Television – Image signal processing circuitry specific to television – Transition or edge sharpeners
Reexamination Certificate
1998-03-27
2001-05-15
Kostak, Victor R. (Department: 2611)
Television
Image signal processing circuitry specific to television
Transition or edge sharpeners
C348S627000
Reexamination Certificate
active
06233022
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the processing of analogue and digital video signals and is particularly concerned with restoration of picture sharpness, ie. with recovery of wanted high frequency signal components lost or reduced in the process of video recording or transmission. Traditionally, such processing is known as aperture correction or edge enhancement.
A well known enhancement technique is to determine the second differential of the signal, multiply it by a coefficient (which determines the degree of correction) and add the result to the input signal. The frequency response of such an enhancer is linked to its pulse response by the well-known Fourier transform. A fundamental limitation of this kind of enhancer is that it is impossible to boost the high frequency components without visible and annoying overshoots in the pulse and step responses.
There is another known, non-linear edge enhancement technique based on the synthesis of high frequency components by extension of the low-frequency part of the signal spectrum. This approach includes the calculation of the first and second derivatives of an input signal, then multiplication of said first derivative by the clipped second derivative and, finally, summation of the resulting correction signal with the incoming signal. Such processing provides improved pulse response, although the pulse response is not completely free from unwanted overshoots or ripples, and periodic picture textures are degraded.
BRIEF SUMMARY OF THE INVENTION
The object of this invention is to provide an edge enhancer which offers improvement in the sharpness of picture edges. A further object is to provide a peak enhancer, which offers improvement in the contrast of lines and high-frequency textures without visible artefacts. It is a further object of this invention to provide an improved enhancer capable of combining both signal steepness correction and “peaking” on sinusoidal textures and the tops of short pulses.
Accordingly, the present invention consists in a method of video signal processing comprising the steps of forming left and right difference signals and deriving an enhancement signal through non-linear combination of said difference signals, wherein the enhancement signal is constrained to be zero if either the left or the right difference signal is zero.
The separate use of left and right difference signals gives superior and more flexible correction than prior-art enhancers.
Advantageously, the enhancement signal comprises an edge enhancement signal when the polarities of the left and right difference signals are the same and a peak enhancement signal when the polarities of the left and right difference signals are different.
It is a still further object of this invention to provide an improved enhancer in the form of a circuit which calculates the best of the output correction signals from several constituent enhancers differing in the size of their “correction windows”, ie. the delay between the signal samples involved in the left/right difference calculations.
Preferably, the method further comprises the step of deriving a plurality of enhancement signals with respect to different time intervals T
l
to T
N
and selecting one enhancement signal for use.
Both analogue and digital implementations are possible. Particularly in the case of analogue implementation, the processing can be done in many ways with equivalent results; the variants shown below must be regarded only as examples.
REFERENCES:
patent: 5023919 (1991-06-01), Wataya
patent: 5404180 (1995-04-01), Kitano
patent: 5479216 (1995-12-01), Reime
patent: 5521649 (1996-05-01), Reime
patent: 5696852 (1997-12-01), Minoura et al.
patent: 0505999 A2 (1992-09-01), None
patent: 2222267 (1990-09-01), None
Steinberg Avigdor
Weston Martin
Allen Kenneth R.
Kostak Victor R.
Snell & Wilcox Limited
Townsend and Townsend / and Crew LLP
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