GB2253964A

GB2253964A - Treatment of video signals produced by aperture correctors

Info

Publication number: GB2253964A
Application number: GB9105894A
Authority: GB
Inventors: Trevor Malcolm Osborne
Original assignee: Rank Cintel Ltd
Current assignee: Rank Cintel Ltd
Priority date: 1991-03-20
Filing date: 1991-03-20
Publication date: 1992-09-23
Also published as: GB9105894D0

Abstract

To reduce artefacts in signals produced by aperture correctors a limiting signal C' is formed which clips the correction signal at a given amplitude (Fig 2 (iii)). The limiting signal is derived from a user input which, after processing in a polarity selector 11 is subtracted from the correction signal by subtractor 10. The output of the subtractor is clipped 12 to eliminate any part of the signal which has crossed the zero axis and the clipped signal, which is the limiting signal, is subtracted from the correction signal in subtractor 14. <IMAGE>

Description

IMPROVEMENTS IN APERTURE CORRECTION This invention relates to aperture correction, a technique used in film to video transfer to compensate for high frequency signal losses incurred by mechanisms such as lens distortions or scanning spot size.

Aperture correction is a well established technique and boosts high frequencies to compensate for losses as mentioned above. Level dependent aperture correction has been proposed.

This technique limits the amount of enhancement as a function of the output signal level.

A problem with existing aperture correction systems is that there is a tendency for large amplitude signal detail to acquire unwanted artefacts. These occur as a side effect of obtaining a proper reproduction of fine detail, small amplitude parts of the signal (for example facial texture). The artefacts are due to a number of different factors, for example, overloading enhancement of inter-field flicker in the interlaced scanning system.

The present invention aims to overcome the problem outlined above and accordingly the invention treats the aperture correction signal, clipping large amplitude parts of the signal.

More specifically, the invention provides a method of treating a video signal processed by an aperture corrector, comprising deriving an amplitude limiting signal based on a predetermined limit level and the correction signal from the aperture corrector, and subtracting the limiting signal from the correction signal to form an amplitude limited correction signal.

The invention also provides apparatus for treating a video signal processed by an aperture corrector, comprising means for forming an amplitude limiting signal from a predetermined limit level and the correction signal from the aperture corrector, and means for subtracting the limiting signal from the correction signal to form an amplitude limited correction signal.

The invention operates on the correction signal derived internally by the aperture corrector from the input video signal, and applied to that signal to produce an aperture corrected signal. The invention provides an improved corrected output signal.

The invention has the advantage of providing a signal which retains the advantages of conventional aperture correctors such as high frequency, low amplitude detail retention but avoids the artefacts produced by conventional aperture correctors.

A preferred embodiment of the invention can be used in conjunction with a conventional aperture corrector without need for adjustment of the corrector.

A preferred embodiment of the invention derives, on a pixel-by-pixel basis, a limiting signal which clips the large amplitude portions of the correction signal generated by the aperture corrector. A user defined limit is entered which is formed into a limiting value according to the polarity of the correction signal. The limiting value is subtracted from the correction signal and the resultant signal is clipped where it moves through the zero axis, forming the limiting signal which is then subtracted from the correction signal to form the amplitude limited correction signal.

An embodiment of the invention will now be described, by way of example, and with reference to the accompanying figures, in which: Figure 1 shows, in block form, a circuit for limiting the correction signal produced by the aperture corrector; and Figure 2 (i) - (iii) show, respectively, the waveforms at points (i), (ii) and (iii) in figure 1.

The correction signal produced by the aperture corrector (not shown) is represented in Figure 2(i) as a sawtooth wave form. In essence, the circuit of Figure 1 clips this signal to produce a limited signal shown in Figure 2(iii). This is achieved by deriving a limiting signal on a pixel-by-pixel basis (Figure 2(ii)) based on a user supplied limit.

Referring to Figure 1, a subtractor 10 is provided with the aperture correction signal A and a limiting value B' which is subtracted from the correction signal to provide an output C.

Value B' is obtained from a user-supplied limit B which is passed to a polarity selector 11 which provides an output B' according to the polarity of the correction signal supplied to it, and on a pixel-by-pixel basis.

The limiting signal C output from the subtractor 10 represents the correction signal moved towards the zero axis by an amount B. If the signal A is moved through the zero axis, the signal C is clipped to zero. This is achieved by clipper circuit 12, which compares the polarity of the correction signal A with that of the limiting signal C. The output is limiting signal C if the polarities are the same and zero if not. The final limiting signal C' is illustrated in Figure 2(ii).

The adjusted limiting signal is then supplied to a further subtractor 14 which subtracts C' from the original correction signal A to produce an output A' shown in Figure 2(iii). As can be seen from that figure, the amplitude of the correction signal has been limited to a clipping level equal to +/- B and signals of amplitude less than the clipping level pass unaffected.

The embodiment described can be applied to any situation where aperture correction is used, for example in photoconductive cameras or flying spot telecine. It will be appreciated that apparatus embodying the invention can be added to existing aperture correction systems.

Claims

CLAIMS:

1. A method of treating a video signal processed by an aperture corrector, comprising deriving an amplitude limiting signal based on a predetermined limit level and the correction signal derived by the aperture corrector from the input video signal, and subtracting the limiting signal from the correction signal to form an amplitude limited correction signal.

2. A method according to claim 1, wherein the limiting signal is derived on a pixel-by-pixel basis.

3. A method according to claim 1 or 2 wherein the limiting signal is formed by subtracting from the correction signal a limiting value derived from the limit level and the polarity of the correction signal.

4. A method according to claim 3, wherein the result of the subtraction of the limiting value and correction signal is clipped to zero where it moves through the zero axis to form the limiting signal.

5. A method of reducing artefacts in a video signal produced from photographic film, comprising limiting the amplitude of a correction signal produced by an aperture corrector by the method of any of claims 1 to 4.

6. Apparatus for treating a video signal processed by an aperture corrector, comprising means for forming an amplitude limiting signal from a predetermined limit level and the correction signal from the aperture corrector, and means for subtracting the limiting signal from the correction signal to form an amplitude limited correction signal.

7. Apparatus according to claim 6, wherein the means forming the limiting signal operates on a pixel-by-pixel basis.

8. Apparatus according to claim 6 or 7, comprising a subtractor for subtracting a limiting value from the correction signal, and means for deriving the limiting value from the limit level and the polarity of the correction signal.

9. Apparatus according to claim 8, comprising means for clipping that part of the output of the subtractor which crosses the zero axis to form the limiting signal.

10. Apparatus for reducing artefacts in a video signal produced from photographic film, by limiting the amplitude of a correction signal produced by an aperture corrector, comprising the apparatus of any of claims 6 to 9.

11. A method of treating a video signal processed by an aperture corrector, substantially as herein described with reference to the accompanying drawings.

12. Apparatus for treating a video signal processed by an aperture corrector, substantially as herein described with reference to the accompanying drawings.