Abstract:
An adaptive signal separation system and method for quadrature amplitude modulated signals that is particularly well-suited for YC separation of a composite television signal. An adaptive approach for separating overlapping signals wherein the adaptive approach selects between notch or equivalent filtering and a comb filtering system, wherein the chrominance is first demodulated to the IQ domain prior to comb filtering. The demodulated comb filtered IQ data streams are then re-modulated to reconstruct the luminance portion of the signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an improved comb filter system and method, and in particular an improved system and method for separation of composite signals. 
     2. Background 
     Due to limitations on available bandwidth and the increased demand to transmit additional information on existing bandwidth, it is often necessary to multiplex or combine two or more information signals into a single composite signal. 
     A color television signal is an example of a composite signal. A color television signal comprises a luminance (brightness) component and a chrominance (color) component. These components are often represented as Y and C components wherein Y represents the luminance component and C represents the chrominance component. 
     Originally, broadcast television in the United States began with black and white broadcast and therefore lacked the chrominance component, C, of modern television&#39;s composite signal. Television standards and technology required that the black and white television signal, that is, the luminance component (Y), reside within 6 megahertz (MHz) of bandwidth space. 
     Eventually, however, technology advanced to provide color television. To allow black and white televisions to receive the new color signal broadcast, the color signal standard located the color information within the same 6 MHz of bandwidth space allotted to each channel of the black and white signal. Under this standard, the color information overlaps with the luminance information. 
     FIG. 1 illustrates a composite television signal on a coordinate system in which the horizontal axis  100  represents frequency and the vertical axis  102  represents amplitude. Signal line  104  represents the luminance information (Y) while line  106  represents the chrominance information (represented as I and Q) of the composite signal. As shown, the frequencies of these signals  104 ,  106  overlap. In an NTSC (National Television Standards Committee) system, the luminance information occupies the range DC to 5.5 MHz of bandwidth while the chrominance signal is bandlimited to the range approximately 0.6 to 1.3 MHz and is modulated onto a carrier at 3.58 MHz. The audio portion of the signal is at 4.5 MHz. While these two data signals conveniently fit within the 6 MHz of bandwidth space they are allotted, obvious decoding challenges are presented in order to separate the luminance information from the chrominance information. 
     The first decoding scheme adopted to separate the overlapping luminance (Y) and chrominance (C) signals comprises simple notch filtering in combination with band pass filtering. FIG. 2 illustrates a block diagram of a basic notch filter  152  and band pass filter  154 . An incoming composite signal on line  150  is presented to both of the notch filter  152  and the band pass filter  154 . 
     FIG. 3 illustrates the frequency response of a notch filter  152  and a band pass filter  154 . The output of the notch filter generally comprises the luminance portion  174  of the composite signal while the output of the band pass filter generally comprises the chrominance portion  176  of the composite signal. 
     In particular, for NTSC video, the notch filter removes a portion of the composite signal centered at 3.58 MHz, but allows the remainder  174  to pass. While the notch filter  152  allows the majority of the luminance information  174  to pass, it undesirably removes components of the luminance signal having frequencies within the range of the notch filtered frequencies  177 . The notch filtered frequencies that are removed range from 2.5 to 4.5 MHz. Stated another way, the notch filter allows the frequency band below 2.5 MHz and the frequency band above 4.5 MHz to pass. 
     The band pass filter  154  configured to operate in accord with the NTSC standard video allows a 2 MHz portion of the composite signal centered at 3.58 MHz to pass while removing portions outside of this band. This portion of the composite signal contains all the chrominance information. Undesirably, however, the output of the band pass filter also contains luminance components having frequencies within the band pass filter&#39;s frequency band. 
     Notch and band pass filtering suffers from numerous drawbacks as can easily be understood with reference of FIG.  3 . In particular, the band pass filtered chrominance portion of the composite signal also contains luminance information, i.e., band pass filtering does not remove all luminance information from the chrominance signal. The unwanted luminance information in the chrominance signal introduces artifacts into the video image. This is most noticeable in pictures that contain closely spaced black and white lines, such as when the video display is of person is wearing a herringbone jacket. 
     Likewise, notch filtering the composite signal to remove the chrominance information from the composite signal to obtain the luminance information removes valuable portions of the luminance signal. A loss of luminance information is especially critical due to the human eye&#39;s sensitivity to brightness and contrast variations in a projected image. 
     Therefore, a need exists for a method and apparatus for video separation that is more robust than prior systems, requires less memory, and more completely separates the components of the composite signal. 
     SUMMARY OF THE INVENTION 
     In accordance with the purpose of the invention as broadly described herein, there is provided a method and apparatus for separating overlapping components in a composite signal, such as for example, separating the chrominance and luminance components in a quadrature amplitude modulated (QAM) signal. A novel technique is employed in which the composite signal is notch filtered to create a luminance signal containing all but a portion of the luminance components. The composite signal is also band passed filtered to create a chrominance signal containing all the chrominance components and the luminance components missing from the luminance signal. 
     Next, the chrominance signal is demodulated from the subcarrier frequency. Thereafter, the demodulated chrominance signal is comb filtered to separate the luminance components from the chrominance components. The chrominance signal is thus isolated. 
     Next, the isolated chrominance signal is subtracted from the original chrominance signal that contains the luminance components to yield a signal comprising only luminance components. 
     This luminance components signal is next remodulated and added back to the original luminance signal that is missing these luminance components. The entire luminance signal is thus created. 
     In an alternative embodiment, the sampling rate, i.e. format, of the demodulated chrominance signal is modified prior to comb filtering to reduce memory and processing requirements. This significantly reduces memory requirements for each line store in the comb filter. After comb filtering, the original sampling rate is restored to facilitate the combination of the luminance components with the notch filtered luminance signal. 
     In alternative embodiments, the number of taps or line delays in the comb filter is varied depending on one or more design parameters. 
     In an alternative embodiment, the signal separation system adopts an adaptive signal weighting scheme to dynamically adjust the portion of the final signal that is derived from a particular filtering scheme. In one configuration, a line difference detector forms a luminance coefficient and a chrominance coefficient depending on the amount of change occurring between video scan lines. The value of the luminance coefficient controls the ratio of notch filter separated luminance signal to comb filter separated luminance signal in the final output of the luminance signal. The value of the chrominance coefficient controls the ratio of band pass filter separated chrominance signal to comb filter separated chrominance signal in the final output of the chrominance signal. 
     Thus, the adaptive system dynamically selects which type of signal separation method is used at the output based on the detected difference between scan lines on a pixel by pixel basis. The adaptive system increases the percentage of notch and band pass filtered signal in the final output during periods when the signal represents motion or a change in color in the video image. Conversely, the adaptive system increases the percentage of comb filtered signal in the final output during periods when the signal represents reduced motion or consistent color in the video image. 
     Other features and advantages of the invention, as well as the structure and operation of particular embodiments of the invention, are described in detail below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a frequency plot of the components of a composite video signal. 
     FIG. 2 illustrates a block diagram of a combined notch and band pass filter. 
     FIG. 3 illustrates a plot of the frequency response of a notch and band pass filter configured to filter a composite video signal. 
     FIG. 4 illustrates a plot of sine and cosine modulation signals in accordance with quadrature amplitude modulation. 
     FIG. 5 illustrates a plot of the frequency content of a quadrature amplitude modulated composite video signal. 
     FIG. 6 illustrates a block diagram of a basic comb filter. 
     FIG. 7A illustrates a plot of the frequency response of a comb filter configured to separate the luminance components of a composite signal. 
     FIG. 7B illustrates a plot of the frequency response of a comb filter configured to separate the chrominance components of a composite signal. 
     FIG. 8 illustrates a block diagram of an adaptive signal separator of a first embodiment. 
     FIG. 9 illustrates an operational flow diagram of one exemplary method of operation of the adaptive signal separator. 
     FIG. 10 illustrates a block diagram of the modified signal separator in accordance with the first embodiment. 
     FIG. 11 illustrates a block diagram of a chrominance demodulator. 
     FIG. 12A illustrates the position of unique data samples in a 4:4:4 format signal. 
     FIG. 12B illustrates the position of unique data samples in a 4:2:2 format signal. 
     FIG. 13 illustrates an operational flow diagram of one exemplary method of operation of the modified signal separator of the first embodiment. 
     FIG. 14 illustrates a plot of two signals suffering from line to line subcarrier phase differences. 
     FIG. 15 illustrates a block diagram of a data decoder for use in conjunction with the second, third and fourth embodiments of the subject invention. 
     FIG. 16 illustrates a block diagram of a vertical scaler and two tap comb filter configured in accordance with the second embodiment of the subject invention. 
     FIG. 17 illustrates an operational flow diagram of the luminance subsystem of the vertical scaler and two tap comb filter of the second embodiment. 
     FIG. 18 illustrates an operational flow diagram of the chrominance subsystem of the vertical scaler and two tap comb filter of the second embodiment. 
     FIG. 19 illustrates a plot of a chrominance signal with luminance components. 
     FIG. 20 illustrates a plot of a time delayed chrominance signal with luminance components. 
     FIG. 21 illustrates a plot of the combination of the chrominance signal with luminance components and the time delayed chrominance signal with luminance components. 
     FIG. 22 illustrates a plot of the isolated luminance components. 
     FIG. 23 illustrates a block diagram of a vertical scaler and three tap comb filter configured in accordance with the third embodiment of the subject invention. 
     FIG. 24 illustrates a block diagram of an adaptive output weighting system in accordance with the fifth embodiment of the subject invention. 
     FIG. 25 illustrates method of operation of an adaptive output weighting system in accordance with a sixth embodiment of the subject invention. 
     FIG. 26 illustrates a block diagram of an exemplary embodiment of the subject invention. 
     FIG. 27 illustrates the pinouts of an exemplary embodiment of the subject invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Background On Comb Filters 
     The following background on comb filters is provided for the purpose of facilitating understanding of the subject invention. More specifically, it is being provided for purposes of comparing and contrasting with the subject invention although it incorporates some of its features. Since it incorporates some of the features of the subject invention, it is not entitled to prior art effect. Comb filtering is best understood in reference to FIGS. 4 and 5. 
     To aid in the understanding of comb filters, a brief discussion of quadrature amplitude modulation is first provided. Quadrature amplitude modulation (QAM), based on basic amplitude modulation, is widely adopted for television transmission. However, QAM improves on the performance of basic amplitude modulation. The QAM technique comprises simultaneously transmitting two carrier signals, each of which are at the same frequency, but separated by a 90° phase shift. The mathematical form of the transmitted signal is as follows: 
     
       
           S ( t )= A ×Sin ( Wc×t )+ B ×Cos ( Wc×t )  
       
     
     In this equation, A and B represent the amplitude of the two carrier signals. 
     FIG. 4 graphically illustrates the relationship between two quadrature signals, A and B, each transmitted with a 90 degree phase separation  208 . A horizontal axis  200  represents time and a vertical axis represents amplitude  202 . For an NTSC video signal, the I component of the chrominance portion of the composite signal is transmitted on a cosine carrier wave  204 , the Q component of the chrominance portion of the composite signal is transmitted on a sine carrier wave  206 , and the luminance portion of the composite signal is transmitted directly. The sine and cosine waves are 90 degrees out of phase. 
     When considered over multiple lines, the luminance frequency components cluster around harmonics of the horizontal line frequency. The subcarrier frequency has been chosen such that the chrominance frequency components cluster around the midpoints between the harmonics of the horizontal line frequency. This has been achieved by selecting a subcarrier frequency that is 227.5 times the horizontal line frequency. Therefore, there are 227.5 subcarrier cycles per line, and the subcarrier phase at each point in one line will be 180 degrees away from the subcarrier phase at that same point in the preceding line and the following line. The advantage of this is further illustrated below in conjunction with a discussion of comb filtering. 
     This is further illustrated in FIG. 5 which represents the frequency content of the luminance and chrominance components of a composite signal to better illustrate the differences in frequency between these signal components. As shown, luminance frequency components  260  are interleaved with the chrominance frequency components  262 . 
     FIG. 6 illustrates a basic comb filter. In operation, a composite signal arrives at input  230  and branches into a line store  232 , a first summing point  234  and a second summing point  236 . The line store delays the incoming composite signal for a time equivalent to the period of one line. 
     In regions of the video image where the Y, I, and Q components on one line are the same as the previous line, the composite signal for one line is similar to the composite signal for the previous line. However, differences in phase do exist. The luminance portion of the composite signal is the same for both lines, but the chrominance portion of the composite signal for one line is phase shifted by 180 degrees compared to the chrominance portion of the composite signal for the previous line. This occurs because the subcarrier is phase shifted by 180 degrees relative to the previous line. With reference to FIG. 6, these phase differences and the additive and subtractive properties of the feed around loop cause the unwanted portions of the chrominance and luminance signals to cancel. The luminance signal is provided on a line  238  and the chrominance signal provided on a line  240 . 
     Comb filters have a frequency response configured to filter out a particular repeating frequency pattern in signals that are offset in time. The frequency response of a comb filter is illustrated in FIGS. 7A and 7B. The horizontal axis  250  represents frequency and the vertical axis  252  represents filter response in dB. As shown in FIG. 7A, the comb filter frequency response for the luminance portion (Y) of the signal is represented by line  254 . Similarly, FIG. 7B illustrates the comb filter frequency response for the chrominance portion (C) of the signal, represented by line  256 . In effect, the comb filter acts as a notch filter with a plurality of combs or teeth centered on or aligned with the frequency components of the desired signal. 
     Because of its configuration, the comb filter excels in situations where the composite signal is stable or consistent from line-to-line such as, for example, in an area of solid color. Undesirably, however, at portions of a signal representing vertical transitions between colors or areas of motion in the video image, the comb filtering technique is undesirable because comb filtering creates artifacts at vertical transitions between differing colors and adjacent to moving objects. 
     A further drawback of comb filters is that comb filters are sensitive to imperfections in the line-to-line subcarrier phase difference. This sensitivity results from the line store  232  and the summing points  234 ,  236  in the comb filter. Subcarrier phase differences occur when the phase between the current signal and a line stored signal is other than 180 degrees out of phase. In such a situation, rather than perfectly adding or canceling at the summing points  234 ,  236 , the signals, being out of phase, combine inaccurately and provide inaccurate luminance and chrominance signals. 
     Yet another drawback of digital comb filters concerns memory requirements. Comb filters include line stores which require memory to store a line of video information. Comb filters may be implemented as 2 tap, 3 tap, 4 tap, etc. comb filters. The more taps, the greater the number of line stores, and the greater number of line stores, the greater the memory requirements for storing a line of memory intensive video information. Excessive memory requirements are undesirable because they increase the cost and size of a video device. 
     2. Example Environment of the Subject Invention 
     An example environment of the subject invention is in a video decoder. An example of a situation in which such a video decoder might be beneficially used is in a television receiver configured to receive a composite television signal. In such an environment, the composite signal, shown in FIG. 1, comprises overlapping luminance and chrominance components. Hence, one use of the present invention is in the separation of chrominance and luminance components in a television signal. 
     Another example environment of the subject invention is in video processing systems for computers. In such an environment, the subject invention separates composite video signals into the appropriate components for use in the video processing and display apparatus of a personal computer. For example, the signal separation system of the present invention, when presented with a composite video signal, separates the luminance and chrominance components so that each component may be encoded into a format compatible with computer video systems, such as an RGB color space. 
     It is contemplated that numerous other environments exist for the subject invention, including but not limited to all environments in which comb filters are presently in use. 
     First Example Embodiment 
     FIG. 8 illustrates a first embodiment of the subject invention. As shown, FIG. 8 illustrates a block diagram of an adaptive signal separation circuit. As shown, an input line  300  configured to carry a composite signal connects to a filter or filter system  302 . A composite signal is defined to mean a signal having unique overlapping signal components. In this embodiment, the composite signal may comprise any type of composite signal, although for purposes of discussion, reference is made to a composite video signal. 
     The filter system 302 separates the majority of the two or more overlapping signals that comprise the composite signal. In one configuration, the filter system  302  comprises a notch and band pass type filter. 
     Two output lines  304 ,  306  exit the filter system. Output line  304  connects to a modified signal separator  310  and a luminance multiplexer  312 . Output line  306  connects to the modified signal separator  310  and a chrominance multiplexer  314 . It is contemplated that, if the signal separation circuitry of the subject invention is configured to separate a composite signal comprising more than two signal components, then the circuitry would include an output line  304 ,  306  and a multiplexer  312 ,  314  for each component of the composite signal. 
     The modified signal separator  310  includes two outputs lines  316 ,  318  connected to the luminance multiplexer  312  and chrominance multiplexer  314  as shown. 
     A line difference detector  320  connects to the luminance multiplexer  312  and the chrominance multiplexer  314 . 
     The operation of the adaptive signal separation system illustrated in FIG. 8 is preferably discussed in conjunction with the operational flow chart illustrated in FIG.  9 . The operation of the adaptive signal separation system shown in FIG. 8 comprises first receiving a composite signal at step  350 . As can be appreciated, the signal arriving at input line  300  comprises some form of composite or combined signal containing signal components that must be separated. 
     Moving to step  352 , the signal enters the filter system  302 . The filter system  302  in this first embodiment comprises a notch filter and a band pass filter. These types of filters are known by those of skill in the art and accordingly need not be discussed in detail herein. Basically, in one embodiment, the notch filter separates the majority of the luminance signal from the composite signal although a portion of the luminance information is missing, while the band pass filter separates all or substantially all of the chrominance signal, although the luminance information missing from the luminance signal is riding on or otherwise associated with the chrominance signal. 
     Next, at step 356, the chrominance (C) component is output on the output line  306 . As shown, the band pass filtered signal is provided to the modified signal separator  310  (MSS) and the chrominance multiplexer  314 . 
     Simultaneously or at least generally concurrently, at step  354 , the luminance signal is provided to the modified signal separator  310  and the luminance multiplexer  312 . 
     Thereafter, at step  360 , the modified signal separator  310  performs modified comb filtering in accordance with the subject invention as is discussed below in conjunction with FIG.  16 . For purposes of the present discussion, the modified signal separator  310  outputs a comb filtered luminance signal (Y 3 ) and a comb filtered chrominance signal (C 3 ). In one embodiment, the comb filtered luminance signal (Y 3 ) and the comb filtered chrominance signal (C 3 ) comprise the complete versions of each signal. 
     Next, in one embodiment, at step  364 , the operation outputs the separated luminance signal (Y 3 ) to the luminance multiplexer  312  on a line  316  simultaneously or at least generally concurrently with, at step  362 , forwarding the chrominance signal (C 3 ) to the chrominance multiplexer  314 . The operation of the modified signal separator  310  is discussed in greater detail in conjunction with FIG.  13 . 
     Thereafter, the operation progresses to step  366 , wherein the line difference detector  320  compares on a pixel by pixel basis the incoming video signal with a time delayed version for differences. 
     If the line difference detector  320  detects a line-to-line difference, the operation progresses to step  368  and each of the luminance multiplexer  312  and the chrominance multiplexer  314  is directed to output the respective signals generated by the notch and band pass filters. For example, if the color assignment as detected between pixels is changing, or if there is a change in rumination between pixels, then the preferred signal separation technique is notch and band pass filtering. 
     Alternatively, if at step  366 , the line difference detector  320  does not detect a difference between the current pixel and the time delayed pixel of the previous line, or the size of the difference is below a threshold level, the operation progresses to a step  370  wherein the difference detector  320  instructs each of the luminance multiplexer  312  and chrominance multiplexer  314  to output the respective signals from the modified signal separator  310 . Thus, if the color assignment as detected between pixels is not changing and if there is no change in rumination between pixels, or if the size of the difference is below a threshold level, then the preferred signal separation technique is comb filtering. 
     In summary, when the line difference detector  320  determines a substantial change in pixel values in a line to line comparison, then it instructs the multiplexers  312 ,  314  to output on lines  322 ,  324  the output of the filter system  302 . Conversely, if the line difference detector  320  determines no change in pixel value in a line to line comparison, or if the size of the change is below a threshold level, it instructs the multiplexers  312 ,  314  to output on lines  322 ,  324  the output of the modified signal separator  310 . 
     FIG. 10 illustrates a detailed block diagram of the modified signal separator  310 . In relation to FIG. 8, like elements in FIG. 10 are numbered with like reference numerals. The modified signal separator  310  receives a notch filtered luminance (Y) input  304  and a band pass filtered chrominance (C) input  306 . In one embodiment, the inputs  304 ,  306  are modulated. In the case of a composite television or a video signal, the information is generally imposed on a subcarrier signal to facilitate transmission. The notch filtered Y input  304  is coupled to a summer unit  410 , the function of which will be discussed in greater detail below. 
     The band pass filtered chrominance input  306  is coupled to a chrominance demodulator  400 . As known by those of ordinary skill in the art, the demodulator  400  reverses the effect of modulation, thereby eliminating the controlled frequency variation and phase and/or amplitude variation of a carrier wave. 
     FIG. 11 illustrates an example of a chrominance demodulator. The demodulator comprises an input line  306  coupled to the demodulation circuitry. For NTSC composite video, the incoming chrominance signal is multiplied by 2 cosine wT, which is supplied by cosine input in multiplier  454  for the I component of the chrominance signal. Likewise, the chrominance input is multiplied at multiplier  456  by 2 sine wT, which is supplied by a sine input  452 , for the Q component of the chrominance signal. 
     The incoming chrominance signal is multiplied by the cosine and sine signals arriving from the subcarrier generator because an NTSC chrominance signal is represented by 
     
       
           C=Q  sin  wT+I  cos  WT    
       
     
     where w=2πf SC , f SC =3.579545 MHz, and f SC  represents the carrier frequency. This yields the demodulated format of the I and Q signals. 
     Next, as shown by the following equations, the two times subcarrier frequency components (2wT) are removed by low pass filtering resulting in the I and Q signals being recovered. 
     
       
         ( Q  sin  wT+I  cos  WT )×(2 sin  WT )= Q−Q  cos 2 WT+I  sin 2 WT    
       
     
     and 
     
       
         ( Q  sin  wT+I  cos  WT )×(2 cos  WT )= I+I  cos 2 WT+Q  sin 2 WT    
       
     
     In one NTSC example, I has a range of 02±78 and Q has a range of 02±68. In another configuration, the multipliers  454 ,  456  have saturation logic to ensure that overflow and underflow conditions are saturated to the maximum and minimum values respectively. 
     Each multiplier  454 ,  456  connects to a low pass filter  460 ,  462 . The I and Q signals undergo low pass filtering to remove two times subcarrier frequency components (2f SC ), i.e., the 2wT components. Thereafter, in one implementation, the output of the low pass filters  460 ,  462  are rounded by rounding mechanisms  464 ,  466  to a minimum of 8 bits plus a sign bit to conserve processing resources in subsequent aspects of the digital receiver. After demodulation, the chrominance signal is composed of two components, I and Q, or transformations of these two components, e.g. U and V, or Cr and Cb, both of which are discussed below. 
     It is fully contemplated that the principles of the subject invention are applicable to other video standards beyond basic NTSC including but not limited to NTSC (M), NTSC (4.43), NTSC without 7.51RE pedestal, and PAL (B, D, G, H, M, N and N combination). 
     With this basic understanding of a chrominance demodulator as shown in FIG. 11, the discussion of FIG. 10 is continued. With reference to FIG. 10, the output of the demodulator  400  is coupled to a decimator  402 . A decimator  402  is a video processing device that reduces the amount of digital information that represents an image. 
     FIGS. 12A and 12B illustrates the effect of decimation on a video image in relation to the storage requirements for the image. As is known in the art, two common video formats are the 4:4:4 YCrCb format and the 4:2:2 YCrCb format. The Cr and Cb format is simply a different color space representation of the chrominance signal and, while not the focus of the present discussion, is briefly discussed for purposes of understanding. 
     Color space is a term common to video processing and simply denotes a mathematical representation for a color. Television decoders utilize the mathematical color representation to generate the various colors in the displayed image. The YCrCb color space is the color space defined by Recommendation ITU-R BT.601. In comparison, U-V color space is the mathematical format used to define color in the NTSC standard. The Cr and Cb components are color difference signals that, when presented together, represent a color. These values can be thought of as scaled versions of U and V in the YUV color space. 
     Relevant to the present discussion is the difference between the 4:4:4 format and the 4:2:2 format. These format representations indicate the number of samples that represent four video pixels. By way of example, if a format is denoted as X:Y:Z, the X position represents the number of data samples that represent the luminance information for four pixels. Likewise, the Y position represents the number of data samples that represent four pixels of Cr data while the Z position represents the number of samples that represent four pixels of the Cb data. 
     Thus, in a 4:4:4 format, for every four Y samples there exist four Cr samples and four Cb samples. Each pixel is represented by an independent Y, a Cr, and a Cb value. This is illustrated in FIG. 12A. A portion of the display comprising a first line  500  and a second line  502  is shown. The first line  500  includes pixels  504 ,  505  and  508 . As shown by key  509  and in accordance with the 4:4:4 format, each pixel is represented by an eight bit Y sample value, an eight bit Cr sample value and an eight bit Cb sample value. 
     Conversely, in a 4:2:2 format, for every four Y samples there are two Cr samples and two Cb samples. Thus, the 4:2:2 advantageously reduces the memory requirements needed to store the data representing an image. The 4:2:2 format is illustrated in FIG. 12B where, in relation to FIG. 12A, like elements are referenced with like reference numerals. As shown, and with reference to the key  509 , an independent sample value for each of Y, Cr and Cb represent the pixel  510 . However, pixel  512  only has an independent sample value for the luminance component (Y). The Cr and Cb sample values must be derived from other Cr and Cb values in the image data. This pattern repeats as shown in all the lines in the display. In one configuration, the sample values for Cr and Cb for pixel  512  are obtained using interpolation between the corresponding values of pixels  510  and  513 . 
     With reference to FIG. 10, in one embodiment, decimator  402  reduces a video signal in a 4:4:4 format to a 4:2:2 format. As can readily be understood from the forgoing discussion, altering the format of a video signal to a 4:2:2 format from a 4:4:4 format significantly reduces the amount of memory required to store a digitized image. 
     Although discussed in terms of decimation from 4:4:4 format to 4:2:2 format, other types of signal manipulation may be undertaken to reduce the storage requirements. One example of a different signal manipulation is reduction to a 4:1:1 format. Another method of reducing the memory requirements is to reduce the number of bits utilized to represent each of the Y, Cr and Cb values, such as from 8 bits per sample to 7, 6, 5 , or 4 bits. 
     Decimation to a 4:2:2 format is performed in several ways. One method is to simply discard the unwanted samples. However, this method may introduce image artifacts in the video signal. An alternative method is to use a decimation filter which reduces artifacts to an acceptable level by smoothing in areas of the image containing fine or detailed image information or by smoothing in areas of the signal that represent movement in the image. Decimation filters provide excellent results in areas where the color information is unchanging from one sample to the next. 
     With reference to FIG. 10, the output of the decimator  402  is coupled to a comb filter  404 . Comb filters will be discussed in greater detail in conjunction with FIG.  16 . 
     Comb filters take advantage of the difference in phase shift from one line to the next line in the chrominance signal versus that in the luminance signal to remove unwanted portions of the luminance information from the chrominance signal. The subcarrier frequency has been selected such that its phase shifts by 180 degrees from one line to the next line. When this subcarrier is used to modulate the I and Q components, it produces a chrominance signal that is 180 degrees phase shifted from one line to the next line, assuming the I and Q components are the same for the two lines. This modulated chrominance signal is combined with the luminance signal, which is not modulated and has no phase shift from one line to the next line, to produce the composite signal. During the decoding process, the band pass filtered data, which contains chrominance and luminance components, is demodulated, and the line to line phase shift is eliminated from the chrominance components, but introduced into the luminance components. It is this difference in phase shift characteristics that enables comb filters to be used to improve Y/C separation. The operation of the comb filter is discussed in great detail in conjunction with FIGS. 17 through 18. 
     The comb filter  404  includes two outputs. The first output comprises a combed chrominance signal on signal line  318 . In one implementation, the chrominance signal C 3  on line  318  contains only chrominance information because the comb filter removes or at least substantially removes the luminance components in accordance with the principles of comb filters, discussed in great detail below. 
     The second output of the comb filter  404  on signal line  405  connects to an interpolator  406 . The signal on line  405  contains the luminance components missing from signal Y 2  on line  304 , and combed from the chrominance signal C 2 . As will be seen, in accordance with the subject invention, the combed luminance components on line  405  are eventually combined with the luminance signal Y 2  on line  304 . 
     The output of the comb filter  404  is coupled to an interpolator  406 . As known by those of skill in the art, the interpolator  406  reverses, or at least substantially reverses, the effects of the decimator  402 , thereby restoring the video signal to a 4:4:4 format. The interpolation averages adjacent pixel sample values to restore the information that was discarded or filtered by the decimator  402 . Interpolation to a 4:4:4 format is necessary so that the luminance components may be accurately restored to the luminance signal. 
     The output of the interpolator  406  is coupled to a modulator  408 . The modulator  408  reverses or at least substantially reverses the effects of the demodulator  400  thereby modulating the combed luminance components to the subcarrier frequency. In the case of NTSC video, the luminance component riding on the chrominance signal was originally centered at 3.58 MHz. However, the demodulator  400  removed or at least substantially removed the effect of the 3.58 MHz sub-carrier. Thus, to properly re-combine the comb filtered luminance information, the signal is re-modulated to 3.58 MHz. Modulation techniques for video signals are known by those of ordinary skill in the art and accordingly are not discussed in great detail herein. However it should be noted that the proper phase relationship and timing with the sub-carrier is maintained to ensure proper synchronization. 
     The output of the modulator  408  is coupled to summing point  410 , which is configured to add the signal arriving from the modulator  408  to the notch filtered luminance signal (Y 2 ) arriving over line  304 . The summing point  410  provides an output (Y 3 ) on a line  316  that represents the total luminance signal. The signal Y 3  is the complete luminance signal. 
     FIG. 13 illustrates an operational flow diagram of the modified signal separator as illustrated in FIG.  10 . At step  550 , the modified signal separator  310  receives at its inputs  304 ,  306  the notch filtered luminance signal, denoted Y 2 , and the band pass filter chrominance signal, denoted C 2 . 
     Next, at step  552 , the luminance value is forwarded to a summing point  410  and the chrominance value is forwarded to the demodulator  400 . The operation of the summing point  410  is discussed in more detail below. 
     The demodulator  400  eliminates, or at least substantially eliminates, the carrier signal from the incoming chrominance information, thereby restoring information to a demodulated state. 
     Thereafter, at step  554 , the demodulated chrominance signal enters the decimator  402 . The decimator  402  operates to reduce the number of bits utilized to store the chrominance signal. In this embodiment, the decimator  402  modifies the chrominance signal from a 4:4:4 format to a 4:2:2 format. Reducing the storage requirements to a 4:2:2 format reduces the memory required to store the information for each line of video information. For example, in a 4:4:4 format, 24 bits are required to store the information for each pixel. Thus, the storage requirements for four pixels equates to 4×24 bits=96 bits. Consequently, memory space to store 96 bits is required to store 4 pixels of video in a 4:4:4 format. 
     Conversely, in a 4:2:2 format, every four pixels of video is comprised of four luminance samples and two chrominance samples. Accordingly, to store four pixels of video in a 4:2:2 format, 32 bits of luminance information is stored (4 luminance samples×8 bits each), 32 bits of chrominance information (2 Cr samples×8 bits each), and (2 Cb samples×8 bits each) is stored. This equates to 64 bits of memory space required to store the chrominance information and luminance information in a 4:2:2 format. The advantages of reducing the storage requirements for the video information will become apparent in the following discussion for comb filters. 
     Next, at step 556, the operation forwards the 4:2:2 format chrominance information to the comb filter  404 . The comb filter utilizes line stores in combination with summing points to manipulate the chrominance signal to isolate or at least substantially isolate the luminance components from the chrominance signal. Upon isolation, these luminance components are subtracted from the chrominance signal to provide the combed chrominance signal (C 3 ) that does not contain unwanted luminance components. 
     Next, at step  558 , the operation forwards the comb filtered luminance components to an interpolator  406 . Thereafter, at step  560 , the interpolator restores the format of the comb filtered luminance components to a 4:4:4 format. It is necessary to restore the format of the comb filtered luminance components, such as those on line  405  in FIG. 10, to the format of the notch filtered luminance signal (Y 2 ) prior to combination. 
     After conversion to 4:4:4 format, the operation progresses to step  562 , wherein the modulator  408  re-modulates the combed luminance components to the subcarrier frequency so that the combed luminance components are at the proper frequency. For NTSC, the 3.58 MHz subcarrier frequency is added to the frequency of the luminance components. 
     Next, at step  564 , the operation combines the comb filtered luminance components with the luminance signal (Y 2 ) in summing point  410 . This process restores the luminance components that were removed from the composite signal by the notch filter to provide a restored or complete luminance signal. The restored luminance signal is denoted Y 3 . 
     At step  566 , the operation outputs the complete luminance signal (Y 3 ) on a line  316 . This completes the operational process of the modified comb filter. 
     Advantages Of Subject Invention 
     The subject invention&#39;s method of signal separation possesses several advantages. First, a signal separation system configured in accordance with the subject invention more accurately separates the signals by more completely recreating the luminance signal. Systems operating in accordance with one embodiment of the subject invention restore or at least substantially restore the comb filtered luminance components that are missing from the notch filtered luminance signal (Y 2 ). 
     Second, as discussed above, reducing the format of the signal prior to comb filtering reduces the memory requirements of each line store in the comb filter. In comb filters having greater than one line store, memory requirements are further reduced. 
     Third, systems that operate in accordance with the subject invention and that demodulate the signal prior to comb filtering significantly reduce distortion arising from line-to-line subcarrier phase differences. 
     Line-to-line subcarrier phase differences are best explained with reference to FIG. 14, which illustrates a plot of two band pass filtered signals that are in a modulated state, one of which is suffering from line-to-line subcarrier phase differences. An in-phase signal  570  is not suffering from line-to-line subcarrier phase distortion and as such has a zero value as it crosses the horizontal axis. Also shown is an out-of-phase signal  571  that is offset from the in-phase signal  570 . When the in-phase signal  570  is delayed by the line store in a comb filter and combined with non-delayed out-of-phase signal  571 , the two signals do not completely cancel due to the phase differences. This leads to distortion and artifacts in the video image. This is a problem with systems that do not adopt the principles of the subject invention. 
     Systems operating in accordance with the subject invention demodulate the chrominance signal prior to comb filtering. Demodulating the chrominance signal eliminates the subcarrier modulation from the chrominance signal. By eliminating the subcarrier, the line-to-line phase differences are advantageously also eliminated from the chrominance information. Thus, the subject invention overcomes the problems of distortion caused by subcarrier line-to-line phase differences. 
     Data Decoder for Second, Third and Fourth Embodiments 
     FIG. 15 illustrates one configuration of a video data decoder path as found in video decoders adopting the signal separation techniques of the subject invention. This decoder is provided by way of example only and the scope of the subject invention is not limited to the configuration shown. The data decoder provides the supporting circuitry to facilitate video signal separation in accordance with the subject invention. For purposes of discussion, the data decoder shown in FIG. 15 is configured for operation with signals following the NTSC standard. It is contemplated that the principles of the present invention could be applied to other video formats including but not limited to PAL and SECAM. 
     FIG. 23 illustrates a third example embodiment, including the componentry and configuration for the block labeled vertical scaler and comb filter. This embodiment interfaces with the hardware of the data decoder shown in FIG.  15 . 
     With reference to FIG. 15, a digitized composite video signal is provided over line  700  to a notch filter  702  and band pass filter  704 . 
     When configured for NTSC standard signals, the design of the notch filter  702  removes or at least substantially removes signal components having a frequency between 2.5 to 4.5 MHz. For an NTSC signal, the signal portion which is allowed to pass through the notch filter  702  contains luminance information. Undesirably, however, as previously discussed, the notch filter also removes portions of the signal within this frequency range that contain luminance information. 
     The band pass filter  704  allows to pass portions of the composite signal having frequency components generally between 2.5 to 4.5 MHz. This portion of the composite signal contains primarily chrominance information, but as previously discussed, this portion or band of the signal allowed to pass also contains luminance information. 
     The band pass filter output is coupled to a chrominance demodulator  706 . The chrominance demodulator  706  receives sine and cosine modulated signals at the subcarrier frequency from a subcarrier signal generator  708 . The demodulator  706  utilizes these subcarrier signals to demodulate the chrominance signal. During demodulation, the demodulator separates or at least substantially separates the chrominance signal into I and Q components. 
     The output of the demodulator  706  is coupled to a format conversion module  710 . The format conversion module  710  of the second embodiment changes the signal format from a 4:4:4 format to a 4:2:2 format. This advantageously reduces the storage requirements of the signal. Reducing the format to a 4:2:2 format does not result in data loss or reduced resolution because the signal is demodulated prior to format conversion. Demodulating the signal reduces the highest frequency of the signal and because the highest frequency is reduced, the sampling rate may also be reduced without data loss. This principle is exemplified by the Nyquist sampling rate theory that states the sampling rate need only be twice the highest frequency of the sampled signal in order to accurately recreate the signal. This relationship is expressed below: 
     
       
         Nyquist Rate=Freq sampling rate ≧2×Freq sampled signal    
       
     
     Sampling rates greater than the Nyquist rate do not add to the resolution of the signal when the digital signal is restored to an analog format. Therefore, by demodulating and reducing the format of the signal prior to comb filtering, the memory requirements are reduced without sacrificing the resolution or integrity of the signal. 
     The dual outputs of the format conversion module  710  are coupled in tandem with the output of the notch filter  702  to a horizontal scaler  712 . The horizontal scaler  712  adjusts the horizontal dimensions of the video image in accordance with a predetermined ratio or based on user input. 
     The horizontal scaler outputs  714 ,  716  are coupled to a vertical scaler and comb filter  718 . The vertical scaler and comb filter  718  also receives as input the sine and cosine subcarrier signals from the signal generator  708 . In this embodiment, the vertical scaler and comb filter  718  receives as a first input the horizontally scaled, notch filtered luminance signal  714  and as a second input the horizontally scaled, demodulated band pass filtered chrominance signal  716  in a reduced memory format. 
     Turning now to FIG. 16, a second embodiment of the vertical scaler and comb filter  718  is provided. For purposes of discussion the vertical scaler and comb filter  718  is separated into a luminance subsystem and a chrominance subsystem. Each is discussed below. 
     Luminance Subsystem 
     Through a signal line  750 , the horizontally scaled luminance signal is provided to a line store  754 . The line store  754  stores the input for a time equivalent to one line trace period, i.e. the time it takes to trace one line on a display, before providing its output to a vertical luminance scaler  756 . One configuration of a line store comprises a series of registers configured to store the data samples until prompted by a control signal. In another configuration, the line store comprises computer memory accessed with a software pointer. 
     The vertical luminance scaler  756  also receives as an input the undelayed horizontally scaled luminance signal on line  750 . The vertical luminance scaler interpolates the delayed and undelayed luminance signals to adjust the vertical dimension of the luminance signal. A luminance summing point  762  is coupled to the output of the vertical luminance scaler  756  over line  760 . The signal on line  760  comprises a luminance signal that is missing certain luminance components due to notch filtering. For purposes of discussion, this signal is designated (Y−Y′) where Y is the complete luminance signal and Y′ represent the luminance components missing from the luminance signal. 
     A luminance output line  798  provides to the output of the luminance summing point  762 . 
     Chrominance Subsystem 
     Through signal line  752 , the horizontally scaled chrominance signal is provided to a line store  770 , a first multiplier  774  and a summing point  784 . The signal on line  752  is designated (C t +Y t ′) where C t  is the chrominance signal and Y t ′ is the luminance signal riding on the chrominance signal. 
     The line store  770  delays the signal for the period of a line trace. The output of the line store  770  connects to a second multiplier  772 . Both multipliers  772 ,  774  reduce their received signals by one-half or about one-half through a multiplication process. This may be implemented in various ways, including but not limited to a simple shift of the data one bit at a time. 
     A first comb filter summing point  780  is coupled to the output of the first multiplier  774  and the output of a second multiplier  772 . The first comb filter summing point  780  adds the two inputs and provides this combined signal on a line  786 . This signal comprises the combed chrominance information (I/Q) and is provided at an output line  800 . 
     The output of the summing point  780  also connects to an inverting input of a second summing point  784 . The second summing point  784  also receives as a non-inverted input the horizontally scaled chrominance signal directly from line  752 . The inverting input to the second comb filter summing point  784  negates the sign of the signal arriving on that input. The output of the second comb filter summing point  784  on a line  788  comprises the recovered portions of the luminance signal that undesirably remained with the chrominance signal after band pass filtering. These diagrams assume the I and Q data is multiplexed on a clock by clock basis, so one clock period is I data and the next clock period is Q data. Multipliers  772  and  774  and adders  780  and  784  therefore process both I and Q data. If the I and Q data is not multiplexed, then those four components must be duplicated so that the I data and the Q data can each have dedicated multipliers and adders. 
     A second format conversion module  790  is coupled to the output of the second summing point  784 . The second format conversion module  790  restores the recovered luminance components to the 4:4:4 format, which includes separation of the multiplexed I and Q components into a non-multiplexed format. The second format conversion module  790  includes two outputs because the interpolation process creates twice as many data samples thereby requiring additional means to carry the data. 
     Next, a modulator  792  is coupled to the outputs of the second format conversion module  790  and re-modulates the signal using the appropriate sine and cosine signals at the subcarrier frequency. A summing point  794  connects to the dual outputs of the modulator  792  to combine the outputs of the modulator into a single signal on line  796 . 
     The luminance summing point  762  is coupled via line  796  to the output of the summing point  794 . Output line  798  is coupled to the output of the luminance summing point  762  and carries the combined luminance signal. 
     Operation of Second Embodiment 
     FIGS. 17 and 18 illustrate an operational flow diagram of the vertical scaler and two tap comb filter as shown in FIG.  16 . FIG. 17 illustrates the operation of the luminance subsystem, while FIG. 18 illustrates the operation of the chrominance subsystem. 
     Operation of Luminance Subsystem 
     With reference to FIG. 17, at a step  900 , the luminance subsystem receives the horizontally scaled luminance signal, denoted as (Y−Y′). Next, at a step  902 , the operation stores one line of video for vertical luminance scaling. At a step  904 , the operation performs vertical luminance scaling to adjust the vertical dimensions of the video image. 
     At a step  906 , the luminance subsystem obtains the combed luminance components, denoted as Y′, from the chrominance subsystem, the operation of which is presented in FIG.  18 . 
     Next, at a step  908 , the operation combines or adds the output of the vertical scaler, denoted Y−Y′, with the input from the chrominance subsystem, denoted Y′. The following equation represents the combination. 
     
       
         ( Y−Y′ )+ Y′=Y    
       
     
     Y represents the complete luminance signal. Finally, at a step  910 , the complete luminance signal Y is output on a line  798 . 
     Operation of Chrominance Subsystem 
     FIG. 18 illustrates an operational flow diagram of the chrominance subsystem. At a step  920 , the chrominance subsystem receives the horizontally scaled chrominance signal, which for purposes of discussion, is denoted (C t +Y t ′), which is composed of I and Q components, [(I t +Y I ′), (Q t +Y Q ′)]. 
     An example of this (C t +Y t ′) signal is shown in FIG.  19 . The C t  portion is signal  912  while the Y′ portion is denoted as signal  914 . The signal C t +Y′ t  contains the chrominance signal C t  and a portion of the luminance signal Y′ t  riding on the chrominance signal. 
     Next, at a step  922 , the chrominance signal C t +Y t ′ is time delayed in line store  770  to form C t−1 .+Y′ t−1 . An example of this time delayed signal  915 ,  916  is illustrated in FIG.  20 . As shown, the C t−1  signal  915  remains in-phase with the signal C t  while the Y′ t  signal  916  is now out of phase by 180 degrees, or at least about 180 degrees, with Y′ t−1 . 
     At a step  924 , the operation reduces by one-half, or at least about one-half, the amplitude of the signal (C t +Y t ′) and the signal (C t−1 +Y t−1 ′). A one-half amplitude reduction is advantageous because at a step  926  the signals are added together in the first comb filter summing point  780 . The following equation represents this operation. 
     
       
         ½( C   t   +Y   t )+½( C   t−1   +Y′   t−1 )  
       
     
     The signal component ½C t  overlaps in-phase with the ½C t−1  component to form C t . The signal component ½Y′ t  is 180 degrees out of phase with the component ½Y′t −1  causing the Y′ t  component to cancel with the Y′ t−1  component. Hence the resulting signal is purely or at least substantially the chrominance information, denoted C t . 
     At a step  930 , the pure or at least substantially pure chrominance signal C t  is output on a line  800 . 
     Simultaneously, the operation isolates or at least substantially isolates the luminance components Y′ t  from the combined signal C t +Y′ t . At a step  928 , the second comb filter summing junction  784  subtracts the pure chrominance signal C t  from the original chrominance signal (C t +Y′ t ). This is shown in the equation below and illustrated in FIG.  21 . 
     
       
         ( C   t   +Y′   t )− C   t   =Y′   t    
       
     
     As shown in FIG. 21, the original chrominance signal  912  and the luminance information  916  are added to the negative of the pure chrominance signal  917 . This in effect subtracts the pure chrominance C t  signal from the combined signal (C t +Y′ t ), an operation which leaves only the luminance portion of the signal (Y′ t ). FIG. 22 illustrates the luminance component Y′ t , shown as line  916 , that has successfully been isolated. 
     Next, at a step  932 , the format of the luminance signal Y′ t  is restored to a 4:4:4 format and the I and Q components are demultiplexed for purposes of re-modulation. This is denoted by Y′ t (I) and Y′ t (Q). 
     At a step  934 , the operation re-modulates each component of Y′ t (I) and Y′ t (Q) to the carrier frequency centered at 3.58 MHz, the general center frequency of the notch filter. This advantageously aligns at the proper frequency the combed Y′ t  components of the luminance information with the original luminance signal (Y t −Y′ t ). Absent the re-modulating alignment process, the combed luminance signal Y′ t  would, when combined in a step  938 , not be combined at the right frequency position in the luminance signal. 
     Next, at a step  936 , the operation combines in modulator summing point  794  the Y′ t (I) and Y′ t (Q) components to form a remodulated Y′ t  signal. 
     At a step  938 , the combed luminance components Y′ t  are combined with the original luminance signal (Y t −Y′ t ). The following equation represents this combination. 
     
       
         ( Y   t   −Y′   t )+ Y′   t   =Y   t    
       
     
     Thus, at a step  940 , the complete luminance signal Y t  is output on line  798 . 
     This process advantageously shares the benefits discussed above regarding reduced memory requirements in the line store in the comb filter and generally eliminates susceptibility to interference from line-to-line subcarrier phase differences. Further, this method and apparatus restores or at least substantially restores the luminance signal to its original composition by adding in the otherwise absent Y′ t  component. 
     Third Embodiment—Multiple-Tap Comb Filter 
     FIG. 23 illustrates a third embodiment of the vertical scaler and comb filter configured with a three tap comb filter. With reference to FIG. 16, like elements are numbered with like reference numerals. As shown in FIG. 23, a three tap comb filter replaces the two tap comb filter of FIG.  16 . The configuration of the three tap comb filter is now discussed. 
     The input line  752  is coupled to a first line store  820  and a first ¼ multiplier  822 . The ¼ multiplier  822  reduces the amplitude of the received signal by one quarter and provides it to the first comb filter summing point  780 . 
     The first line store  820  delays or stores the incoming signal for a period equal to the line trace time and provides the output on a line  824 . Line  824  connects to a ½ multiplier unit  832 , a second line store  830 , and the second comb filter summing junction  784 . The ½ multiplier unit  832  reduces the amplitude of the incoming signal by one half. The output of the ½ multiplier unit  832  connects to the first comb filter summing point  780 . 
     The second line store  830  delays the signal on line  824  a time equal to a line trace and provides the twice delayed output on a line  831  to a second ¼ multiplier unit  834 . The output of the second ¼ multiplier unit  834  connects to the first comb filter summing point  780 . 
     Operation of three tap comb filters is known by those of skill in the art and accordingly is not described in detail herein. The weighting applied by the multipliers  822 ,  832 , and  834  in conjunction with the first comb filter summing point  780  ensures proper cancellation between overlapping signals. The other aspects of the operation of this embodiment do not differ significantly from the operation of the second embodiment. In particular, the re-formatting, re-modulation and combination of the combed luminance components do not significantly differ from the embodiment shown and described in FIG.  16  and FIGS. 17 and 18. 
     The advantages gained from restoring the combed components of the luminance signal to the notch filtered luminance signal are realized regardless of the number of taps of the comb filter. Indeed, the greater the number of taps, the greater the number of line stores. The greater the number of line stores in the comb filter, the greater the memory savings that result from format conversion of the chrominance signal. It is fully contemplated that the principles of the present invention may be applied to comb filters having any number of taps. 
     Fourth Embodiment—No Format Conversion Module 
     In a fourth embodiment, the subject invention is embodied without the format conversion module identified as element  710  (FIG. 15) and  790  (FIG.  16 ). Configuring the subject invention without the format conversion module advantageously reduces the cost of the system by eliminating this componentry. Absent the format conversion module  710 ,  790 , the separation system does not enjoy the memory saving attributes associated with presenting a signal sampled at a lower sampling rate to the line stores in the comb filter. However, inclusion of the format conversion module is not a requisite to practice the subject invention. 
     Fifth Embodiment Adaptive Filtering 
     FIG. 24 illustrates adaptive circuitry configured to integrate with the circuitry of the third embodiment shown in FIG.  23 . With reference to FIG. 23, like elements in FIG. 24 are referenced with like reference numerals. 
     In summary, the adaptive circuitry dynamically generates variable coefficients based on the degree of difference between pixel values separated by one or more lines of video. The coefficients control the degree of weighting assigned to the notch and band pass filtered signals and the comb filtered signals during the formulation of the final output signal. 
     In particular, a line difference detector  950  taps onto line  752  and line  831  to obtain the signals thereon. Line  752  provides the horizontally scaled chrominance signal at t=0 while line  831  provides the horizontally scaled chrominance signal after two line delays, i.e. at a time t−2. 
     The line difference detector  950  compares the signals on these two lines on a pixel-by-pixel basis for differences. It is contemplated that an evaluation on other than a pixel-by-pixel basis may be undertaken. The pixels subject to comparison are separated by two lines or a multiple of two lines, to ensure that the subcarrier phase is the same for both pixels. In this embodiment, the chrominance portion of the signal has been demodulated (block  706 , FIG.  15 ). Because the chrominance signal includes certain luminance components (C t +Y′ t ) in a demodulated state, the chrominance signals, when compared line-to-line, are in-phase while the luminance component riding thereon are out of phase between adjacent lines by 180 degrees. Thus to provide accurate line difference detection for both the chrominance components and the luminance components riding on the chrominance signal, the difference detector  950  must compare non-adjacent video lines. Stated another way, the difference detector  950  must compare pixels at C t  to pixels at C t−2 . 
     If the line difference detector  950  compared adjacent lines, that is lines at t=0 and t=1, the out of phase luminance components would provide inaccurate results. The results would be inaccurate because the luminance components that are 180 degrees out of phase line-to-line would be different thereby giving the false impression of a change in pixel values. 
     The line difference detector  950  compares the difference between the signal at t=0 and t=2 and, depending on the amount of difference, generates a Y coefficient (Ycoeff) and C coefficient (Ccoeff). In this embodiment, each of the Ycoeff and Ccoeff vary between 0 and 1 in value. The line difference detector  950  also performs mathematical manipulation to generate a value representing one minus the Ycoeff (1−Ycoeff) and one minus the Ccoeff (1−Ccoeff). Thus, as the Ycoeff increases the value for (1−Ycoeff) decreases. 
     Luminance Weighting System 
     A multiplier  960  is coupled to the Ycoeff output via a line  952  and to the notch filter output (Y t −Y′ t ) on line  750 . The multiplier  960  multiplies the notch filter output (Y t −Y′ t ) by the value of the Ycoeff and outputs this value to a summing point  970 . 
     A multiplier  962  is coupled to the (1−Ycoeff) output via a line  954  and to the complete luminance output Y t  on line  798 . The multiplier  960  multiplies the comb filtered Y output (Y t ) by the value of (1−Ycoeff) and outputs this value to a summing point  970 . 
     The summing point  970  adds each of the weighted portions of the inputs provided from the multipliers  960 ,  962  and outputs the sum on the luminance adaptive output line  974 . This is the luminance signal of the adaptive signal separation system of the fifth embodiment. 
     Chrominance Weighting System 
     A multiplier  964  is coupled to the Ccoeff output via a line  956  and to the band pass filtered chrominance output (C t +Y′ t ) from line  752 . The multiplier  964  multiplies the band pass filtered chrominance output (C t +Y′ t ) by the value of the Ccoeff. and outputs this value to a summing point  972 . 
     A multiplier  966  is coupled to the (1−Ccoeff) output via a line  958  and to the comb filtered chrominance output C t  on line  800 . The multiplier  966  multiplies the comb filtered chrominance output C t  by the value of (1−Ccoeff) and outputs this value to a summing point  972 . 
     The summing point  972  adds each of the weighted portions of the inputs provided from the multipliers  964 ,  966  and outputs the sum on the chrominance adaptive output line  976 . This is the chrominance signal of the adaptive signal separation system of the fifth embodiment. 
     The adaptively adjusted luminance output on line  974  and the adaptively adjusted chrominance output on line  976  represent the dynamically adjusted output of 1) the notch and band pass filtered composite signal and 2) the modified comb filter operating in accordance with the subject invention. Thus, this circuit dynamically selects, on a pixel by pixel basis or on a line by line basis, the amount of 1) a signal separated using basic notch and band pass filtering and 2) a signal separated using the subject invention&#39;s comb filtering process. 
     Signal Selection Process 
     By way of example, if the line difference detector  950  finds a difference between compared pixels, it increases the value of the Ycoeff. and the Ccoeff. This in turn causes the values of 1−Ycoeff and 1−Ccoeff to decrease. In turn, the multipliers  960 ,  962 ,  964 ,  966  provide greater weight to the signals obtained from basic notch and band pass filtering and less weight to the signals obtained from the comb filter. The summing points  970 ,  972  form the final output on lines  974  and  976  based on the more heavily weighted signals from notch and band pass filtering. Thus, in instances of motion or color change in alternating lines, the adaptive system of the fifth embodiment predominately utilizes the notch and band pass filtered outputs. Because of the drawbacks of comb filtering in relation to video signals that represent motion or color changes, the system advantageously adjusts the output thereof as appropriate, and more specifically, the filtering method used to form the output. The following table provides a more complete but general illustration of this point: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Ycoeff. 
                 1-Ycoeff. 
                   
               
               
                   
                 Line 
                 and 
                 and 
                 Predominate 
               
               
                 Video Image 
                 Difference 
                 Ccoeff. 
                 1-Ccoeff 
                 Separation 
               
               
                 Properties 
                 Detector 
                 Values 
                 Values 
                 Method 
               
               
                   
               
             
             
               
                 motion or 
                 Difference 
                 Increase 
                 Decreases 
                 Notch and Band 
               
               
                 color change 
                 Detected 
                   
                   
                 Pass Filtering 
               
               
                 no motion or 
                 No 
                 Decrease 
                 Increases 
                 Comb Filtering 
               
               
                 color change 
                 Difference 
               
               
                   
                 Detected 
               
               
                 moderate 
                 Moderate 
                 set to 
                 equates to 
                 Combination of 
               
               
                 motion 
                 Difference 
                 about .5 
                 about .5 
                 Both Notch and 
               
               
                 or color 
                 Detected 
                   
                   
                 Band Pass and 
               
               
                 change 
                   
                   
                   
                 Comb Filtering 
               
               
                   
               
             
          
         
       
     
     Sixth Embodiment—Adaptive Filtering with Adjustable Thresholds 
     In another embodiment, the signal selection or signal weighting performed in relation to the selection of the filtering method utilized to form the output is selected based on a threshold value, in contrast to a sliding scale as employed in the fifth embodiment. 
     FIG. 25 illustrates a method of operation for adjustable threshold adaptive filtering. At a first step  1000 , the adaptive system obtains a chrominance threshold value (Ctv). The Ctv represents the threshold value that marks the jump point at which the system changes the filtering method for the chrominance signal. 
     At a step  1002 , the system obtains a luminance threshold value (Ytv). The Ytv represents the threshold value that marks the jump point at which the system changes the filtering method for the luminance signal. 
     In one preferred embodiment, the system obtains the Ctv and Ytv from a user input such as from a computerized user interface or manual adjustment. In an alternative embodiment, the Ctv and Ytv values are permanently configured for a particular system. 
     At a step  1012 , the system detects the change or delta (a) in the chrominance value between the input pixel one line prior to the current output pixel and the input pixel one line after the current output pixel. The change in chrominance is represented (C1−C2). The chrominance value for each pixel is composed of two components, I and Q, so the difference between two chrominance value s is also composed of two components. Since the chrominance value for the input pixels to the comb filter contain the chrominance as well as some of the luminance for that pixel, changes in that chrominance value will reflect changes in the chrominance as well as changes in the luminance that passed in to the chrominance channel. 
     Operation of Luminance Signal Filter Selection 
     At a decision step  1006 , the change in chrominance value (C1−C2) is compared to the threshold value Ytv. This comparison can be performed in various ways including but not limited to comparing the vector length, or comparing the sum of the vector components, or comparing each component of the vector separately. 
     If the change in chrominance value (C1−C2) is greater than the threshold Ytv, the operation progresses to a step  1008 . At step  1008 , the system outputs the luminance signal from the filter that is optimized for changes in chrominance value that are above Ytv. 
     If the change in chrominance value (C1−C2) is less than or equal to the threshold Ytv, the operation progresses to a step  1010 . At step  1010 , the system outputs the luminance signal from the filter that is optimized for changes in chrominance value that is at or below Ytv. 
     By way of example, if (C1−C2)&gt;Ytv then the adaptive system utilizes tile output from the notch filter because in instances of change or at least significant change in chrominance values, which may represent changes in chrominance or luminance, the notch filtering method is superior over comb filtering. However, if (C1−C2) is &lt;Ytv, then the adaptive system utilizes the output from the comb filter because, in instances when there is little change in chrominance values, comb filtering provides superior results over notch filtering. Ytv is selected to optimize the output and may conveniently be adjusted by the user based on personal preference. 
     After steps  1008  and  1010 , the system progresses to decision step  1028 , discussed below in greater detail. 
     Operation of Chrominance Signal Filter Selection 
     At a decision step  1022 , the change in chrominance value (C1−C2) is compared to the threshold value Ctv. This comparison could be performed in various ways including but not limited to comparing the vector length, or comparing the sum of the vector components, or comparing each component of the vector separately. 
     If the change in chrominance value (C1−C2) is greater than the chrominance threshold Ctv, the operation progresses to a step  1026 . At step  1026 , the system outputs the chrominance signal from the filter that is optimized for changes in chrominance value that are above Ctv. 
     If the change in chrominance value (C1−C2) is less than or equal to the chrominance threshold Ctv, the operation progresses to a step  1024 . At step  1024 , the system outputs the chrominance signal from the filter that is optimized for changes in chrominance value that are at or below Ctv. 
     By way of example, if (C1−C2)&gt;Ctv, then the adaptive system utilizes the output from the band pass filter because, in instances of change or at least significant change in chrominance values, the band pass filtering method is superior over comb filtering. However, if (C1−C2) is &lt;Ctv, then the adaptive system utilizes the output from the comb filter because, in instances when there is little change in chrominance values, comb filtering provides superior results over band pass filtering. Ctv is selected to optimize the output and may conveniently be adjusted by the user based on personal preference. 
     After steps  1008  and  1010 , the system progresses to decision step  1028 . At decision step  1028 , the system queries the Ctv values and the Ytv values to evaluate whether the threshold values have changed. 
     If there is a new Ctv value or Ytv value, the operation progresses to a step  1030 , which returns to step  1000  to obtain the updated Ctv and/or Ytv values. In one configuration, the system obtains these values from a computerized user interface. In another configuration, they are manually input by a user. 
     Alternatively, if at decision step  1028  the system does not detect new Ctv or Ytv values, the operation progresses to a step  1032 . At step  1032 , the system jumps to Point A wherein the operation progresses with the same Ctv and Ytv values to steps  1004  and step  1020 , both of which are discussed above. 
     Implementation Example 
     One example implementation of the subject invention is embodied in a video capture processor and scaler for television and VCR input. In this example embodiment, the subject invention is embodied in the Bt835 VideoStreaM™ III Decoder available from Conexant Systems, Inc. of San Diego, Calif., formerly Rockwell Semiconductor Systems. The Bt835 is a high quality single chip composite NTSC/PAL/SECAM video and S-video decoder having low power consumption requirements. The subject invention embodied therein integrates with a 3-line adaptive comb filter in accordance with the principles of the subject invention to overcome the disadvantages of traditional comb filter artifacts. 
     FIG. 26 illustrates a block diagram of the configuration of the Bt835, including the 3-line adaptive comb filter for luma-chroma separation. The block diagram of the Bt835 will now be described. 
     Input port  600  connects to an analog video multiplexer  602 . The analog video multiplexer provides means to simultaneously receive input from multiple video sources, such as a receiver, VCR, camcorder, or antenna. The output of the multiplexer  602  connects to a first 40 MHz analog to digital converter  608 . The 40 MHz analog to digital converter  608  (A/D) converts the analog input into a digital output. The output of the 40 MHz A/D converter  608  connects to a decimation low pass filter  611 . A second 40 MHz analog to digital converter  610  obtains an analog chrominance signal from a C input, and, after conversion to a digital format, outputs the digital signal to the decimation low pass filter  611 . The decimation low pass filter  611  performs two times decimation on the outputs of the over-sampled A/D converters  608 ,  610  so that a simpler anti-aliasing filter may be utilized in the analog domain. 
     The output of the decimation low pass filter  611  connects to a phase lock and clock generation module  604 , a 3-line adaptive luma-chroma separation and chroma demodulation module  606 , and an automatic gain control (AGC)  652 . The phase lock and clock generation module  604  synchronizes the phase of the video signal and generates appropriate clock signals for operation of the digital circuitry. 
     The 3-line adaptive luma-chroma separation and chroma demodulation module  606  separates the luminance and chrominance components from the received signal by adaptively utilizing either of a notch and band pass filter combination or a 3 tap comb filter depending on the attributes of the received signal. The 3-line adaptive luma-chroma separation and chroma demodulation module  606  also includes a chroma demodulation capability to demodulate the chrominance portion of the signal in accordance with the subject invention. Advantageously, the combed portions of the luminance signal are re-formatted, re-modulated and combined with the notch filtered luminance signal to prevent unwanted artifacts in the output and provide a more robust and complete luminance signal. 
     The AGC  652  enables the Bt835 to compensate for reduced amplitude in the analog circuit received at input  600 . 
     The output of the phase lock and clock generation module  604  connects to a video timing unit  620 . The video timing unit detects sync pulses in the incoming signal and generates internal and external timing signals to insure accurate timing of the video output. 
     The output of the 3-line adaptive luma-chroma separation and chroma demodulation module  606  connects to a spatial and temporal scaling module  622  that is responsible for adjusting the size and timing of the video display. The temporal scaling module  622  also receives input from a digital video input  624  in conjunction with a digital input video clock and timing input  626 . These inputs provide means for the Bt835 to receive digital video input. 
     The outputs of the video timing unit  620  and the spatial and temporal scaling module  622  connect to an output formatting module  630 . The output formatting module  630  manipulates the image signal to a format compatible for transmission to a display monitor or video processing card. The output formatting module  630  includes a 16 bit digital video output  634  and a video timing output  632 . These outputs  634 ,  632  connect to a display or video processing card. An output control line  640  provides input to the output formatting module  630  to partially control the output of the output formatting module  630 . 
     The Bt835 also includes and an I 2 C bus  650  and a JTAG module  653 . The I 2 C bus (Inter-IC bus)  650  is a multi-master bus used to interconnect integrated circuits. The JTAG module provides compatibility with the JTAG testing standard (Joint Test Action Group). 
     FIG. 27 illustrates the pin outs for the Bt835. Additional information regarding the function of each pin out and the Bt835 in general is available in a document entitled Advance Information on the Bt835 VideoStream III Decoder, from Conexant Systems, Inc. (formerly Rockwell Semiconductor Systems) of San Diego, Calif., which is hereby fully incorporated by reference herein as though set forth in full. 
     While particular embodiments and examples of the present invention have been described above, it should be understood that they have been presented by way of example only and not as limitations. Those of ordinary skill in the art will readily appreciate that other various embodiments or configurations adopting the principles of the subject invention are possible. The breadth and scope of the present invention is defined by the following claims and their equivalents, and is not limited by the particular embodiments described herein.