Decoder and method for separating luminance and chrominance information from a composite video signal

A decoder and a method for separating luminance and chrominance information from a target pixel in a composite video signal are provided. The decoder comprises a delay module, a filter module, a weighting factor generator, and a separator. The delay module is configured to delay a plurality of horizontal lines of the composite video signal to output a plurality of pixels including the target pixel. The filter module is coupled to the delay module and configured to filter the composite video signal in a horizontal direction and a vertical direction to generate a plurality of frequency components. The weighting factor generator is configured to generate a first weighting factor of the target pixel according to the horizontal direction and a second weighting factor of the target pixel according to the vertical direction. The separator is configured to separate luminance information from the target pixel in accordance with the frequency components, the first weighting factor, and the second weighting factor.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to video signal processing; more particularly, the present invention relates to decoders and methods for separating luminance and chrominance information from a composite video signal.

2. Descriptions of the Related Art

Video, a technology which is first developed for TV systems, processes a sequence of still images and represent them in motion. Video has been further developed in various formats to allow different media, such as internet or disks, for storage and distribution.

A composite video signal is composed of luminance and chrominance components, wherein luminance is a photometric measure of the density of luminous intensity and chrominance is used to represent the color information of the pictures. For example, a luminance component of a TV video signal carries the primary gray-level information and is suitable for display in a monochrome TV, and the chrominance component carries color information of the TV video signal and is mixed with the luminance component to generate an RGB signal for TV display.

More specifically, the TV video signal can be a NTSC color television signal which includes a luminance component ranging in frequency from direct current to a nominal bandwidth of 4.2 MHz, and a 3.58 MHz subcarrier which is modulated in phase and amplitude to represent hue and saturation of the image. Typically, the subcarrier is demodulated to produce color difference signals which are combined with the luminance component for reproduction of red, blue, and green color information. As is well known, simple filters have long been used to separate the chrominance from the luminance in television receivers. In particular, the composite video signal is typically fed to a low pass filter to extract the luminance information, and to a band pass filter to extract the chrominance information.

However, cross luminance artifacts, such as hanging dots and sot crawls, usually appear after separating chrominance and luminance from the composite video signal because of the cross luminance and cross chrominance (cross color). In summary, because video display quality is important to viewers, a method of luminance and chrominance separation without producing cross luminance artifacts is desirable.

SUMMARY OF THE INVENTION

The primary objective of this invention is to provide decoders and methods for separating luminance and chrominance information of a target pixel in a composite video signal. An embodiment of the decoders comprises a delay module, a filter module, a weighting factor generator, and a separator. The delay module is configured to delay a plurality of horizontal lines in the composite video signal to output a plurality of pixels including the target pixel. The filter module is coupled to the delay module and configured to filter the composite video signal in a horizontal direction and a vertical direction to generate a plurality of frequency components. The weighting factor generator is configured to generate a first weighting factor of the target pixel according to the horizontal direction and a second weighting factor of the target pixel according to the vertical direction. The separator is configured to separate luminance information from the target pixel in accordance with the frequency components, the first weighting factor, and the second weighting factor.

An exemplary embodiment of the methods comprises the following steps: filtering a composite video signal with a plurality of horizontal lines composed of a plurality of pixels in a vertical direction and a horizontal direction to generate a plurality of frequency components; generating a first weighting factor of a target pixel according to the horizontal direction; generating a second weighting factor of the target pixel according to the vertical direction; and generating a first separated information of the target pixel in accordance with the frequency components, the first weighting factor, and the second weighting factor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A video signal composed of a plurality of pictures, and a picture is composed of a plurality of pixels, separating luminance and chrominance information of a video signal is realized by separating the luminance and chrominance information from each of the pixels. In general, a filter processes a target pixel of a sampled composite video signal by first acquiring target pixel and pixels surrounding the target pixel. For example, as shown inFIG. 1, a target pixel P7and its surrounding pixels P0to P14are acquired by line delay units and pixel delay units. The number of pixels acquired by the filter for separating chrominance and luminance of a target pixel is not necessary to be 15 as shown inFIG. 1as it can be varied according to the filter design.

FIG. 2illustrates an embodiment of a decoder2of the present invention. The decoder2comprises a delay module21, a filter module23, a weighting factor generator25, and a separator27. The decoder2receives a composite video signal20and the delay module21comprises a plurality of line delay units and a plurality of pixel delay units (not shown), and is configured to output a plurality of horizontal lines composed of a target pixel and a plurality of neighboring pixels.FIG. 3shows an example of the delay module21, which comprises line delay units300and301, and pixel delay units302to313. In this embodiment, three horizontal lines each comprising a plurality of pixels are retrieved for calculation. Pixels of a first horizontal line are retrieved through two line delay units300and301and respectively through pixel delays302to305, pixels of a second horizontal line are retrieved through one line delay300and respectively through pixel delays306to309, and pixels of a third horizontal line are respectively retrieved before pixel delay310and after pixel delays310to313.

In the hardware, pixels of the picture are inputted into the delay module21one by one. To hold 15 sampled pixels for conducting luminance and chrominance separation for a target pixel (e.g. pixel321), the delay module21uses two line delay units300and301, and twelve pixel delay units302to313. These 15 pixels314to328as shown inFIG. 3are provided to a processing unit, such as the filter module23as shown inFIG. 2. It is noted that the number of line and pixel delays may depend on the processing ability of the processing unit, it is however not a limitation of the present invention.

The filter module23coupled to the delay module21filters the composite video signal in a horizontal direction and a vertical direction to generate a plurality of frequency components.FIGS. 4A-4Dshow a series of schematic diagrams illustrating an exemplary process of filtering the composite video signal.FIG. 4Aillustrates a two-dimensional frequency domain of an unfiltered composite video signal, where the vertical and horizontal axes respectively represent the vertical and horizontal frequency.

As shown inFIG. 4B, the filtering module23first filters the composite video signal20in the horizontal direction to generate a low horizontal frequency component40and a high horizontal frequency component41.

As shown inFIG. 4C, the filtering module23filters the low horizontal frequency component40in the vertical direction to generate a first frequency component400with high vertical frequency and a second frequency component401with low vertical frequency.

Finally, as shown inFIG. 4D, the filtering module23filters the high horizontal frequency component41in the vertical direction to generate a third frequency component410with high vertical frequency and a fourth frequency component411with low vertical frequency.

More specifically, the filter module23filters the composite video signal20in the horizontal direction to generate a low horizontal frequency component40corresponding to the target pixel321(P321). The following equations show an example of conducting horizontal filtering for the composite video signal
A1=(P314+P315*0+P316*2+P317*0+P318)/4
A2=(P319+P320*0+P321*2+P322*0+P323)/4
A3=(P324+P325*0+P326*2+P327*0+P328)/4,

wherein P314to P328represent composite information including luminance information and chrominance information of the pixels314to328respectively, and A1to A3respectively represent upper, medium, and lower lines in the low horizontal frequency component40.

The filter module23may also generate a high horizontal frequency component41corresponding to the target pixel321based on the low horizontal frequency component40.
B1=P316−A1
B2=P321−A2
B3=P326−A3,

wherein B1to B3respectively represent upper, medium, and lower lines in the high horizontal frequency component41.

The first frequency component400and second frequency component401are generated by vertical filtering the low horizontal frequency component40, and the following equations show an example of conducting vertical filtering on the low horizontal frequency component40.
D=(A1+A2*2+A3)/4
C=A2−D,

wherein D and C represent the second frequency component401and first frequency component400, respectively.

Similarly, the low horizontal frequency component41is further divided into the third frequency component410and fourth frequency component411.
F=(B1+B2*2+B3)/4
E=B2−F,

wherein F and E represent the fourth frequency component411and third frequency component410, respectively. In summary, the second frequency component401is filtered to contain low frequency components at both vertical and horizontal directions, the third frequency component410is filtered to contain high frequency components at both vertical and horizontal directions, the first frequency components400contains high vertical frequency and low horizontal frequency components, and the fourth frequency component411contains high horizontal frequency but low vertical frequency components. To separate luminance information, the second frequency component401and weighted first and fourth frequency components400and411are reserved, and the third frequency component410is removed in this embodiment.

The weighting factor generator25of this embodiment is configured to generate a first weighting factor of the target pixel321according to the horizontal direction.FIG. 5shows a weighting factor generator25comprising a subtractor250, a calculation module251, and a storage module252. The storage module252is configured to store a lookup table (not shown) comprising a plurality of predetermined weighting factors. The subtractor250is configured to subtract two pixels (i.e. pixels319and323) which are arranged in the same horizontal line with the target pixel321. The calculation module251is configured to generate a first absolute value of the subtracted result. The first weighting factor is retrieved in accordance with the first absolute value from the lookup table stored in the storage module252.

In addition, the weighting factor generator25can be further configured to generate a second weighting factor of the target pixel321according to the vertical direction. The subtractor250is further configured to subtract an upper pixel (i.e. pixels316) of the target pixel321from a lower pixel (i.e. pixels326) of the target pixel321. The calculation module251is further configured to generate a second absolute value of the subtracted result of the two pixels. The second weighting factor is retrieved in accordance with the second absolute value from another lookup table stored in the storage module252. In some other embodiments, the weighting factor generator25is realized by two sets of weighting factor generator comprising a subtractor, a calculation module, and a look up table, where one set of weighting factor generator generates the first weighting factor for the first frequency component400and the other generates the second weighting factor for the fourth frequency component411.

The separator27is configured to separate luminance information from the target pixel321in accordance with the frequency components, the first weighting factor, and the second weighting factor. More particularly, the separator27retrieves the luminance information according to the following equation:
L=D+Kc*C+Kf*F+0*E,
wherein L is the luminance information, Kc is the first weighting factor, and Kf is the second weighting factor.

The separator27is configured to separate chrominance information from the target pixel321according to the luminance information of the target pixel321and the target pixel321. In fact, the chrominance information of the target pixel321is separated by subtracting the luminance information of the target pixel321from the composite information of target pixel321, which can be represented as the following equation:
C=P321−L,

wherein C is the chrominance information.

FIG. 6is a flow chart illustrating an embodiment of a method, for separating luminance and chrominance information from a composite video signal. More specifically, the method illustrated inFIG. 6can be implemented by various modules of the decoder2. Step60is executed to delay a plurality of horizontal lines in the composite video signal to obtain a plurality of pixels. Step61is executed to filter the composite video signal in a horizontal direction and a vertical direction to generate a plurality of frequency components.

Step62is executed to subtract composite information of a first pixel from composite information of a second pixel, the first and second pixels are arranged in the same horizontal line with the target pixel. Step63is executed to generate a first absolute value of the subtracted result of Step62. Step64is executed to retrieve a first weighting factor from a lookup table in accordance with the first absolute value. Step65is executed to subtract composite information of an upper pixel from composite information of a lower pixel, the upper and lower pixels are arranged in the same vertical line with the target pixel. Step66is executed to generate a second absolute value of the subtracted result of Step65. Step67is executed to retrieve a second weighting factor from a lookup table in accordance with the second absolute value. Specifically, the first and second weighting factors are between 0 and 1, and more specifically, a greater first absolute value typically results in a smaller first weighting factor, and a greater second absolute value typically results in a smaller second weighting factor.

After the first and the second weighting factors are generated, step68is executed to separate luminance information of the target pixel in accordance with the frequency components, the first weighting factor, and the second weighting factor. Finally, step69is executed to separate chrominance information of the target pixel by subtracting the luminance information of the target pixel from the composite information of the target pixel.

FIG. 7illustrates the details of generating a plurality of frequency components as stated in Step61ofFIG. 6. Step610is executed to filter the composite video signal to generate a low horizontal frequency component. Likewise, step611is executed to filter the composite video signal to generate a high horizontal frequency component. Step612is executed to filter the low horizontal frequency component to generate a first frequency component with a high vertical frequency. Likewise, step613is executed to filter the low horizontal frequency component to generate a second frequency component with a low vertical frequency. Then, step614is executed to filter the high horizontal frequency component to generate a third frequency component with a high vertical frequency. Finally, step615is executed to filter the high horizontal frequency component to generate a fourth frequency component with a low vertical frequency. An embodiment of filtering the composite video signal to generate the four frequency components is described in the above content and thus omitted for brevity.

According to the aforementioned descriptions, the luminance and chrominance information is separated from a target pixel in a composite video signal through summing the frequency components with some weighting factors. The weighting factors are mainly used to decide the importance of the frequency components and to prevent cross luminance artifacts such as hanging dot and dot crawls.