CA1323686C - Apparatus for coding a digital component video signal - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

Apparatus for coding a digital component video signal, comprising first means for modulating a first carrier signal with one of the component chrominance signals, the first carrier signal having a four-field sequence such that the phase of the first carrier signal is inverted at every line intervals, second means for modulating a second carrier signal with the other of the component chrominance signals, the second carrier signal having a two-field sequence such that the phase of the second carrier signal is inverted at every field intervals, and means for combining a luminance signal with the modulated chrominance signals to thereby reduce the bandwidth of the combined component video signal.

Description

IITLE OF THE INVENTION 1 ~ h ~

APPARATUS FOR CODING A DIGITAL COMPONENT
VIDEO SIGNAL

BACKGROUND OF THE INVENTION

Field of the Invention The present invention relates to an apparatus for coding a digital component video signal.

Description of the Prior Art In the CCIR601 standard coding scheme (4:2:2), out of a total bandwidth of 13.5 MHz, the frequency band 6.75 MHz is allotted to the luminance component and the remaining frequency band 3.375 MHz is allotted to each of the chrominance components. In order to reduce the total bandwidth to half, it was proposed to halve the allotted bandwidths to the lurninance component and chrominance components. Although the chrominance component thus reduced in bandwidth can still keep an adequate quality, the luminance component suffers from lack of bandwidth because of the maximum frequency 3.375 MHz. To achieve quality reproduction, it needs S MHz at least.

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, : , '- . :' SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus for coding a digital component video signal capable of reducing the transmission bandwidtll to half snd yet capable of securing a sufficient bandwidth for the luminance signal.
According to the present invention an apparatus is provided for coding a digital component video signal which includes a luminance signal and a first and a second chrominance component signal modulated by a carrier signal llaving a frequency lligher than the frequency spectrum range of the luminance signal, in which the first chrominance component signal has a four-field sequence such that its phase is inverted at every line intervals and the second chrominance component signal has a two-field sequence such that its phase is inverted at every field intervals.
The luminance component and the modulated chrominance component signal are arranged in frequency interleaved relation. Therefore these signals can be separated by a comb filter. Since the carrier signal for the chrominance component is of a frequency outside the range of the bandwidtll of the luminance signal the separation of the luminance component and the chrominance component is very easy. The two chrominance components can be separated time-wise.
The first and second color difference components are both in interleaved relation with the luminance component. Further, since the first color difference component and the second color difference component are made different as to modulation phases such that the one has a four-field sequence and the other has a two-field sequence, these components can be also separated by mea~s of a comb i ilter. As the comb filter, either of the h,vo kinds of comb filters, one utilizing correlation in the directions of both field and line and the other utilizing correlation in the direction of line, can be used. Hence, it is possible to selectively switch these comb filters from one to the other in response to abrupt change in color.

BRIEF DESCRIPIlON OF THE DRAWINGS
FIG. 1 is a frequency spectrum chart showing necessary bandwidth for each of the lurninance component and chrominance component of a component video signal;
FIG. 2 is a frequency spectrum chart sho~,ving a composite signal which the luminance component and chrominance component are combined;

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~ ~ ~ 3 ~ ~ 0 FIG. 3 is a block diagram of an encoder in an embodiment of the present invention;
FIG. 4 are schematic diagrams showing modulation phases of chrominance components;
~ IG. 5 are schematic diagrams showing a three dimensional representation and its three projections of a digital video signal;
FIG. 6 is a v-t projection showing positions of each components in a coded digital video signal;
~ IG. 7 are schematic diagrams showing frequency response and filter factor of comb filters for separating each component;
FIG. 8 are schematic diagrams showing frequency response of comb filters for separating two color difference components;
FIG. 9 is a block diagram showing an adaptive decoder;
FIG. 10 is a block diagram of a select filter for use in the adaptive decoder;
~ IG. 11 is a schematic diagram useful for explanation of operation of a select filter; and FIG. 12 are schematic diagrarns showing frequency response of the select filter.

~r ,3 DESCRI~ION OF TH~ PREFERRED EMBODIMENT
In a preferred embodiment of the present invention, a digital component video signal of (4:2:2) form is converted into a composite signal of (4:0:0) form. As the carrier signal used for modulation of the chrominance component signals is used a signal having a frequency outside the range of the bandwidth of the chrominance component.
FIG. 1 are charts showing necessary bandv~idths for the luminance component signal and the chrominance component signals in a component video signal. The luminance component Y requires a bandwidth with an upper limit frequency of 6.75 MHz, ~vhile the chrominance components C
require a bandwidth with an upper limit frequency of 3.375 MHz.
FIG. 2 shows a frequency spectrum of a composite signal obtained by combination of the aforesaid luminance component Y and the chrominance components C. The two chrominance components are modulated with the carrier signal having a frequency of approximately 6.75 MHz. The spectral components of the luminance component and the modulated chrominance components are arranged in interleaved relation whereby unwanted interference is minimized. The interleaving relationship is similar to that practiced in NTSC system.

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, Fundamental difference between the coding scheme shown in ~IG.

2 and that of the PAL or NTSC system is that the carrier signal for the chrominance components lies outside the range of the bandwidth of the luminance component. Thereby, it is easy to separate the luminallce component from the chrominance components.
FIG. 3 is a block diagram of an encoder in an embodiment of the present invention. Referring to FIG. 3, an input terminal denoted by 1 is supplied with a luminance component Y, an input terminal denoted by 2 is supplied with a color difference component CB being (B - Y) component, and an input terminal denoted by 3 is supplied with a color difference component CR being (R - Y) component.
The luminance component Y is supplied through a scaling circuit 4 to an adder circuit 5. The color difference component CB is supplied through a scaling and modulating circuit 6 to an adder circuit 8. The color difference component CR is supplied through a scaling and modulating circuit 7 to the adder circuit 8. The output signal of the adder circuit 8 is supplied to the adder circuit 5, wherefrom an output terminal 11 is led out.
The scaling circuit is used for suppressing increase in amplitude of the output signal as a result of the adding process. The scaling and modulating circuit 6 is supplied with a carrier signal from a terminal 9, the . .

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phase of which is inverted from frame to frame and has a two-field sequence of phase inversions. Likewise, the scaling and modulating circuit 7 is supplied with a carrier signal from a terminal 10, the phase of which is not changed, whereby the modulated carrier signal has a four-field sequence of phase inversions.
The color difEerence components CB and CR modulate these carrier signals, whereby the polarities of the modulated color difference components with respect to the luminance signal will be such one as shown in ~IG. 4A and FIG. 4B. In ~lG. 4, sign + indicates (Y + C) and sign -indicates (Y - C).
As seen from FIG. 4A and FIG. 4B, the carrier signal for the chrominance component CB is a signal of a two-field sequence inverted at every field intervals. And, the carrier signal for the chrominance component CR is a signal of a four-field sequence inverted at every line intervals.
The most general way to show interrelations of the three components Y, CB, and CR utilizes a three-dimensional representation of Nyquist limit of a digital video signal as shown in FIG. SA and its three . . .

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projections as shown in F~G. 5B. The three axes are vertical axis (V), hoAzontal axis (h), and time axis (t). The broken lines ~ fc virtually are coincident with the positions of the coior subcarriers in the NTSC system.
As far as the above described video coding scheme is concerned, the region of bandwidth between +fc and -fc includes only the luminance component Y and the regions between +fc and hoAzontal Nyquist frequency +fN and between -fc and -fN include the three components.
FIG. 6 is a projection on the v-t plane showing positions of the luminance component and chrominance components of the digital video signal in the vicinity of the Nyquist frequencies. The luminance component Y gathers around the origin 0, and the chrominance component CB converges on the regions of corners, while the chrominance component CR converges at the intermediates of the oblique lines.
The separation of the components Y, CB, and Cr as shown in FIG.
6 is performed by comb filters. FIG. 7 shows examples of frequency response and simple filter factor of the comb filters separating each component.
FIG. 7A shows the frequency response and filter factor of the comb filter for separating the luminance component Y. The component video '~

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and FIG. 4B is applied to the comb filter having the filter factor in FIG.
7A, the color difference components CB and CR are cancelled and only the luminance component Y is separated therefrom. ~G. 7B shows the frequency response and filter factor of the comb filter for separating the color difference component CB. By the use of this comb filter, the luminance component Y and the color difference component CR are cancelled and only the color difference component CB is separated therefrom. FIG. 7C shows the frequency response and filter factor of the comb filter for separating the color difference component CR.
The two color difference components can also be separated by utilizing the fact that the carrier signal for the color difference component CB is not inverted at every line intervals, while the carrier signal for the color difference cornponent ClR is inverted at every line intervals. ~G.
8A and 8B show comb filters only for vertical direction (line comb filters).
FIG. 8A shows frequency response of the comb filter for separating color difference component CB, while FIG. 8B shows frequency response of the comb filter for separating color difference component CR. In ~IG. 7 and FIG. 8 the hatched portions indicate the regions providing large output signals (passing regions). The line comb filters shown in FIG. 8 may contain a certain amount of cross color due to high-frequency component of the luminance signal. However, different from the comb filters shov,1n in FIG. 7, there is an advantage that the filter need not use data of other fields.
Since the two types of the comb filters are available as described above, an adaptive configuration of decoder can be considered which will use the comb filters as shown in FIG. 7 in the case where there exists a field or frame correlation, but will use the comb filters as shown in FIG. 8 in the case of no existence of field or frame correlation.
FIG. 9 shows an example configuration of such an adaptive decoder. Referring to FIG. 9 an input terminal denoted by 21 is supplied with a coded digital video signal which is for example, reproduced from a magnetic tape and performed such processes on the reproduction side as error correction or error concealment. The input coded signal is supplied to a digital low-pass filter 22 and a delay circuit 23. The delay circuit 23 has a delay amount equal to that of the digital low-pass filter 22. The luminance signal is obtained from the output signal of the low-pass filter 22. The output signals of the low-pass filter 22 and the delay circuit 23 are supplied to a subtractor circuit 24. As the output of the subtractor circuit 24 is obtained a high-pass component. The output signal of the low-pass ,Or~.
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filter 22 is supplied through a delay circuit 25 to an adder circuit 26.
The output terminal of the subtractor circuit 24 is connected with a cascade connection of delay circuits 27, 28, 29, 30, 31, and 32. The delay amount of the delay circuits 27, 29, 30, and 32 is one H (one horizontal period). The delay amount of the delay circuits 28 and 31 is 261H (525 system) or 311H (625 system). The output terminal of the subtractor circuit 24 and the output terminals of the delay circuit 27, 28, 29, 30, 31, and 32 are selectively connected with comb filters 40, 41, 42, 43, and 44, as specifically shown in FIG. 9.
T~e comb filter 40 is that for separating the luminance component Y having the characteristic as shown in ~IG. 7~ The comb filter 41 is that for separating the color difference component CB as shown n FIG.
7B. The comb filter 42 is the line comb i ilter for separating the color difference component CB having the characteristic as shown in FIG. 8A.
The comb filter 43 is that for separating the color difference component CR having the characteristic as shown in FIG. 7C. The comb filter 44 is the line comb filter for separating the color difference component CR
having the characteristic as shown in FIG. 8B. The output signals of these comb filters 40, 41, 42, 43, and 44 are supplied respectively, to PROMs 50, 51, 52, 53 and 54.

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The output signal of the PROM 50 (the high-frequency component of the luminance component Y) is supplied to an adder circuit 26 to be added therein with the low-frequency luminance component passed through the compensation delay circuit 25. The output signal of the adder circuit 26 is obtained at an output terminal 61 as a coded luminance component Y.
The output signals of the PROM 51 and PROM 52 are supplied to an adder circuit 55, and the output signals of the PROM 53 and PROM 54 are supplied to an adder circuit 56. The PROMs 50 - 54 are provided for giving desired switching characteristics when the ffve comb fflters 40 - 44 are changed in response to the ffeld correlation. One of the switching characteristics is a simple changeover switching and another is a more complicate switching characteristic such as cross fade. In order to control the svvitching characteristics of the PROMs 50 - 54, a select fflter 45, to be described later, is provided.
The output signal of the adder circuit 55 is supplied to a scaling and modulating circuit 57. From a terrninal 58 is supplied a carrier signal for demodulation. At an output terminal 62, a decoded color difference component CB is obtained. The output signal of the adder circuit 56 is supplied to a scaling and demodulating circuit 59. From a terminal 60 is "~
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- ' ' ' ' - -' , , ' ' ~ ' ` ' ' ' ~ : ' i3 supplied a carrier signal for demodulation. At an output terminal 63, a decoded color difference component CR is obtained.
FIG. 10 shows an example of the select filter 45. Referring to ~IG.
10, the input terminal denoted by 71 is connected with a cascade connection of delay circuits 72, 73, 74, and 75. The delay amount of tbe delay circuits 72 and 75 is one H and the delay amount of the delay circuits 73 and 74 is 262 H (525 system) or 312 H (625 system). The output signal of the delay circuit 72 and tbe output signal of the delay circuit 74 are supplied to a subtractor circuit 76. Tbe output signal of the subtractor circuit 76 is the output signal of a filter 1. Tbe output signal of tbe delay circuit.

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75 and the input signal are supplied to a subtractor circuit 77. The output signal of the subtractor circuit 77 is the output signal of a filter 2.
The output signal of the subtractor circuit 76 is supplied to an absolute value circuit 78 and the output signal of the absolute value circuit 78 is supplied to an adder circuit 80. Likewise, the output signal of the subtractor circuit 77 is supplied to an absolute value circuit 79 and the output signal of the absolute value circuit 79 is supplied to the adder circuit 80. From the adder circuit 80 is led out an output terminal 81.
FIG. 11 shows the data which are dealt with by the filter 1 and the filter 2 as the objects of processing. The filter 1 and the filter 2 each are for providing a differential between data at the same modulation phase included in each of the preceding and succeeding fields of the field for whicll decoding is being performed. The output signsls of these filter 1 and filter 2 indicate whether there is produced a sudden change in color by movement of a subject or the like.
When a sudden change in color is produced, the amplitude of each of the outputs of the filter 1 and the filter 2 becomes larger. In such a casej output signals of the ~ 3~j comb filters ~2, 44 are preferred to those of the comb filters 41, 43, and therefore, tlle select filter 45 generates a control signal for selecting such output signals.
FIG. 12~ and FIG. 12~ show the frequency responses of the filter 1 and the filter 2, respectively. In FIG. 12, the regions indicated by oblique lines are where output si~nals are generated at a high level. There is no need of using both tl-e filter 1 and the filter 2 but use of one of these is enough.
In fact, only the filter 1 is used in the adaptive decoder shown in FIG. 9.
According to the present invention, by coding a componeDt digital video signal into the form of a composite video signal, the bandwidth necessary for transmission can be reduced to half. Further, by appropriately devising the modulating systems for the first color difference component and the second color difference component, it is enabled to separate these two color difference components with the use of two types of comb filters, one utilizing both the interline and the interfield correlations and the other utilizing only the line correlation, and to thereby structure an adaptive decoder.

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Claims (5)

1. Apparatus for coding a digital component video signal, comprising:
first means for modulating a first carrier signal with one of the component chrominance signals, said first carrier signal having a four-field sequence such that the phase of the first carrier signal is inverted at every line intervals;
second means for modulating a second carrier signal with the other of the component chrominance signals, said second carrier signal having a two-field sequence such that the phase of the second carrier signal is inverted at every field intervals; and means for combining a luminance signal with the modulated chrominance signals to thereby reduce the bandwidth of the combined component video signal.

2. Apparatus according to claim 1, in which a frequency of said first and second carrier signals is selected to be higher than the frequency spectrum range of the luminance signal.

3. Apparatus according to claim 2, in which said one of the component chrominance signal comprises a blue difference signal (B - Y).

4. Apparatus according to claim 2, in which said other of the component chrominance signal comprises a red difference signal (R - Y).

5. Method for coding a digital component video signal, comprising the steps of:
modulating a first carrier signal with one of the component chrominance signals, said first carrier signal having a four-field sequence such that the phase of the first carrier signal is inverted at every line intervals;
modulating a second carrier signal with the other of the component chrominance signals, said second carrier signal having a two-field sequence such that the phase of the second carrier signal is inverted at every field intervals; and combining a luminance signal with the modulated chrominance signals to thereby reduce the bandwidth of the component video signal.