FR2473240A1 - COLOR CAMERA COMPRISING A VERTICAL OPENING CORRECTION CIRCUIT - Google Patents

Abstract

A. TELEVISION CAMERA. B. CAMERA CHARACTERIZED BY A DELAY MEANS 11 BY THE VIDEO SIGNAL WHICH IS COMBINED WITH THE ORIGINAL SIGNAL EA, THE SIGNAL OBTAINED BEING WRITTEN 24 AND AX 21 AND THE TWO SIGNALS OBTAINED ARE COMBINED 3. C. THE INVENTION CONCERNS THE PROCESSING SIGNALS IN VIDEO SHOOTING.

Description

The present invention relates to a color camera with a correction circuit

  vertical opening to accentuate

  changes in the level of clarity of the video image of an inter-

  from one line to another while reducing any effect of

  line mismatch that can be specific to the video signal. In the color television picture provided by many color television cameras, the accuracy of the

  edge or the sensitivity of the image is not well defined when

  that the picture comes from a black and white television camera.

  The transition from the clarity or contrast of a horizontal line interval to the next interval may not have the desired level of precision. As a result, the viewer can

  do not perceive the precise details in the vertical direction.

  This loss of precision or sharpness in the vertical direction, that is to say in the direction perpendicular to the scanning line, is analogous to the aberrations of opening of an optical system. Various proposals have been made to improve

  this precision. Such correction or compensation systems

  have been generally referred to as compensation systems

  vertical opening. In such a system of compensation

  the luminance signal, which may be the tele-

  provided by a black-and-white television camera, or the luminance component of a color television signal, composed, provided by a color television camera, is delayed by a horizontal scan line interval and the difference between the delayed luminance signal and that which is not delayed. If the luminance level in successive line intervals is approximately the same,

  the difference signal mentioned above has an ampli-

  relatively low study. However, if the level of clarity

  changes from one line interval to another, this signal of difference

  will be more accentuated. As a result, the difference signal can be used as an indication of accuracy or sharpness

  relative variations in clarity in the vertical direction.

  To accentuate variations in the level of clarity

  in the vertical direction that is to say to obtain a

  vertical aperture, we add a predefined part

  the difference signal to the original luminance signal, ie to the undelayed luminance signal. The added signal is thus a luminance signal whose opening

vertical is corrected.

  The vertical aperture correction technique

  mentioned above is accompanied by an interfering phenomenon

  inconvenient when this signal is used in a Trinicon color television camera. In a Trinicon type camera,

  the target in the image tube comprises a set of electrodes of

  dexage. These indexing electrodes receive an indexing signal whose polarity is inverted for each horizontal scan interval so as to overprint an indexing signal that is alternating with the photoelectric target. This indexing signal appears as a periodically fluctuating voltage level that is added to the luminance signal derived from the Trinicon camera. When using the vertical aperture compensation described above for such a luminance signal, the level of

  periodic fluctuations that we add.

  Therefore, in addition to the indication of changes

  level of clarity of a line interval at the next interval, the vertical aperture compensation luminance signal has a periodically fluctuating level,

  added and accented, resulting in an effect of

  line in the final video image.

  In addition to this periodic fluctuation caused by the indexing signal, the operation of a converter

  Continuous-DC characteristic used in the TV camera

  vision can add another alternative component to the luminance signal. This DC-DC converter includes a television camera for generating various DC control voltages from a single DC voltage. In general, during normal operation, the DC / DC converter receives a relatively large power and it is known to introduce an AC component into this signal.

  relatively low level luminance. This alternative component

  tive may appear as a noise and typically a striped trace on the reproduced television image. To reduce this noise to a minimum, the training frequency of the

  DC / DC converter can synchronize on the half

  horizontal scan period. However, this is the precise frequency of the indexing signal that gives a level

  periodic fluctuation, added to the luminance signal. So when

  using the vertical aperture compensation technique

  described above, the AC component related to the operation of the DC / DC converter is

  by the overlapping line effect mentioned above.

  It has already been proposed to eliminate this line overlap effect caused by the addition of the index signal and probably the line overlap effect caused by the AC component generated by the DC / DC converter.

  continuous using the solution described in US Patent 4,160,265.

  According to this document, the difference between the delayed luminance signal and the undelayed luminance signal is an indication of the variations in the level of line-to-line clarity, and which also accentuates the superimposed periodic fluctuations at the luminescent signal, this difference is squared (multiplied by itself), then the difference signal thus squared is mixed with the sum of the delayed video signal and the undelayed video signal. The output of the mixer is a vertical aperture correction luminance signal which practically no longer contains the periodic level fluctuations existing in the indexing signal of the Trinicon camera and caused by the AC component generated by the DC / DC converter.

  The object of the invention is to create a circuit of

  to eliminate any line overlap effect and to reduce the complexity of the correction circuit to

  using the squaring circuit mentioned above.

  Another object of the invention is to create a vertical aperture correction circuit applicable to a camera of

  color television virtually eliminates the effect of

  line processing, to process a video signal and ensure its vertical aperture correction by practically eliminating or minimizing periodic fluctuations which

  can exist in the amplitude of the video signal.

  It is also an object of the invention to provide a vertical aperture correction circuit applicable to a color shooting device which provides a video signal on which a periodic fluctuation index signal is superimposed so that the correction circuit vertical opening eliminates the line overlap that may be generated by this fluctuating signal, this correction circuit must be relativemez

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  simple and inexpensive, to apply in particular to a color shooting device generating a video signal

  superimposed on a periodically fluctuating indexing signal

  diques. To this end, the invention relates to a color television camera comprising a video signal processing circuit formed by a delay circuit to give a delay.

  the video signal it receives and a combination circuit

  sound to combine the delayed video signal relatively and a non-delayed signal to give a combined signal. A clipping circuit passes at least a portion of the combined signal that exceeds the predetermined clipping level, and a clipping circuit passes at least a portion of the combined signal.

  which is below the predetermined clipping level. A mixture

  the video signal to the respective parts which have passed through the clipping circuit and the switching circuit for

  give a corrected output video signal, containing no practical

  more periodic fluctuations affecting the amplitude of the

video signal provided.

  According to a preferred characteristic, the video signal is provided by a color-taking device which superimposes a periodic fluctuation index signal on the video signal, this fluctuating signal being likely to cause a

  line overlap in the video image permanently reproduced

  from this video signal. In the corrected video signal, obtained, the level of clarity variations from line to line

are accentuated. -

  The present invention will be described in more detail with the aid of the accompanying drawings, in which - figures LA-1D are timing diagrams for

  explain the operation of an opening correction circuit

characteristic vertical ture.

  FIGS. 2A-2D are timing diagrams for

  explain the operation of an opening correction circuit

  vertical character, a characteristic applied to a color shooting device in which the indexing signal

  periodic fluctuation is superimposed on the video signal.

  FIG. 3 is a block diagram of one embodiment of

of the invention.

  FIGS. 4A-4G are timing diagrams used to

  explain the operation of the embodiment of Figure 3.

  FIG. 5 is a block diagram of a variant of the

circuit of the invention.

  - Figures 6A-6H are timing diagrams for explaining the operation of the embodiment of Figure 5.

  DETAILED DESCRIPTION OF DIFFERENT PREFERRED EMBODIMENTS

petitive

  Figures lA-lD represent timing diagrams

  characteristics generated by an opening correction circuit.

  vertically enhancing the vertical sharpness of the video image.

  Figure LA represents the luminance signal Ea whose amplitude is a function of the clarity of the video scene. In a simplified manner, the luminance signal E has two transitions for a constant level of clarity. Figure 1B shows the delayed version Eb of the luminance signal. The delayed luminance signal E b is delayed by a horizontal scanning period or a line interval with respect to

  luminance signal E a. The difference between the light signal

  and the delayed luminance signal Eb is shown as the difference signal Ec by the timing diagram of FIG. 1C. It should be noted that the curve shown in FIG. 1C accentuates the variations in the level of clarity in the luminance signal from one line interval to the next interval. A predetermined fraction of the difference signal Ec

  (FIG. 1C) is added to the non-delayed video signal Ea, of origin

  gine; the signal obtained by addition is represented in FIG.

  1D; it constitutes the luminance signal with correction of opening

  It should be noted that the difference signal E can be applied to a suitable adder by a divider of

  voltage that divides the difference signal from a division ratio

  predetermined resolution. o. Thus the luminance signal E can

write as Ey = Ea + eEc.

  The timing diagrams of FIGS. 2A-2D show the operation of a typical vertical aperture correction circuit in the case of a Trinicon type color television camera. US Pat. No. 3,784,737 describes the Trinicon camera which comprises at its target, a set of alternating electrodes receiving different continuous levels which are

  reversed in synchronism with the horizontal scanning frequency.

  This results in a fluctuating signal induced in the target photo-

  conductive superimposed in turn a indexing signal fluctuating with the luminance signal, the indexing signal having a frequency equal to half the horizontal scan frequency. This luminance signal, on which is superimposed a periodic fluctuation level, is represented by the luminance signal E in FIG. 2A. As before, this signal

  luminance is delayed in the opening correction circuit.

  vertically, following a period of horizontal sweeping; the retarded luminance signal is shown as delayed signal Eb in FIG. 2B. The delayed signal is subtracted from the original signal, ie the undelayed luminance signal Ea, to give the difference signal Ec (FIG. 2C). This difference signal Ec has accentuated levels of fluctuations. When the difference signal Ec is added to the original luminance signal Ea, the corrected luminance signal Ey shown in FIG. 2D is obtained. This corrected luminance signal superimposed a periodic fluctuation signal which is derived from the indexing signal

  superimposed on the photoconductive target of the color camera.

  This fluctuating signal, superimposed, changes level at each inter-

  successive horizontal scanning range, to give a line overlapping effect in the video image which is reproduced ultimately from this signal. The periodic change in the level of clarity of the luminance signal Ey (FIG. 2D) is

  thus perceived as overlapping of line.

  The above disadvantages, in particular the fluctuating level which is superimposed on the vertical aperture correction luminance signal, are avoided by virtue of the present invention of which FIG. 3 gives an embodiment. In this figure, the video signal processing circuit is coupled to a color video camera 1, Trinicon type; the processing circuit of the

  video signal gives an output video signal, corrected

  It is noted that camera 1 can be made as described in US Pat. This document as well as US Pat. No. 4,160,265 show that the luminance signal E can be derived from the combined video signal provided by the video camera.

  This luminance signal E is amplified by an ampli-

  appropriate video indicator 2, and is then supplied to the processing circuit.

  the video signal according to the invention.

  The video signal processing circuit works

  as a vertical aperture correction circuit; this cir-

  bake consists of a delay circuit 11, a subtractor 12, a hash circuit 21, a clipping circuit 24 and a mixer 3. The delay circuit 11 may be a circuit of FIG. conventional delay giving a delay equal to a horizontal scan interval to the video signal it receives. The delay circuit 11 is coupled to the output of the amplifier 2 and receives the luminance signal E. The output of this delay circuit is coupled to the subtracter 12 which subtracts the signal

  Delayed video Eb at non-delayed luminance signal Ea of origin.

  The output of the subtractor 12 is coupled in common to the hash circuit 21 and the clipping circuit 24. The hash circuit 21 receives predetermined threshold levels provided by a suitable means not shown to pass only the portions of the video signal. lying between the thresholds-. Preferably, as will be described in detail later, the thresholds are equal and opposite + L and -L, on either side of the average level of the difference signal supplied by the subtractor 12. In a variant, if the output signal of the subtractor 12 is a positive (or negative) signal, the hash circuit 21 can

  receive a positive (or negative) threshold and let alone

  the positive (or negative) part of the output signal of subtractor 12, lower threshold. The output of the hashing circuit 21, that is to say the part of the difference signal supplied by the subtractor 12 and which is lower than the respective threshold applied to this hashing circuit, is inverted by the inverter 22 to supply the mixer 3. by means of an amplitude adjustment circuit 23. This amplitude adjustment circuit 23 is composed according to the figure of a variable resistor such as a potentiometer and is adjusted to provide the mixer 3 with a predetermined part. of the inverted output signal provided by the hash circuit 21. This fraction P can be modified by appropriately adjusting the control circuit

amplitude 23.

  The clipping circuit 24 passes the portion of the difference signal Ec which is greater than a threshold. If the thresholds + L and -L are set on the hash circuit 21, the same thresholds can be provided on the clipping circuit 24 so that this circuit only allows the portions of the difference signal Ec which exceed the threshold + L and those which exceed the threshold -L (the excess is taken for the absolute values). The amplitude adjusting circuit 25, which may be analogous to the amplitude adjusting circuit 23, supplies the output signal of the clipping circuit 24 to the mixer 3. It should be noted that the amplitude control circuit 25 is used to set the fraction of the output signal of the clipping circuit that is

supplied to the mixer 3.

  The mixer 3 receives the luminance signal Ea, the desired fraction of the inverted output of the hash circuit 21

  and the desired fraction of the output of the clipping circuit 24.

  The mixer 3 can function as adder to add the different signals it receives. The output signal of the mixer 3 is supplied to the output terminal as

  luminance signal E with vertical aperture correction.

  The role of the inverter 22 is to subtract the output signal of the hashing circuit 21 from the sum of the video signal Ea and the output signal of the clipping circuit 24. The case

  where appropriate, the inverter 22 can be suppressed and the

  3 using the respective circuits that perform the additions and subtractions above. For example, it is possible to add the luminance signal Ea to the output of the clipping circuit 24 in the adder, and then to subtract the output of the hash circuit 21 from the sum of the signals supplied by a

additional subtractor.

  The operation of the vertical aperture correction circuit according to Figure 3 will be described below using the curves of Figures 4A-4G. The luminance signal Ea provided by the video camera 1 is shown in FIG. 4A. The periodic fluctuation index signal is represented as superimposed on the brightness level of the luminance signal. Figure 4B shows the delayed luminance signal of a horizontal scan interval. This delayed Eb luminance signal is

  obtained at the output of the delay circuit 11.

  The subtractor 12 subtracts the delayed luminance signal E b from the undelayed luminance signal Ea to give the difference signal E (FIG. 4C). It can be seen that this difference signal E increases the brightness variations of the luminance signal from a horizontal line interval to the next interval; it also increases the levels of periodic fluctuations of the luminance signal. This difference signal Ec whose brightness variations and the periodically fluctuating levels are amplified, is supplied to the hashing circuit 21 as well as to the clipping circuit 24. FIG. 4C also shows the thresholds + L, -L by mixed lines; these thresholds are provided as reference voltage levels to the hash and clipping circuits. The levels of the thresholds + L and -L are for example equal and opposite, on either side of the average level of the difference signal E. The hashing circuit 21 serves to pass the portion of the difference signal E c which is found between the thresholds + L and -L. This part of the difference signal which passes through the circuit 21 is represented by the signal E a d

  Figure 4D. This passing part Ed is reversed by the invert-

  22, its level is adjusted by the amplifier control circuit.

  Study 23, then it is added in the mixer 23 to the luminance circuit Ea. This has an effect equivalent to the subtraction of the output Ed of the hash circuit 21 (whose amplitude is

  adjusted accordingly) from the luminance signal.

  FIG. 4E shows this operation and indicates the signal Ee that would be obtained if the output signal Ed of the hash circuit 21 was retracted from the luminance signal Ea. FIG. 4E shows the curve obtained by doing the operation E e = Ea-Ed. It should be noted that the proportion of the output signal Ed supplied by the hashing circuit 21 and which is applied to the mixer 3 is determined by the circuit amplitude adjustment 23 to attenuate the periodic fluctuations shown in FIG. 4D, so that they are

  substantially equal to the periodic fluctuations which are super-

  posed to the luminance signal Ea (FIG. 4A). Thus, as represented in FIG. 4E, the signal Ee is obtained by subtracting the amplitude-adjusted output Ed from the hashing circuit 21 and

  the luminance signal Ea has substantially more fluctuation

periodic amplitude statements.

  The clipping circuit 24 can also receive the thresholds + L and -L. The clipping circuit passes the positive part of the difference signal E which exceeds the threshold + L as well as the negative part of the difference signal Ec which in absolute value exceeds the absolute value of the threshold -L. The exit

  Ef of the clipping circuit 24 is shown in FIG. 4F.

  After its correct adjustment in amplitude by the amplitude adjustment circuit 25, the output Ef is added to the signal E (FIG. 4E). The sum signal Ey = Ee + Ef (FIG. 4G) is supplied to the output terminal 4 by the mixer 3. In other words, the correct luminance signal Ey 1 supplied by the mixer 3 can be represented by the following mathematical formula: Ey = Ea - Ed + Ef. We see that the luminance signal, corrected

  presents variations in the level of clarity amplified

  that is, the level of clarity of a line interval is magnified relative to the clarity level of the interval of

  following line while showing virtually no fluc-

  periodic amplitude oscillations specific to the luminance signal E a. The comparison between the curves of FIGS. 4G and 2D shows the improvement of the invention. So the luminance signal at

  opening correction Ey (Figure 4G) does not produce an overlap

  troublesome line in the video image permanently reproduced. According to the invention, the difference signal Ec (FIG.

  4C) is further processed before being mixed with the light signal.

  nance Ea. For this processing, the corrected luminance signal would appear as in Figure 2D. If the difference signal

  E was used directly as a correction signal for

  vertically, the "corrected" luminance signal would have variations in levels of clarity, amplified and contain

  levels of increased fluctuations as in Figure 2D. Toute-

  time according to the invention shown in FIG. 3, the signal of

  luminance Ey corrected in vertical aperture (Figure 4G) similar

  more closely to the corrected luminance signal shown in Figure ID, the latter being derived from the

  color television that does not superimpose levels of color.

  periodicals at the video sSinal.

  The hash circuit 21 can receive only one

  threshold + L and can process the absolute value of the difference signal.

  ec to pass the signal whose absolute value is below the threshold. Similarly, the clipping circuit 24 can receive only one threshold + L and pass the value

  absolute of the difference signal E c which exceeds this threshold.

  The different settings provided by the circuits of

  amplitude adjustment 23, 25 are assumed to be different from each other.

  The amplitude adjusting circuit 23 serves to attenuate the level of the signal applied thereto, by using a coefficient (0, the amplitude adjustment circuit 25 serves to attenuate the signal it receives by the coefficient -Y. The attenuation ratio A serves to suppress the level of fluctuations of the luminance signal Ea, the attenuation coefficient Y serves to accentuate

  variations in the level of clarity.

  The embodiment of Figure 3 is a relatively simplified version of the invention. A version closer to reality is shown in FIG. 5 in which the same references serve to designate the same elements and whose

  the description will not be repeated. The embodiment of

  Figure 5 differs from that of Figure 4 by an additional delay circuit 13 cascaded on the delay circuit 11; the output signal of the delay circuit 13 is added to the luminance signal Ea in the adder 14. The addition signal Eh supplied by the adder 14 is subtracted from the delayed luminance signal Eb supplied by the delay circuit 11, in the subtractor 15. In the embodiment of FIG. 5, the delay circuits 11, 13 each give a delay equal to a horizontal line interval.

  the luminance signal E may be supplied to the subtraction

  aeur 15 by the delay circuit 11; this luminance signal can also be supplied directly to the adder 14 as shown as well as to this adder by another delay circuit (not shown) which gives a delay equal to two horizontal line intervals. In this variant, the delay circuit 13 is eliminated and the output of the delay circuit 11 is connected directly to the subtractor 15 (as well as to the

mixer 3).

  In operation, the luminance signal Ea corresponds to

  to the timing diagram of Figure 6A. This luminance signal

  undergoes a first delay equal to a horizontal line interval

  tale provided by the delay circuit 11 and thus giving a delayed luminance signal Eb (Figure 6B). This delayed video signal Eb is further delayed by another horizontal line interval by the delay circuit 13, resulting in a delayed luminance signal E9 of two periods of horizontal line 2H (FIG. 6G). As indicated, this delay of 2H can alternatively be obtained by a single delay circuit which gives a delay equal to two horizontal line intervals for the luminance signal E e The luminance signal E delayed by 2H is added to the signal luminance E not delayed in the adder 14, which gives the sum signal Eh (FIG. 6B). Preferably, the delayed signal E is attenuated by a factor of 1/2 and the undelayed luminance signal Ea is attenuated in the same way by a coefficient 1/2. Thus the sum signal Eh shown in FIG. 6D can be written as follows Eh = 1/2 (Ea + Eg).

  The addition signal Eh (FIG. 6D) is subtracted from the

  luminance signal Eb delayed by a period 1H in the

  tractor 15, which gives the difference signal Ec (Figure 6E).

  This difference signal Ec is Ec = Eb - Eh. This difference signal E. accentuates the variations in brightness level of the luminance signal from a line interval to

  the next interval and in addition the level of

  Periodic tuition that has been added to the luminance signal Ea * As indicated above, the difference signal Ec is supplied to the hash circuit 21 which passes the portion of the difference signal Ec between the thresholds + L and -L. . The signal Ed which passes through the hash circuit 21 is

  shown in Figure 6F. This Ed signal is inverted by the

  21 to be adjusted in amplitude by the amplitude adjustment circuit 23, then added to the luminance signal

delayed Eb by the mixer 3.

  The difference signal Ec is supplied to the clipping circuit 24 which passes the portion of the difference signal which exceeds the thresholds + L and -L. The output signal Ef of the clipping circuit 24 is shown in FIG. 6G. This Ef signal which passes through is adjusted in amplitude by the amplitude adjustment circuit 25, then is added in the mixer 3 to the delayed luminance signal Eb and the inverted signal Ed whose amplitude is set. The output signal of the mixer 3 appears as the vertical aperture correction luminance signal E (FIG. 6H). This corrected luminance signal exhibits variations of clarity levels which are accentuated and further the level of fluctuations which has been superimposed on the original luminance signal E is suppressed (FIG. 6A). We have a correction of vertical opening without component

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line overlap.

  It should be noted that according to FIG. 3, the delay circuit 11 and the subtractor 12 function as combined circuits to bring together the relatively delayed luminance signal Eb and the undelayed luminance signal E. Likewise in FIG. 5, the circuits 1.1 and 13 in conjunction with the adder

  14 and the subtractor 15 function as combinational circuits.

  to combine the non-delayed luminance signal Ea and the delayed luminance signal E; these combined signals are

  again combined with the retardance signal Eb.

  In both embodiments, the combination circuits

  serve to accentuate the variations in the brightness of the light signal.

  nance of a horizontal line interval to the next interval and to accentuate periodic

  superimposed on the luminance signal due to the function

the camera 1.

Claims (4)

REVENDICATI 0 NS) Television camera comprising a video signal processing circuit, camera characterized in that it comprises means (1) providing a video signal (E a), delay means (11) giving the video signal ( E) a delay, combining means (12) for combining the delayed video signal (Eb) and the undelayed video signal (Ea) to give a combined signal (E), a clipping means (24) coupled with by means of combination (12) to pass only that part of the combined signal (E c) which exceeds a predetermined clipping level (L), hashing means (21) connected to the combining means (12) for leaving passing at least the portion of the combined signal (E) which is less than the clipping level (L), and a mixer (3) for mixing the video signal (E a) with the respective portion which has passed through the clipping means (24) and the hashing means (21) to provide a corrected video signal (E y), not containing practically ent more periodic fluctuations that may exist in the amplitude of the input video signal.   2) Camera according to claim 1, characterized in that the delay means gives a delay at least equal to one   horizontal scan interval (1H).   Camera according to claim 1, characterized in   the mixer (3) comprises an adder for adding   the respective signals it receives and a means (22) for inverting the portion from the hashing means (21), so that the video signal whose part of the combined signal has passed through the clipping means (24); ) and the inverted portion of the combined signal having passed through the clipping means (21) is added. 40) A camera according to claim 1, characterized in that the mixer (3) comprises means for adding the portion which has passed through the clipping means (24) to the video signal and for subtracting the portion which has passed through the means of hashing (21).   50) Camera according to claim 1, characterized in that the combining means (3) comprises a subtractor giving the combined signal as a function of the difference between   the supplied video signal and the delayed video signal.   Camera according to claim 5, characterized in that the delay means (11) communicates a delay equal to one 2473240 horizontal scan interval.   A camera according to claim 1, characterized in that the combining means compasses another delay means providing another delayed video signal, the adder for adding said further delayed signal and the received video signals provides a sum signal and the subtractor. gives the combined signal according to the difference between the signal   delayed video and sum signals.   Camera according to claim 7, characterized in that the second delay means is connected in cascade to the first delay means and each gives the signal a delay.   equal to a horizontal scan interval.   Camera according to claim 1, characterized in that it comprises means (25) for adjusting the amplitude of the part of the combined signal which has passed through at least the circuit clipping (24).   A camera according to claim 1, characterized in that the means for providing the video signal is a color imaging device generating a video signal and having means for overprinting a fluctuating index signal   periodicals to the video signal thus generated.   ) A color television camera generating a video signal on which a fluctuating - periodic index signal is superimposed giving a video signal whose level undergoes periodic fluctuations with a line overlap effect in the video image definitively obtained from this video signal, camera characterized in that it comprises a vertical aperture correction circuit comprising a delay means (11) for giving a relative delay to the video signal, means receiving the delayed video signal (Eb) and the signal non-delayed video for emphasizing video-clarity variations of a horizontal scanning interval to the next, and for emphasizing the periodically fluctuating levels, clipping means (24) for passing the portions of the output signal of the medium which exceed the predetermined clipping levels, a hash means (21) passing the portion of the output signal of the d which is between the predetermined clipping levels and a mixer for mixing the video signal and the portions having passed through the clipping means and the chopping means, the parts 16 2473240   having passed through the hash means being cut off from the sum of the video signal and the portions having passed through the clipping means. A camera according to claim 11, characterized in that the predetermined clipping levels are respectively above and below the average level of the levels at   periodic fluctuations, accentuated.   Camera according to claim 11, characterized in that the delay means communicates a delay equal to a horizontal scan interval and the emphasis means comprises a subtractor giving an output signal which is a function of the difference between the delayed video signal of a horizontal scan interval and the undelayed video signal. 140) Camera according to claim 11, characterized   in that the delay means gives a delay equal to   horizontal scan range and the boosting means consists of a delay-additional means giving a total delay to the video signal, equal to two horizontal scan intervals, the adder for adding the delayed video signal of two scan intervals horizontal to the undelayed video signal and the subtractor giving an output signal which is a function of the difference between the delayed video signal of a horizontal scan interval and the output provided by the adder.    Camera according to claim 13, characterized in that the mixer (3) is composed of an inverter (22) coupled to the chopping means (21) for reversing the portion having passed through the chopping means (21) and an adder (3) coupled to the inverter (22) and the clipping means as well as to the input for receiving the undelayed video signal and adding the inverted portions having passed through the hash means, the portions having passed through the means of 'clipping and video signal not delayed.   16) Camera according to claim 14, characterized in that the mixer consists of an inverter (22) coupled to the hashing means (21) for inverting the portion having passed through the chopping means (21) and an adder coupled to the means delay, the inverter (22) and the clipping means (24) for adding the delayed video signal by a scanning interval 17 2473240   horizontally, the inverted part having passed through the means of   hash and the portion having passed through the clipping means.   17) Camera according to claim 11, characterized in that it comprises a level adjusting means for selectively adjusting the ratio of the amplitudes of the signals supplied in the mixer (3).