Abstract:
A digital signal processing system and method applied for chroma transition, wherein the method has the acts of: performing a difference process on an original chroma signal C to obtain a first difference signal C′; calculating an absolute value |C′| of the first difference signal C′; performing a difference process on the absolute value |C′| to obtain a second difference signal Ca′; determining whether the second difference signal Ca′ is a positive signal or a negative signal; wherein based on a determined result, an optimized chroma signal is generated by either mixing the original input chroma signal C with a k-delayed chroma signal, or mixing the k-delayed chroma signal C[n−k] with a 2k-delayed chroma signal C[n−2k], where k is a constant.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a digital signal processing system and method applied for chroma transition, and more particularly to a system that selectively mixes previous, present and delayed chroma signals to generate an optimized chroma signal to improve color sharpness in a television. 
   2. Description of Related Art 
   In usual video signal (NTSC or PAL standard) processing, a video signal is separated into picture information and non-picture information. A chrominance (chroma) signal and a luminance signal which comprise the picture information are then subsequently extracted. Based on the chroma signal, two orthogonal factors can be further derived from the chroma signal. The chroma signal together with the two orthogonal factors are usually referred to as Y, U, V signals (or Y, I, Q signals or Y, Cb, Cr signals). The chroma signal is carried over the luminance signal, wherein the phase of the chroma signal can represent a unique color. During chroma transition processes, the phase of the chroma signal will accordingly be changed. With the increase in the phase of the chroma signal, the time of the chroma transition will be extended, which detracts from color sharpness. 
   Picture signals including those of the NTSC, PAL and SECAM type all can be represented by combinations of the chroma and the luminance signal. The chroma signal bandwidth is narrow in comparison with the luminance signal bandwidth. Because of the limited frequency bandwidth of the chroma signal, chroma signal transitions (transients) are relatively slow. That is to say, the slope of a transition representing color edges has only a moderate slope, which when displayed for viewing detracts from sharp color demarcations. 
   Some chroma transition approaches, which are directly performed on the chroma signal, have been proposed to improve the color sharpness. For example, with reference to  FIG. 6 , U.S. Pat. No. 5,920,357 issued to Ohara entitled “Digital color transient improvement” mainly uses two time delay circuits ( 61 )( 62 ), a band pass filter ( 63 ) and a median filter ( 64 ) to improve the chroma transition. With reference to  FIG. 7 , another similar approach is disclosed in U.S. Pat. No. 5,936,682 entitled “Circuit for enhancing chrominance transitions in real-time video reception” that utilizes multiple time delay circuits ( 71 ), a band pass filter  72  and a comparison circuit ( 73 ) to improve the chroma transition. 
   The two prior arts are indeed able to enhance the color sharpness. However the implementation of the digital filter is rather complex and expensive. Further, the output waveform of the median filter ( 64 ) is not ideal. 
   Another known technique is shown in  FIG. 8 , U.S. Pat. No. 6,008,862 entitled “Perceived color transient improvement”, which adjusts the luminance signal based on the detected status of the chroma signal without directly modifying the chroma signal. The chroma signal (Cin) and the luminance signal (Yin) are respectively input to two edge detectors ( 81 )( 82 ). A multiplier (not numbered) multiplies the logical signals from the edge detectors ( 81 )( 82 ) and furnishes a logical control signal to an artificial peaked signal generator ( 83 ). Preferably, the edge detector ( 81 ) furnishes edge parameters like width and steepness of the edge to the artificial peaked signal generator ( 83 ). An output signal from the generator ( 83 ) is applied to an adder (not numbered), which also receives a peaked luminance signal from an optional luminance peaking circuit ( 84 ) to which the input luminance signal Yin is applied. The adder supplies the output luminance signal Yout. The output chroma signal (Cout) is identical to the input chroma signal (Cin). 
   The architecture of  FIG. 8  is mainly applied to the color display apparatus that receives video signals composed of three primary color signals, red, blue and green. The three primary color signals can be converted into Y, Cb and Cr signals through linear converting functions. Therefore, if the luminance signal has any change, the chroma signal is basically supposed to be affected. However, as shown in  FIG. 9 , the output chroma signal (Cout) is identical to the input chroma signal (Cin). That means there is no improvement in performance of the chroma signal. 
   With reference to  FIG. 10 , U.S. Pat. No. 6,571,224 entitled “Fuzzy logic based color transition improvement method and system” adopts the fuzzy theory to optimize color transition. The system uses two parameter tables to control the color transition, wherein the first table is established based on first-order difference signals and second-order difference signals, and the second table is established based on the time when each of the first-order signals takes place. Even though the system does not need any filters, these fuzzy logic blocks ( 91 )( 92 ), calculation blocks ( 93 )( 94 ) and signal differential circuits are still complex. 
   SUMMARY OF THE INVENTION 
   The objective of the present invention is to provide a digital signal processing system and method applied for chroma transition, wherein the system selectively mixes previous, present and delayed chroma signals to generate an optimized chroma signal thus improving color sharpness. 
   Preferably, the system is performed by: 
   a first difference circuit, which receives an original chroma signal and generates a first difference signal of the original chroma signal; 
   a first absolute value circuit coupled to the first difference circuit to calculate an absolute value of the first difference signal; 
   a second difference circuit, which generates a second difference signal based on the reception of the absolute value from the first absolute value circuit; 
   a symbol determining circuit, which determines that the second difference signal is either a positive signal or a negative signal, wherein a determined result is used as a selecting signal; 
   a switching circuit, which receives the original chroma signal and a 2k-delayed chroma signal of the original chroma signal, wherein the switching circuit selectively outputs either the original chroma signal or the 2k-delayed chroma signal based on the selecting signal; and 
   a signal calculation circuit, which receives an output signal of the switching circuit and a k-delayed chroma signal of the original chroma signal, and then generates therefrom an optimized chroma signal. 
   Other objects, advantages, and unique features of the invention will become more apparent from the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a digital chroma signal transition system in accordance with the present invention; 
       FIG. 2  is a circuit block diagram of  FIG. 1  according to a first embodiment; 
       FIG. 3  illustrates waveforms of  FIG. 2  in accordance with the present invention; 
       FIG. 4  is a circuit block diagram of  FIG. 1  according to a second embodiment; 
       FIG. 5  illustrates waveforms of  FIG. 4  in accordance with the present invention; 
       FIG. 6  is a circuit block diagram of a conventional circuit for color transient improvement of U.S. Pat. No. 5,920,357; 
       FIG. 7  is a circuit block diagram of a conventional circuit for color transient improvement of U.S. Pat. No. 5,936,682; 
       FIG. 8  is a circuit block diagram of a conventional circuit for color transient improvement of U.S. Pat. No. 6,008,862; 
       FIG. 9  illustrates waveforms of  FIG. 8 ; and 
       FIG. 10  is a circuit block diagram of a conventional circuit for color transient improvement of U.S. Pat. No. 6,008,862. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1 , a digital chroma signal transition system in accordance with the present invention comprises a first difference circuit ( 101 ), a first absolute value circuit ( 102 ), a second difference circuit ( 103 ), a symbol determining circuit ( 104 ), a switching circuit ( 105 ), a signal calculation circuit ( 106 ), a second absolute value circuit ( 107 ) and an optional modifying circuit ( 108 ). 
   The first difference circuit ( 101 ) generates a difference signal C′ of first order based on an original input chroma signal C[n]. 
   Upon reception of the first difference signal C′, the first absolute value circuit ( 102 ) calculates an absolute value |C′| of the first difference signal C′. 
   When the second difference circuit ( 101 ) receives the absolute value |C′|, the second difference circuit ( 101 ) then generates a difference signal Ca′ (or referred to as “delta” hereinafter) of a second order. 
   The difference signal of second order Ca′ is then input to the symbol determining circuit ( 104 ). The symbol determining circuit ( 104 ) determines that the difference signal of second order Ca′ is either a positive signal or a negative signal. 
   The switching circuit ( 105 ) simultaneously receives two signals, i.e. the original chroma signal C[n] and a 2k-delayed chroma signal C[n−2k] of the chroma signal C[n], where k is a constant. The output of the symbol determining circuit ( 104 ) is employed as a selecting signal furnished to the switching circuit ( 105 ). Therefore, based on the output of the symbol determining circuit ( 104 ), the switching circuit ( 105 ) selectively outputs either the original chroma signal C[n] or the 2k-delayed chroma signal C[n−2k]. 
   The second absolute value circuit ( 107 ) receives the difference signal Ca′ and calculates its absolute value |Ca′|. The absolute value |Ca′| is output to the signal calculation circuit ( 106 ) through the modifying circuit ( 108 ). The modifying circuit ( 108 ) can apply a proper weight factor on the absolute value |Ca′| before transmission to the signal calculation circuit ( 106 ). 
   The signal calculation circuit ( 106 ) receives the output signal from the switching circuit ( 105 ) and a k-delayed chroma signal C[n-k] of the chroma signal C[n]. Based on the received weight factor (kappa), the signal calculation circuit ( 106 ) based on the two received signals generates an output chroma signal Cout. 
   With reference to  FIG. 2 , a detailed implementation according to the first embodiment of the foregoing system is illustrated. The original chroma signal C[n] is input to the first difference circuit ( 101 ) composed of a first delay circuit ( 101 A) and a first subtractor ( 101 B). After the original chroma signal is delayed by k, the k-delay chroma signal C[n−k] is output from the first delay circuit ( 101 A). The subtractor ( 101 B) takes in the k-delayed chroma signal C[n−k] and the original chroma signal C[n], and then generates therefrom the difference signal C′ of the first order. The first absolute value circuit ( 102 ) calculates the absolute value |C′| of the difference signal C′. 
   The second difference circuit ( 103 ) comprises a second delay circuit ( 103 A) and a second subtractor ( 103 B). The absolute value signal |C′| is input to the second delay circuit ( 103 A) thus deriving a delayed absolute value signal (Alpha). The second subtractor ( 103 B) receives the absolute value |C′| and the delayed the absolute value (Alpah), and then generates therefrom the difference signal (Delta) of second order. 
   The symbol determining circuit ( 104 ) then determines that the difference signal Delta is either a positive signal or a negative signal. The determined result is employed as a selecting signal applied to the switching circuit ( 105 ). 
   The k-delayed chroma signal C[n−k] is also input to a third delay circuit ( 109 ) thereby obtaining a 2k-delay chroma signal C[n−2k]. The k-delayed chroma signal C[n−k] and the 2k-delay chroma signal C[n−2k] are simultaneously input to the switching circuit ( 105 ) composed of a multiplexer. Based on the selecting signal provided by the symbol determining circuit ( 104 ), the multiplexer selects either the k-delayed chroma signal C[n] or the 2k-delayed chroma signal C[n−2k] as an output signal. For example, in the case that the selecting signal is a negative signal, the multiplexer chooses the 2k-delayed chroma signal C[n−2k] as the output signal. On the contrary, if the selecting signal is a positive signal, the multiplexer chooses the chroma signal C[n] as the output signal. The output signal from the multiplexer and the k-delayed chroma signal C[n−k] are then input to the signal calculation circuit ( 106 ), and generate therefrom an optimized chroma signal Cout. 
   With reference to  FIG. 3 , the original input chroma signal C[n] is numbered as ( 201 ), the k-delayed chroma signal C[n−k] is numbered as ( 202 ) and the 2k-delayed chroma signal C[n−2k] is numbered as ( 203 ). The difference signal of the first order, beta, between the k-delayed chroma signal C[n−k] and the original input chroma signal C[n] is numbered as ( 204 ), wherein the absolute value of beta signal is denoted with ( 205 ). The delayed absolute value of beta signal, i.e. the |alpha| signal is denoted with ( 206 ). By subtracting the |beta| from the |alpha|, the Delta signal ( 207 ) is obtained. The Delta signal ( 207 ) can be further modified to become the signal “kappa” ( 208 ). The possible waveform of the output optimized chroma signal Cout is shown as waveform of ( 210 ). 
   With reference to  FIGS. 4 and 5 , another embodiment of the system comprises a first difference circuit ( 301 ), a second difference circuit ( 302 ), a first absolute value circuit ( 303 ), a second absolute value circuit ( 304 ), a subtractor ( 305 ), a symbol determining circuit ( 306 ), a switching circuit ( 307 ), a signal calculation circuit ( 308 ), a third absolute value circuit ( 309 ) and a modifying circuit ( 310 ). 
   The first difference circuit ( 301 ) is composed of a delay circuit ( 301 A) and a subtractor ( 301 B), wherein the subtractor ( 301 B) calculates a difference signal “beta” between the original input chroma signal C[n] and the k-delayed chroma signal C[n-k], wherein the absolute value of the difference signal “beta” is calculated by the first absolute value circuit ( 303 ). 
   The second difference circuit ( 302 ) comprises a delay circuit ( 302 ) and a subtractor ( 302 B), wherein the subtractor ( 302 B) calculates a difference signal “alpha” between the k-delayed chroma signal C[n-k] and the 2k-delayed chroma signal C[n−2k], wherein the absolute value of the difference signal “alpha” is obtained by the second absolute value circuit ( 304 ). 
   The subtractor ( 305 ) receives the two absolute values of beta and alpha signals and calculates a difference signal “Delta” between them. The symbol determining circuit ( 306 ) determines that the Delta signal is a positive signal or a negative signal. 
   The switching circuit ( 307 ), composed of a multiplexer, receives the original input chroma signal C[n] and the 2k-delayed chroma signal C[n−2k] and selectively outputs one of the two signals based on the determined result of the symbol determining circuit ( 306 ). 
   The signal calculation circuit ( 308 ) receives the output signal from the multiplexer and the k-delayed chroma signal C[n−k], wherein the output of the signal calculation circuit ( 308 ) is the processed chroma signal Cout. 
   The aforementioned Delta signal is further supplied to the third absolute value circuit ( 309 ) to obtain its absolute value; the acquired absolute value is then input to the modifying circuit ( 310 ) to generate a weighted signal “kappa”. The weighted “kappa” signal is furnished to the signal calculation circuit ( 308 ). The output of the signal calculation circuit ( 308 ) is the optimized chroma signal Cout. 
   Based on the foregoing description, the system in accordance with the present invention does not need any filter. Therefore, the circuit implementation is relatively simple and inexpensive. For video signals of CCIR656 standard, which has a C channel composed of chroma signals Cb and data samples Cr alternatively arranged, the present invention also suits processing such video signals without modifying any circuit implementation. 
   It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only. Changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.