GB2067045A - Vertical aperture correction of a video signal - Google Patents

Abstract

A video signal Ea, such as may be generated by a television camera 1, is subjected to a delay 11, and then subtracted 12 from a relatively undelayed version thereof, the difference signal Ec being supplied to a clipping circuit 24, which passes at least that portion thereof that exceeds a predetermined clipping level, and also to a slicing circuit 21, which passes at least that portion thereof that is less than the predetermined clipping level. The sliced signal Ed is inverted at 22 and attenuated at 23 prior to being applied to a mixing circuit 3 which mixes it with the video signal Ea and the output Ef of the clipping circuit 24, thereby to provide a vertical aperture-corrected output video signal Ey which is substantially free of periodic fluctuations that might be present in the amplitude of the original video signal Ea. A preferred use of this vertical aperture correction circuit is with a colour image pick-up 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. <IMAGE>

Description

SPECIFICATION Video signal processing This invention relates to video signal processing and more particularly to video signal processing circuits and to colour image pick-up devices including a vertical aperture correction circuit.

In a colour television picture as derived from many colour television cameras, it is quite common for the so-called edge sharpness, or sensitivity, of the picture not to be 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 made 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 colour television signal generated by a colour television camera, is delayed by one horizontal line interval, and the difference between the delayed and undelayed luminance signals is then 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 horizontal 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, undelayed, luminance signal. The summed signal is thus a vertical aperture corrected luminance signal.

The aforementioned vertical aperture correction technique is accompanied by undesired interference when used in a Trinicon colour television camera. In the Trinicon camera, the target end of the pick-up 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 line 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-corrected luminance signal has an emphasized, superimposed periodic fluctuating level which results in a line-crawling effect in the video picture utilimately 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 as 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 which, presumably, will also eliminate the line-crawling effect due to the AC component derived from the DC-DC converter, is disclosed in US patent 4,160,265. According to this patent, the difference between the delayed and undelayed luminance signal, which is indicative of lineto-line brightness-level changes and which also emphasizes the periodic fluctuations superimposed onto the luminance signal, is squared, that is multiplied by itself, and the squared difference signal is then 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 line 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.

According to the present invention there is provided a video signal processing circuit comprising: means for supplying a video signal; delay means for imparting a relative delay to the supplied video signal; combining means for combining the relatively delayed video signal with a relatively undelayed version of the video signal to produce a combined signal; clipping means coupled to said combining means for passing at least that portion of said combined signal which exceeds a predetermined clipping level; slicing means coupled to said combining means for passing at least that portion of said combined signal which is less than said predetermined clipping level; and mixing means for mixing the video signal with the respective portions passed by said clipping means and said slicing means, thereby to provide a corrected output video signal substantially free of periodic fluctuations that might be present in the amplitude of said supplied video signal.

According to the present invention there is also provided a colour image pick-up device of the type which generates a video signal having a superimposed periodic, fluctuating index signal thereon resulting in periodic fluctuating levels of said video signal having a linecrawling effect on the video picture ultimately reproduced from said video signal; the device including a vertical aperture correction circuit comprising: delay means for imparting a relative delay to said video signal; means responsive to relatively delayed and undelayed video signals for emphasizing changes in brightness of the video signal from one horizontal line interval to the next and for emphasizing said period fluctuating levels; clipping means for passing those portions of the output of said means for emphasizing which exceed predetermined clipping levels; slicing means for passing that portion of said output of said means for emphasizing which lies between said predetermined clipping lev els; and mixing means for mixing the video signal and the portions passed by said clipping and slicing means, whereby the portions passed by said slicing means are subtracted from the sum of the video signal and the portions passed by said clipping means.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figures 1A to 1D are waveform diagrams which are useful in understanding the operation of typical vertical aperture correction circuits; Figures 2A to 2D are waveform diagrams which are useful in understanding the operation of a typical vertical aperture correction circuit with a colour image pick-up device of the type which superimposes a periodic, fluc tuating index signal on the video signal; Figure 3 is block diagram of one embodiment of the invention; Figures 4A to 4G are waveform diagrams which are useful in understanding the operation of the embodiment of Fig. 3; Figure 5 is a block diagram of another embodiment of the invention; and Figures 6A to 6H are waveform diagrams which are useful in understanding the opera- tion of the embodiment of Fig. 5.

Referring now to the drawings, the waveform diagrams of Figs. 1A to 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 a luminance signal Ea whose amplitude is a function of the brightness of the video scene. For the purpose of simplification, the luminance signal Ea is illustrated as having two transitions in a constant brightness level. Fig. 1 B represents a delayed version Eb of the luminance signal. That is, the delayed luminance signal Eb is delayed by one horizontal line interval from the luminance signal E2. The difference between the undelayed luminance signal Ea and the delayed lumi nance signal Eb is illustrated as a difference signal Ec in Fig. 1C. it is noted that the waveform shown in Fig. 1C emphasizes brightness-level changes in the luminance sig nal from one line interval to the next.A predetermined proportion of the difference signal Ec (Fig. 1 C) is added to the original, undelayed video signal Ea, the summed sig nals being illustrated in Fig. 1 D as an aperture-corrected luminance signal Ey. The differ ence signal Ec may be supplied to a suitable adding circuit by a voltage-divider circuit which divides the difference signal by a pre determined dividing ratio a. Hence, the lumi nance signal Ey may be represented as Ey = Ea + alec.

The waveforms illustrated in Figs. 2A to 2D represent the manner in which a typical verti cal aperture correction circuit operates with a Trinicon colour television camera. The Trini con camera is described in US patent 3,784,737 and includes, at its target end, a set of alternating electrodes which are sup plied 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 il lustrated as a luminance signal Ea in Fig. 2A.

As before, the luminance signal Ea is delayed in the typical vertical aperture correction cir cuit by one horizontal line period, the delayed luminance signal being shown as a delayed signal Eb in Fig. 2B. The delayed signal Eb is subtracted from the original, undelayed, lumi nance signal Eat resulting in a difference signal Ee shown in Fig. 2C. It will be noted that difference signal Ec has emphasized fluctuating levels. When the difference signal Ec is added to the original luminance signal Ea a resultant corrected luminance signal Ey appears as shown in Fig. 2D. The corrected luminance signal Ey has superimposed thereon a periodic fluctuating signal which is derived from the index signal that had been superimposed onto the photoconductive target of the colour camera.This superimposed fluctuating signal has its level changed over at each successive horizontal scanning interval, thereby resulting in a "line crawling" effect in the video picture which is ultimately reproduced therefrom. That is, the periodic change in the brightness level of the luminance signal Ey (Fig. 2D) is readily perceived as a line crawl.

The foregoing disadvantages, particularly the superimposed fluctuating level on the vertical aperture corrected luminance signal, can be avoided by embodiments of the invention, one embodiment of which is illustrated in Fig.

3. A video signal processing circuit is coupled to a Trinicon-type colour 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 the camera 1. The camera 1 may be constructed as a colour image pick-up device in accordance with the disclosure of aforementioned US patent 3,784,737. As is apparent from that disclosure, and as is also described in US patent 4,160,265, a luminance signal Ea may be derived from the composite colour video signal produced by the camera 1. The luminance signal Ea is amplified by a suitable video amplifier 2 and then supplied to the video signal processing circuit.

The video signal processing circuit functions as a vertical aperture correction circuit and comprises a delay circuit 11, a subtracting circuit 12, a slicing circuit 21, a clipping circuit 24 and a mixing circuit 3. The delay circuit 11 may comprise a conventional delay circuit arranged to impart a delay equal to one horizontal line interval to the video signal supplied thereto. The delay circuit 11 is coupled to the output of the amplifier 2 and receives the luminance signal E,,. The output of the delay circuit 11 is coupled to the subtracting circuit 1 2 wherein a delayed video signal Eb is subtracted from the original, undelayed, luminance signal Ea The output of the subtracting circuit 1 2 is coupled in common to the slicing circuit 21 and to the clipping circuit 24.The slicing circuit 21 is supplied with predetermined threshold levels by suitable means (not shown), and is arranged to pass only those portions of the video signal supplied thereto which lie between these 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 the subtracting circuit 1 2.

As an alternative, if the output of the subtracting circuit 1 2 is a positive (or negative) signal, the slicing circuit 21 may be arranged to pass only the positive (or negative) portions of the output of the subtracting circuit 1 2 which are less than the threshold level. The output of the slicing circuit 21, that is, that portion of the difference signal supplied thereto by the subtracting circuit 12, which is less than the respective threshold levels supplied thereto, is inverted by an inverter circuit 22 and supplied to the mixing circuit 3 via an amplitude adjustment circuit 23. The amplitude adjustment circuit 23 is illustrated as an adjustable resistor, such as a potentiometer, and is settable to supply to the mixing circuit 3 a predetermined portion of the inverted output of the slicing circuit 21.By suitably adjusting the amplitude adjustment circuit 23, this proportion ss may be varied as desired.

A clipping circuit 24 is arranged to pass that portion of the difference signal Ec which exceeds a threshold level. If the threshold levels + L and - L are supplied to the slicing circuit 21, these same threshold levels may be supplied to the clipping circuit 24, whereupon the clipping circuit 24 passes those portions of the difference signal Ec which exceed the threshold level + L and which exceed the threshold level - L. An amplitude adjustment circuit 25, which may be similar to the amplitude adjustment circuit 23, supplies the output of the clipping circuit 24 to the mixing circuit 3. The amplitude adjustment circuit 25 serves to adjust the proportion of the output of the clipping circuit 24 that is supplied to the mixing circuit 3.

The mixing circuit 3 is supplied with the luminance signal Ea, the desired proportion of the inverted output of the slicing circuit 21, and the desired proportion of the output of the clipping circuit 24. The mixing circuit 3 functions as a summing circuit, and the output of the mixing circuit 3 is supplied to an output terminal as a vertical aperture corrected luminance signal Ey.

The function of the inverting circuit 22 is to subtract the output of the slicing circuit 21 from the summation of the video signal Ea and the output of the clipping circuit 24. If desired, the inverting circuit 22 may be omitted, and the mixing circuit 3 may comprise respective circuit devices which carry out the aforementioned summing and subtracting operations. For example, the luminance signal Ea may be summed with the output of the clipping circuit 24 in an adding circuit, and the output of the 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 will now be described with reference to the waveforms shown in Figs 4A to 4G. The luminance signal Eaw derived from the 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 line interval. The delayed luminance signal Eh is derived at the output of the delay circuit 11.

The subtracting circuit 1 2 subtracts the delayed luminance signal Eb from the undelayed version of the luminance signal Ea to produce the difference signal Ec shown in Fig.

4C. This difference signal Ec emphasizes the changes in brightness of the luminance signal Ea from one horizontal line interval to the next, and also emphasizes the periodic fluctuating levels in the luminance signal Ea This difference signal Ec, having the emphasized brightness changes and periodic fluctuating levels, is supplied to the slicing circuit 21 and also to the clipping circuit 24.

Fig. 4C also illustrates the threshold levels + L and - L, which are represented by the broken lines, and are supplied as threshold reference voltage levels to the slicing and clipping circuits 21 and 24. The threshold levels + L and - L are, for example, equal and opposite threshold levels disposed on opposite sides of the mean level of the difference signal Ec. The slicing circuit 21 serves to pass that portion of the difference signal Ec which lies between the threshold levels + L and - L. This portion of the difference signal that is passed by the slicing circuit 21 is illustrated as a signal Ed in Fig. 4D.The signal Ed is inverted by the inverting circuit 22, leveladjusted by the amplitude adjustment circuit 23, and then summed in the mixing circuit 3 with the luminance signal E This has the equivalent effect of subtracting the signal Ed from the slicing circuit 21 (suitable amplitudeadjusted) from the luminance signal Ea. Fig.

4E represents this operation, and illustrates a signal Ea which would be produced if the signal Ed from the slicing circuit 21 were subtracted from the luminance signal Ea. That is, Fig. 4E illustrates a waveform which is formed by the operation Ec = Ea - Ed. It will be appreciated that the proportion of the signal Ed from the slicing circuit 21 that is supplied to the mixing circuit 3 is determined by the 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 the luminance signal Ea (Fig. 4A).

Hence, and as shown in Fig. 4E, the signal Ee which would be produced by subtracting the amplitude-adjusted signal Ed from the slicing circuit 21 from the luminance signal Ea exhibits substantially no periodic amplitude fluctuations.

The clipping circuit 24 also may be supplied with the threshold levels + L and - L.

The clipping circuit 24 functions to pass the positive portion of the difference signal Ec which exceeds the threshold level + L, and also passes the negative portion of the difference signal Ec which exceeds the threshold level - L. The output Ef of the clipping circuit 24 is illustrated in Fig. 4F. This output Ef, after being suitably amplitude adjusted by the- amplitude adjustment circuit 25 is added to the signal Ee (Fig. 4E). The summed signal Ey = Ee + E, is shown in Fig. 4G and is supplied to an output terminal 4 by the mixing circuit 3. The corrected luminance signal Ey produced by the mixing circuit 3 may be mathematically represented as Ey = Ea - Ed + Ef.It is seen that the corrected luminance signal Ey is provided with emphasized brightness level changes, that is, changes in the brightness level from one horizontal line interval to the next are emphasized, but is substantially free of the periodic amplitude fluctuations inherent in the luminance signal Ea. A comparison between the waveforms shown in Figs. 4G and 2D illustrates the improvement obtained. Thus, the aperture corrected luminance signal Ey (Fig. 4G) does not give rise to the undesired line crawling effect in the video picture ultimately reproduced therefrom.

It is seen that the difference signal Ec (Fig.

4C) is further processed before being mixed with the luminance signal Ea. But for this further processing, the "corrected" luminance signal would appear as shown in Fig. 2D.

That is, if the difference signal Ec 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, using the embodiment shown in Fig. 3, the vertical aperture-corrected luminance signal Ey (Fig.

4G) more closely resembles the corrected lu minance signal shown in Fig. 1 D, this latter signal being derived from a colour television camera which does not superimpose a periodic, fluctuating level onto the video signal.

The slicing circuit 21 may be supplied with a single threshold level + L and may be operative to pass the absolute value of the difference signal Ec which is less than the threshold level + L. Likewise, the clipping circuit 24 may be supplied with a single threshold level + L and may be operative to pass the absolute value of the difference sig nal Ec which exceeds the threshold level + L.

The respective settings of the amplitude adjustment circuits 23 and 25 are assumed to differ from each other. The amplitude adjust ment circuit 23 serves to attenuate the level of the signal supplied thereto by a factor ss and the amplitude adjustment circuit 25 serves to attenuate the signal supplied thereto by the factor y. The attenuating ratio ss serves to eliminate the fluctuating level of the luminance level Ea, and the attenuating ratio y serves to provide desired emphasis of brightness-level changes.

The embodiment shown in Fig. 3 is a relatively simple embodiment. A more practical version is illustrated in Fig. 5, wherein like reference numerals are used to identify like elements. The embodiment of Fig. 5 differs from that of Fig. 3 in that an additional delay circuit 1 3 is connected in cascade with the delay circuit 11, the output of the delay circuit 1 3 being summed in a summing circuit 14 with the luminance signal Ea. The summed signal Eh produced by the summing circuit 1 4 is subtracted in a subtracting circuit 1 5 from the delayed luminance signal Eb produced at the output of the delay circuit 11.In the embodiment of Fig. 5, the delay circuits 11 and 1 3 each impart a delay equal to one horizontal line interval. If desired, the luminance signal Ea may be supplied to the subtracting circuit 1 5 by the delay circuit 11, and the luminance signal Ea may be supplied directly to the summing circuit 14, as illustrated, and also to the summing circuit 14 via another delay circuit (not shown) which serves to impart a delay equal to two horizontal line intervals. In this alternative arrangement, the delay circuit 1 3 is omitted, and the output of the delay circuit 11 is connected only to the subtracting circuit 1 5 (and also to the mixing circuit 3).

In operation, the luminance signal Ea appears as shown in Fig. 6A. The luminance signal Ea is subjected to a first delay, equal to one horizontal line interval, by the delay circuit 11, resulting in the delayed luminance signal Eb shown in Fig. 6B. The delayed video signal Eb is further delayed by another horizontal line interval (H) in the delay circuit 13, resulting in a 2H delayed luminance signal Eg, 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 the luminance signal Ea The 2H delayed luminance signal Eg is summed with the undelayed luminance signal Ea in the summing circuit 14, resulting in the summed signal Eb shown in Fig. 6B. Preferably, the delayed signal Eg is attenuated by a factor of 1 /2, and the undelayed luminance signal Ea is likewise attenuated by the factor of 1/2.Hence, the summed signal Eh shown in Fig. 6D may be represented as Eh = 1/2 (Ea + The summed signal Eh (Fig. 6D) is subtracted from the 1 H delayed luminance signal Eb in the subtracting circuit 15, resulting in the difference signal Ec, shown in Fig. 6E.The difference signal Ec may be represented as Ec= Eb - Eb. It is appreciated that the difference signal Ec emphasizes the brightness-level changes in the luminance signal Ea from one line interval to the next, and, moreover, emphasizes the periodic fluctuating level which has been superimposed onto the luminance signal Ea As before, the difference signal Ec is supplied to the slicing circuit 21, which passes that portion of the difference signal Ec which lies between the threshold levels + L and - L. The signal Ed passed by the slicing circuit 21 is shown by Fig. 6F.The signal Ed is inverted by the inverting circuit 22, amplitude adjusted by the amplitude adjustment circuit 23, and then summed with the delayed luminance signal Eb in the mixing circuit 3.

The difference signal Ec is supplied to the clipping circuit 24 which passes that portion of the difference signal that exceeds the threshold levels + Land - L. The signal Ef from the clipping circuit 24 is illustrated in Fig. 6G. The signal Ef is amplitude adjusted by the amplitude adjustment circuit 25, and then summed in the mixing circuit 3 with the delayed luminance signal Eb and the amplitude-adjusted, inverted signal Ed. The output of the mixing circuit 3 appears as a vertical aperture-corrected luminance signal Eyt as shown in Fig. 6H. The corrected luminance signal Ey has the brightness-level changes emphasized therein and, moreover, the fluctuating level that had been superimposed onto the original luminance signal Ea (Fig. 6A) is eliminated.Thus, vertical aperture correction is attained without a line crawling component.

It is appreciated that, in Fig. 3, the delay circuit 11 and the subtracting circuit 1 2 function as a combining circuit for combining the relatively delayed luminance signal Eb and the undelayed luminance signal Ea Likewise, in Fig. 5, the delay circuits 11 and 13, together with the summing circuit 14 and the subtracting circuit 1 5 function as a combining circuit for combining the undelayed luminance signal Ea with the delayed luminance signal Eg, these combined signals being further combined with the delayed luminance signal Eb. In both embodiments, the combining circuits serve to emphasize changes in the brightness of the luminace signal Ea 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 Ea by the inherent operation of the camera 1.

Claims (16)

1. A video signal processing circuit comprising: means for supplying a video signal; delay means for imparting a relative delay to the supplied video signal; combining means for combining the relatively delayed video signal with a relatively undelayed version of the video signal to produce a combined signal; clipping means coupled to said combining means for passing at least that portion of said combined signal which exceeds a predetermined clipping level; slicing means coupled to said combining means for passing at least that portion of said combined signal which is less than said predetermined clipping level; and mixing means for mixing the video signal with the respective portions passed by said clipping means and said slicing means, thereby to provide a corrected output video signal substantially free of periodic fluctuations that might be present in the amplitude of said supplied video signal.

2. A circuit according to claim 1 wherein said delay means imparts a delay of at least one horizontal line interval.

3. A circuit according to claim 1 wherein said mixing means comprises summing means for summing the respective signals supplied thereto; and the circuit further comprises means for inverting the portion supplied from said slicing means to said summing means, whereby the video signal, the portion of said combined signal passed by said clipping means, and the inverted portion of said combined signal passed by said slicing means are summed.

4. A circuit according to claim 1 wherein said mixing means comprises means for adding the portion passed by said clipping means to the video signal and for subtracting therefrom the portion passed by said slicing means.

5. A circuit according to claim 1 wherein said combining means comprises subtracting means for producing said combined signal as a function of the difference between said supplied and relatively delayed video signals.

6. A circuit according to claim 5 wherein said delay means imparts a delay equal to one horizontal line interval.

7. A circuit according to claim 1 wherein said combining means comprises further delay means for producing a further delayed video signal, summing means for summing said further delayed and supplied video signals to produce a sum signal, and subtracting means for producing said combined signal as a function of the difference between said delayed video and sum signals.

8. A circuit according to claim 7 wherein said further delay means is connected in cascade with said delay means, and each imparts a delay equal to one horizontal line interval.

9. A circuit according to claim 1 further comprising means for adjusting the amplitude of the portion of said combined signal that is passed by at least said clipping means.

10. A circuit according to claim 1 wherein said means for supplying a video signal comprises a colour image pick-up device for generating said video signal and having means for superimposing a periodic, fluctuating index signal on said generated video signal.

11. A colour image pick-up device of the type which generates a video signal having a superimposed periodic, fluctuating index signal thereon resulting in periodic fluctuating levels of said video signal having a linecrawling effect on the video picture ultimately reproduced from said video signal; the device including a vertical aperture correction circuit comprising: delay means for imparting a relative delay to said video signal; means responsive to relatively delayed and undelayed video signals for emphasizing changes in brightness of the video signal from one horizontal line interval to the next and for emphasizing said period fluctuating levels; clipping means for passing those portions of the output of said means for emphasizing which exceed predetermined clipping levels; slicing means for passing that portion of said output of said means for emphasizing which lies between said predetermined clipping levels; and mixing means for mixing the video signal and the portions passed by said clipping and slicing means, whereby the portions passed by said slicing means are subtracted from the sum of the video signal and the portions passed by said clipping means.

1 2. A circuit according to claim 11 wherein said predetermined clipping levels are respectively above and below the mean level of said emphasized periodic fluctuating levels.

1 3. A circuit according to claim 11 wherein said delay means imparts a delay equal to one horizontal line interval and said means for emphasizing comprises subtracting means for producing an output which is a function of the difference between the video signal delayed by one horizontal lie interval and the undelayed video signal.

14. A circuit according to claim 11 wherein said delay means imparts a delay equal to one horizontal line interval, and said means for emphasizing comprises additional delay means to impart an overall delay to said video signal equal to two horizontal line intervals, summing means for summing the video signal delayed by two horizontal line intervals and the undelayed video signal, and subtracting means for producing an output which is a function of the difference between the vide6 signal delayed by one horizontal line interval and the output produced by said summing means.

1 5. A circuit according to claim 1 3 wherein said mixing means comprises inverter means coupled to said slicing means for inverting the portions passed by said slicing means, and adding means coupled to said inverter means and said clipping means and also coupled to receive the undelayed video signal for adding the inverted portions passed by said slicing means, the portions passed by said clipping means, and said undelayed video signal.

16. A circuit according to claim 14 wherein said mixing means comprises inverter means coupled to said slicing means for inverting the portions passed by said slicing means, and adding means coupled to said delay means, said inverter means and said clipping means for adding the video signal delayed by one horizontal line interval, the inverted portions passed by said slicing means and the portions passed by said clipping means.

1 7. A circuit according to claim 11 further comprising level adjusting means for selectively adjusting the ratio of the amplitudes of the signals supplied to said mixing means.

1 8. A video signal processing circuit substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

1 9. A video signal processing circuit substantially as hereinbefore described with referene to Fig. 5 of the accompanying drawings.