Abstract:
In certain video scrambling systems, composite video is imperfectly separated into luminance and chrominance and scrambled in such a way that an unstable residual chroma color subcarrier remains in the luminance channel. When this unstable residual subcarrier subsequently is summed with stabilized chroma, the resultant composite color signal has small but visible amounts of color subcarrier instability that causes a noisy color signal when descrambled or when displayed on a television display device. A coring circuit is disclosed which provides means for substantially removing the unstable residual chroma subcarrier from the luminance channel, thereby substantially reducing color subcarrier instabilities. An improved coring technique also is disclosed using adaptive chroma coring, which is achieved by adjusting the amount of coring applied in accordance with the amplitude of the chrominance signal. Thus, if the program video input has high levels of color saturation, the chroma coring is electronically turned up. Conversely, if the input video program is essentially in black and white (no color content), then the coring circuit essentially is electronically turned off. The coring techniques also are applicable to enhance the luminance-chrominance separation of video comb filters frequently used in video circuits such as, for example, television sets and video recorders.

Description:
RELATED PATENTS 
     This application is a nonprovisional application based on the copending provisional application No. 60/011,584, filed Jun. 17, 1996. This application is related to U.S. Pat. No. Re. 35,078 by J. O. Ryan issued on Oct. 31, 1995, U.S. Pat. No. 5,438,620 by J. O. Ryan et al. issued on Aug. 1, 1995 and U.S. Pat. No. 5,504,815 by J. O. Ryan et al. issued on Apr. 2, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to video signal scrambling systems and, more particularly, to a coring system for providing a substantially stable color subcarrier signal while retaining full luminance resolution, in line time shifting or position modulating video scrambling apparatus. 
     The U.S. Pat. Nos. Re. 35,078 (&#39;078), 5,438,620 (&#39;620) and 5,504,815 (&#39;815) of previous mention, incorporated herein by reference, disclose typical line positional shifting video scrambler processes and apparatus. Such line positional or time shifting video scramblers include a video comb filter or equivalent luminance/chrominance separator to separate the composite input signal into luminance and chrominance components (see FIG. 1A) or luminance and demodulated chrominance R-Y and B-Y components (see FIG.  1 B). The separated signals then are shifted in time or position via suitable memories, such as first-in-first-out (FIFO) memories, or variable delay lines. In FIG. 1A, the chrominance signal first is time shifted (that is, scrambled) and then is color subcarrier stabilized in both phase and frequency (for example, via a heterodyning process), before being added to the scrambled luminance signal, to provide a composite color stabilized time or position modulated scrambled video output signal. In FIG. 1B, where the R-Y and B-Y demodulated chrominance components are provided, the time shifted or position modulated R-Y and B-Y components are re-encoded with a stable color subcarrier frequency and phase before being added to the time shifted or position modulated luminance signal to provide a composite color stabilized time shifted or position modulated scrambled video output signal. 
     However, the video comb filter or luminance/chrominance separator of the above scrambling systems does not provide perfect separation of the chrominance (chroma) and luminance (luma) components. Therefore residual chroma remains in the luma channel in the scrambling systems described above. Normally, imperfect luminance-chrominance separation is not a problem in equipment such as television sets. That is, in such equipment, the residual chrominance in the luminance channel still is stable chrominance and thus does not contribute to chrominance instabilities in, for example, the television set. However, once the luma channel is time shifted by the scrambling process, the residual chroma becomes unstable in phase and frequency. When the time shifted or (low frequency) position modulated luminance channel signal with the unstable residual chroma is added to the stabilized time shifted chroma channel signal, a composite video signal is produced with small, but visible, unstable chroma phase and amplitude errors when scrambled and later descrambled. These unstable chroma phase and amplitude errors cause low frequency color streaking or hue and saturation noise throughout the television field. 
     FIG. 1A illustrates a basic video path for a scrambling system  10  which employs a signal wobbling technique, such as described in the &#39;620 and &#39;815 patents of previous mention. A program video signal, such as a composite video signal, is supplied via an input lead  12  to a comb filter  14 . The elements  76 ,  78  and  80  shown in FIG. 4 of the &#39;620 and &#39;815 patents exemplify elements which can be used to form the comb filter  14  in FIG.  1 A. The comb filter  14  provides outputs of a luma signal with residual chroma, and a chroma signal. The luma signal along with its residual chroma are supplied to a time shift element  16  to effect the scrambling process, whereby the element  16 , provides a shifted luma signal with shifted unstable phase residual chroma. The chroma signal is supplied to a second time shift element  18  which provides the previously mentioned scrambling of the chroma signal. It is to be understood that the time shift elements  16 ,  18  could be position modulation elements as well. Both of the time shift (or position modulation) elements shift the video signal by an equal amount as part of the particular scrambling process used. The output of the time shift element  18 , comprising the time shifted chroma signal, is supplied to a chroma subcarrier stabilizer  20 . The chroma subcarrier stabilizer  20  is comparable to the hetrodyne element  100  in FIG. 4 of the &#39;620 and &#39;815 patents, and provides a shifted chroma signal with stable phase. The outputs of the chroma subcarrier stabilizer  20  and of the time shift element  16  are supplied to the inputs of an adder circuit  22 , which produces a scrambled video signal having chroma subcarrier instabilities on an output lead  24 . 
     FIG. 1B illustrates a basic video path for a scrambling system  30  which employs a signal wobbling technique such as described in the &#39;078 patent of previous mention. A program video signal, such as a composite video signal, is supplied via an input lead  32  to a comb filter/chroma demodulator circuit  34 . The element  16  shown in FIG. 4A of the &#39;078 patent exemplifies an element which can be used as the comb filter/chroma demodulator circuit  34  in FIG.  1 B. The comb filter/chroma demodulator circuit  34  provides outputs of a luma signal with some residual chroma, a R-Y component and a B-Y component. The luma signal along with its residual chroma are supplied to a time shift or position modulation element  36 , which provides a scrambled luma signal formed of a shifted luma signal with shifted unstable phase residual chroma. The R-Y and B-Y signals are supplied to a two channel time shift element  38  which provides scrambled R-Y and B-Y component signals. The time shift elements  36 ,  38  shift the respective video signal by an equal amount as part of the scrambling process. The outputs of the time shift element  38 , comprising the time shifted or position modulation R-Y and B-Y signals, are supplied to a chroma encoder  40 . The chroma encoder  40  is comparable to the encoder element  25  in FIG. 4A of the &#39;078 patent of previous mention, and provides a scrambled chroma signal with stable phase. The outputs of the chroma encoder  40  and of the time shift element  36  are supplied to the inputs of an adder circuit  42 , which produces a scrambled video signal having chroma subcarrier instabilities on an output lead  44 . 
     One solution for removing the unstable residual chroma from the positionally shifted luma channel includes the application of a notch filter in the luminance channel, wherein the filter has a frequency band around the color subcarrier frequency and its color sidebands. However, this solution severely degrades the luminance resolution and thus degrades the frequency response of the video scrambler&#39;s and thus the descrambler&#39;s output. The degraded resolution eliminates much of the advantage that video comb filters provide, since comb filters normally aid in gaining full luminance resolution. Accordingly, there is a need for a solution that eliminates the unstable chroma phase and amplitude errors without degrading the luminance frequency response and the resultant resolution of the signal scrambling system. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to retain full luminance resolution of video scrambling systems while providing a substantially stable color subcarrier. The stable subcarrier phase and amplitude appear in the resulting composite video output signal, corresponding to the time shifted or position modulated scrambled video signal. The increased stability substantially reduces or eliminates chroma noise due to imperfect luminance/chrominance signal separation. That is, it is the intent of the present invention to substantially remove the unstable residual chroma color subcarrier caused by the small amount of chroma signal which leaks into the luma channel, so when the signal in the positionally modulated or time shifted (wobbled) luminance channel is summed with the signal in the positionally modulated or time shifted stable chroma color subcarrier channel, color instabilities are substantially reduced to a minimum. 
     To this end, the method and apparatus of the present invention employs a chroma coring system to remove the unstable residual chroma color subcarrier, that is, the unstable residual chroma, in the luma channel. An alternative embodiment of the invention provides further improvement by including an adaptive chroma coring system. The adaptive chroma coring is provided by variably adjusting the amount of coring applied in accordance with the chroma amplitude sensed in the chrominance signal in the scrambled stabilized chroma color subcarrier channel, or in the signal derived from a video comb filter in the chrominance channel. For example, if the program video input signal has large areas of highly saturated color components, the chroma coring is electronically turned up. At the other extreme, if the program video signal is essentially in black and white (no color content), then the coring circuit essentially is electronically turned off. Thus, the adaptive coring technique maximizes the frequency and pulse response for black and white signals. 
     It is to be understood that the invention also can be used to enhance the luminance-chrominance separation of video comb filters frequently used in other video circuits and systems such as television sets and video recorders. 
     These and other aspects, features and advantages of the invention will become apparent upon review of the succeeding description taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a block diagram of a prior art positionally shifted video scrambler using luminance/chrominance comb filtering; 
     FIG. 1B is a block diagram of a prior art positionally shifted video scrambler using comb filtering with luminance and demodulated chroma, R-Y and B-Y components; 
     FIG. 1C is a schematic diagram depicting a basic typical coring circuit; 
     FIG. 1D is a graph illustrating a transfer function of the coring circuit of, for example, FIG. 1C; 
     FIG. 2A is a block diagram illustrating the invention incorporated into a device such as depicted in FIG. 1A; 
     FIG. 2B is a block diagram illustrating the invention incorporated into a device such as depicted in FIG. 1B; 
     FIG. 2C is a graph illustrating the chroma coring effect on the luma channel signal as a function of input signal level; 
     FIG. 3 is a block diagram illustrating an embodiment of a chroma coring circuit of the invention; 
     FIG. 4 is a block diagram illustrating another embodiment comprising an adaptive version of the invention of FIG. 3; 
     FIG. 5 is a schematic diagram illustrating further details of the invention of FIGS. 3,  4 . 
     FIG. 6 is a schematic diagram illustrating another embodiment of an adaptive chroma coring circuit of the invention; and 
     FIG. 7 is a block diagram illustrating a further embodiment comprising a multiband coring circuit configured to include adaptive and/or fixed coring around the chroma frequencies. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Signal coring is a process whereby low level signals below a selected amplitude are denied passage through a specific circuit, and signals greater than the selected amplitude are allowed passage through the circuit. In the present invention, coring is used to enhance the luminance and chrominance separation of the comb filter. A complementary class B or C transistor amplifier with crossover distortion is an example of a coring circuit as depicted in FIG.  1 C. To this end, any signal between about +0.7 volt to −0.7 volt are denied passage whereas all other signals greater than about 0.7 volt in absolute value are allowed passage. See for example FIGS. 1C,  1 D. The coring circuit can be placed before or after the time shifting element (i.e., the memory, FIFO, delay line, etc.) of the video scrambler&#39;s luminance channel to meet the intent of this invention. 
     As depicted in FIGS. 2A,  2 B, in the preferred embodiment the coring is done after the luminance channel has been time shifted to provide scrambling. Also the coring is done by the inverted summing (subtraction) of a portion of the output signal of an amplifier that limits or clips an input signal. Thus, in accordance with the invention, at low input signal levels, such as, for example, +0.1 to −0.1 volt signals (or smaller), the subtraction is complete because the amplifier is operating in its linear range. However, at selected higher signal levels over the 0.1 volt absolute, (or smaller than 0.1 volt), subtraction is limited and accordingly most of the higher input signals are allowed passage. FIG. 1C depicts a typical coring circuit as discussed above. FIG. 1D depicts generally the transfer characteristic of a coring circuit. For a circuit such as illustrated in FIG. 1C, the deadband voltage, V, is about 0.7 volt. In a coring circuit using a transistor differential pair, the deadband voltage, V, is 0.1 volt or less. 
     More particularly, FIG. 2A depicts a system level block diagram of how an embodiment of the present invention is implemented into a scrambling system  50  such as that described in FIG.  1 A. The composite program video signal on an input lead  52  is supplied to a comb filter  54  similar to comb filter  14  in FIG.  1 A. As in FIG. 1A, the comb filter  54  supplies outputs comprising a luma signal on a lead  57  that contains residual chroma, and a chroma signal on a lead  55 . These two signals are supplied to a time shift element  58  and a time shift element  60 , respectively. In accordance with the invention, a chroma coring circuit  62  preferably is inserted after the element  58  and thus between points A and B in the luma channel of FIG.  2 A. The placement of the chroma coring circuit at this location in the scrambling system, provides a time shifted luma signal with zero, or a much reduced, time shifted unstable residual chroma. However, as an alternative, the chroma coring circuit  62  may be inserted prior to the element  58  at a location  56  between the comb filter  54  and the time shift element  58 , as depicted in dashed lines. The chroma channel contains a time shift element  60  and a color subcarrier stabilizer  64  which perform respective functions of the time shift element  18  and the color subcarrier stabilizer  20  in FIG.  1 A. The stabilized time shifted chroma and the time shifted luma with minimized time shifted residual chroma, are combined in an adder  66  to produce a scrambled video signal having a stable noise free chroma component on an output lead  68 . 
     FIG. 2B depicts a system level block diagram of how an embodiment of the present invention is implemented into a scrambling system  70  such as that of FIG. 1B. A coring circuit  84  is depicted inserted after a time shift element  80  and thus between points A and B in the luma channel. As in FIG. 2A, however, the coring circuit may be inserted prior to the element  80  as depicted in dashed lines at  76 . As previously described, small amounts of unstable chroma component due to the imperfect comb filter are removed by the chroma coring circuit  84 . 
     Thus, the chroma coring circuit of FIGS. 2A and 2B remove from the luma signal, low level signals whose frequencies are in the region of the color subcarrier frequency. These low level signals constitute the chroma which is not removed from the luma signal by the comb filters  54  and  74 , or an equivalent circuit. Larger level signals pass through the chroma coring circuit. For example, the chroma coring circuit of FIGS. 2A,  2 B is set to remove no more than 3% of the nominal signal level around 3.58 MHz. This means if the scrambled luma channel has a residual chroma component of 3% or less, the chroma coring; circuit will completely or substantially remove the unstable chroma component. FIG. 2C depicts the chroma coring effect on the luma channel as a function of input level. In practice, depending on cost, comb filters in general produce a residual chroma component in the luma channel from about 3% to about 0.3%. 
     It should be noted that the chroma coring system also reduces random noise in the luminance channel caused by quantizing noise or noise in the video frequency band. In general, the chroma coring can be more generic. By coring small signals above 500 kHz, the circuit not only eliminates residual (unstable) chroma from the scrambled luminance channel, but also reduces video noise present in the luminance channel. 
     FIG. 3 illustrates an embodiment  100  of a chroma coring circuit of the invention, such as circuits  62 ,  84  of FIGS. 2A,  2 B. The signal at point A is the time shifted luminance signal with an unstable residual chroma component, supplied via a lead  102  to an amplifier  104 , which has for example a gain of two. The output of the amplifier  104  is supplied to a bandpass filter  106 . The filter  106  can be a low Q bandpass filter set at the color subcarrier frequency of 3.58, for example, or some other filter like a high pass filter set at about 500 kHz, wherein the bandpass filter cores the unstable residual chroma while the high pass filter removes other video noise. The output of the filter  106  is amplified by an amplifier  108  with for example a gain of five, and then is supplied to an amplifier with limited headroom (or positive and negative clippers) such as a limiting amplifier  110 . This limiting amplifier can be a transistor differential pair amplifier with maximum input of about +/−100 millivolts, as further described below. The differential pair amplifier  110  may have a gain of minus unity for inputs less than +/−100 millivolts and thus limits or clips signals having greater input levels. The output of the differential pair amplifier  110  is attenuated in this example by ⅕ in an attenuator  111 , and is supplied to a first input of a summing amplifier  112 . The output of the amplifier  104  also drives a delay line  113  (or low pass filter) to match the delay caused by the filter  106 , amplifier  108 , limiting amplifier  110  and the ⅕ attenuator  111 . The output of the delay line  113  is supplied to a second input of the summing amplifier  112 . Typically the luminance level at A is 700 millivolts. The output of the delay line  113  thus is 1400 millivolts and the output of the  115  attenuator  111  is 40 millivolts. The output of the summing amplifier  112  then will subtract up to 40 millivolts (about 3% or {fraction (40/1400)}) of residual chroma from the luma channel signal provided on output lead  114  corresponding to point B of FIGS. 2A,  2 B. 
     FIG. 4 illustrates an embodiment  120  of an adaptive coring circuit of the invention which uses the chroma signal amplitude in the chroma channel, that has been time or position shifted, to modulate the amount of coring applied to the luma channel. In FIG. 4, the components in the figure are similar to the respective components in FIG. 3, although numbered differently. Coring depth is controlled by varying the maximum output level of a limiting amplifier  140  (which is similar to the limiting amplifier  110  of FIG.  3 ), while keeping its small signal gain constant. To this end, the chroma channel signal from, for example, the color subcarrier stabilizer  64  or chroma encoder  86  of FIGS. 2A,  2 B, respectively, is supplied via a lead  142  to an amplifier  144  of an adaptive coring control circuit  145 . The amplified signal is supplied to a full wave rectifier (or envelope detector)  146 , and the resulting signal is smoothed via a capacitor  148 /resistor  151  network. An amplifier  152  supplies a voltage from the control circuit  145  which is proportional to the chroma signal amplitude, to control the output level of the limiting amplifier  140 . Thus, the higher the color saturation, the higher the clipping level of the limiting amplifier  140  is raised. This raises the amount of chroma frequency coring. Conversely, if there is a lack of color in the program video, the clipping level is reduced which provides very little, or zero, coring of the signal in the luminance channel. The resulting adaptively cored signal is supplied on an output lead  160  corresponding to the point B. 
     FIG. 4 illustrates one of various ways of providing adaptive coring using an adaptive coring control circuit. Alternatively, for example, it is possible to supply the output of the amplifier  152  to control the circuits  130  and  156  instead, such that a value K2 of circuit  130  varies inversely with the chroma amplitude in the time shifted chroma channel to keep the limiting amplifier  140  at a corresponding fixed clipping level. For instance, if the chroma level is low, the value K2 should be large, for example, K2≈10, for about 1.5% of coring. If the chroma level is higher, K2 should be lower, for example, about 5 for about 3% of coring. 
     FIG. 5 illustrates a modified embodiment  170  of a coring circuit with fixed level of coring. The components illustrated in FIG. 5 correspond to those of the previous figures, though shown in more detail. As previously discussed, the coring circuit of FIG. 5 or its equivalent also can be inserted either prior to or after the time shift element of, for example, FIGS. 2A and 2B, to achieve the goal of removing chroma instabilities in the scrambled composite video signal. 
     To this end, the time shifted luma with unstable residual chroma is supplied via a lead  168  and is amplified by a feed back amplifier  172 , with a gain of two set via resistors  169 ,  171 . The output of the amplifier  172  is supplied to a chroma bandpass filter of Q less than two, formed of a resistor/inductor/capacitor network  173 . The output of chroma bandpass filter  173  is supplied to a non-inverting input of an amplifier  174 . Amplifier  174  is set at a gain which determines the coring depth. For instance if amplifier  174  is set for a gain of five via resistors  175 ,  177  coupled to its inverting input, the coring depth is about 3% to 3.5%. If the amplifier  174  is set for a gain of seven, the coring depth is about 2% to 2.5%. Diodes  179  limit the amplifier  174  output to about 1.4 volts peak-to-peak, to insure that the output of amplifier  174  does not reverse breakdown Q 1  and Q 2  base emitter junctions of a limiting amplifier  176 . The differential amplifier circuit of Q 1  and Q 2  is used specifically herein as the limiting amplifier  176 , with limiting occurring when the voltage at the base of Q 1  exceeds about 100 millivolts positive or negative. The inverting output of the amplifier  176  is provided at a collector of Q 1  via a resistor  181  and an adjustable resistor  183 . A gain of minus 1 is derived via the base of Q 1  to the slider of resistor  183  at signals less than 100 millivolts peak into the base of Q 1 . Amplifier  180  and amplifier  182  form a summing amplifier, with a resistor  185  coupled to the inverting input of amplifier  180  receiving about 1.4 volts of video signal (not including the sync signals) via a delay line formed of a resistor/inductor/capacitor network  187  and a buffer amplifier  186 . The delay line is necessary to match the delay in the chroma bandpass filter  173  and the limiting amplifier  176 , so that maximum nulling or coring can occur at about the chroma frequency. Because the limiting amplifier  176  outputs ⅕ or {fraction (1/7)} of a maximum of plus/minus 100 millivolts (200 millivolts peak-to-peak), the maximum subtraction of signals around the chroma frequency via the resistor  185  and a resistor  184  is 200 millivolts/5=40 millivolts (or 200 millivolts/7=28.5 millivolts). Thus the coring depth referenced to the video signal of 1400 millivolts is then {fraction (40/1400)} or approximately 3%, or {fraction (28.6/1400)} or approximately 2%. 
     It should be noted that any coring depth can be achieved by setting the gain of the amplifier  174 . Usually it is preferable to use the minimum coring depth needed for an acceptable stable chroma output, because excessive coring will cause an undesirable decrease of luminance detail along with a desirable reduction in luminance noise. If the Q of the chroma bandpass filter  173  is raised to greater than  2 , the coring depth can be increased without losing much, or as much, luminance detail, since coring in this case will be around a narrower band in the region of the chroma frequency. However, care must be taken to insure that the cored signal on an output lead  188 , and thus the output of the scrambler system, still is acceptably free of residual chroma instabilities. 
     FIG. 6 illustrates a further embodiment  190  of an adaptive chroma coring circuit employing an adaptive coring control circuit  209 . In this embodiment, the coring depth again is adjusted, as in FIG. 4, by sensing the chroma chanel&#39;s signal amplitude. As previously mentioned, the coring circuit of FIG. 6, whether adaptive or fixed, may be inserted after the comb filter and prior to a respective time shift element. The FIG. 6 embodiment further is applicable when the invention is used to increase luminance-chrominance separation for comb filters used in television sets, video recorders, etc. 
     FIG. 6 includes components similar to those of FIG. 5 except that a limiting amplifier  204  in FIG. 6, employs transistors Q 1 , Q 2 , Q 3  and Q 4  to form a compound feedback differential amplifier. The gain in limiting amplifier  204  is substantially independent of emitter tail current via the collector current of a transistor Q 5  of transistor pair Q 5 , Q 6 , but the output clipping level is proportional to the emitter tail current via the Q 5  collector current. The forming of a very high transconductance amplifier via Q 1 , Q 2 , Q 3 , Q 4  and resistors  197 ,  199 , and local feedback resistors  193 ,  195 , provides an overall transconductance for the amplifier of about 1/resistor  193 . Note that resistors  193 ,  195  preferably are of equal resistance. The peak clipping output level at resistor  197  and/or resistor  199  is proportional to the emitter tail current (Q 5  collector current) multiplied by the resistance of resistor  197 . Resistors  197 ,  199  are output load resistors for the amplifier  204 , whose outputs herein are fed to (an optional) differential amplifier  200 , which rejects the chroma channel&#39;s envelope signal supplied via the output of an amplifier  212  in the adaptive coring control circuit  209 . An adjustable resistor  201  is adjusted for the maximum of coring (subtraction) with the output of the limiting amplifier  204 . A summing amplifier  202  is coupled to the adjustable resistor  201  and to a delay line  211  similar to the delay lines of FIGS. 3-5. The output of the amplifier  202  on an output lead  214  then is the luminance signal with minimized unstable chroma. 
     As in FIG. 4, the adaptive coring control circuit  209  of FIG. 6 includes an amplifier  208  which amplifies the chroma channel signal on a lead  206  and then supplies the signal to a full wave rectifier  210  (or an envelope detector). The output of the full wave rectifier or envelope detector is smoothed via a charge capacitor  203 , which is discharged via a resistor  205 . An amplifier  212  outputs a voltage proportional to the chroma signal amplitude. Additionally, a DC offset is supplied at  213  to amplifier  212  to bias a voltage to current convertor circuit  215  formed of a bias resistor  207  and the transistors Q 3  and Q 4 . The Q 3  collector current then is proportional to the chroma signal amplitude. The higher the color saturation, the higher the Q 3  collector current. In turn, a higher clipping level is supplied by the limiting amplifier  204 , which raises the amount of chroma frequency coring. If there is no color in the program video signal, then the Q 3  collector current approaches zero and the clipping level at resistor  197  or  199  is near zero (for near zero output) and thus there is zero, or very little, coring performed on the luminance channel signal. 
     FIG. 7 illustrates an embodiment  220  of a multiple band frequency coring system, where coring is done at the chroma frequency via a bandpass filter  226  in a first coring path, and at other frequencies for further noise reduction of the luminance channel signal via a high pass filter  228  in a second parallel coring path. FIG. 7 also illustrates the alternative of adaptive coring on the residual chroma frequencies in the first coring path by means of an adaptive coring control circuit  255 , while providing fixed coring at other frequencies by utilizing the high pass filter  228  in the second parallel coring path. It should be noted that the high pass filter  228  may include a notch filter set at the frequency of the bandpass filter  226  so there is no phase interaction at summing circuits  238  and/or  240 . 
     The additional components  224 ,  230 ,  234  are similar to respective components of the previous FIGS. 3-6, and the components  231 ,  236  in the second parallel coring path of the high pass filter  228  are similar to the respective components in the first coring path of the bandpass filter  226 . Likewise, a delay line  242  is similar to the delay lines  113 ,  128 ,  187 - 186 ,  191 - 204 , respectively, of FIGS. 3-6. In similar fashion, the adaptive coring control circuit formed of the components  246 ,  248 ,  250  and  249 - 251  also are similar to respective components of the adaptive coring circuits of FIGS. 4 and 6. 
     It should be reiterated that this invention can be used to enhance video comb filter performance in television sets. Video tape recorders can utilize the concepts to improve their performance via noise reduction and increased luma-chroma separation. 
     Although the invention has been described herein relative to specific embodiments, various additional features and advantages will be apparent from the description and drawings, and thus the scope of the invention is defined by the following claims and their equivalents.