Television – Format conversion – Specified chrominance processing
Patent
1992-09-17
1994-07-26
Groody, James J.
Television
Format conversion
Specified chrominance processing
348471, 348624, 348493, 348663, H04N 964, H04N 978
Patent
active
053330145
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the Weston PAL video signal (W-PAL), and in particular to improvements in the assembler and splitter which respectively, assemble and split luminance Y and chrominance (U+V/U-V) components.
BACKGROUND TO THE INVENTION
The Weston Clean PAL system is described in GB Nos. 1534268 to 1534270 and in BBC Research Department Report No: 1980/1. The contents of these four documents are incorporated herein by reference. W-PAL obtains separation of luminance and chrominance by phase segregation, whereby the luminance is sampled at a frequency of twice the colour subcarrier frequency thereby causing it to appear as double sideband modulation of the colour subcarrier. At the same time the two chrominance components are formed into a single signal consisting of the quantities (U+V)/.sqroot.2 and (U-V)/.sqroot.2 on alternate lines which modulates the subcarrier directly. If PAL compatibility were not required the two modulation processes could be in quadrature so enabling the subcarrier to carry both luminance and chrominance without interaction in spite of their occupying the same spectral space. FIG. 1 shows the two-dimensional spectrum of the alternate-line chrominance and the sampled luminance, from which it can be seen that there is a considerable overlap of the luminance and chrominance spectra, amounting to the region 3.36-5.5 MHz in system I PAL. This region is the region in which aliasing will occur at 2 Fsc sampling (8.86 MHz-5.5 MHz=3.36 MHz) and varies with different PAL standards.
Compatibility with normal PAL, however, requires that the signals are further processed before combination. In particular, the combined chrominance signal must be phase shifted so that spectral components at 3/4 line offsets are in phase quadrature with components at 1/4 line offsets, restoring the phase quadrature between U and V. This must be done in such a way as to preserve the potential phase separation between chrominance and luminance. At the receiver, the UV phase separation is removed and the luminance and chrominance are separated by synchronous demodulation. These complex functions are provided by so-called assembler and splitter circuits which lie at the core of the Weston Clean PAL system. These circuits are described in GB Nos. 1534268 and 1534270.
SUMMARY OF THE INVENTION
The present invention aims to provide an improved assembler and splitter through improved filter design.
We have appreciated that cross colour effects may be eliminated without affecting the luminance bandwidth and independently of the link filter in the PAL transmission channel. This may be achieved by ensuring that filters in the W-PAL assembler and splitter have the correct interrelationship.
We have appreciated that the filter in the luminance channel of the splitter must have an amplitude which is a shifted mirror of the filter in the chrominance path of the assembler. Moreover, the filter in the chrominance path of the splitter must have an amplitude which is a shifted mirror of the amplitude of the filter in the luminance path of the assembler. Furthermore, the product of the filters in the luminance paths in the assembler and splitter must be a Nyquist filter. Similarly, the product of the filters in the chrominance path of the assembler and splitter must also be a Nyquist filter.
The phase of the luminance and chrominance filters in the splitter may be the anti-mirror of the phase of the chrominance and luminance filters respectively in the assembler.
Preferably, the filter in the luminance channel of the assembler extinguishes below the frequency of the link filter in the PAL channel. This means that variations in link bandwidth affect the horizontal chrominance bandwidth only and not the ability to eliminate cross colour effects or the luminance bandwidth.
The invention is set out in the independent claims to which reference should be made.
Various modifications, further improvements and preferred features are set out in the dependent claims.
DESCRIPTION OF DRAWINGS
An embodiment
REFERENCES:
patent: 4051516 (1977-09-01), Weston
patent: 4212028 (1980-07-01), Drewery
patent: 4288810 (1981-09-01), Drewery et al.
patent: 4322739 (1982-03-01), Drewery et al.
patent: 4399454 (1983-08-01), Warnock et al.
EBU Review/Technic, J. O. Drewery, "A Compatible Improved PAL System", No. 215, Feb. 1986.
Drewery John O.
Wells Nicholas D.
British Broadcasting Corporation
Groody James J.
Metjahic Safet
O'Connell Robert F.
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