Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Patent
1992-11-16
1995-07-18
Groody, James J.
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
348665, H04N 1116
Patent
active
054346274
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the Weston Clean PAL (W-PAL) video signal and in particular the design of practically realisable assembler and splitters in a W-PAL system.
BACKGROUND TO THE INVENTION
The W-PAL coding and decoding system (also known as Weston Clean PAL) is described in GB 15343268, GB1534269 and GB1534270 and in BBC Research Department Report 1980/1 February 1980 all of which are incorporated herein by reference. It is a form of "clean PAL" which uses comb-filters and modulation of the luminance at a frequency of twice sub-carrier (2fsc), to reduce cross talk between the luminance and chrominance signals.
All W-PAL systems, proposed, contain a band-pass filter which limits the action of the comb filter to the sub-carrier band (approx. 3.3 to 5.5 MHz) for system I PAL. Cross Talk is completely eliminated at all frequencies within this band, where the band-pass filter has unity gain, and also at all frequencies outside the band, where the filter has zero gain. But some cross talk remains in the transition band, where the response is neither unity nor zero.
SUMMARY OF THE INVENTION
The invention aims to reduce or eliminate this residual cross talk between luminance and chrominance signals.
The invention is defined by the independent claims to which reference should now be made.
In essence, the invention replaces the band-pass filter with a high-pass filter and introduces a further filter to compensate for the unbalance between the various signal paths in the transition band.
Other advantageous features of the invention are set out in the dependent claims.
DESCRIPTION OF DRAWINGS
Embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings in which:
FIGS. 1a) and 1b) are, respectively, block schematic diagrams of a Weston Clean PAL (W-PAL) coder and decoder;
FIG. 2 is a block diagram of an ideal W-PAL system wholly eliminating cross-talk;
FIG. 3a) is a block schematic diagram of a conventional W-PAL assembler;
FIG. 3b) is a block schematic diagram of a conventional W-PAL splitter;
FIGS. 4a) to e) shows frequency responses under certain conditions of the band pass filter of FIG. 3;
FIGS. 5a) to e) shows frequency responses under the same conditions with the band-pass filter replaced by a high-pass filter:
FIGS. 6a) to g) shows frequency responses for a compensated high-pass filter;
FIGS. 7a) and b) show, respectively, implementations of the W-PAL assembler and splitter for the filter of FIG. 6;
FIGS. 8a) and b) show, respectively, an alternative implementations of the assembler and splitter to that shown in FIG. 7;
FIG. 9 shows frequency responses for the individual components of a practical filter; and
FIG. 10 shows overall frequency responses under four test conditions.
DESCRIPTION OF BEST MODE
FIGS. 1(a) & (b) are overall block diagrams of the W-PAL coder and decoder. In the coder, the incoming luminance signal (Y) and the two chrominance signals (U & V) are separately pre-filtered, at 10, 12, 14 and in the decoder, the outgoing Y,U&V signals are separately post-filtered at 20, 22, 24. These pre and post filters determine the overall characteristics of the system, but they have no effect whatsoever on the cross-talk. So they may be ignored for the sake of the current analysis.
Similarly the combination of the two chrominance signals into a single signal (C=U+V/U-V) which is a feature of W-PAL creates cross-talk between the two chrominance signals (which is reduced by the pre & post filters). But this combination has no effect on the cross-talk between luminance and chrominance, and so may also be ignored.
This only leaves the 2fsc modulation shown at 30, assembler 32, splitter 34, and re-modulator 36 as contributing to cross-talk. We would like these to be equivalent to a single modulator in the luminance path, and a band-pass filter in the chrominance path. The overall system then would be equivalent to that shown in FIG. 2, in which cross-talk is clearly impossible.
The following description shows how th
REFERENCES:
patent: 4291331 (1981-09-01), Devereux
patent: 4597007 (1986-06-01), Reitmeier et al.
patent: 4638351 (1987-01-01), Clarke
patent: 4683490 (1987-07-01), Strolle et al.
patent: 5333014 (1994-07-01), Drewery et al.
C. K. P. Clarke, "Pal Decoding: Multi-Dimensional Filter Design For Chrominance-Luminance Separation", BBC Research Department Report, No. 11, (1988), pp. 1-25.
The British Broadcasting Corporation Research Department Report 1980/1.
British Broadcasting Corporation
Fournier Kevin J.
Grant Chris
Groody James J.
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