WO1998014013A1

WO1998014013A1 - Comb filter with bypass mode

Info

Publication number: WO1998014013A1
Application number: PCT/US1997/017034
Authority: WO
Inventors: Mark Francis Rumreich; John Alan Hague
Original assignee: Thomson Consumer Electronics, Inc.
Priority date: 1996-09-27
Filing date: 1997-09-23
Publication date: 1998-04-02
Also published as: AU4650197A

Abstract

A video signal processing system for recovering a plurality of frequency interleaved information components modifies a comb filtering function during a particular mode of operation to increase the resolution associated with the recovered information components. For example, during a first mode of operation, comb filtering operates to recover luminance and chrominance components of a color television signal. During a second mode of operation corresponding to absence of a chrominance component, the comb filtering function is modified by bypassing portions of the filtering function. During the second mode of operation, substantially the full bandwidth of a luminance component is recovered.

Description

COMB FILTER WITH BYPASS MODE

FIELD OF THE INVENTION

The invention relates to comb filters in general and, more particularly, to a comb filter apparatus and method for modifying the combing function in response to changes in the spectral content of the signal to be filtered.

BACKGROUND

Electronic signals which are periodic in nature may be processed advantageously by encoding replicas of the signal which are separated in time by the repetition periods. The signal is recovered by extracting the encoded replicas and combining them to recreate the information content of the signal. For example, television systems require the separation or recovery of luminance and chrominance components of, e.g., an NTSC encoded color television signal. An NTSC color television signal includes a brightness or luminance frequency (Y) signal ranging in frequency from direct current to a nominal bandwidth of 4.2 MHz, and a 3.58 MHz color subcarrier which is modulated in phase and amplitude to represent hue and saturation of the image. The luminance information contained in an image is represented by signal frequencies which are concentrated about integral multiples of the horizontal line scanning frequency. The chrominance information is encoded or inserted in a portion of the luminance signal spectrum around frequencies which lie halfway between the multiples of the line scanning frequency (i.e., at odd multiples of one-half the line scanning frequency). The chroma and luma information as described above may be appropriately recovered by using a simple filter or a comb filter. In a typical simple filter arrangement the composite signal is fed to a low pass filter to provide the luminance or luma, and to a band pass filter to provide the chrominance or chroma. Unfortunately, the simple filter approach has several problems, including bandwidth limitation of the luma output, low and high frequency chroma appearing in the luma signal and high frequency luma appearing in the chroma signal. The bandwidth limitation undesirably reduces the sh.arpness or vertical detail of the reproduced television image. The appearance of luma in the chroma signal or chroma in the luma signal produces undesirable patterns and distortions of the reproduced television image.

In a typical comb filter approach a composite video signal is subtracted from a second composite video signal which has been delayed by one horizontal scan. Since two successive lines (with respect to horizontal synchronization pulses) of NTSC chroma subcarrier are 180 out of phase, the chroma inputs combine as a subcarrier sum. Since the lines of luminance are originally in phase, the combination of the opposite phase luminance components result in cancellation of luminance. Thus, a comb filtered chroma output signal is produced in which stationary luma components have been phase canceled. In addition, the separated chroma is subtracted (without further phase reversal) from the composite video input signal (luma+chroma) to produce separated luma. Since the lines of chrominance are in phase, the phase cancellation of chrominance components produces a separated luminance output signal.

Unfortunately, the luminance channel diagonal resolution is affected by the above-described comb filter because the sidebands surrounding the line frequency harmonics spread into the areas attenuated by the filter. Thus, the sharpness or vertical detail in the vicinity of 3.58 MHz is undesirably reduced. This reduction in diagonal resolution is manifested as, e.g., blurred edges in the reproduced television image .and occurs even if the video signal subjected to combing does not contain chrominance information (i.e., a black and white signal). In addition, the performance of comb filters is related to the line to line correlation of the composite video signal lines being operated upon. Specifically, luma and chroma information from a video signal representing an image including, e.g., edges or motion may not be properly separated by a comb filter. This is because the chroma and/or luma information from one line may not be sufficiently similar to the respective chroma and/or luma information contained in the next line. Thus, the additive or subtractive combing action will not produce the most advantageous result.

SUMMARY OF THE INVENTION

The invention resides, in part, in the recognition by the inventor that in a comb filtering arrangement a reduction in recovered bandwidth of a first information component is an unnecessary performance penalty in a case where a second information component is not present. An aspect of the invention involves providing a comb filtering apparatus and method for recovering a plurality of frequency interleaved information components such that when fewer than all of the plurality of information components are to be recovered the comb filtering function is modified to increase the bandwidth of the recovered information component(s).

Another aspect of the invention involves enhancing a high frequency component of the first information component regardless of the presence or absence of additional information components.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described with reference to the accompanying Drawing wherein like elements are denoted by like reference designators and in which:

FIGURE 1 is a block diagram of a television apparatus embodying the invention; and FIGURE 2 is a block diagram of another television apparatus embodying the invention.

DETAILED DESCRIPTION

FIGURE 1 illustrates a television apparatus 100 embodying the invention, which includes a video source 102 for providing a video signal SI including at least luminance components. An analog to digital (A/D) converter 107 converts the analog video signal SI into a digital video signal S2. A comb filter 160 separates the luminance and chrominance components of the video signal S2 to produce an output luminance signal LUMA and an output chrominance signal CHROMA. The output luminance LUMA and chrominance CHROMA signals are coupled to respective digital to analog (D/A) converters 135,137 to form analog luma Y and chroma C signals. The analog chroma signal C is demodulated by chrominance demodulation unit 139 to form color difference signals B-Y, R-Y. A matrix processor and driver unit 141 processes the analog luma signal Y and the color difference signals B-Y, R-Y to produce RGB signals to drive a display .unit 143. A black and white (b&w) detector unit 150 receives video signal S2, determines if the video signal S2 is monochromatic and produces an output signal VIDEO=B&W indicative of the presence ("0") or absence ("1") of valid color information in the video signal S2. The b&w detector 150 output signal VIDEO=B&W is coupled to comb filter 160. In the exemplary embodiment, b&w detector 150 determines if the video signal S2 is a color video signal by determining if the video signal S2 includes a color reference (color burst) which exceeds a predetermined threshold level. The comb filter 160 operates in a "normal" mode if the video input signal S2 is a color signal (VIDEO=B&W = "0") and in a "bypass" mode if the video input signal S2 is a monochrome signal (VIDEO=B&W = "1"). In the bypass mode the comb filter recovers substantially the full luminance signal bandwidth. For television receiver applications the video source 102 may include, e.g., a conventional tuner, IF amplifier and detector. The source may also include baseband video inputs. For television monitor applications the tuner may be omitted. The demodulation unit 139, matrix process and driver unit 141, display unit 143 and the associated control circuitry may be of conventional design, the details of which are largely omitted. Moreover, the comb filter 160, b&w detector 150, A/D converter 107 and D/A converters 135,137 may be advantageously included in a single integrated circuit 1000.

Referring to FIGURE 1, the digital video signal S2 from A/D converter 107 is coupled to respective inputs of a single line (1H) delay element 109, a first subtracter 117 and a second subtracter 125. Delay element 109 produces at an output a signal S3 which is a replica of the input signal S2 but delayed in time by one horizontal scanning interval (approximately 63.5 microseconds). Delay element 109 may be implemented in a known manner by, e.g., a buffer memory. First subtracter 117 subtracts the video signal S3 (representing the previous scanning interval) from the video signal S2 (representing the present scanning interval) to produce a first video difference signal S4. The first video difference signal S4 includes chroma information and any line to line luma variations. These line to line luma variations, which represent vertical detail (VD) in the video signal, are spectrally located primarily below the subcarrier frequency of 3.58 MHz. The normal mode chroma signal CHROMA is developed as follows. The output signal S4 of first subtracter 117 is filtered by band pass filter (BPF) 119, thereby removing the line to line luma variations (i.e., vertical detail). The resultant BP filtered signal S5 contains the recovered chroma information from the input video signal S2. The BP filtered signal S5 also contains any luminance vertical detail information within the passband of the BPF 119.

A switch 131 couples the BP filtered signal S5 to the chroma D/A converter 137 during the "normal" comb filter operating mode (i.e., VIDEO=B&W = "0"). Additionally, the switch 131 couples the BP filtered signal S5 to an input of subtracter 125. Note that in FIGURE 1 switch 131 is shown in the "bypass" mode position (i.e., VIDEO=B&W = "1").

The normal mode luma signal LUMA is developed as follows. Subtracter 125 subtracts the BP filtered (chroma) signal S5 from the input video (luma+chroma) signal S2 to produce a second difference signal S9, which contains primarily luminance components. Second difference signal S9 is essentially the luminance component without the luminance vertical detail within the passband of BPF 119. This second difference signal S9 is coupled to a first input of an adder 127. The first video difference signal S4 (chroma+VD) is low pass filtered (LPF) by LPF 121, thereby removing the chroma information while leaving the low frequency line to line luma variations (i.e., low frequency vertical detail information). The resultant low frequency low frequency vertical detail signal S7 is coupled to a second input of adder 127. Adder 127 adds the vertical detail signal S7 and the second difference signal S9 (luma) to produce a signal LUMA containing luminance information with enhanced vertical detail information. The output signal LUMA from adder 127 is coupled to the luma D/A converter 135.

It should be noted that in the exemplary embodiment, BPF 119 has a gain of 1/2 such that the BP filtered output signal S5 coupled to subtracter 125 is of the same relative magnitude as the comb input signal S2. This is because subtracter 117 has the effect of doubling the chrominance and vertical detail information containing in first difference signal S4. The gain compensation may be made elsewhere in the circuit. The bypass mode operation will now be explained. As previously stated, comb filter 160 operates in the "bypass" mode when the input video signal S2 is a monochrome signal. In this mode switch 131 couples a "zero" chroma signal S6 (shown as a "0" in FIGURE 1) to chroma D/A 137 and subtracter 125. The zero chroma signal S6, which should be zero (no saturation) during the display portion of the line, may be formed, e.g., by coupling a zero amplitude color signal to switch 131 during the viewable portion of input video signal S2. In the normal combing mode, the second difference signal S9

(luma) does not include any of the luminance information which was contained in the output chroma signal CHROMA. This is because the output chroma signal CHROMA may include luminance vertical detail information which is within the pass band of BPF 119 and the second difference signal S9 is the result of subtracting the output chroma signal CHROMA from the input video signal S2. Thus, the output luma signal LUMA will not include the band-passed luminance vertical detail components from the chroma channel.

In the bypass combing mode the second difference signal S9 (luma) includes substantially all the luminance components of the comb input signal S2. This is because subtracter 125 subtracts only the zero chroma signal S6 from the comb input signal S2. Thus, the output luma signal LUMA includes all of the luma information contained in the input video signal. As previously stated, the comb filter operates in either a normal mode or a bypass mode, depending upon whether the input video signal S2 is, respectively, a color signal or a monochrome signal. This determination is made by black and white detector 150. Referring to FIGURE 1, the exemplary b&w detector 150 receives input video signal S2, determines if the video signal S2 is a black and white video signal and produces an output signal VIDEO=B&W indicative of whether or not the video signal S2 is black and white. A black and white detector of the type shown in the exemplary embodiment is described below.

B&W detector 150 includes a burst accumulator (or quadrature phase detector) 110, which sorts and totalizes the even and odd polarity corrected samples of the video signal S2 occurring during the burst interval into a first group of samples X (which occur at the burst peaks) and a quadrature group of samples Y (which occur at the burst zero crossings). The numbers X and Y represent the burst vector coordinates in a Cartesian (rectangular) coordinate system.

The X and Y coordinates of the burst vector are next applied to phase and magnitude calculator 111 (a rectangular to polar coordinate converter) which converts the XY coordinates from rectangular to polar coordinate form (R,) having a magnitude term R and a phase angle term (not shown here). One approach to providing this conversion would be to apply the X and Y values to the address inputs of a read only memory (ROM) programmed with corresponding radius and angle values. Such an arrangement, however, would require a relatively large memory. An approach used in the exemplary embodiment eliminates the need for large memory by calculating the angles using sine, cosine or tangent trigonometric approximations.

The magnitude term R is coupled to a filter 113, which provides a filtered magnitude term R^VG ^{at an} output. The filter 215 improves system performance by reducing errors caused by spurious or erroneous magnitude terms by recursively filtering the magnitude term R, thus providing a rolling average RAVG °f ^{tne most} recent magnitude terms. A suitable implementation of filter 215 would be, e.g., a recursive filter providing a rolling average of the eight most recent magnitude terms.

The filtered magnitude term RAVG ^{is tnen} coupled to an input

B of a digital comparator 114. A threshold level signal provided by a NO BURST threshold level signal source 112 is coupled to an input A of the digital comparator 114. Comparator 114 produces a substantially bi-level output signal VIDEO=B&W which will be set to a low (i.e., 0) level if the signal at input B (RAVG) ^is 8^reater than the signal at input A (threshold).

The threshold level, which may be adjustable, represents a minimum magnitude level of a valid color burst. If the magnitude of RAVG ^{is less} than the magnitude of the threshold level, then the output of comparator 213 will produce a high (i.e., 1) output signal MAIN=B&W indicating that the main video signal CVM is not a color signal. A low output signal indicates that the main video signal CVM is a color signal.

Several enhancements may be made to the comb filter 160. Referring to FIGURE 1, comb filter 160 includes an optional vertical peaking unit 123 and an optional chroma notch unit 133. The purpose of these units is to enhance the luminance recovery performance of the comb filter.

The optional vertical peaking unit 123 is an adjustable gain block which amplifies the low frequency vertical detail signal S7 to produce an enhanced low frequency vertical detail signal S8. The enhanced signal S8 is coupled to adder 127 such that the amount of vertical detail included in the output luma signal LUMA is increased. The amount of amplification may be controlled by the user as, e.g., a "sharpness" control selectable via a remote control unit (not shown).

The optional chroma notch unit 133 is a digital notch filter at the subcarrier frequency which removes any residual chroma information from the luma signal LUMINANCE. The chrominance notch unit also reduces chrominance vertical detail information which may have entered the luminance processing channel. The chroma notch unit receives the b&w detector 150 output signal VIDEO=B&W and is selectively disabled or enabled depending upon whether the input video signal S2 is, respectively, a black and white or color signal. When the input signal is black and white there is no reason to perform the chroma notch function since, presumably, there is no chroma vertical detail information to remove. Therefore, by disabling the chroma notch unit the luminance signal diagonal resolution is enhanced. To summarize, comb filters operated in an additive or subtractive manner to recovery frequency interleaved components from an information signal. In the case where an information signal (e.g., composite video) having frequencies of a first component (e.g., luminance motion artifacts) overlapping the spectrum of a second component (e.g., chrominance), the recovered frequencies of the first component will not include the overlapping frequencies. Thus, the full bandwidth of the first component will not be recovered. By determining if the spectrum of the overlapping frequencies is not otherwise occupied (e.g., no chrominance component), the operation of the comb filter may be modified such that the overlapping frequencies may also be recovered. In this manner the full bandwidth of the first component is recovered. This technique also applies to comb filter extraction of more than two components.

FIGURE 2 is an alternate embodiment of the invention described above. In the arrangement of FIGURE 2 a video source 102 provides a video signal SI to an A/D converter 107 for conversion into a digital video signal S2. The video signal S2 is coupled to a comb filter 160, a first input 1 of a switch 200 and a b&w detector 150. Comb filter 160 separates the luminance and chrominance components of video signal S2. The luminance component is coupled to a second input 0 of the switch 200 and the chrominance component CHROMA is coupled to a chroma D/A converter 137. B&W detector 150 determines if the video signal S2 is a monochrome signal and produces an indicative output signal VIDEO=B&W. Switch 200 is responsive to the output signal VIDEO=B&W of b&w detector 150 to couple the separated luminance component to a luma D/A converter 135 when the input video signal S2 is a color signal. Switch 200 couples the input video signal S2 to the luma D/A converter 135 when it is not a color signal. The remainder of the circuit operates as previously described with respect to FIGURE 1. Note, a delay element (not shown) may be inserted into the circuit path carrying the video signal S2 to the switch 200. The added delay to the video signal S2 equalizes the delay imparted by the comb filter to its chrominance output CHROMA signal. This equalization may be required by, e.g., the chrominance demodulator 139 or the matrix processor and driver 141 circuit.

It will be apparent to those skilled in the art, that although the invention has been described in terms of specific examples, modifications and changes may be made to the disclosed embodiments without departing from the essence of the invention. It is, therefore, to be understood, that the appended claims are intended to cover all modifications which naturally flow from the foregoing description and examples.

The exemplary embodiment is described above with respect to a television system, where the recovered luminance channel diagonal resolution is reduced because the luminance sidebands surrounding the line frequency harmonics spread into the areas attenuated by a comb filter. That is, luma and chroma information from a video signal representing an image including, e.g., edges or motion may not be properly separated by the additive or subtractive operation of a comb filter.

It is important to note that while the invention has been described in terms of comb filter for use in a television system, the invention is applicable to any system in which interlaced information components .are susceptible to being decoded or recovered by comb filter techniques

Claims

1. Signal processing apparatus, comprising: means (160) for filtering an input signal (S2) containing combined first and second information components (C,Y) having interleaved frequency spectra for providing a separated first information component (C) and for providing a separated second information component (Y) responsive to said separated first information component; means (150) responsive to said input signal (S2) for providing an indicator signal (VIDEO=B&W) when said first information component of said input signal has an amplitude below a threshold value; a switch (200) responsive to said indicator signal for bypassing said filtering means (160) when said indicator signal is present for providing said separated second information component in response to said input signal and substantially independent of said separated first information component; and means (139) for demodulating said separated first information component (C).

2. Apparatus as recited in Claim 1 wherein said switch (200) having a first input coupled to receive said input signal (S2), having a second input coupled to receive said separated second information component (Y), and having a control input coupled to receive said indicator signal (Video=B&W).

3. Apparatus as recited in Claim 1 wherein: said filtering means (160) comprising means (125) for subtracting said separated first information component (C) from said input signal for providing said separated second information component (Y); said switch (131) being coupled to said subtracting means (125) and being responsive to said indicator signal (Video=B&W) for preventing said subtracting means (125) from subtracting said separated first information component (C) from said input signal (S2) when said indicator signal is present.

4. Apparatus as recited in Claim 3 wherein said switch ( 131) being responsive to said indicator signal for applying a reference level signal ("0", S6) to an input of said demodulator (139) and to an input of said subtracting means (125) when said indicator signal (Video=B&W) is present.

5. Apparatus as recited in Claim 4 wherein said switch ( 131 ) having a first input coupled to receive said separated first information component, having a second input coupled to receive a reference level ("0") signal (S6), having a control input coupled to receive said indicator signal (Video=B&W), and having an output coupled to said demodulator (139) and to said subtracting means (125).

6. Apparatus as recited in Claims 2, 3, 4 or 5 further comprising peaking means (121,123,127) responsive to said separated first information component (S4) and coupled to said subtracting means (125) for applying peaking to said separated second information component (Y).

7. Apparatus as recited in Claims 2, 3, 4, 5 or 6 further comprising: means (133) for attenuating frequency components of said second separated information component (Y) occurring within a frequency range characteristic of said first separated information component (C); and means for disabling said attenuating means (133) in response to the presence of said indicator signal.

8. A signal processing method comprising the steps of: filtering (109,117,119) an input signal (S2) containing combined first and second information components (C,Y) having interleaved frequency spectra for providing a separated first information component (C) at an output; subtracting (125) said separated first information component (C) from said input signal (S2) for providing a separated second information component (Y); providing (150) an indicator signal (Video=B&W) when said first information component (C) of said input signal has an amplitude below a threshold (112); preventing subtraction of said separated first information component from said input signal when said indicator signal is present; and demodulating (139) said separated first information component (C) at said first output.

9. A method as recited in Claim 8 further comprising the step of substituting (131) a reference level signal ("0", S6) for said separated first information component at an output when said indicator signal (Video=B&W) is present.

10. A method as recited in Claims 9 or 10 further comprising the step of attenuating (133) frequency components of said second separated information component (Y) occurring within a frequency range characteristic of said first separated information component (C), the step of attenuating being performed only when said indicator signal is not present.