Abstract:
A method, apparatus, and system for reducing chrominance artifacts in a luminance signal obtained from a composite NTSC television signal is disclosed. Chrominance artifacts are reduced by detecting chrominance artifacts in the luminance signal of a current line and a previous line, weighting the luminance signal of the current line and the luminance signal of the previous line based on the detected chrominance artifacts, and combining the weighted luminance signal of the current line and the weighted luminance signal of the previous line for use as the luminance signal for the current line. Reducing chrominance artifacts reduces the occurrence of “hanging-dots” displayed on a television monitor, which are due to incompletely canceled chrominance artifacts in the luminance signal.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of consumer electronics and, more particularly, to reducing chrominance artifacts in a luminance signal obtained from a composite NTSC television signal.  
         BACKGROUND OF THE INVENTION  
         [0002]    In a color television (TV) system (such as NTSC), the luminance and chrominance components (“luma” and “chroma,” respectively) of a composite color television signal are disposed within the video frequency spectrum in a frequency interleaved relation. The luma components are positioned at integral multiples of the horizontal line scanning frequency and the chroma components are positioned at odd multiples of one-half this frequency. In the NTSC system, the upper portion (i.e., about 2.1 to 4.2 MHz) of the video frequency spectrum (0 to 4.2 MHz) is shared by chroma components and high frequency luma components. The lower portion (below about 2.1 MHz) of the video frequency spectrum is occupied solely by luma components. The video frequency spectrum is located within a 6 MHz NTSC television channel and begins at 1.25 MHz within this channel. Thus, 2.1 MHz in the video frequency spectrum corresponds to 3.35 MHz in the 6 MHz NTSC television channel. Additionally, in accordance with the NTSC system, from horizontal-line to horizontal-line (“adjacent lines”), the luma components are in-phase with one another and the chroma components are 180 out-of-phase with one another.  
           [0003]    Comb filters are frequently used to separate the luma and chroma components from one another. Comb filters operate on the premise that the composite video signals of adjacent lines are highly correlated. Since the luma components of adjacent lines are in-phase and the chroma components are out-of-phase, adding the composite signal for the previous line to the composite signal for the immediately preceding line yields the luma components of a current line. This effectively removes the chroma components, leaving only the luma components. Likewise, subtracting the composite signal of a previous line from the current line yields the chroma components of the current line. This effectively removes the luma components, leaving only the chroma components.  
           [0004]    When the composite video signal from adjacent lines is not highly correlated, anomalies occur in the reproduced images. The anomalies result from imperfect cancellation of chroma in the luma signal. For example, if there is an abrupt change in the amplitude of chroma between adjacent lines, serrations will occur along the horizontal edges displayed in the image for a line combed filtered (hereinafter “combed”) signal. These serrations (called “hanging dots”) are due to incompletely cancelled chroma components (called “artifacts”) in the luma signal.  
           [0005]    Various techniques have been developed to avoid hanging dots. Typically, these techniques examine a composite signal or a luma signal separated from the composite signal and use different filtering techniques based on this examination. Decision circuits examine these signals and actuate switches to select the appropriate technique. When the decision circuitry has difficulty deciding what to do, switching artifacts may be introduced to the display area of a television. Examples of these types of filters can be found in U.S. Pat. No. 4,814,863 to Topper et al. entitled DETECTION AND CONCEALING ARTIFACTS IN COMBED VIDEO SIGNALS and U.S. Pat. No. 4,179,705 to Faroudja entitled METHOD AND APPARATUS FOR SEPARATION OF CHROMINANCE AND LUMINANCE WITH ADAPTIVE COMB FILTERING IN A QUADRATURE MODULATED COLOR TELEVISION SYSTEM.  
           [0006]    Accordingly, there is a need for methods, apparatus, and systems for separating chroma and luma components from a composite signal that have reduced chroma artifacts in the luma signal and address the limitation of the prior art. The present invention fulfills this need among others.  
         SUMMARY  
         [0007]    The present invention provides a luminance/chrominance (Y/C) separation method, apparatus, and system that satisfies the aforementioned need by detecting chroma artifacts in a luma signal separated from a composite NTSC television signal for a current line and a previous line. The detected chroma artifacts are then used to weight the luma signal for the current and previous lines. The weighted signals are then combined to form a replacement luma signal for the current line. The lines are weighted such that if the chroma artifacts in the current line are larger than the chroma artifacts in the previous line, the previous line will receive more weight in forming the replacement luma signal, and vice versa. Weighting the line with the smaller chroma artifact more heavily effectively removes the chroma artifact without the need of switching, thereby avoiding the generation of switching artifacts associated with such techniques. The detected chroma artifacts may additionally be used to weight the chroma signal for the current and previous lines. The weighted chroma signals are then combined to form a replacement chroma signal for the current line.  
           [0008]    A method for reducing chroma artifacts in a luma signal of a current line in accordance with the present invention includes detecting chroma artifacts in the luma signal of a current line and a previous line, weighting the luma signal of the current line and the luma signal of the previous line based on the detected chroma artifacts, and combining the weighted luma signal of the current line and the weighted luma signal of the previous line for use as a replacement luma signal for the current line.  
           [0009]    An apparatus for reducing chroma artifacts in a luma signal of a current line in accordance with the present invention includes a detection circuit which detects chroma artifacts in the luma signal of a current line and the luma signal of a previous line, a first weighting circuit which weights the luma signal of the current line and the luma signal of the previous line based on the detected chroma artifacts, and a first combiner which combines the weighted luma signal of the current line and the weighted luma signal of the previous line for use as a replacement luma signal for the current line.  
           [0010]    A system for reducing chroma artifacts in a luma signal of a current line in accordance with the present invention includes means for detecting chroma artifacts in the luma signal of a current line and a previous line, first weighting means for weighting the luma signal of the current line and the luma signal of the previous line based on the detected chroma artifacts, and means for combining the weighted luma signal of the current line and the weighted luma signal of the previous line for use as a replacement luma signal for the current line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The invention is best understood from the following detailed description when read in connection with the accompanying drawings. This emphasizes that according to common practice, the various features of the drawings are not drawn to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following features:  
         [0012]    [0012]FIG. 1 is a block diagram of a Y/C separation apparatus in accordance with the present invention; and  
         [0013]    [0013]FIG. 2 is a block diagram of an artifact detector for use in the Y/C separation apparatus of FIG. 1.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    In the DRAWINGS, the lines interconnecting various blocks represent either single conductor connections carrying analog signals or multi-conductor buses carrying multi-bit parallel binary digital signals. Those of skill in the TV signal processing art will appreciate that the invention may be practiced on either digital or analog representations of the composite video signal. For the purposes of the detailed description, however, it will be assumed herein that the composite video signal is a digital signal and that the composite video signal is in the NTSC format. Additionally, it will be assumed that the composite video signal is sampled at a sampling rate equal to four times the frequency of the color subcarrier (four times 3.58 MHz or approximately 14.3 MHz). Under these conditions there will be 4 sample intervals for one complete color-difference cycle and there will be a total of 910 samples per line. The four sample intervals may be represented by Y+I, Y+Q, Y−I, and Y−Q, where Y is luma, I is an in-phase component of chroma, and Q is a quadrature-phase component of chroma. Each sample of an interval includes Y and either I or Q, with I and Q alternating from sample to sample. One-half of one color-difference cycle includes one sample of I and one sample of Q, which together form a color-difference pair.  
         [0015]    [0015]FIG. 1 depicts a luminance/chrominance (Y/C) separation apparatus  100  for separating a composite video signal into a chroma signal (C) and a luma signal (Y) in accordance with one embodiment of the present invention. The composite video signal is received from the output port of a video detector stage (not shown). The composite video signal is applied to an analog-to-digital (A/D) converter  102 . The A/D converter  102  samples the incoming composite video signal at four times the color subcarrier frequency (4 fsc) and converts it into a digital signal.  
         [0016]    The digital composite video signal at the output port of the A/D converter  102  is applied to a separator circuit  104  for separating the composite video signal into intermediate chroma and luma signals. In the illustrated embodiment, the separator circuit  104  separates the composite video signal into an intermediate chroma signal (C′), an intermediate low frequency luma signal (Ylf), and an intermediate high frequency luma signal (Yhf). The illustrated separator circuit  104  is a conventional comb filter including a high pass filter (HPF)  106 , a first subtractor  108 , a delay element  110 , a second subtractor  112 , and a summer  114 .  
         [0017]    Due to the overlap of luma and chroma components in the composite video signal at high frequencies, chroma artifacts may be present in Yhf after separation. Artifacts arise when the separator circuit  104  is unable to fully separate the composite signal into its luma and croma components. At frequencies where the luma and chroma components overlap, e.g., greater than 3 MHz in a 6 MHz NTSC television channel, the chroma components are typically much larger than the luma components. Thus, if present, the chroma artifacts overpower the luma components within Yhf, which appear as a pattern of dots on a television display.  
         [0018]    The high pass filter (HPF)  106  is operative to pass frequencies above a predefined level. Preferably, the HPF  106  passes frequencies of the composite video signal in which luma and chroma components overlap (i.e., frequencies greater than approximately 3.0 MHz). The output signal of the HPF  106  is subtracted from the composite video signal by the first subtractor  108  to obtain Ylf. Since chroma components are not contained in the low frequency portion of the composite signal (i.e., frequencies less than approximately 3.0 MHz), the resultant Ylf contains low frequency luma components and is free of chroma artifacts. Accordingly, no further processing of Ylf is performed.  
         [0019]    The high frequency signal passed by the HPF  106  contains all the chroma components and high frequency luma components. This high frequency signal is applied to the delay element  110 . Preferably, the delay element  110  is a 1-H delay element, which delays the signal by one horizontal line scanning period to develop a delayed signal representing corresponding components from the previous horizontal line. The output signal of the delay element  100  is subtracted from the output signal of the HPF  106  by the second subtractor  112  to develop an intermediate chroma signal, C′. Additionally, the output signal of the delay element  100  is added to the output signal of the HPF  106  by summer  114  to develop Yhf. As described above, in the NTSC system, for adjacent horizontal lines, the luma components are in-phase and the chroma components are 180 degrees out-of-phase. In addition, typically, the luma components and the chroma components for adjacent horizontal lines do not vary substantially. Thus, adding two adjacent horizontal lines typically yields luma components at twice the amplitude of the luma components in a single line and subtracting one horizontal line from an adjacent horizontal line yields chroma components at twice the amplitude of the chroma components in a single line. If the chroma components change from line to line, artifacts of the chroma components may be found in Yhf.  
         [0020]    In an exemplary embodiment, as described in detail below, to accommodate the presence of chroma artifacts in Yhf, current and previous lines of Yhf are weighted based on chroma artifacts in the current and previous lines for Yhf to produce a weighted Yhf. After weighting, Yhf is combined with Ylf to produce the luma signal Y. In addition, to compensate for the errors in the intermediate chroma signal, C′, caused by the changes in the chroma signal from line to line, current and previous lines of C′ are also weighted based on the chroma artifacts for Yhf to produce the chroma signal C.  
         [0021]    Yhf is applied to a delay element  116  and C′ is also applied to a delay element  118 . Preferably, the delay elements  116 ,  118  are 1-H delay elements, which delay Yhf and C′, respectively, by one horizontal line scanning period to develop delayed signals. The non-delayed signals represent the current lines and the delayed signals represent the previous lines for corresponding horizontal positions of the lines, i.e., pixels that are vertically adjacent  
         [0022]    The Yhf signals for the current and previous lines are passed to an artifacts detector  120 . The illustrated artifacts detector  120  includes a first artifact detector  122  and a second artifact detector  124 . The first artifact detector  122  detects the presence of chroma artifacts in Yhf for the current line and the relative strength of these chroma artifacts. The second artifact detector  124  detects the presence of chroma artifacts in Yhf for the previous line and the relative strength of these chroma artifacts.  
         [0023]    As described in detail below, in a preferred embodiment, the relative strength of the chroma artifacts in Yhf for the current and previous lines is used to weight Yhf for the current and previous lines to develop the high frequency portion of Y. Preferably, the relative strength of the chroma artifacts is also used to weight C′ for the current and previous lines to develop the chroma signal, C. In an alternative embodiment, C′ is not weighted and C is essentially C′.  
         [0024]    [0024]FIG. 2 depicts an exemplary artifact detection circuit  200  suitable for use as an artifact detector  122 ,  124  (FIG. 1) for processing Yhf of the current and previous lines, respectively, to develop signals representing the relative weights of the chroma artifacts within these lines. The illustrated artifact detection circuit  200  includes an absolute value circuit  202 , a delay element  204 , a maximum circuit  206 , and a register  208 . For descriptive purposes, the artifact detection circuit  200  is described in terms of detecting chroma artifacts in Yhf for the current line (i.e., as the artifact detector  122  of FIG. 1). The use of the artifact detection circuit  200  for detecting chroma artifacts in Yhf for the previous line will be readily apparent from the description for detecting chroma artifacts in Yhf for the current line.  
         [0025]    The absolute value circuit  202  rectifies the individual samples of the color-difference cycles within Yhf since their arithmetic sign alternates from one-half color-difference cycle to the next. By rectifying the individual samples, the arithmetic sign can be ignored, leaving the magnitude of individual samples within the color-difference cycles.  
         [0026]    The rectified individual samples are applied to the delay element  204 . The delay element  204  introduces a one sample delay. Because the composite video signal is sampled at 4 fsc, the individual samples for a Yhf signal containing chroma artifacts of I and Q alternate between having an I artifact and a Q artifact. When an I artifacts is at the input port of the delay element  204 , a Q artifact is at the output port, and vice versa.  
         [0027]    The maximum circuit  206  processes adjacent rectified individual samples. Therefore, if chroma artifacts containing I and Q artifacts are present, the maximum circuit  206  processes a Q artifact of a sample and an I artifact of an adjacent sample. Because the samples are rectified by rectifier  202 , the maximum circuit  206  can compare the magnitude of I and Q artifacts from adjacent individual samples within a single one-half color-difference cycle or spanning two one-half color-difference cycles. In the illustrated maximum circuit  206 , the maximum circuit  206  produces a non-additive mix of the adjacent rectified individual samples at an output port. Thus, if the I artifact is larger than the Q artifact, the magnitude of the I artifact will be produced by the maximum circuit  206 , and vice versa.  
         [0028]    The register  208  processes the output signal of the maximum circuit  206 . Preferably, the register  208  is clocked at one-half the individual sample rate. By clocking the register  208  at one-half the individual sample rate, the output signal produced by a color-difference pair (i.e., one I artifact and one Q artifact) is presented by the register  208  for two individual samples. Thus, one value is produced for both the individual samples of the color-difference pair. This value represents the relative weight, W, of the chroma artifacts within the line signal being processed.  
         [0029]    Referring back to FIG. 1, the signals representing the relative weights of the chroma artifacts within Yhf of the current and previous lines are passed to a weighting circuit  126 . The illustrated weighting circuit  126  includes a weight generator  128 , a first weight block  130 , a first summer  132 , a second weight block  134 , and a second summer  136 . The first weight block  130  weights the current and previous lines of Yhf based on a weight determined by weight generator  128 . The weighted current and previous lines of Yhf are then combined at the first summer  132  to produce the high frequency luma components of the signal Y, which is combined with the low frequency luma components of Y (i.e., Ylf) at summer  150  to produce the signal Y. The second weight block  134  weights the current and previous lines of C′ based on the weight determined by weight generator  128 . The weighted current and previous lines of C′ are then combined at the second summer  136  to produce C. It will be apparent to those of skill in the art that in embodiments of the present invention where C′ is not weighted, the second weight block  134  can be eliminated.  
         [0030]    The weight generator  128  in the illustrated embodiment generates the weight value, G, based on the relative weights of the chroma artifacts within the current and previous lines of Yhf as determined by the artifacts detector  120 . In the illustrated embodiment, the weight generator generates a value representing the ratio of the relative weight of the chroma artifacts within the current line for Yhf to the sum of the relative weights of the chroma artifacts within the current and previous lines for Yhf. Thus, if the relative weight of artifacts in the current line is high (low) and the relative weight of artifacts in the previous line is low (high), G will approach one (zero). Accordingly, G will vary between 0 and 1 depending on the relative weights of the chroma artifacts on the two lines. In accordance with certain exemplary embodiments, if the relative weights of the chroma artifacts within Yhf for each of the current and previous lines are below a threshold valve, e.g., below two % of full scale video, G is set to zero. The weight generator  128  may be implemented using discrete components, integrated circuits, ASICs, or essentially any device capable of processing digital or analog signals.  
         [0031]    The first weight block  130  in the illustrated embodiment includes a first amplifier  138  and a second amplifier  140 . The first amplifier  138  amplifies the signal Yhf for the current line and the second amplifier  140  amplifies the signal Yhf for the previous line. In a preferred embodiment, the first amplifier  138  amplifies Yhf for the current line by 1-G and the second amplifier  140  amplifies Yhf for the previous line by G. Thus, if G is zero (one), Yhf for the current line is multiplied by one (zero) and Yhf for the previous line is multiplied by zero (one). Additionally, values of G between zero and one result in the amplification of Yhf for the current and previous lines by values between zero and one. Specifically, the previous line is amplified by G and the current line is amplified by 1-G. The first weight block  130  may be implemented using a conventional addressable memory block.  
         [0032]    The second weight block  134  in the illustrated embodiment includes a first amplifier  142  and a second amplifier  144 . The first amplifier  142  amplifies the signal C′ for the current line and the second amplifier  144  amplifies the signal C′ for the previous line. In an exemplary embodiment, the first amplifier  142  amplifies Yhf for the current line by 1-G and the second amplifier  144  amplifies Yhf for the previous line by -G. This is essentially identical to the processing performed by the first weight block  130 , with the exception that G has a negative arithmetic sign, resulting in the inversion of the previous line.  
         [0033]    Additional processing circuitry is provided to correct the magnitude of the signals and to ensure proper delay periods. This circuitry includes a first divider  145 , a delay element  146 , and a second divider  148 . The first divider  145  divides Yhf by two to correct for a doubling of the magnitude of Yhf by the separator circuit  104 . The delay element  146  delays Ylf to compensate for delay introduced to Yhf by the separator circuit  104  and the weighting circuit  126  such that the samples of Ylf coincide with corresponding samples of Yhf when combined at the summer  150 . The second divider  148  divides the signal C by two to correct for a doubling of the magnitude of C′ by the separator circuit  104 . The necessary components for correcting magnitude and delay periods are readily apparent to those of skill in the art of television signal processing.  
         [0034]    In an exemplary use, the illustrated Y/C separation apparatus  100  operates in the following manner. The separator circuit  104  separates a composite signal into an intermediate chroma signal C′, a low frequency luma signal Ylf, and a high frequency luma signal Yhf. The high frequency luma signal Yhf may contain chroma artifacts that are not completely removed by the separator circuit  104 . Yhf for the current line and Yhf for a previous line are supplied to an artifacts detector  120  that produces a weight value which is indicative of the relative level of chroma artifacts in Yhf. Yhf for current and previous lines are then weighted based on this weight value and the weighted lines are combined to form a replacement for the high frequency component of Y. Preferably, the ratio of the amplitudes from the artifact detectors  122 ,  124  are computed and used to weight the current and previous lines of Yhf such that lines having smaller detected values (i.e., less chroma artifacts) are weighted more heavily in creating the replacement for the high frequency component of Y for the current line.  
         [0035]    The current and previous lines of Yhf for the illustrated embodiment are weighted as follows:  
         [0036]    If the chroma artifacts in the current line and the previous line are essentially identical, the weight generator  128  produces a weight value, G, of one-half. If the weight value is one-half, Yhf for the previous line is amplified by one-half (i.e., G) and Yhf for the current line is amplified by one-half (i.e., 1-G). Thus, the previous and current lines each contribute equally to produce a replacement Yhf for the current line.  
         [0037]    If the chroma artifacts detected in the current line are greater that the chroma artifacts detected in the previous line (a condition which may result in the appearance of “hanging-dots” on a television display), or vice versa, the weight generator  128  produces a weight value, G, proportional to the difference in the detected artifacts. If the weight value is one, Yhf for the previous line is amplified by one and Yhf for the current line is amplified by zero. Thus, the current line containing a high level of chroma artifacts is essentially discarded and the previous line is used to produce the replacement Yhf for the current line. If the weight value is zero, Yhf for the current line is amplified by one and Yhf for the previous line is amplified by zero. Thus, the previous line containing a high level of chroma artifacts is essentially discarded and the current line is used to produce the replacement Yhf for the current line. Values of G between one-half and one result in both previous and current lines contributing to the production of the replacement Yhf with the previous line being more heavily weighted than the current line. Likewise, values of G between zero and one-half result in both lines contributing to the replacement Yhf with the current line being more heavily weighted than the previous line.  
         [0038]    If no chroma artifacts are detected in either the current line or the previous line, or the detected artifacts are below a predefined threshold value, the weight generator  128  produces a weight value, G, of zero. Thus, the current line is used to produce the replacement Yhf for the current line.  
         [0039]    The current and previous lines of C′ for the illustrated embodiment are weighted essentially as described above for Yhf, with the exception that the previous line is inverted in addition to being amplified.  
         [0040]    While a particular embodiment of the present invention has been shown and described in detail, adaptations and modifications will be apparent to one skilled in the art. Such adaptations and modifications of the invention may be made without departing from the scope thereof, as set forth in the following claims.