Television – Image signal processing circuitry specific to television – Transition or edge sharpeners
Patent
1999-08-13
2000-10-24
Hsia, Sherrie
Television
Image signal processing circuitry specific to television
Transition or edge sharpeners
348630, 348649, H04N 964, H04N 521
Patent
active
061375403
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to colour television video processors utilising a luminance stretch function. More particularly, the invention relates to the colour matching function used in connection with the luminance or white stretch function in a television video processor.
BACKGROUND ART
With the use of a luminance or white stretch function in a television video processor as part of a picture enhancement feature, there arises a need to provide some automatic adjustment in the amplitude of the colour difference signals to compensate for the effect of the stretch of the luminance signal. If compensation is not provided then the dematrixed colour signals will be incorrect and the picture will appear de-saturated. The provision of this compensation is called colour matching. The general principle of colour matching is that any percentage change in the amplitude of the luminanc signal due to the white stretch effect, must be balanced by the same percentage changes in the colour difference signal so that the ratio of the colour signals (R G B signals) can be maintained after matrixing. Existing circuits provide a compensation for the colour difference signals, however, the performance is only satisfactory for a luminance level below 50IRE and tends to over compensate above this level.
DISCLOSURE OF THE INVENTION
It is an object of this invention to provide an improved method and apparatus for correcting colour difference signals, which will overcome the limitations of the prior art.
Throughout the following description and claims use of the words "compensation" and "compensating" in relation to colour difference signal is to be taken as meaning application of an additional gain factor to the relevant signal.
Accordingly, in one aspect this invention provides a method of correcting colour difference signals in a colour television video processor utilising a luminance stretch function; said method comprising the steps of detecting the level of the input luminance signal; compensating the colour difference signals by a fixed amount when the level of the input luminance signal is below a selected input luminance threshold and compensating the colour difference signals by a varying amount that decreases with increasing input luminance signal level when the level of the input luminance signal is above said selected threshold.
In a second aspect, this invention also provides an apparatus for correcting colour difference signals in a colour television video processor utilising a luminance stretch function, said apparatus comprising means to detect the level of the input luminance signal; and means to apply a substantially constant amount of compensation to the colour difference signals when the level of the input luminance signal is below a selected input luminance threshold and to apply a varying of compensation to the colour difference signals that decreases with increasing input lumiance signal level when the level of the input luminance signal is above said selected threshold.
Preferably, the amount of compensation decreases linearly with increasing input luminance signal level. It is also preferred that the amount of compensation is reduced to zero at AND above a selected maximum luminance signal level. The maximum luminance level is preferably about 100IRE.
In the preferred embodiment, the input luminance threshold corresponds to the maximum luminance stretch of said video processor. Typically, this corresponds to a threshold luminance signal level of about 50IRE.
In one form of the invention, the input luminance signal is compared to a reference signal correponding to the input luminance threshold to determine whether the input luminance signal level exceeds the input luminance threshold.
The invention is preferably applied to an existing video processor providing a constant compensation for the colour difference signals by applying a correction to adjust the amount of compensation.
BEST MODE FOR CARRYING OUT THE INVENTION
One embodiment of the invention will now be described, by way
REFERENCES:
patent: 4757373 (1988-07-01), Van Rooy
patent: 4831434 (1989-05-01), Fuchsberger
patent: 5068718 (1991-11-01), Iwabe et al.
Chua Loo Kah
Desprez-Le Goarant Yann
Carlson, Esq. David V.
Galanthay, Esq. Theodore E.
Hsia Sherrie
STMicroelectronics Asia Pacific PTE Limited
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