Vertical aperture correction circuit

A video signal, such as may be generated by a television camera, is subjected to a delay, and then combined with a relatively undelayed version thereof, the combined signal being supplied to a clipping circuit, which passes at least that portion thereof that exceeds a predetermined clipping level, and also to a slicing circuit, which passes at least that portion thereof that is less than the predetermined clipping level. A mixing circuit mixes the video signal with the outputs of the clipping and slicing circuits, thereby to provide a vertical aperture-corrected output video signal which is substantially free of periodic fluctuations that might be present in the amplitude of the original video signal. A preferred use of this vertical aperture correction circuit is with a color image pickup device of the type which generates a video signal having a superimposed periodic, fluctuating index signal thereon that produces a line-crawling effect in the ultimately produced video picture.

BACKGROUND OF THE INVENTION 
This invention relates to a vertical aperture correction circuit and, more 
particularly, to such a circuit which can be used in a color image pickup 
device to emphasize changes in the brightness level of the video picture 
from one line interval to the next while minimizing any line-crawling 
effect which might be inherent in the video signal. 
In the color television picture which is derived from many color television 
cameras, the so-called edge sharpness, or sensitivity, of that picture is 
not as well-defined as in the picture derived from black-and-white 
television cameras. That is, a transition in brightness, or contrast, from 
one horizontal line interval to the next may not exhibit a desirable level 
of sharpness. Consequently, a viewer may not perceive accurate detail in 
the vertical direction. This loss of sharpness in the vertical direction, 
that is, in the direction perpendicular to the direction of line scanning, 
is analogous to aperture aberrations in an optical system. 
Various proposals have been suggested for improving this sharpness. Such 
compensation or correction systems have been referred to generally as 
vertical aperture compensation systems. In one type of vertical aperture 
compensation system, the luminance signal, which may be the television 
signal generated by a black-and-white television camera or the luminance 
component of a composite color television signal generated by a color 
television camera, is delayed by one horizontal scanning, or line, 
interval, and the difference between the delayed and undelayed luminance 
signals then is derived. If the luminance level in successive line 
intervals is approximately the same, the aforementioned difference signal 
exhibits a relatively low amplitude. However, when the brightness level 
changes from one line interval to the next, this difference signal will be 
more pronounced. Consequently, the difference signal can be used as a 
relatively accurate indication of brightness changes in the vertical 
direction. 
To emphasize such brightness level changes in the vertical direction, that 
is, to obtain vertical aperture compensation, a predetermined proportion 
of the difference signal is added to the original, i.e. undelayed, 
luminance signal. The summed signal thus is a vertical aperture-corrected 
luminance signal. 
The aforementioned vertical aperture-correction technique is accompanied by 
undesired interference when used in a Trinicon color television camera. In 
the Trinicon camera, the target end of the pickup tube is provided with a 
set of index electrodes. These index electrodes are supplied with an index 
signal whose polarity is reversed at each horizontal scanning interval, 
thereby superimposing an alternating index signal onto the 
photoelectroconductive target. This index signal appears as a periodic 
fluctuating voltage level superimposed onto the luminance signal derived 
from the Trinicon camera. When the aforementioned vertical 
aperture-compensation technique is used with this luminance signal, the 
superimposed periodic, fluctuating level is emphasized. Therefore, in 
addition to providing an indication of brightness level changes from one 
line interval to the next, the vertical aperture-compensated luminance 
signal is provided with an emphasized, superimposed periodic fluctuating 
level which results in a line-crawling effect in the video picture 
ultimately reproduced therefrom. 
In addition to this periodic fluctuating level derived from the index 
signal, another AC component may be introduced into the luminance signal 
due to the operation of the typical DC-DC converter that is used with the 
television camera. This DC-DC converter is provided in the television 
camera for the purpose of generating various DC control voltages from a 
single supplied DC voltage. Generally, during normal operation thereof, 
the DC-DC converter is supplied with relatively large amounts of power, 
and this has been known to introduce an AC component into the relatively 
low-level luminance signal. Such an AC component may appear as noise, 
typically a striped pattern, on the reproduced television picture. To 
minimize this noise, the driving frequency of the DC-DC converter may be 
synchronized to one-half the horizontal scanning frequency. However, this 
is the very same frequency of the index signal that results in a 
superimposed periodic, fluctuating level on the luminance signal. Thus, 
when the vertical aperture-compensation technique discussed above is used, 
the AC component derived from the DC-DC converter is manifested in the 
aforementioned line-crawling effect. 
One technique which has been proposed for eliminating the line-crawling 
effect due to the superimposed index signal and, presumably, also will 
eliminate the line-crawling effect due to the AC component derived from 
the DC-DC converter, is disclosed in U.S. Pat. No. 4,160,265. According to 
this patent, the difference between the delayed and undelayed luminance 
signal, which is indicative of line-to-line brightness-level changes and 
which also emphasizes the periodic fluctuations superimposed onto the 
luminance signal, is squared, or multiplied by itself, and the squared 
difference signal then is mixed with the sum of the delayed and undelayed 
video signals. The output of the mixing circuit is a vertical 
aperture-corrected luminance signal that is substantially free of 
undesired periodic level fluctuations which may be due to the superimposed 
index signal of the Trinicon camera or may be due to the AC component 
produced from the DC-DC converter. 
In accordance with the present invention, vertical aperture-correction is 
attained in the absence of any line-crawling effect, and the complexity of 
the correction circuit due to the aforementioned squaring circuit is 
reduced. 
OBJECTS OF THE INVENTION 
Therefore, it is an object of the present invention to provide an improved 
vertical aperture-correction circuit for use with a color television 
camera which is substantially free of the aforementioned line-crawling 
effect. 
Another object of this invention is to provide a video signal processing 
circuit which operates upon a video signal to provide vertical 
aperture-correction therefor while substantially eliminating or minimizing 
periodic fluctuations that might be present in the amplitude of that video 
signal. 
A further object of this invention is to provide a vertical 
aperture-correction circuit for use with a color image pickup device of 
the type which generates a video signal having a superimposed periodic, 
fluctuating index signal thereon, which vertical aperture-correction 
circuit cancels or eliminates "line crawling" which may be caused by such 
superimposed fluctuating signal. 
An additional object of this invention is to provide an improved, 
relatively simple and inexpensive vertical aperture-correction circuit 
which is particularly useful with a color image pickup device of the type 
which generates a video signal having a superimposed periodic, fluctuating 
index signal thereon. 
Various other objects, advantages and features of the present invention 
will become readily apparent from the ensuing detailed description, and 
the novel features will be particularly pointed out in the appended 
claims. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a video signal processing circuit is 
comprised of a delay circuit for imparting a relative delay to a video 
signal supplied thereto, and a combining circuit for combining the 
relatively delayed video signal with a relatively undelayed version 
thereof to produce a combined signal. A clipping circuit passes at least 
that portion of the combined signal which exceeds a predetermined clipping 
level, and a slicing circuit passes at least that portion of the combined 
signal which is less than the predetermined clipping level. A mixing 
circuit mixes the video signal with the respective portions passed by the 
clipping and slicing circuits, thereby to provide a corrected output video 
signal substantially free of periodic fluctuations that might be present 
in the amplitude of the supplied video signal. In a preferred application, 
the video signal is supplied by a color image pickup device of the type 
which superimposes a periodic, fluctuating index signal on the video 
signal, this superimposed, fluctuating signal having a line-crawling 
effect on the video picture ultimately reproduced from the video signal. 
The resultant, corrected video signal has its line-to-line brightness 
level changes emphasized.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
Referring now to the drawings, the waveform diagrams illustrated in FIGS. 
1A-1D are typical of the waveforms which are generated in a vertical 
aperture-correction circuit by which vertical sharpness in the video 
picture is improved. FIG. 1A represents the luminance signal E.sub.a whose 
amplitude is a function of the brightness of the video scene. For the 
purpose of simplification, luminance signal E.sub.a is illustrated as 
having two transitions in a constant brightness level. FIG. 1B represents 
a delayed version E.sub.b of the luminance signal. That is, delayed 
luminance signal E.sub.b is delayed by one horizontal scanning, or line, 
interval from luminance signal E.sub.a. The difference between the 
undelayed luminance signal E.sub.a and the delayed luminance signal 
E.sub.b is illustrated as difference signal E.sub.c in FIG. 1C. It is 
appreciated that the waveform shown in FIG. 1C emphasizes brightness-level 
changes in the luminance signal from one line interval to the next. A 
predetermined proportion of difference signal E.sub.c (FIG. 1C) is added 
to the original, undelayed video signal E.sub.a, the summed signals being 
illustrated in FIG. 1D as aperture-corrected luminance signal E.sub.y. It 
may be appreciated that difference signal E.sub.c may be supplied to a 
suitable adding circuit by a voltage-divider circuit which divides the 
difference signal by a predetermined dividing ratio .alpha.. Hence, 
luminance signal E.sub.y may be represented as E.sub.y =E.sub.a 
+.alpha.E.sub.c. 
The waveforms illustrated in FIGS. 2A-2D represent the manner in which a 
typical vertical aperture-correction circuit operates with a Trinicon 
color television camera. The Trinicon camera is described in U.S. Pat. No. 
3,784,737 and includes, at its target end, a set of alternating electrodes 
which are supplied with different DC levels that are reversed in 
synchronism with the horizontal scanning rate. Because of this, a 
fluctuating signal is induced on the photoconductive target which, in 
turn, superimposes onto the luminance signal a fluctuating index signal 
having a frequency equal to one-half the horizontal scanning rate. This 
luminance signal with the superimposed periodic fluctuating level is 
illustrated as luminance signal E.sub.a in FIG. 2A. As before, this 
luminance signal is delayed in the typical vertical aperture-correction 
circuit by one horizontal scanning period, the delayed luminance signal 
being shown as delayed signal E.sub.b in FIG. 2B. The delayed signal 
E.sub.b is subtracted from the original, undelayed luminance signal 
E.sub.a, resulting in the difference signal E.sub.c shown in FIG. 2C. It 
may be appreciated that difference signal E.sub.c is provided with 
emphasized fluctuating levels. When this difference signal E.sub.c is 
added to the original luminance signal E.sub.a, the resultant corrected 
luminance signal E.sub.y appears as shown in FIG. 2D. This corrected 
luminance signal has superimposed thereon a periodic fluctuating signal 
which is derived from the index signal that had been superimposed onto the 
photoconductive target of the color camera. This superimposed fluctuating 
signal has its level changed over at each successive horizontal scanning 
interval, thereby resulting in the "line crawl" effect in the video 
picture which ultimately is reproduced therefrom. That is, the periodic 
change in the brightness level of the luminance signal E.sub.y (FIG. 2D) 
is readily perceived as a line crawl. 
The foregoing disadvantages, particularly the superimposed fluctuating 
level on the vertical aperture-corrected luminance signal, are avoided by 
the present invention, one embodiment of which is illustrated in FIG. 3. 
in this Figure, a video signal processing circuit is coupled to a 
Trinicon-type color video camera 1, the video signal processing circuit 
serving to provide a corrected output video signal substantially free of 
periodic fluctuations that are present in the amplitude of the video 
signal derived from camera 1. It may be appreciated that camera 1 may be 
constructed as a color image pickup device in accordance with the 
disclosure of aforementioned U.S. Pat. No. 3,784,737. As is apparent from 
that disclosure, and as is also described in U.S. Pat. No. 4,160,265, a 
luminance signal E.sub.a may be derived from the composite color video 
signal produced by color camera 1. This luminance signal E.sub.a is 
amplified by a suitable video amplifier 2 and then supplied to the video 
signal processing circuit which is the subject of the present invention. 
The video signal processing circuit functions as a vertical 
aperture-correction circuit and is comprised of a delay circuit 11, a 
subtracting circuit 12, a slicing circuit 21, a clipping circuit 24 and a 
mixing circuit 3. Delay circuit 11 may comprise a conventional delay 
circuit adapted to impart a delay equal to one horizontal scanning 
interval to the video signal supplied thereto. Delay circuit 11 is coupled 
to the output of amplifier 2 and receives luminance signal E.sub.a. The 
output of this delay circuit is coupled to subtracting circuit 12 wherein 
the delayed video signal E.sub.b is subtracted from the original, 
undelayed luminance signal E.sub.a. The output of subtracting circuit 12 
is coupled in common to slicing circuit 21 and to clipping circuit 24. The 
slicing circuit is supplied with predetermined threshold levels by 
suitable means (not shown), and is adapted to pass only those portions of 
the video signal supplied thereto which lie between the threshold levels. 
Preferably, and as will be described in greater detail below, these 
threshold levels are equal and opposite levels +L and -L, respectively, 
disposed on opposite sides of the mean level of the difference signal 
produced by subtracting circuit 12. As an alternative, if the output of 
subtracting circuit 12 is a positive (or negative) signal, slicing circuit 
21 may be supplied with a positive (or negative) threshold level and may 
be adapted to pass only the positive (or negative) portions of the output 
of subtracting circuit 12 which are less than the threshold level. The 
output of slicing circuit 21, that is, that portion of the difference 
signal supplied thereto by subtracting circuit 12, which is less than the 
respective threshold levels supplied thereto, is inverted by an inverter 
circuit 22 and supplied to mixing circuit 3 via an amplitude adjustment 
circuit 23. This amplitude adjustment circuit is illustrated as an 
adjustable resistor, such as a potentiometer, and is settable to supply to 
mixing circuit 3 a predetermined proportion of the inverted output of 
slicing circuit 21. By suitably adjusting amplitude adjustment circuit 23, 
this proportion .beta. may be varied, as desired. 
Clipping circuit 24 is adapted to pass that portion of difference signal 
E.sub.c which exceeds a threshold level. If threshold levels +L and -L are 
supplied to slicing circuit 21, these same threshold levels may be 
supplied to clipping circuit 24, whereupon the clipping circuit passes 
those portions of difference signal E.sub.c which exceed threshold level 
+L and which exceed threshold level -L. An amplitude adjustment circuit 
25, which may be similar to amplitude adjustment circuit 23, supplies the 
output of clipping circuit 24 to mixing circuit 3. It is appreciated that 
amplitude adjustment circuit 25 serves to adjust the proportion of the 
output of the clipping circuit that is supplied to the mixing circuit. 
Mixing circuit 3 is supplied with luminance signal E.sub.a, the desired 
proportion of the inverted output of slicing circuit 21, and the desired 
proportion of the output of clipping circuit 24. Mixing circuit 3 may 
function as a summing circuit so as to sum the respective signals supplied 
thereto. The output of mixing circuit 3 is supplied to an output terminal 
as a vertical aperture-corrected luminance signal E.sub.y. 
It is appreciated that the function of inverting circuit 22 is to subtract 
the output of slicing circuit 21 from the summation of video signal 
E.sub.a and the output of clipping circuit 24. If desired, inverting 
circuit 22 may be omitted, and mixing circuit 3 may be comprised of 
respective circuit devices which carry out the aforementioned summing and 
subtracting operations. For example, luminance signal E.sub.a may be 
summed with the output of clipping circuit 24 in an adding circuit, and 
the output of slicing circuit 21 then may be subtracted from the summed 
signals in an additional subtracting circuit. 
The manner of operation of the vertical aperture-correction circuit shown 
in FIG. 3 now will be described with reference to the waveforms shown in 
FIGS. 4A-4G. Luminance signal E.sub.a, derived from video camera 1, is 
illustrated in FIG. 4A. The periodic fluctuating index signal is shown to 
be superimposed onto the brightness level of the luminance signal. FIG. 4B 
illustrates the luminance signal delayed by one horizontal scanning 
interval. This delayed luminance signal E.sub.b is derived at the output 
of delay circuit 11. 
Subtracting circuit 12 subtracts the delayed luminance signal E.sub.b from 
the undelayed version of the luminance signal E.sub.a to produce 
difference signal E.sub.c shown in FIG. 4C. It is seen that this 
difference signal E.sub.c emphasizes the changes in brightness of the 
luminance signal from one horizontal line interval to the next, and also 
emphasizes the periodic fluctuating levels in the luminance signal. This 
difference signal E.sub.c, having the emphasized brightness changes and 
periodic fluctuating levels, is supplied to slicing circuit 21 and also to 
clipping circuit 24. 
FIG. 4C also illustrates threshold levels +L and -L, these threshold levels 
being represented by the broken lines, and being supplied as threshold 
reference voltage levels to the slicing and clipping circuits. Threshold 
levels +L and -L are, for example, equal and opposite threshold levels 
disposed on opposite sides of the mean level of difference signal E.sub.c. 
Slicing circuit 21 serves to pass that portion of difference signal 
E.sub.c which lies between threshold levels +L and -L. This portion of the 
difference signal that is passed by slicing circuit 21 is illustrated as 
signal E.sub.d in FIG. 4D. This passed portion E.sub.d is inverted by 
inverting circuit 22, level-adjusted by amplitude adjustment circuit 23, 
and then summed in mixing circuit 3 with luminance signal E.sub.a. This 
has the equivalent effect of subtracting the output E.sub.d of slicing 
circuit 21 (suitably amplitude-adjusted) from the luminance signal. FIG. 
4E represents this operation, and illustrates a signal E.sub.e which would 
be produced if the output E.sub.d from slicing circuit 21 is subtracted 
from luminance signal E.sub.a. That is, FIG. 4E illustrates a waveform 
which is formed by the operation E.sub.e =E.sub.a -E.sub.d. It may be 
appreciated that the proportion of the output E.sub.d from slicing circuit 
21 that is supplied to mixing circuit 3 is determined by amplitude 
adjustment circuit 23 such that the periodic fluctuations shown in FIG. 4D 
are attenuated so as to be substantially equal to the periodic 
fluctuations that are superimposed onto luminance signal E.sub.a (FIG. 
4A). Hence, and as shown in FIG. 4E, the signal E.sub.e which would be 
produced by subtracting the amplitude-adjusted output E.sub.d of slicing 
circuit 21 from luminance signal E.sub.a exhibits substantially no 
periodic amplitude fluctuations. 
Clipping circuit 24 also may be supplied with threshold levels +L and -L. 
The clipping circuit functions to pass the positive portion of difference 
signal E.sub.c which exceeds threshold level +L, and also passes the 
negative portion of difference signal E.sub.c which exceeds threshold 
level -L. The output E.sub.f of clipping circuit 24 is illustrated in FIG. 
4F. This output E.sub.f, after being suitably amplitude adjusted by 
amplitude adjustment circuit 25 is added to the signal E.sub.e (FIG. 4E). 
The summed signal E.sub.y =E.sub.e +E.sub.f is shown in FIG. 4G and is 
supplied to output terminal 4 by mixing circuit 3. Stated otherwise, this 
corrected luminance signal E.sub.y produced by mixing circuit 3 may be 
mathematically represented as E.sub.y =E.sub.a -E.sub.d +E.sub.f. It is 
seen that this corrected luminance signal is provided with emphasized 
brightness level changes, that is, changes in the brigtness level from one 
line interval to the next are emphasized, but is substantially free of the 
periodic amplitude fluctuations inherent in luminance signal E.sub.a. A 
comparison between the waveforms shown in FIGS. 4G and 2D illustrates the 
improvement obtained by the present invention. Thus, aperture-corrected 
luminance signal E.sub.y (FIG. 4G) does not give rise to the undesired 
line crawling effect in the video picture ultimately reproduced therefrom. 
In the present invention, it is seen that difference signal E.sub.c (FIG. 
4C) is further processed before being mixed with luminance signal E.sub.a. 
But for this further processing, the corrected luminance signal would 
appear as shown in FIG. 2D. That is, if difference signal E.sub.c is used 
directly as a vertical aperture-correction signal, the "corrected" 
luminance signal would have brightness level changes emphasized, and also 
would contain enforced fluctuating levels, as shown in FIG. 2D. However, 
in accordance with the embodiment shown in FIG. 3, the vertical 
aperture-corrected luminance signal E.sub.y (FIG. 4G) more closely 
resembles the corrected luminance signal shown in FIG. 1D, this latter 
signal being derived from a color television camera which does not 
superimpose a periodic, fluctuating level onto the video signal. 
Slicing circuit 21 may be supplied with a single threshold level +L and may 
be operative to pass the absolute value of difference signal E.sub.c which 
is less than this threshold level. Likewise, clipping circuit 24 may be 
supplied with a single threshold level +L and may be operative to pass the 
absolute value of difference signal E.sub.c which exceeds this threshold 
level. 
The respective settings of amplitude adjustment circuits 23 and 25 are 
assumed to differ from each other. Amplitude adjustment circuit 23 serves 
to attenuate the level of the signal supplied thereto by a factor .beta. 
and amplitude adjustment circuit 25 serves to attenuate the signal 
supplied thereto by the factor .gamma.. The attenuating ratio .beta. 
serves to eliminate the fluctuating level of luminance level E.sub.a ; and 
the attenuating ratio .gamma. serves to provide desired emphasis of 
brightness-level changes. 
The embodiment shown in FIG. 3 is a relatively simplified version of the 
present invention. A more practical version is illustrated in FIG. 5, 
wherein like reference numerals are used to identify like component parts. 
The embodiment of FIG. 5 differs from that of FIG. 3 in that an additional 
delay circuit 13 is connected in cascade with delay circuit 11, the output 
of delay circuit 13 being summed in a summing circuit 14 with luminance 
signal E.sub.a. The summed signal E.sub.h produced by summing circuit 14 
is subtracted in a subtracting circuit 15 from the delayed luminance 
signal E.sub.b produced at the output of delay circuit 11. In the 
embodiment of FIG. 5, delay circuits 11 and 13 each impart a delay equal 
to one horizontal line interval. If desired, luminance signal E.sub.a may 
be supplied to subtracting circuit 15 by delay circuit 11, and this 
luminance signal may be supplied directly to summing circuit 14, as 
illustrated, and also to this summing circuit via another delay circuit 
(not shown) which serves to impart a delay equal to two horizontal line 
intervals. In this alternative arrangement, delay circuit 13 is omitted, 
and the output of delay circuit 11 is connected only to subtracting 
circuit 15 (and also to mixing circuit 3). 
In operation, luminance signal E.sub.a appears as shown in FIG. 6A. This 
luminance signal is subjected to a first delay, equal to one horizontal 
line interval, by delay circuit 11, resulting in the delayed luminance 
signal E.sub.b shown in FIG. 6B. This delayed video signal E.sub.b is 
further delayed by another horizontal line interval in delay circuit 13, 
resulting in a 2H delayed luminance signal E.sub.g, shown in FIG. 6C. As 
mentioned above, this 2H delay may, alternatively, be produced by a single 
delay circuit which imparts a time delay equal to two horizontal line 
intervals to luminance signal E.sub.e. 
2H delayed luminance signal E.sub.g is summed with undelayed luminance 
signal E.sub.a in summing circuit 14, resulting in the summed signal 
E.sub.h shown in FIG. 6B. Preferably, delayed signal E.sub.g is attenuated 
by a factor of 1/2, and undelayed luminance signal E.sub.a likewise is 
attenuated by the factor of 1/2. Hence, summed signal E.sub.h shown in 
FIG. 6D may be represented as E.sub.h =1/2 (E.sub.a +E.sub.g). 
Summed signal E.sub.h (FIG. 6D) is subtracted from 1H delayed luminance 
signal E.sub.b in subtracting circuit 15, resulting in the difference 
signal E.sub.c, shown in FIG. 6E. This difference signal E.sub.c may be 
represented as E.sub.c =E.sub.b -E.sub.h. It is appreciated that this 
difference signal E.sub.c emphasizes the brightness-level changes in the 
luminance signal from one line interval to the next and, moreover, 
emphasizes the periodic fluctuating level which has been superimposed onto 
luminance signal E.sub.a. 
As before, this difference signal E.sub.c is supplied to slicing circuit 
21, which slicing circuit passes that portion of difference signal E.sub.c 
which lies between threshold levels +L and -L. The signal E.sub.d passed 
by slicing circuit 21 is shown in FIG. 6F. This passed signal E.sub.d is 
inverted by inverting circuit 21, amplitude adjusted by amplitude 
adjustment circuit 23, and then summed with delayed luminance signal 
E.sub.b in mixing circuit 3. 
Difference signal E.sub.c is supplied to clipping circuit 24 which passes 
that portion of the difference signal that exceeds the threshold levels +L 
and -L. The output E.sub.f of clipping circuit 24 is illustrated in FIG. 
6G. This passed signal E.sub.f is amplitude adjusted by amplitude 
adjustment circuit 25, and then summed in mixing circuit 3 with delayed 
luminance signal E.sub.b and amplitude-adjusted, inverted signal E.sub.d. 
The output of mixing circuit 3 appears as a vertical aperture-corrected 
luminance signal E.sub.y, as shown in FIG. 6H. This corrected luminance 
signal has the brightness-level changes emphasized therein and, moreover, 
the fluctuating level that had been superimposed onto the original 
luminance signal E.sub.a (FIG. 6A) is eliminated. Thus, vertical aperture 
correction is attained in the absence of a line crawling component. 
It is appreciated that, in FIG. 3, delay circuit 11 and subtracting circuit 
12 function as a combining circuit for combining the relatively delayed 
luminance signal E.sub.b and undelayed luminance signal E.sub.a. Likewise, 
in FIG. 5, delay circuits 11 and 13, together with summing circuit 14 and 
subtracting circuit 15 function as a combining circuit for combining 
undelayed luminance signal E.sub.a with delayed luminance signal E.sub.g, 
these combined signals being further combined with delayed luminance 
signal E.sub.b. In both embodiments, the combining circuits serve to 
emphasize changes in the brightness of the luminance signal from one 
horizontal line interval to the next, and also serve to emphasize the 
periodic fluctuating levels which had been superimposed onto the luminance 
signal by the inherent operation of camera 1. 
While the present invention has been particularly shown and described with 
reference to certain preferred embodiments, it should be readily apparent 
to those of ordinary skill in the art that various changes and 
modifications in form and details may be made without departing from the 
spirit and scope of the invention. It is intended that the appended claims 
be interpreted as including such changes and modifications.