The present invention relates to the signal processing of a quadrature modulated subcarrier color television signal. More particularly, the present invention relates to the control of bandwidths of the chrominance and luminance paths in a quadrature modulated color television chroma/luminance separator in order to improve the performance thereof.
U.S. Pat. No. 4,504,853 is related to the present invention in the sense of increasing apparent bandwidth of chrominance, although by quite different methods and for different reasons.
A composite video signal contains both luminance and chrominance information. Decoding and many other processes require the separation of the luminance and chrominance signals from the composite signal. All methods of chrominance separation use a bandpass or high pass filter somewhere in the chrominance signal processing structure, and all methods of luminance separation use a notch or low pass filter somewhere in the lumnance signal processing structure.
Because of the spectral overlap of the luminance and chrominance information it is impossible completely to separate the two signals. The incomplete separation of these two signals results in luminance component contamination in the chrominance signal component, and this is called "cross-color". The concomitant chrominance component contamination present in the luminance signal component is known as "cross-luminance". Cross-color is seen in the picture display as a rainbow artifact which occurs at certain luminance transitions, and cross-luminance is seen as dot crawl and occurs at certain chrominance transitions.
The width of the bandpass filter in a chrominance separator determines the chrominance resolution/cross-color tradeoff; and the width of the notch filter in a luminance separator determines the luminance resolution/cross-luminance tradeoff. As the width of the bandpass filter in the chrominance separator increases, chroma resolution increases and cross-color contamination increases. As the width of the notch filter in the luminance separator decreases, luminance resolution increases and cross-luminance contamination in the chroma component likewise increases.
Wide chroma bandwith is very desirable under certain picture conditions. For example, in the instance of a horizontal domain transition between a green bar and a magenta bar of a color bar test pattern (maximum color phase shift), the chroma transition between the green bar and the magenta bar should be as sharp as possible, and such high resolution requires a wide chroma bandwidth. Narrow chroma bandwidth is desirable under certain other picture conditions. Cross-color pollution can be reduced when chroma sharpness is not as important. At low chroma amplitude levels or at low chroma transition levels, the present inventors have discovered that the sharpness of chroma transitions are less important to overall picture quality, and that the bandwidth of the chroma path may be made quite narrow in order to reduce cross-color pollution in the picture.
Wide luminance bandwidth (narrow notch) is very desirable under most picture conditions. Maximum resolution or picture sharpness is almost always desirable. However, the visibility of cross-luminance is greatly increased if the luminance notch bandwidth is decreased. The narrower the luminance notch bandwidth, the much more likely will be the production of unwanted cross-luminance artifacts in the reproduced picture images. The present inventors have observed that frequently cross-luminance (dot-crawl) is very objectionable, even to the point of hinding a luminance transition. At these times a narrower luminance bandwidth (wider notch) is desirable to reduce the cross-luminance even in some instances at the expense of luminance sharpness. The present inventors have also observed that luminance sharpness is less important when the luminance is superimposed with a high chroma level in the composite signal.
Narrow chroma bandwidth is much less susceptible to generation of cross-color artifacts. At low chroma levels and at low chroma transition levels, the present inventors have discovered that the bandwidth of the chroma path may be made quite narrow in order to reduce cross-color pollution of the picture with little degradation otherwise upon chroma performance in the reproduced picture image.
Television receiving sets of the consumer variety are typified by narrow chroma path bandwidth, and such sets are not very sensitive to cross-color problems. However, in the case of large screen television color projectors, the situation is different. These projectors typically employ wide chroma bandwidths and have significant problems with cross-color artifacts visible in the reproduced and projected picture images.
Comb filters implemented by recursive processing of successive scan lines developed by line scan delay lines are increasingly found in color television decoders which decode quadrature modulated color television signals in which a chrominance component is interleaved between energies of a luminance component in the vicinity of a chroma subcarrier, typically 3.58 MHz in the NTSC format. The primary purpose of a comb filter decoder is to separate the chroma component and the luminance component from the composite color signal. Each component should have as full a bandwidth as possible, and the luminance component should have as few chroma artifacts as possible.
One of the most significant artifacts in the luminance component are rows of horizontal dots. These dots are typically present whenever the chroma component changes in the vertical direction, and they are taken care of by processing in the vertical domain, as is taught for example by U.S. Pat. Nos. 4,179,705 and 4,240,105.
In the process of decoding the luminance component with comb filter techniques, luminance combing is commonly limited to high frequencies located in the vicinity of the chroma subcarrier. Therefore, some chroma sideband energy at low frequencies (2-MHz) may not necessarily be perfectly eliminated in the luminance path; and, as a result, columns of vertical dots in the picture for horizontal rate transitions (which appear as vertical lines in the picture) will appear whenever there is a change in the chroma at the horizontal rate.
Thus, a hitherto unsolved need has arisen to eliminate cross-luminance, artifacts, such as the vertical dots in the luminance path which accompany horizontal domain chroma transistors, and to eliminate cross-color artifacts, such as the rainbow pattern which appars at the instance of horizontal domain luminance transistions.