DE3342530A1 - ARRANGEMENT AND METHOD FOR COMB FILTERING TELEVISION SIGNALS ON FULL-SCREEN BASE - Google Patents

Description

BCi 72 881 Ks / Ri

UeS. Serial Nos. 444.381 / 444.724

Filed: November 26, 1982

RCl Corporation
New York, NY, V.St.vA

Arrangement and method for chimney filtering of television signals on a full-message basis

The invention relates to a full-frame based comb filter for separating the luminance component (Y) and the chrominance component (C) of a composite television signal, and more particularly relates to a device to correct signal distortion caused by a Comb filters of this type result when there is movement between successive television frames.

If an electrical signal is periodic in nature, then one can favorably emphasize its informational content, by creating individual copies of the signal that are spaced apart in time according to the repetition period the periodicity are apart, stores and then combined the saved counterparts. The usual television broadcasting systems (and also most video recording and playback systems) are for example designed in such a way that a large part of the brightness information (luminance) contained in an image is determined by signal frequencies is represented, which is concentrated by integer multiples of the horizontal deflection frequency (line frequency) are. The color information (chromaticity) is in a part of the

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Luminance signal spectrum in the vicinity of frequencies inserted between the multiples of the line i / frequency (i.e. with odd multiples of half the line frequency).

A separation of the chromaticity and luminance a formation and The detailed information responsible for the subtleties of the image can be improved by suitable comb filtering of the spectrum of the composite television signal is possible. Known comb filters take advantage of the fact that because of the above-mentioned special relationship (odd multiples) between the frequencies of the components of the chrominance signal and at half the line frequency, the chrominance components for corresponding image areas in successive lines are 180 out of phase with one another, while the components of the luminance signal for corresponding image areas in successive lines have essentially the same phase. In the first case one speaks of frequency-wise "nested" components, iro the latter case of "non-nested" components.

In a comb filter, one or more subsections of the composite image signal generated mutually are delayed by at least one period of the horizontal or line deflection frequency (so-called 1H delay) and thus represent similar images or reproductions of the signal. The signals from a line are marked with Signals from a previous line added, whereby the interleaved frequency components (e.g. the chrominance components) cancel each other out, while the non-interleaved frequency components (e.g. the luminance components) reinforce each other. By subtracting the signals from two consecutive lines (e.g. by Inxrertation of the signals from one line and then

7.ς send addition with the signals from the other line) an erasure of the non-nested components and a mutual reinforcement of the nested com-

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components. In this way, the luminance and chrominance signals can be mutually combed out and thereby separated from each other be separated.

The combing out described above is possible because the signals of a typical television scene naturally come from Are quite similar row to row, i.e. have a high level of redundancy. Returns the composite television signal a still picture again, then a signal is that a certain pixel of this scene of full picture to full screen would have a redundancy of practically equal to $ 100. If in this case you can use the comb filtering Would perform full image basis, then the remaining portion of the luminance signal would be in the comb-filtered chrominance signal significantly less than with comb filtering carried out on a line basis. The remainder would be similar the chrominance signal in the comb-filtered luminance signal is significantly smaller than in the case of a comb filterunp; on a row basis.

In detail, the comb-filtered chrominance signal contains z.E. no more accompanying information about vertical image refinements. The chrominance signal of a composite NTSC signal is synchronized in such a way that its phase jumps by 180 ° from frame to frame. Therefore, the luminance component can be extract by extracting sections of the composite signal that represent successive frames, added linearly to each other. By linear subtraction of consecutive The chrominance component can use full frames extracted.

The frequency response curve of a comb filter working on a line basis has zeros or "teeth" at intervals accordingly the line frequency (15734 KHz for the NTSC system), while a comb filter working on a frame basis, teeth are spaced apart according to the frame rate (30 Hz) has. The closer spacing of teeth or zeros

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"in the voir-image-wise comb filtering leads to the Comb filter effect is more complete in all spatial directions of the image than with line-by-line comb filtering.

With full-screen comb filtering, image notges, which do not stand still lead to distortions in the reproduced images. These distortions are due to the fact that in image areas where there is movement, the extinction of the luminance components in the comb-filtered chrominance signal and the chrominance components in the comb-filtered luminance signal is not complete and also an effective one Reduction of the bandwidth of the luminance signal occurs. There is then a visible movement of the scene to speak if as a result of object movements or Pans result in differences in the image content within a time of 1/30 seconds or less.

Those caused by motion with full-frame comb filtering Distortion is two-dimensional and entirely due to scene movement going from one to the other Take place in full screen. The distortions are in both the horizontal and vertical directions Image level visible and express themselves as double or ghost images in the reproduced scenes. The double vision have a spatial distance from one another, which corresponds to the speed of movement, and can also be accompanied by a wrong hue on the edges of moving objects.

In contrast, the line-based comb filters produce one-dimensional distortions that are caused by the The presence of vertical structures within a scene can be caused even when the scene is stationary. the Distortions that occur with line-by-line comb filtering appear as an effective reduction in bandwidth the person responsible for the vertical scene structure Signal and make vertical transitions in the picture blurred. _ 9 _

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The object of the invention is to reduce the disruptive effects of image movement on a frame-by-frame basis working comb filtering to decrease. An inventive Device and a method according to the invention for solving this problem are set out in the claims 1 and 8. Advantageous embodiments of the invention are characterized in the individual subclaims.

Contains a comb filter for television signals according to the invention delay means for delaying the television signal by one picture period. A first combining circuit combines the television signal with the delayed one Television signal for forming the sum of the two signals, which corresponds to a first component of the television signal. A second combining circuit combines the television signal and the delayed television signal to form the Difference between the two signals, which corresponds to a second component of the television signal. With the second unification circuit the input of a phase linear low pass filter is connected to the second component of the television signal to recieve. The low pass filter normally suppresses this from the second component of the television signal occupied frequency spectrum. With the first combining circuit and with the output of the low-pass filter is a third merging circuit connected which the first component of the television signal combined with the low-pass filtered signal to eliminate those errors of the first Correct components that occur as a result of motion from picture to picture.

In a particular embodiment of the invention, a movement detector is connected to the output of the low-pass filter connected, which is responsive to the low-pass filtered signal, to detect the presence of a picture of picture taking place To capture movement. The motion detector generates at Presence of such movement a control signal.

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-ΙΟΙ A fourth merging circuit that has a first and has a second input and an output, receives at its first input the second component of the television signal s from the second merging circuit. A switching device is also connected to the motion detector, responsive to the control signal for selectively switching the first component of the television signal to the second To input the fourth merging circuit when motion from picture to picture is detected.

According to an advantageous embodiment of the invention, it is in series with the output of the fourth combining circuit connected to a phase linear band filter. This bandpass filter is designed so that it only uses the frequency spectrum of the chrominance signal lets through.

The invention is explained in more detail below using an exemplary embodiment with reference to drawings.

Fig. 1 shows the block diagram of a frame based comb filter as used in the art is known in television signal processing;

Fig. 2 shows with waveforms a) to d) the time course of the amplitude of certain signals in the Circuit according to FIG. 1, while waveform e) for a modification of the circuit according to FIG is applicable;

7,0 Figures Ja to 3c show the structure of a in block form comb filter operating on a frame basis according to the invention.

The arrangement shown in Fig. 1 is on full screen state-of-the-art comb filter. This comb filter is in connection with an operating mode explains where the luminance and chrominance

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sign all e from a composite television signal (television signal mixed.) separates. However, the arrangement is not limited to this particular application but can e.g. can also be used to comb filter a luminance signal, in order to include a chrominance signal in it to form a television signal.

The arrangement of FIG. 1 contains a memory device 10 with a capacity for storing the electrical representation of a television frame (e.g. two fields of the NTSC format). The memory device 10 receives an input signal and sequentially at the input terminal 11 outputs the same signal delayed by the duration of one frame period to the output terminal 14-. The establishment can be a digital memory, e.g. a memory with direct access and the associated auxiliary electronics, or an analog memory such as a serial charge transfer device (CTD) such as a charge coupled device (CCD) with the necessary auxiliary circuits.

In principle it does not matter whether the device is a digital or an analog circuit; in the The following assume that it is a digital circuit is. Accordingly, it is assumed that the other associated circuits are also digital in nature in order not to need any digital / analog conversion between the circuits.

The input signal from the terminal 11 and the delayed signal from the terminal 14- are in an adder circuit

^ O 12 summed together. The luminance components of the Signals that are in phase with each other add up, while the chrominance components of both signals, which are phase-shifted by 180 ° to each other, cancel each other out, so that a luminance signal at connection 15 Y appears which is essentially free of chrominance components. The input signal and the delayed signal become also given to a subtracting circuit 13, wherein

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the chrominance components of the two full images lied to one another sum building up, while the luminance components cancel each other out, so that at terminal 16 a A chrominance signal C appears which is essentially free of luminance components (at least if the signals consecutive frames are representations of the same still scene).

FIG. 2 graphically illustrates the effects of a "movement of the scene" on the one described above has full frame comb filtering. 2a shows part of the composite signal fed to terminal 11 in an arbitrarily chosen time interval. Fig. 2b shows the composite video signal of the same scene exactly a frame period later and after there has been movement in the scene. Assume that the Signal according to FIG. 2a, in its part lying between times T1 and T3, an object with a luminance signal parameter L2 and a chrominance signal parameter C2. Also assume that this object is in a uniform environment with a luminance signal parameter L1 and a chrominance signal parameter C1.

A spatial movement of the object takes place between the full images, which means that the object represented Shifts part of the video signal in time such that it comes to lie between the times T2 and T4-, as it does Fig. 2b shows. Although from FIG. 2a to FIG. 2b there is a time shift of the shown by L2 and C2 Object takes place, the relationships of the various luminance and chrominance signal parameters L and 0 to one another remain same. The phase of the chrominance component of the signal, however, is between Figures 2a and 2b (i.e. between the full images) shifted by 180 °.

The sum of the signals according to FIGS. 2a and 2b is shown in FIG. 2c, and the difference between the two signals

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Is shown in Fig «, 2d. It can be seen that for those periods of time in which the chrominance component of both Signals 2a and 2b have the same absolute value, practically complete cancellation of the chrominance component in the sum signal (Fig. 2c) takes place. In a similar way, practically complete cancellation takes place in the differential signal (FIG. 2d) the luminance component for those periods of time in which the amplitude of the luminance component is the same in both signals 2a and 2b. Me Fig. 2a shows that at the connection 11 and FIG. 2b that at the connection 14 of the arrangement according to FIG. 1, while FIG. 2c shows that at connection 15 and FIG. 2d shows that at connection 16 represents the signal appearing.

During the times in which a movement takes place between frames, ie between times T1 and T2 and between times T3 and TM-, the differentially combining signals are unequal, so that the respectively undesired components are incompletely extinguished .

In addition, the desired signal is distorted by the movement. The movement causes a Kind of averaging of the signal in the moving sections takes place. These mean value sections are shown in Fig. 2c is denoted by (L1 + L2), which is equivalent to the expression (2L1 + 2L2) / 2. Usually the signal comes from the both full images -weighted with a factor 1/2 before being combined. The amplitude of the comb filtered signal is equal to half of the values given in the figure, and the signal during the movement sections is the same (L1 + L2) / 2. The mean value sections (L1 + L2) in the luminance signal have the consequence that signal transitions take place in two stages creating a double or ghost image that surrounds the moving object. The mean values (C1 + C2), which occur in the comb-filtered chrominance signal during the sections moved from frame to frame, falsify

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- 14 the hue at the edges of the moving object.

In the Pig. 1 at connection 16 Provides a low-pass filter, then the comb-filtered chrominance signal according to FIG. 2d becomes the signal shown in FIG. 2e. In the signal according to Fig. 2e, the spectrum of the chrominance signal is removed, so that only that luminance component appears, which was not extinguished from the chrominance signal as a result of the movement. If the signal of Fig. 2e added or subtracted from the signal according to FIG. 2c, then the edges of the luminance signal are again produced, and a luminance signal is produced which is equivalent to a non-comb-filtered luminance signal is. Adding the signal according to FIG. 2e brings the luminance signal according to FIG. 2c between the points in time T1 and T2 to level 2L2 and reduces the luminance signal between times T3 and T4 to the level 2L1, as illustrated by the dashed lines.

The circuitry of Figure 3a is on a frame basis working comb filter, which is constructed according to the invention and distortions such as double images or edge smearing corrected which by. Movement can occur between consecutive frames. In the Fig. 3a are parts which have the same function as parts fulfilled in Fig. 1, denoted by the same reference numerals as there. At connections 15 and 16 of the circuit after. 3 a also appear comb-filtered Luminance and chrominance signals, each of which is due to both frame-to-frame movement undeleted residual components of the other signal (cross components) as well as incorrectly comb-filtered signal components contains.

Comb-filtered chrominance signal appearing at connection 16 is applied to a phase-linear low-pass filter 27 in order to remove the chrominance signal spectrum and

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deleted to leave low-frequency luminance components. The low-pass filtered signal from the filter 27 is, in an adding circuit 31, again with that at the connection 15 appearing comb-filtered luminance signal added back, to a corrected luminance signal at connection 32 T to deliver. At terminal 32, the corrected luminance signal according to the principles of the present Invention developed. The remaining unquenched chrominance component in that coming from port 15 Comb-filtered luminance signal causes only a second order distortion, which is normally not visible unless you look closely at the reproduced image.

The low-pass filter 27 must be phase linear so that the movement detail is correctly inserted into the comb-filtered luminance signal. This is because this insertion takes place. then not correct if there are phase differences between the movement-affected sections of the comb-filtered Luminance signal and the movement detail extracted from the comb-filtered chrominance signal. In this In this case, the transient response of the comb filter is impaired.

The remaining chrominance components (C2-C1, C1-C2) that result from movement from frame to frame contained in the luminance signal do not cause any significant distortion in the reproduced image. on the other hand however, the color beard mean value (C1 + C2), which is in the Comb-filtered chrominance signal appears to be interfering temporarily during the moving sections of this signal Color adulteration. The chrominance signal is a vector quantity. Thus, the vector sum can be C2 + C1 during the moving sections Result in colors completely different from those represented by each of signals C1 and C2 Colours. In order to maintain the color purity at the edges of moving objects, it is often desirable to use the

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Comb filtering through. Movement arising errors also the Correct chrominance component.

The information necessary to correct the comb-filtered chrominance signal is in the comb-filtered luminance signal contain. Re-insertion of this information can be done in a manner similar to restoration of the movement details in the comb-filtered luminance signal. However, the restoration of the chrominance component should do not take place in a continuous process but only during movements that are taking place. The reason for this is the relatively high energy density of the luminance signal compared to the cross component of the chrominance signal. One Continuous addition of the comb-filtered luminance signal with the comb-filtered chrominance signal would result in comb filtering practically nullify. However, if you have that the comb-filtered luminance signal is added to the comb-filtered chrominance signal during periods of movement, then you get a better signal than without chromaticity reproduction.

Normally it is necessary to bandpass filter the comb filtered chrominance signal in order to remove the rest of the chrominance signal To eliminate luminance signal; that is, the luminance motion detail associated with the comb filtered chrominance signal added comb-filtered luminance component is essentially derived from the corrected chrominance signal eliminated. In the corrected chrominance signal, only those luminance components remain that are in the frequency

"0 band of the chrominance signal occur. The energy of the luminance however, sign than in this volume is relatively low.

The luminance signal is composed of components of the chrominance signal, ie, red, blue and green signals, and as ^3-r> her shows the amplitude of the luminance signal, the amplitude of the chrominance signal on. If the signal coming from the low-pass filter 27 exceeds a predetermined value, then

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this indicates the presence of movement, and the extent of this overstepping is a measure of the amplitude of the chromatic aberrations. By measuring the amplitude of the vom
The luminance signal passed through the low-pass filter 27 can thus be determined as to whether there are chrominance signal errors which are large enough to cause disruptive distortions in the reproduced image.

In the arrangement according to FIG. 3a, the output of the low-pass filter is connected 27, a motion detector 28 is connected, which is the low-pass filtered version of the comb-filtered chrominance signal compares to a threshold. The threshold is set so that it corresponds to a signal level from which disruptive chrominance distortions occur. When the low-pass filtered signal from filter 27 exceeds this threshold value exceeds, the motion detector 28 generates a control signal which closes the switch 29.

Figures 3b and 3c show examples of an analog and a digital motion detector. The analog embodiment 28 'consists of a differential amplifier 40 with a high gain factor and a reference voltage source 41. The signal coming from the filter 27 becomes a
applied to the non-inverting input 42 of the amplifier 40, and the signal supplied by the reference voltage source 41 is applied to the inverting input of the amplifier 40. If the voltage at input 42 exceeds the reference voltage, then the output of amplifier 40 goes high, otherwise it stays low. A field effect transistor 29 'of the N conductivity type, which can take over the function of the switch 29 in FIG. 3a, is coupled to the output of the amplifier 40. With high potential
at the gate electrode G of the transistor 29 'the switch is closed, and when the potential is low it is open.

The digital embodiment 28 "of the motion detector
according to Fig. 3c consists of a binary reference value transmitter

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/] 4-5, a subtracter circuit 4-3 (Vfergleicher), a Sign detector 44 and an AND element 29 "as a switch. (In the drawing, the Connections represent parallel bit lines.) The subtracting circuit 4-3 subtracts that consisting of a binary number Reference value (output variable of the encoder 4-5) from the binary signal delivered at the output 4-2 of the filter 27. The resulting Difference value is determined by the pre eh. En detector 4-4 checked to see if it is positive or negative.

If the difference value is positive, the sign detector delivers a positive output pulse, which activates the AND gate 29 ″ to the signal at its data input to let through. The subtraction circuit works with Two's complement arithmetic, then the sign detector can be omitted and replaced by an inverter circuit directly between the terminal of the output lines leading the most significant bit (sign bit) the subtraction circuit and the AND circuit is inserted.

The circuits according to Figures 3b and 3c only speak on input signals of one polarity "One can, however, itfie known per se to connect two such circuits in parallel, to get a response to signals of both polarities. Another alternative is as a Comparison device to use a so-called "window comparator".

The switch 29 couples the comb-filtered luminance signal from the connection 15 optionally to an input of a Adder circuit 30. A second input to this circuit receives the comb filtered chrominance signal directly from the terminal 16. When movement detail in excess of the threshold is felt, switch 29 closes, and the comb-filtered chrominance and luminance components are added to one another in the adding circuit 30. If no detail of movement is felt, switch 29 remains open, and the comb filtered chrominance signal from

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Terminal 16 is used without change by the adder circuit let through.

The output signal of the adder circuit 30 is applied to a phase-linear bandpass filter 34- which only has the Frequency band of the chrominance signal at its output terminal 35 passes.

The circuits 31 and 30 can be either adding or subtracting circuits, as is indicated by the sign symbol ~ at their inputs in FIG. 3a. However, the two circuits must be of the same type, that is, both must be adding circuits or both must be subtracting circuits. Whether adding or subtracting circuits are to be selected depends on which of the two motion-distorted sections of the signal (the front or the rear section) is to be restored / restored and which is to be deleted.

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Claims (10)

1. Comb filter for processing television signals, with a delay device for delaying the television signal by one picture period, further comprising a first combining circuit which converts the television signal and the delayed television signal coming from the delay device combined in such a way that the corresponding to a first component of the television signal Sum of these two signals is obtained, and with a second combining circuit that the television signal and the delayed television signal coming from the delay device is combined with one another in such a way that that the difference between these two signals corresponding to a second component of the television signal is obtained is indicated by a - ρ - BATH ORIGINAL linear phase low-pass filter (27) connected to the second Combination circuit (13) for receiving the second Component of the television signal is connected and that normally occupied by the second component Frequency spectrum essentially suppressed, and one third merging circuit (31) associated with the first Combining circuit (12) and coupled to the output of the low-pass filter to produce the first component of the television signal with the low-pass filtered signal and thereby the first component of the To free the television signal from those errors caused by a movement taking place from picture to picture will. 2. Comb filter according to claim 1, characterized by: a motion detector (28) which is connected to the output of the low-pass filter (27) and is responsive to the low-pass filtered signal in order to detect the presence of scene movement occurring from frame to frame and to generate a corresponding control signal in the event of such movement; a fourth merging circuit (30) one connected to the second merging circuit (13) has a first input for receiving the second component of the television signal and a second input and one Has output; a switching device (29) which is coupled to the motion detector and responds to the control signal, in order to selectively the first component upon detection of a scene movement taking place from image to image of the television signal to the second input of the fourth combining circuit. 3. Comb filter according to claim 2, characterized in that the television signal is a composite signal, wherein a Luminance signal forms the first component (Y) and a chrominance signal forms the second component (C), and «Απ that the delay device has a memory device (10) for storing at least one frame of the television signal. 4-, comb filter according to claim. 3 _? characterized in that the output of the fourth combining circuit (30) is followed in series by a base-linear bandpass filter (34-) which only lets through the frequency spectrum of the chrominance signal. 5. comb filter according to claim 2, characterized in that the motion detector the low-pass filter (2?) and one Encoder for a reference value (4-1; 4-5) includes and one Comparison device (28 '; 28 ") contains the from Compares the signal passed through the low-pass filter with the reference value and always generates the control signal if the sign passed by the low-pass filter :. . differs from the reference value by a predetermined amount. 6. comb filter according to claim 2, characterized in that the third merging circuit (31) and / or the fourth Combining circuit (30) is a circuit for adding signals. 7-comb filter according to claim 2, characterized in that the third merging circuit (31) and / or the fourth merging circuit (30) a circuit for subtracting of signals is. 8. Method for comb filtering television signals below Use of the comb filter according to Claim 1 for separating chromaticity and luminance components, characterized by the following steps: Delay the television signal by the duration of one Full screen; subtractive union of the delayed and the undelayed Television signal to generate a comb BATH ORIGINAL filtered chrominance signal; additive union of the delayed and the undelayed Television signal for generating a comb-filtered luminance signal; pbase-linear low-pass filtering of the comb-filtered Chrominance signal to obtain the chrominance components from this Substantially remove signal; Combination of the low-pass filtered version of the comb-filtered chrominance signal with the comb-filtered Luminance signal during periods in which expresses a motion taking place from frame to frame to produce a comb-filtered luminance signal that is substantially free of the distortion caused by frame-to-frame Scene movement. 9. The method according to claim 1, characterized in that the pre-show is one from frame to frame scene movement taking place is felt and in the case of such movement a control signal is generated and that the comb-filtered chrominance signal when the control signal appears with the comb-filtered luminance signal is united. 10. The method according to claim 9, characterized in that the combined comb-filtered chromaticity and luminance signals be subjected to a bandpass filtering in which only the chrominance signal spectrum is allowed to pass will. - 5 - BATH ORIGINAL