing net-
works. Accordingly, the converter is easily made fast.
In general, the conversion techniques discussed
above uses more components than the laddsr network type
of converter. Ho~ever, components of much lower preci-
- sion can be emplo~ed and the present con~ersion technique
is well suited for int graked circuit implementation.
The composite s~nc and blanking signals are weighted
into the summing net~ork in the same way as the data. In
addition, the presence o~ the blanking signal gates the
data input to O in order to reject any spurious informa-
- tion on the data input during blanking periods.
Character generator 13 of Figure 4 is illustrated
in more detail in Figure 9. This generator includes~ ~or
illustration, from 1 to 8 ~ont storage elements 30 where
each storage element stores the image of 32 characters
or a total o 256 possible cnaracters. The generator
~lso includes decoder 31 to select one font storage
element according to the incoming character code and
control el~ment 32 that will be more thoroughl~ described
below.
, . ..
, ~ 21 ~
X
': ' ; , 1,_., ,,~
Z45
.`;
The various font storage elements 30 of Figur~ 9
axe shown in more detail in Fi~ure 10. A digital video
.` character is produced on character data ~us 41 in response
to the receipt o~ the chaxacter code and vertical position
aata. This in~ormation together forms the address to the
respective xead-only storage elements 33. The image data
for each character has been computed by techniques explain-
ed abo~e and this data stored in the three similar read-
; only storage sections 33 b~ khe usual mask process. It
will be u~derstood that read-write storages may b~ employ-
ed with pro~ision ~or lo~ding the contents from the char-
acter dispLay pro~essor of Figure 4. When a load signal
is produced by ~ND gate 39 of Figure 10, the data from
i respective read-only storage elements 33 are trans~erred
synchronously into their corresponding shift reg1ste~rs
34 rom where they are read out in~a seria1 digital video
data sequence to the ~ideo synthesizer 14 of Figure 4 as
:
~ was described above.
. .
The appearance o~ a start signal to an enabled
storage element also sets the internal selèct flip flop
37 and the storage element will remain selected until
-. another start signal is received with ena~le logical zero.
The signal input to the parallel to serial conversion
shift registers 34 lS a logical zero and the selected
. ~ storage element will simply output zeros to the charac-
ter data bus 41 afte~ the prescribe~ number of picture
elements have been produced from the corresponding
. ~ storage element. In addLtion, the control may disable
; the output at times in oraer to o~erla~ an underline,
crossout text, or cursor s~mbol.
; For variable - pitch characters, it is most natural
to store the character width in the same element as the
~ 22 -
~ Z~5~
- ~ - character image. ~or ~ariable pitch character~, optional
read-only ~torage element 35 may ~e employed which contains
32 four-~it words which are masked to contain the charac-
ter width as a prescri~ed number of picture elements. In
this case, when an ena~led alement is started, the width
of the selected character is transferred synchronously
into a binary down counter 36 and the underflow signal
~rom this counter is gated onto a bus line which returnstothe
control to indica~e the completion o the current characters.
~ontrol element 32 of Figure 9 is illustrated in
more detail in Figure Il. The control element receives
the parameters required to control such features as verti-
cal format slow scrolling, underlining, or crossout text
To this end, initial position and count parameters are
recei~ed respectively by counting registers 42 and 43.
Mask counter 44 receives the mask parametex. These regis-
ters are driven by a systems clock. The function o~ these
parameters w~ e more thoroughly described below~ In
addition, the control element is proYided with a decoder
read-only store 48 which contains the data for underlining
such as required. The last line and ~ield parameters from
miscellaneous register 45 are used to control vertical sync
counter 46 that drives decoder read-only store 47 to supp-
ly 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
stvred image is placed within a larger address space as
illustrated graphically in Figure 12. Th~ addresses are
allowed to "wrap around", i.e., modulus 2S6. For this
3~ reason, the address space is referred to as the "charac-
ter wheel". Because of the interlaced scan, the lowest~
or~er binary digit o~ the address to the character wheel
~i - 23 -
~, j¢
qLS8
.
is the signal called FIELD and this indicates whether~
the upper or lower scan line in the address ~can line pair
is used for displ~.
The manner in which slo~ scrolling or smooth scroll-
ing is achieved ~ill no~ be more specifically described
in relation to ~igures ll and 12. The location of the
character in the character wheel o~ Fi~ure 12 is defined
b~ the three parameters supplied to ~he control element
of Figure ll. As indicated abo~e, thase parameters are
,the initlal position, the cQurlt and the mask. ~he~e
parameters are loaded inko the respecti~e 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 re-
maining scan lines ~utomaticaLl~ moving up with addition-
al scan lines ~eing added at the ~ottom o~ 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 e~cept that portion
being displayed whichl in the case of Figure 12 is each
scan line pairs 0-7. Thus, as the character is moved
upwardly in the character line, it will appear to dis
appear ~ehind the barrier. At the same time, in the
previous character linel a different c,haracter wheel
employed therefor as defined by its own parameters will
give the appearance o~ the character m ving 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. Conversel~,
the character can be made to appear to move downward in
- 2~ -
5~
the same slow smooth manner. This is accomplished because
position registex 42, count register 43, and mask regist~
er 44 are loaded at the beginning of each character line
~ith the actual loc~tion of the character line being deter-
mined by character display p~ocessor 11 of Figure 4 which
fetches the respective character codes and their corres-
ponding parameters from storage 10 in a continuous manner.
Thus, there is no need to have another counter in the
system to count out Yertical synchronization since it is
the number of times pex field that the parameter reglsters
are loaded that determines how ~any character lines there
are plus one more to do the vertical ~ynchronization.
~n Figure 11, mask register 44 need only contain
4-bits for the indi~idual 15 scan lines of the 8 scan
line pairs ~hich 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 ~isible through the mask. When the
high order bit is one~ the ch~racter is invisible as this
causes an output disable signal to be generated as indica-
:, ~
- ~ ted in Figure 11.
~ Slow, smooth scrolling can be achieved without use; - of the mask register and operation by manipulation only
of the initial ~alues of the position and count registers
at the first and last character lines in a scrolling
segment. Ho~e~er, i~ is possible in this case to dis-
play only a single or ~ery small number of scan lines in
a contracting character line, and the small numher of
scan lines may consume so short a period of time that the
~rocessor may be unable to complete filling the next
I character buffer before it is required for display.
Accordingly~ if smooth scrolling is to be performed with-
- ~5 -
- \
S~
out use of the m~sk ~egister, the character buffer must
,
have the capacit~ to stoxe three or more character lines,
and the processor must be able to replenish this speed-
matching ~uf~er mo~e rapidl~ than display depletes the
buffer.
As ~as indicated abo~e~ the two fundamental para-
meters which control the foxmat o~ the display are the
initial position on the character wheel and the count cf
the numbex of scan lines to ~e produced. For example,
starting at address zero with a count o~ eight scan lines
will result in the character lines being spaced verti-
cally by their normal mini~um separation analogous to
single spacing on a typewriter. Starting at address 252
~- ~ith a count of 16 display lines, lines 252 through line
11, is similar to double-spaced typing~ Display of lines
254 through line 9 ~ith a cou~* of 12 scan line pairs
along the chaxacter ~heel is similar to one-and-one-half
~` spacing on a type~xiter.
- The same functional mechanism is employed to provlde
~ program-controlled vertîcal synchronization. For
example, 480 o~ 483 possible visible scan lines for text
displa~ 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 2Z or
- 23 scan lines depending upon which field is completed.
' In this vertical synchronization mode, the display is
; blanked ~vertical blanking~ and the vertical synchroni~a-
i tion signal charac~eristic of the particular ~ideo stand-
ard is produced beginning either one or two half scan
lines later depending on the ~ield completed.
This techni~ue avoids the necessity of ha~ing a
- 26 -
~ I
~%4Sl5
.
s~an line counter and ~et allows pro~ram-controlled
vertical synchronization to accomodate r 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, khe initial value and count parameters stored
in the respective counting registers 42 and 43 and supp-
lies vertical position information to the respective font
storage elements 30 of Figure 9~ ControL element 32 ~
that ~igure detects when the character wheel address is
within the active segment, nominall~ position 0-7 of the
character wheel and initiates each character and advances
the character line buf~e~ one position for the next char-
acter~ The control element also receives the control bits
for FIELD and L~ST LINE which indicates that the next
:
haracter line i~ the ~ertical synchronization interval.
When the control element is to perform the under-
line function or crossout text ~unction, these functions
are produced by decoding the vertical position, disabling
~0 the font storage output and inserting data on the charac-
ter data bus. ~ full bxight scan line with half-bright
scan lines on either side produces very good underlining
~; ~ or crossout which does not flicker. The respective font
storage elements are activated even when their outputs
are not used in order to determine the character width in
the variable-width case.
The mask parameter supplied to mask counter 44 of
` Figure ll is 4 bits for the scale of characters employed
- -' in the present invention. Whenever the high~order bit of
thi~ parameter is zero ~the register counts are 0-7~ dis-
pla~ occurs as usual. When the hiyh-order bit is one
Cthe register contents are 8-15) and the effect is the
~7 -
:
` same as if the chaxacter ~heel ~exe in positions 8-255 and
~he displa~ is masked. If t~e mask parameter was the same
~s the four low-order bits of the initial position on the
character wheel, the display would not be masked. How-
ever~ if the mask paxametex were 12 and the initial posi~
tion were zero, the first four scan lines of the charact-
er line display would be masked.
The timin~ system employed in the present inv ntion
will now be described which system employs U.S~ broadcast
or close-circuit standard ~ideo, the parameters of which
are described in the Electronic Industries Association
Standards RS-170 and RS-330. The report o the National
Television Standards Committee CNTSC1 is the basic stand-
ard applicable to color systems used in the United States.
While the present invention is not adapted as a color
, . .
; system, the features invol~ed can be emplo~ed in such a~
syst~m.
; ~ Digital systems such as employed with the present
- invention can be adapted to different standards by para-
- 2~ metric variation without an~ change in the basic structure
of the system. Such changes require only variations in
such parameters as the modulus of the counter, the funs-
tion produced by the combinatorial network and the fre-
quency of the particular oscillator. As an example of
~- easily accomodated difference in standards, U.S. standards
have 525 scan lines per frame and a~rame rate of 30Hz,
~hile ~he British standards have 625 scan lines per frame
and a frame rate of 25 ~z~ This difference is explained
by the predominance of 60Hz. A.C. power in the United
i 30 States and 50Hz power in England. The number o~ scan
lines produced per second ~15,750 in the United S~ates
and 15 r 625 in England~ is similàr enough that even the
,
l 28 -
,, ~
\
~24~
: same monltors can o~ten be used with either standard by
adjustment onl~ of the ~ertical synchronization a~d si~e.
For the digital production of video signals, the
following analysis is provided ~ased on the U.S. Standards.
There are 525 scan lines per ~rame, but which t~pically
483 are visible and the r~maining 42 used or vertical
synchroniæation and the required ~ertical retrace time.
The scan linas arehorizontal from left to right and the
horizontal~vertical aspect ratio o~ the ~isible image is
4~3. A square geometry coordinate s~stem, in whi.ch the
:~ picture elements are spaced horizontally at the same
' pitch as the scan lines are spaced vertically, is desir-
able both to equalize horizontal and ~ertical resolution
:~ and to simplify digital computation of the coordinate.
- : T~us, the best choice of the number of picture elementsalong each scan line is C4~3~ X 483 - 644 picture elements
pex scan line. The acti~e picture information portion of
the sean is 33~40ths of the time H allowed for each scan,
and the time P ~or each picture element is approximately
1~780ths o~ the scan line time. In the present invention,
the picture element oscillator is approximately 12.27 MHZo
If optional color timing is provided, the color oscillator
~- ~ is at a frequency of approximately 1~o32MHz~
The system timing generator is illustrated in Figure
13. As sho~n therein, the picture element clock 51 dri~es
a 10-b.it binary counter ~2 whose outputs are mapped through
: read-only storage device 53. The counter contents 779
t~rough 1023 cause one output o~ the read~only storage
device to become a logical one, so that when connected
b~ck to the s~nchronous clear input of counter S2, it
causes the count o modulus 780. Yarious other conditions
in the counting sequence from p~to 779 are mapped through
29 -
v
/\
~2~S~3
.
. .
read-only storage 53 and clocked into register S4 in order
to produce timing indications o~ points in the horizontal
scan. These include the horizontal synchronization pulse,
horizontal blanking to define the active ~ideo area and
the color burst gate~ All of these signals are constitu-
t8nt5 of the composite video signal. One other signal
t~PIX-n~ changes ~rom logical 0 to logical 1 n clock
periods be~ore the beginning of the active video period
and changes from a logical l to a logical 0 n clock periods
before the end of the active video period so as to allow
a sufficient number of clock steps in advance to start
and stop the flow of digital video information through
the sequence of intermediate registers described above in
.. . . .
a pipeline fashion. Vertical synchroni2ation, blanking
and the active video area are controlled by display pro-
,
cessor ll of Figure 4, ~he system timing generator of
~; Figure 13 goes no urther than a frequency division to
l~H.
Combinatorial mapping is indicated here as implement-
ed in a read-only storage device 53. However, a g~te net-
work that produces the same set of func~ions of the given
input address lines would ser~e as well. For a different
~ideo standard, the structure of Figure 13 would remain
the same, except only the oscillator frequency and the
contents of read-only storage device 53 would be different.
Also shown in Figure I3 is an optional timing system
for a color system. The output of oscillator 55 is divid-
- - ed by a modulus-4 Gray code counter 56 and 57 in order to
produce a 4-phasa color subcarrier. Division of the out-
j 30 put of oscillatox 55 by 7 in divider 58 can be employed
to synchronize or lock each sixth cycle of the main
picture element oscillator 51.
, ~ 30 -
~`Z4S~
E~ILOGUE
A digital video display s~stem has been described
whexein the vaxious characters to be displayed are dis~-
played in the form o~ images o~ the complete character
rather than the standaxd dot~matrix pattern of the prior
art. A character generatox in the display system stores
signals representing the various characters to be display-
ed which are retrie~ed from sto}age in response to a
character code~ The signals are in the form of a binary
code having a sufficient number o~ bits to represent a
different number of levels of gray~scale or lumlnance
values or the ~arious picture elements making up the-
character image.
The binary codes retriev~d ~rom storage are supp1ied
to a ~ideo synthesi~er that generates the video signal
to display the character images whlch may be displayed in
a number of di~erent modes incIuding white-on-black,
black-on-white, black-on-gray, and white-on-gray as well
as different combinations of such modes to represent a
cursor. The video synthesi er can generate output voltage
levels in discrete steps ~or the different~luminances to
be employed. The ~oltage output signals differ from level
to level in accordance with the 2.2 root of luminance.
The display system is also providsd with a control
element that includes position and count registers to
specify the initial position of a scan line and the number
of scan lines displayed whioh registers are supplied with
incoming data to speci~y the position of each character
lina so that the position of a character image on display
can be adjusted upwardly or downwardly to give the appear-
ance of smooth scrolling.
While but one embodiment has been disclosed it will
,
:J - 31 -
. X
s~
~; ~
- be apparent to t~ose skilled in the art that changes and
modifications can ~e made therein without depar~ing from
the spirit and scope o~ the in~ention as claimed.
. ', .
. . .
.
"~ ,' ' '
2~
... .
.
! 30
-- 32 ~
,
;i .
'' ~X -
