Note: Descriptions are shown in the official language in which they were submitted.
Constant Light Greyscale Generator
For Color Camera System
TECHNICAL FIELD
This invention relates to a method of, and
apparatus for, obtaining a hard copy of an image visible in
a video frame represented by periodic video signals that
define the frame. Stated otherwise, the invention relates
to photographing the screen of a television tube on which
the same information is repeatedly displayed by the periodic
video signals utilizing a stop-motion process that "freezes"
the image on the screen, whereby a color photograph of a
color television image can be obtained.
BACKGROUND ART
One approach to obtaining a color hard copy of a
color television image is to photograph the screen, using
color film. This requires a static image, which can be
achieved by electronically selecting a video signal
representing a television frame and periodically applying
this signal to a color television tube. Alternately, a
static image can be achieved by utilizing a television
camera televising a still scene, or by utilizing the output
of a video disk or videotape recorder operating in a
stop~frame mode, or by utilizing a computer-generated
image. Experience has shown that these approaches will
reproduce the subject matter in a way which, while
satisfactory for some purposes, is of a quality much poorer
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than that o a direct photo~raph of the scene being
televised.
~ well-known technique is f irst to divide the
color television signal into its color-separation video
5 signals, i.e., the red, blue, and green video signals, then
sequentially to display these signals on a monochromatic CRT
(cathode ray tube) whose screen is photographed through
filters in a way that synchronizes the color of the filter
to the color of the video signal being displayed on the
10 screen. In this technique, color film would be exposed,
typically, for 50 seconds through a red filter to the image
on the screen of a monochromatic C~T while the red
color separation image is displayed, for two seconds through
a blue filter while the blue color-separation image is
lS displaced on the CRT, and for six seconds through a green
filter while the green color-separation image is displayed.
In other words, the film will be exposed, typically, to 1500
frames of the red color-separation image, 60 frames of the
blue color-separation image, and 180 frames of the green
20 color-separation image. The exposure times will, of course,
depend on the optics, the phosphors of the CRT and the type
of color film; and the proper exposure time can be
determined by trial and error.
While the quality of images produced by the
25 last-described technique represents an improvement over the
quality of image produced by merely photographing a
still~rame color video picture, the non linear response of
the CRT in terms of light intensity to input signal level
and the non-linear characteristic of most film materials
30 represent inherent deficiencies that will not produce an
image approaching the quality of a direct photograph of the
scene being televised.
It is, therefore, an object of the present
invention to provide a new and improved method of, and an
35 apparatus for, obtaining a hard copy o~ a video frame
represented by periodic video signals where the image
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quality is improved over that obtained with the techniques
known-in~the--prior-art.
DISCLOSURE OF THE INVENTION
According to the present invention, each picture
5 element (pixel) on a photosensitive sheet is exposed to
lic3ht of a predetermined brightness for a period of time
functionally dependent on the level of the video signal
representing the corresponding pixel in the frame. This
distinguishes from the prior art approach described above,
10 where each pixel on the film is exposed for a predetermined
period of time to lic3ht whose brightness depends on the
level of the video signal representing the corresponding
pixel in the frame. In other words, the exposure o a pixel
(i.e., the product of the intensity of light incident on the
15 pixel and the time durinc3 which light of such intensity is
incident on the pixel) in the present invention is
determined by varying the time of exposure to light of
predetermined brightness, while in the prior art, exposure
is determined by varying the brightness of the light. This
20 fundamental change in exposure permits the characteristic
curve (i.e., the ~&H curve, or density/log E curve) of the
film to be taken into account in a way that improves
contrast and achieves a photograph more closely approaching
the quality of a direct photograph of the scene.
In carrying out the present invention, the ranc3e
between reference white and reference black in the video
sic3nal is divided into N levels, and the video frame is
displayed on the screen of a CRT at least N times. For each
level, the frame is displayed in terms of a binary
30 brightness distribution wherein a pixel in the display has a
predetermined brightness if its amplitude exceeds a selectecl
level, or it is dark if its amplitude is below the selected
level. The total time a frame is dis~layed at each level is
selected to take into account the characteristic curve of
35 the film. Thus, in decomposin~ the actual video frame into
N binary hri~htness distributions on the screen of a CRT,
anc~ varyin~ the time each distribution remains on the screen
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as a function o~ -the cnaracte:rls-tic cuxve of the :Eil-rn, an iinL~roved
photograph of -the screen can be ohtained~
Where the set o:E video signals represents a frame of
a color-separation image, the color of the light to which the
photosensitive sheet is exposed should be -the same as the color
of the color-separation image. In such case, a color hard copy
of a color video frame can be obtained by sequentially carrying
out the method described above for the red, blue, and green color-
separation images. In each case, the film is exposed -through
a filter compatible with -the color of the color-separation image.
According to a first broad aspect of the present inven-
tion, there is provided a method Or obtaining a hard copy of
a video frame represented by periodic video signals which lie
within a predetermined range of amplitudes that define N levels
of brightness comprising the steps of: comparing the amplitude
of the video signa~s with a reference signal level to provide
a select output signal when the amplitude of the video signals
is at least equal to the amplitude of said reference signal;
shifting the reference signal level through N levels correspon-
ding to the N levels of brightness defined by the predeterminedrange oE video signal a~plitudes at a frequency functionally
related to and synchronized with the periodic frequency of the
video signals; deflecting a CRT, which is capableof displaying
monochrome, in response to the sync signals in the video signals
so as to cause the beam of the CRT to trace out a raster on the
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screen of the CRT and controlling -the in-tellsity of the beam to
provide a two-level b.rightness distribu-tion with the upper of
said two levels of brightness occurring in response to said select
output signal from said comparator; and expsoing a sheet of pho-to-
sensitive material to the screen of the CRT.
~ pparatus according to the invention comprises a) a
comparator having signal and reference inputs for producing an
output when the signal applied to its signal input exceeds a
threshold defined by the voltage applied to its reference inpu-t;
b) means for applying said video signal to the signal inpu-t of
said comparator; c) means for shiftingsaid voltage applied to
the reference input of said comparator at a frequency function-
ally related to and synchronized with the periodic frequency
of the video si~nal; d) a CRT, which is capable of displaying
monochrome, having a deflection circuit responsive to -the sync
signal in the periodic video signal for causing the beam of the
CRT to trace out a raster on the screen of the CRT, and having
an intensity control responsive to the output of said comparator
for controlling the intensi.ty of the beam whereby a two~level
brightness distribution is produced on the screen; and e) means
for stationing a sheet of photosensitive material for exposure
to the screen of the CRT.
The invention will now be described in greater detail
with reference to the accompanying drawings, in which:
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Fig. 1 is a representation of a television screen having
vertical bars whose brightness is uniform ver-tically, the bright-
ness of the bars increasing from left to right;
Fig. 2 is an idealized representation of the video
signal that will produce lines on the television screen shown
in Fig. l;
Fig. 3 is a schematic showing of the manner in which
the brightness distribution along a given line in the scene shown
in Fig. 1 can be broken up into different levels;
Figs. 4A-H are representations of the television screen
shown in Fig. 1 having a binary brightness distribution in accor-
dance with the levels shown in Fig. 3;
Fig. 5 is a typical characteristic curve ~or a given
photosensitive material exposed to monochromatic light or light
of a predetermined dominant wavelength;
Fig. 6 is a normalized version of the curve shown in
Fig. 5;
Fig. 7 is a chart showing the incremental differences
in exposure required to achieve the various levels; and
Fig. 8 is a block diagram of apparatus in accordance
with the present invention for obtaining a color hard copy of
a color video frame.
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BEST MODE FOR CARRYING OUT THE INVENTION
- - Reference is made to-Figs. 1-3 and 4A-G ~or th~ _
purpose of providing a brief background that will enhance
understanding of the operation of the circuits shown in
Fig. 8. In Fig. 1, reference numeral 10 designates the
screen of a television monitor having an 8x8 matrix of
picture elements (pixels), the brightness of each pixel
being designated by the number associated with the pixel.
The number "0" designates black, while the number "7"
designates the seventh of ei~ht possible levels of
brightness. In other words, Fiq. 1 would show an
arrangement of vertical bars whose brightness is uniform
vertically, the brightness increasin~ from left to right.
In order to create such an image on the television
screen in a manner well known in the art, each line of the
video signal would have a step-shaped orm similar to that
shown in Fig. 2, which is highly idealized, in the region
bètween horizontal sync pulses 11. As the electron beam
sweeps the screen from left to right through columns A to H,
the intensity of the beam is modulated in accordance with
the amplitude of the video signal shown in Fig. 2. For
example, the video signal corresponding to a pixel in row A
of a given line would have no amplitude, and would provide a
reference black level. On the other hand, the video signal
corresponding to a pixel in row H would have a brightness
level of 7, which would be close to the reference white
level.
In order for the picture to remain "froæen" in
time on the television screen, video signals like those
shown in Fig. 2 would be applied over and over again to the
input of the television, an~ the result would be a display
of vertical bars of varying brightness, as indicated in
Fig. 1. In general, a single frame of video signals is
applied to the television screen at the rate of ~hirty times
per second, establishing the bar pattern on the screen,
which would be perceived by an observer as bein~ stationary.
In order to obtain a photoqraph of an image on the
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television screen, trial-and error process could be carried
- out to determine the--number~of-frames that--must-be--displayed- -
on the screen before an acceptable exposure is obtained on
the film. In such case, each pixel on the film would be
exposed to light from the screen for the same period o
time, but the intensity of light incident on a pixel on the
film would vary in accordance with the amplitude of the
video signal representing the corresponding pixel in the
video frame. That is to say, proper exposure of the pixels
in line 4 of the television picture, which has 7 units of
bri~htness, may require, say, 10 seconds or 300 frames.
Elowever, every pixel on the film would be exposed for the
same period of time, and, because of the characteristic
curve of the film, it is unlikely that any of the other
pixels in the film in columns other than column El would be
exposed properly with respect to the pixels in column H.
In order to remedy this situation, the arrangement
shown in Figs~ 3 and 4A-H is utilized according to the
present invention.
The video signal shown in Fig. 2 can be broken
into eight component parts, for instance, as illustrated in
Fig. 3. That is to say, the eight levels shown in Fig. 3
can, when superimposed on each other, reproduce the video
signal shown in Fig. 2. If the level 1 video signal were
applied to a television screen, the result would be as shown
in Fig. 4~, where the pixels in column A would have no
light, while the pixels in columns B-~ would be of uniform
brightness. The result is a thin black bar at the left
margin of the picture. Similarly, when the level 2 video
signal is applied to the television monitor, the pixels in
columns ~ and ~ woul~ be dark, while the pixels in columns
C-~l would be uniformly bri~ht. Thus, the use of the video
signal shown in Fig. 3 would produce eight two-level or
binary brightness distributions on the television screen.
The light on the television screen is integrated
on a film to reproduce the distribution shown in Fig. 1.
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This can be seen by noting from Fig. 4 that the total amount
-of--ligh~ for---example,-in the pixels in column 1I would total
7, which is the value o the pixel in column ~I shown in
Fig. 1.
S There are advantages to the approach described
above in that the television monitor may be an O~l-OFF type
of display such as LC~- or LED-type display where each pixel
in the display has either an off condition in which no light
is produced, or an on condition where a predetermined amount
of light is produced. With the arrangement shown in Figs. 3
and 4, the exposure of the film to light of each level can
be tailored to match the characteristic curve of the filmO
This is illustrated in Figs. 5 and 6.
The film is exposed to light of a predetermined
level for variable periods of time in order to generate
curve 20, which may be termed the characteristic curve of
the film, because it represents the density of the film as a
function of its exposure. Curve 20 is a fictitious curve,
and represents a typical type of a curve, such curve being a
function of the particular film involved, as well as the
dominant wavelength of the light incident on the film. In
other words, where color photography is being utilized, a
characteristic curve would be generated for each of the
three primary colors, red, blue, and green and the curve
shown in Fig. 5 is merely illustrative of any of the curves
that would be obtained.
In preparation for incorporating the
characteristic curve into the exposure system for obtaining
a hard copy from a video image, curve 20 may be redrawn, as
shown in Fig. 6, by normalizing the percent reflectance
shown in Fig. 5. That is to say, the maximum reflectance
shown in Fig. 5 may be, for the example shown, 62.5%, which
would constitute unit normalized reflectance. Curve 30,
shown in FigO 6, is the normalized characteristic curve for
a particular film and a given color. If the normalized
reflectance is divided into eight levels, as indicated in
Fig. h, then the time, i.e., the number of frames over which
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the film must be exposed to obtain proper exposure for each
~ level, is determined by the intercept of the level with
curve 30. The results are summarized in the chart shown in
Fig. 7 where, for example, it can ~e seen that about 90
frames are required properly to expose a pixel whose
brightness is level "1". Properly to expose a pixel whose
brightness is level "2", would require a total of 140
frames. However, because 90 frames have already been
applied to expose the level 1 pixels properly, only 50
additional frames are required for level "2" pixels. Thus,
the right column in Fig. 7 represents the incremental change
in frames required on a level-by-level basis. The
non-linear relationship between the exposure time and the
level is illustrated by chain line 31, shown in Fig. 6.
Actually, where color hard copies of a television
image are to be obtained, a set of curves like those shown
in Figs. 5 and 6 is obtained for each of the colors of the
color-separation images available. That is to say, a red
video si~nal would be applied to the television monitor for
producing light of a predetermined intensity, and such light
would be incident on a film to achieve the red color-
separation image characteristic curve. The process would be
repeated for the blue and the green color-separation images,
and three charts like that shown in Fig~ 7 would be
obtained, the right column of each chart representing the
number of frames the various levels must be incrementally
exposed for each of the color-separation images at each
level.
An embodiment for carrying out the invention to
achieve a color hard copy of a video frame represented by
the three sets of red, blue, and green color-separation
images is shown in Fig. 8.
Video source 4n shown schematically in Fig. 8
represents a source of red, blue, and green color-
separation video signals such as the output of aconventional color television recciver, a videodisk or
videotape recorder, or the output of a computer-generated
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image system. The essential factor in connection with
source 40 is that it produce, either sequentially or
simultaneously, the three color-separation image video
signals, and that the signals be repeated over and over.
Multiplexer 42 receives ~he three color-separation video
signals and, depen~ing upon which of the three selector
lines 4~, 46, 48 is activatedl the output of multiplex 42
will be one of the three color-separation video signals.
The output of multiplexer 42 is applied through nc
restorer 50 to signal channel 52 of comparator 54, which
produces an output when the video signal applied to its
signal input exceeds a threshold defined by a voltage
applied to reference channel 56 of the comparator~ This
reference ~oltage is produced at the output of
digital-to-analog converter ~DAC) 58 in the manner described
below.
Comparator 54 functions to achieve signals like
those shown in Fig. 3, by slicing the video at a level
determined by the output of ~AC 58. Comparator 54 will
produce an output only if the video signal input is in
excess of the voltage established by the output of DAC 58;
and such output operates on intensity control 60 of
monochrome CRT 62 to turn on the beam. When there is no
output from the comparator, the beam will remain turned
off. In this way, a two-level or binary brightness
distribution will be established on screen 64 of CRT 62 in
accordance with the level established by the output of DAC
58 and the information in the video signal~
Light from screen 64 passes through one of the
filters on filter wheel 66, and then through lens 70, where
it is focused onto a film plane containing color film 72.
Filter wheel 66 has three possible positions for locating
any one of a red, blue, or green filter in alignment with
lens 7n. The angular pOsitiO-l of the filter wheel is
established by motor drive 74, whose operation is controlled
by motor control 76; and position sensor 78 senses the
angular position of the filter wheel, and produces a control
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si~nal that identifies the particular filter aligned with
lens 70. In the illustration shown in Fig.--8-,-.red.--.ilter....68
is ali~ned with lens 70, and position sensor 78a produces an
output on line 80 indicative of this condition. This signal
on line 80 is applied to line 44 associated with multiplexer
42, permitting only the red color~separation video image to
be applied to DC restorer 50. In the event that motor con-
trol 76 were operated to index filter wheel 66 until the
blue filter is aligned with lens 70, then position sensor
78a would produce an output in line 82 which would be
applied to line 46 of multiplexer 42, thereby permitting
only the blue video signal to be applied to DC restorer 50,
etc. In this manner, the color of the video signal used to
establish the binary brightness distribution on screen 64 of
CRT 62 is synchronized with the color of the filter aligned
with lens 70.
According to the present invention, apparatus 39
includes level counter 84, the output of which addresses a
read-only memory (ROM) 86, whose output is a number "m" that
establishes the number by which divider 88 operates on the
vertical drive output 90 of sync stripper 92 for producing
an output that increments both counter 84 and sync counter
94, whose contents are identical to the contents of level
counter 84. Read-only memory 86 constitutes a look-up table
in which information obtained experimentally from the
characteristic curves shown in Figs. 5 and 6 is tabulated
for each of the red, blue, and green color-se~aration video
signals. If the right-hand column of the chart shown in
Fig. 7 constitutes the number of vertical frames that must
be applied in order to achieve proper exposure at the
various levels, as indicated in the chart, then the contents
of the ri~ht-hand column of this chart would constitute the
contents of the read-only rnemory for the red color~
separation ima~e.
In a similar way, the blue and green entries in
the look-up table would be determined, and the contents of
the read-only memory could be established. Counter 84
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addresses the read-only memory in accordance with the level
~ under~conside~ration, and-the`output of position sensor 78a
is effective to select which of the red, blue, or green
columns in the read-only memory is to be activated. As a
5 consequence, the contents of level counter 84 and the
selected ilter in the filter wheel 66 determine a cell in
the read-only memory, the contents of which cell establishes
the number of times a frame will appear on screen 64 of CRT
62. This timing is achieved through the use of sync stripper
10 92 and clock 88. The sync stripper is a conventional
a~paratus which operates on the video signals produced at
the output of multiplexer 42, stripping the synchronization
pulses from the video signals for the purpose of providing
horizontal drive 96, as well as vertical drive 90, which are
15 applied to deflection coil 98 of the CRT. The output of
horizontal drive 96 is a pulse train at 15,750 Hz when the
conventional NTSC television system is being utilized, and
provides drive for deflecting the electron beam in CRT 62
hori20ntally across the screen 64. Vertical drive 90 is a
20 train o pulses whose frequency is 60 Hz, this providing the
frame frequency for the system, which in this example is a
non-interlaced system where one frame equals one field. In
this manner, the electron beam of tube 62 is caused to trace
a raster on screen 64 of the tube, the raster being traced
25 60 times per second.
Clock 88, under the control of the selected cell
of read-only memory 86, will divide the vertical drive
L pulses produced by sync stripper 92 by a number whose value
is the number contained in the cell addressed by level
30 counter 84 and the particular output of positio`n sensor 78a,
which is determined by the color of the filter in filter
wheel 66 aligned with lens 70. For example, if the contents
of level counter 84 is the number "3", level "3" of the
video signal will be under investigation. Further, if the
35 red fi1ter is aligned with lens 70, then the cell addressed
by level detector 84 and the "R" line will contain the
number 50, which will be applied to clock 88 ~or the purpose
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of producing an output pulse after fifty vertical pulses-
---- have beerl produced by sync stripper 92. The pulse appearing
at the output of clock 88 serves two purposes, namely, the
incrementing of sync counter 94 to the next level (i.e., 4),
and the incrementing of level counter 84 to the next level.
At this point, a different cell in the read-only memory is
addressed, and clock 88 will then divide by the number
contained in that cellO
The level determined by the contents of sync
counter 94 is converted into an analog signal by
digital-to-analog converter 58, thereby establishing a
threshold for comparator 54. As counter 94 is incremented,
the threshold continues to step upwardly, as indicated by
reference level numerals 1-8 in FigO 6, the time during
which the voltage appears at a given level being determined
by the contents of read-only memory 86.
In operation, an unexposed sheet of color film 72
is placed in position behind lens 70 in the focal plane
thereof. Video source 40 is operated to produce three sets
of periodic video signals, corresponding to the red, blue,
and green color-separation images associated with a color
television picture. Assuming that the red filter is aligned
with lens 70 so that lead 80 of position sensor 78a is
activated, multiplexer 42 will produce, at its output, the
red video frame, as defined by a set of periodic video
signals. This set of signals is repeated until line 44 to
the multiplexer is no longer activated.
After DC restoration, the video signal is applied
to the signal input of comparator 54. The synchronizing
pulses are stripped from the video signal by stripper 92,
and produce the hori~ontal and vertical drive for the
deflection circuit associated with CRT 62. When a "start"
command is applied at input 100, level counter 84, sync
counter 94, and clock 88 are cleared. The "0" level of
counter 84 is applied to read-only memory 86, addressing one
row of the memory, the column addressed being determined by
the output o~ position sensor 78a. Inasmuch as line 80 is
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activated, the "R" line 102 of read-only memory 86 is
enabled, thereby permitting the~~contents of the "0" level
cell in the selected column to be read into and determine
the operation of clock 88. This establishes the divisor for
the vertical sync pulses, so that after the required number
have been applied to clock 88, the latter produces an output
pulse which increments sync counter 94 and level counter
84. During the time that clock 88 is counting, and prior to
its producing an output pulse, DAC 58, operating on the
output of sync counter 94, will produce a voltage
corresponding to the "0" level to the reference input of
comparator 54. Any time that the video signal applied to
the signal input of comparator 54 exceeds the threshold
established at input 56 by the output of D~C 58, comparator
54 will produce an output which will turn on the beam of CRT
62, producing a spot of light on screen 64 at a location
depending upon the location of the pixel in the video si~nal
with respect to the entire frameO Thus, a monochromatic
two-level or binary brightness distribution is produced on
screen 64, much like any of the displays illustrated in
Figs. 4A-H. The display remains on the screen for a period
of time determined by the contents of the addressed cell in
read-only memory 86. When this time expires, the output
produced by clock 88 will change the threshold voltage
applied to comparator 54, and will, at the same time, permit
selection of a new time by reason of the re-addressing of
read-only memory 86. The situation is repeated until
counters 84 and 94 have been incremented through all of the
available levels. At each level, a two-state brightness
distribution will be displayed on the screen, the film being
exposed to the displays on the screen through a filter
compatible with the color of the video color-separation
image.
The contents of counter 94 is decoded so that,
when it reaches its maximum (i.e., the maximum level), an
output pulse appears which operates stop-pulse generator
103, thereby stopping counter 84, an~ applying a turn-on
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si~nal to motor control 76. Motor 74 is thus energized, and
rotates filter wheel 66 until the-next colored filter--is
aligned with lens 70. The position of the filter wheel is
sensed by position sensor 78a, thereby deactivating line 80
and activating, say, line 82 when the blue filter is aligned
with the lens, thereby shift ng the output of multiplexer 42
from the red color-separation video signal to the blue
color-separation video signal. Simultaneously, another
column of the read-only memory is activated in preparation
for exposing the film to the blue color-separation image
when a start signal is applied.
In response to another start signal, the cycles
described above are repeated once again through all the
levels, producing on screen 64, for each brightness level, a
two-level or binary brightness distribution based on the
blue color-separation image. The exposure duration for each
level is determined in accordance with the contents of the
read-only memory; and after this is completed, the changes
indicated above occur once again to enable the green
color-separation image information to be applied to film
72. After the three color-separation images are
sequentially produced on screen 64 and the exposure of film
72 is completed, the film can be developed, and a hard copy
of a video frame will be produced.
In order to speed up obtaining a hard copy, the
circuit indicated generally at 104 can be utilized. This
circuit senses when a level has been reached which contains
no useful information, and is effective to short-circuit or
terminate further operation of counters 84 and 94. As seen
in Fig. 8, flip-flop 106 is set after each frame is
displayed by a vertical drive pulse. If there is no output
at a given level from comparator 54, which means that no
additional information is contained in the video signal
because there are no pixels with amplitudes greater than the
current level under consideration, circuit 104 is effective
to terminate further operation to permit the application of
the next color-separation image to the film. In such case,
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flip-flop 10~ will be in its set state when the next clock
pulse-appears to-increment counkers 84-and 94; and such
clock pulse will appear at AND-gate 108 while flip-flop 106
is in its set state. Consequently, ~ND~gate 108 will
produce what is termed an "early stop signal", which is
effective to stop c]ock 34 and terminate operation.
However, if the video signal contains information that
toggles the intensity control, which is to say that the
video signal has a pixel with a brightness condition which
exceeds the threshold of comparator 5~, flip-flop 106 will
be reset. When the next clock pulse appears corresponding
to the next higher level, AND-gate 108 will not produce an
early stop pulse, and the staircase increase of volta~e at
the reference input to comparator 54 will continue.
15 INDUSTRI~L APPLICABILITY
From the above description, it can be seen that
the present invention provides a method for obtaining a hard
copy of a video frame represen~ed by a set of periodic video
signals, including the step of exposing each pixel on a
photosensive sheet to light of a predetermined brightness
for a period of time functionall~ dependent on the level of
the video signal representing the corresponding pixel in the
frame. As seen in Fig. 8, counters 84 and 94, ROM 86, and
clock 88, together with DAC 58, constitute means for
shifting the reference voltage applied to the reference
input of comparator 54. Counters 84 and 94 constitute means
for establishing the reference voltage; and ROM 8~, as its
operation affects the operation of clock 88, maintains the
voltage at a predetermined level for a period of time
functionally related to the level.
It should be noted that counters 84 and 94 can be
combined into a single counter, because the contents of each
of these counters is identical at any given time. Ilowever,
for simplification purposes, two counters are shown. Thus,
these counters constitute means for shifting the voltage
applied to the reference input of the comparator at a
frequency functionally related to and synchronized with the
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frame frequency of the television system.
- It should be further noted that, although an
analog comparator and an analog video signal are described,
it would also be readily apparent to utilize a digital video
signal with a digital comparator wherein the digital
comparator may comprise both the comparator 54 and the DAC
58.
It is believed that the advantages and improved
results furnished by the method and apparatus of the present
invention are apparent from the foregoing description of the
preferred embodiment of the invention. Various changes and
modifications may be made without departing from the spirit
and scope of the invention as described in the claims that
follow.
