GB2176367A - Circuit arrangement for reducing cross modulation distortion in low frequency chrominance signal - Google Patents
Circuit arrangement for reducing cross modulation distortion in low frequency chrominance signal Download PDFInfo
- Publication number
- GB2176367A GB2176367A GB08612343A GB8612343A GB2176367A GB 2176367 A GB2176367 A GB 2176367A GB 08612343 A GB08612343 A GB 08612343A GB 8612343 A GB8612343 A GB 8612343A GB 2176367 A GB2176367 A GB 2176367A
- Authority
- GB
- United Kingdom
- Prior art keywords
- low frequency
- chrominance signal
- signal
- output signal
- frequency
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/79—Processing of colour television signals in connection with recording
- H04N9/7908—Suppression of interfering signals at the reproducing side, e.g. noise
- H04N9/7912—Suppression of interfering signals at the reproducing side, e.g. noise the interfering signals being intermodulation signals
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/79—Processing of colour television signals in connection with recording
- H04N9/80—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback
- H04N9/82—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback the individual colour picture signal components being recorded simultaneously only
- H04N9/83—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback the individual colour picture signal components being recorded simultaneously only the recorded chrominance signal occupying a frequency band under the frequency band of the recorded brightness signal
Abstract
In a low frequency chrominance signal processing circuit used in color - under heterodyne system of a video tape recorder, video disc player or the like, a comb filter 13 is employed for emphasizing a frequency component 2fs which is twice the chrominance subcarrier frequency in the low frequency chrominance signal. According to one embodiment, the output signal from the comb filter is fed via a limiter 14 to a subtractor 15 which is also responsive to the low frequency chrominance signal so that an output signal from the limiter 14 viz. low amplitude distortion is subtracted from the low frequency chrominance signal. In another embodiment (Fig. 3), an output signal of a delay circuit (11) included in the comb filter (13) is subtracted from the low frequency chrominance signal in a subtractor (16), while the output signal from the comb filter (13) is fed via a slicer (17) to an adder (19) which is also responsive to an output signal from the subtractor (16) to add these signals to each other. With this arrangement cross modulation distortion components, i.e. 2fs, are effectively reduced to prevent undesirable ringing in a reproduced picture. <IMAGE>
Description
SPECIFICATION
Circuit arrangement for reducing cross modulation distortion in low frequency chrominance signal
This invention relates generally to a circuit arrangement for processing a low frequency chrominance signal in a color-under heterodyne system used in video signal processing apparatus, such as a video tape recorder, video d3sc player or the like, and particularly to such circuit arrangement for reducing cross modulation distortion which occurs when a carrier chrominance is frequency converted into a low frequency chrominance signal.
According to standard recording/reproducing system of helical scan magnetic recording reproducing apparatus or video tape recorder a luminance signal and a carrier chrominance signal are separated from a composite color video signal of a standard system, such as NTSC system, PAL system or SECAM system, and then a modulated luminance signal obtained by frequency modulating the luminance signal, and a low frequency chrominance signal obtained by frequency converting the carrier chrominance signal are frequency multiplexed to be recorded on a magnetic tape and to be reproduced therefrom. This is so called color-under heterodyne multiplex system.In such a video tape recorder, a frequency converter is used for converting carrier chrominance signal cos(2nfct) of chrominance subcarrierfc into a signal given bycosf2n(fs + fc)t} (wherein fs < fc) or vice versa. Therefore, when no cross modulation occurs, a signal given by the following equation (1) is derived from the frequency converter: cos(2rtfct) cos{2n(fs + fc)t} = -2cos{2n(2fc + fs)t} + 2cos(2rifst) (1)
When this signal given by Eq. (1) is passed through a low pass filter, we obtain a low frequency chrominance signal cos(2nfst) shown at the second term of the right side of Eq. (1).However, an output signal from such a frequency converter also includes cross modulation distortion because of various reasons including the capacity of circuit elements, such as transistors, forming the frequency converter. Such cross modulation distortion is apt to result in the occurrence of ringing in a reproduced picture.
The present invention has been developed in order to remove the above-described drawbacks inherent in the conventional circuit for processing a low frequency chrominance signal.
It is, therefore, an object of the present invention to provide a new and useful circuit arrangement which is capable of effectively reducing cross modulation distortion.
According to a feature of the present invention a comb filter is employed for emphasizing a frequency component 2fs which is twice the chrominance subcarrier frequency of a low frequency chrominance signal, and then an output signal of the comb filter is processed to reduce the frequency component 2fs in the low frequency chrominance signal. More specifically, according to one embodiment, the output signal from the comb filter is fed via a limiter to a subtractor which is also responsive to the low frequency chrominance signal so that an output signal from the limiter is subtracted from the low frequency chrominance signal.In another embodiment, an output signal of a delay circuit included in the comb filter is subtracted from the low frequency chrominance signal in a subtractor, while the output signal from the comb filter is fed via a slicer to an adder which is also responsive to an output signal from the subtractor to add these signals to each other. With this arrangement cross modulation distortion components, i.e. 2fs, are effectively reduced to prevent undesirable ringing in a reproduced picture.
In accordance with the present invention there is provided a circuit arrangement for reducing cross modulation distortion in a low frequency chrominance signal, comprising: filter means responsive to said low frequency chrominance signal for passing a frequency component 2fs wherein fs is a frequency of chrominance subcarrier in said low frequency chrominance signal, so that said frequency component 2fs is emphasized; amplitude limiting means responsive to an output signal from said filter means for passing only a small-amplitude component; and a subtracting means responsive to an output signal from said amplitude limiting means and to said low frequency chrominance signal for producing an output signal by subtracting the former from the latter.
In accordance with the present invention there is also provided a circuit arrangement for reducing cross modulation distortion in a low frequency chrominance signal, comprising: filter means responsive to said low frequency chrominance signal for passing a frequency component 2fs wherein fs is a frequency of chrominance subcarrier in said low frequency chrominance signal, so that said frequency component 2fs is emphasized, said filter means having a delay circuit responsive to said low frequency chrominance signal for retarding the same by a period of time given by n/(2fs) wherein n is a positive integer, and an adder responsive to an output signal from said delay circuit and to said low frequency chrominance signal; subtracting means responsive to an output signal from said delay circuit and to said low frequency chrominance signal for producing an output signal by subtracting the former from the latter; slicing means responsive to an output signal from said filter means for passing only large-amplitude components; and adding means responsive to an output signal from said slicing means and to an output signal from said subtracting means for producing an output signal by subtracting the former from the latter.
The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which:
Fig. lisa schematic block diagram of a first embodiment of the present invention;
Fig. 2 is a frequency characteristic diagram useful for describing the operation of the first embodiment of Fig. 1;
Fig. 3 is a schematic block diagram of a second embodiment of the present invention;
Figs. 4A to 4D are frequency characteristic diagrams useful for describing the operation of the second embodiment of Fig. 3;
Fig. 5 is a frequency spectrum showing the occurrence of cross modulation distortion components in a low frequency chrominance signal;
Figs. 6A and 6B are waveform charts showing low frequency chrominance signals; and
Fig. 7 is a block diagram showing a conventional noise reducing circuit
The same or corresponding elements and parts are designated at like reference numerals throughout the drawings.
Prior to describing the preferred embodiments of the present invention, the above-described problem to be solved by the present invention will be discussed for a better understanding of the present invention.
Fig. 5 shows a frequency spectrum of an output signal from a frequency converter used for frequency converting chrominance signal in a colorunder heterodyne system. Although an input carrier signal has a band width of fc l 500 kHz actually, the illustrated spectrum shows a case where a single frequency is frequency converted wherein fc is a carrier frequency. In Fig. 5, components at frequencies designated at 2fs, fe - fs, fc, fc + fs and fc + 2fs are cross modulation distortion components wherein fs is chrominance subcarrier frequency of a low frequency chrominance signal.
Among these cross modulation distortion components one whose frequency is 2fs is within a pass band of a low pass filter used for passing the low frequency chrominance signal as shown by a dotted curve I. Therefore, this cross modulation distortion component 2fs is passed through the low pass filter together with the low frequency chrominance signal.
The output signal from the low pass filter is thus given by: cos(2nfst) + a cos(4rfst) = {1 + 2a cos(2rfst)}. cos(2nfst)- a (2) When the low frequency chrominance signal is reproduced, the chrominance subcarrier frequency fsthereof is frequency converted again back to the original carrier frequency fc. From the above Eq. (2) it will be understood thatthe carrier chrominance signal obtained as the result ofthe abovementioned reconversion is amplitude modulated by a frequency fs. Therefore, in the case of including no cross modulation distortion the carrier chrominance signal obtained through reproduction and frequency reconversion has a waveform shown in
Fig. 6A at a solid curve.On the other hand, when including cross modulation distortion the carrier chrominance signal has a waveform shown in Fig.
6B. This cross modulation distortion causes the occurrence of ringing~ because of the change in hue.
In a conventional video tape recorder or the like, a noise reduction circuit is provided in a chrominance signal reproducing system thereof. One example of such a noise reduction circuit is shown in Fig. 7. A chrominance signal, which is obtained by frequency converting a reproduced low frequency chrominance signal into a signal of chrominance subcarrierfrequencyfc, is applied to an input terminal 1, and this input signal is fed to a 1 H delay circuit 2 having a glass delay line or the like so that a chrominance signal retarded by one horizontal scanning period (1 H) is applied to an adder 3. On the other hand, the input chrominance signal is also directly applied to another input of the adder 3 to be added to the delayed signal.Thus, noise and signal components having no line correlation are removed by the adder 3, and an output signal from the adder 3 is fed to a subtractor 5 with its amplitude being limited by a limiter 4. This signal fed from the limiter 4 to the subtractor 5 is subtracted from the input chrominance signal in the subtractor 5. Therefore, a reproduced carrier chrominance signal from which noise has been removed by the subtractor 5 is obtained at an output terminal 6.
Although such noise reduction circuit using line correlation is effective for reducing noise mixed into the reproduced chrominance signal during recording and/or reproducing, it is not possible to reduce ringing caused from cross modulation distortion. Apart from this conventional circuit, none of conventional circuitry is effective for reducing ringing caused from cross modulation distortion included in reproduced low frequency chrominance signal.
Referring now to Fig. 1, a schematic block diagram of an embodiment of the present invention is shown. To an input terminal 10 is fed a low frequency chrominance signal having a low frequency chrominance subcarrier whose frequency is fs, which low frequency chrominance signal is reproduced from a magnetic recording tape via rotary heads of a video tape recorder. This low frequency chrominance signal is fed to a delay circuit 11, an adder 12 and a subtractor 15. Delay time To of the delay circuit 11 is set to 1/(2fs), so that the input low frequency chrominance signal is retarded by a time period corresponding to 1/(2fs) before it is applied to the adder 12. The delay circuit 11 and the adder 12 form a comb filter 13 having pass bands atfrequencies which are even harmonics ofthefrequencyfs and reject bands at frequencies which are odd harmonics of fs. In other words, pass bands and reject bands are alternately arranged in frequency spectrum as teeth of a comb.
A reproduced low frequency chrominance signal having a frequency spectrum shown by a dotted curve II in Fig. 2 is applied to such a comb filter 13 together with cross modulation distortion of a frequency 2fs after being passed through a Y/C separating low pass filter having a frequency response indicated at a dot-dash curve III. A solid curve IV indicates combined or overall frequency response of the combination of such a low pass filter and the comb filter 13.
This the output signal from the comb filter 13, i.e.
the output signal from the adder 12, is emphasized at frequency component 2fs which is a main component of ringing. Although the frequency component 2fs is emphasized, its amplitude is much smaller than that of the entire low frequency chrominance signal. Thus, only a small-amplitude component of the frequency component 2fs is derived by the following limiter 14 so that the low frequency chrominance signal of large amplitude is removed. An output signal from the limiter 14 is fed to the subtractor 15 to which the input low frequency chrominance signal is also fed. Thus, the frequency component 2fs is subtracted from the reproduced low frequency chrominance signal in the subtractor 15. As a result, the frequency component 2fs included in the reproduced low frequency chrominance signal is cancelled or eliminated.In this way, cross modulation distortion components which are main cause of ringing are removed, and thus a reproduced low frequency chrominance signal, from which such cross modulation components have been removed, is obtained at the output terminal 16.
Fig. 3 shows a block diagram of a second embodiment of the present invention. The second embodiment differs from the above-described first embodiment in that a slicer 17 is used is place of the limiter 14 of the first embodiment and in that a subtractor 18 is used to be responsive to an output signal from the delay circuit 11 and to the input low frequency chrominance signal while the subtractor 15 of Fig. 1 is replaced with an adder 19. More specifically, the output signal from the comb filter 13, which is substantially the same as that in the first embodiment, is fed to the slicer 17 having a slicing level V shown by a dotted line in Fig. 4A. A solid curve in Fig. 4A corresponds to the solid curve IV in
Fig. 2.Thus, the output signal from the comb filter 13 having a frequency response indicated by the solid curve in Fig. 4A is sliced so that smallamplitude components smaller than the slicing level
V are removed. Fig. 4B shows a frequency spectrum of an output signal from the slicer 17 which output signal may be obtained when a relatively large amplitude signal having flat frequency spectrum is applied to the input terminal 10. In actual use, the low frequency chrominance signal applied to the input terminal 10 has a frequency spectrum having peak at fs while the amplitude of 2fs component is much smaller than that offs. As a result, the output signal from the slicer 17 exhibits a frequency spectrum having frequency components around fs but no frequency components at frequencies fs, 2fs and at very low frequency.In this way, atthetime of slicing, frequency component 2fs is eliminated.
On the other hand, the output signal from the delay circuit 11 within the comb filter 13 and the input reproduced low frequency chrominance signal are both fed to the subtractor 18 so that the former is subtracted from the latter. Thus, an output signal from the subtractor 18 has a frequency characteristic such that pass bands at odd harmonics ofthefrequencyfs and reject bands at even harmonics ofthefrequencyfs are arranged alternately. Therefore, the output signal from the subtractor 18 has a frequency characteristic shown in Fig. 4C. The output signal from the subtractor 18 is fed to an adder 19. Thus, the output signal from the slicer 17 is added to the output signal from the subtractor 18.As a result, a reproduced low frequency chrominance signal having less 2fs component is produced by the adder 19 to be sent to an output terminal 20. The output signal from the adder 19 has a frequency characteristic shown in
Fig. 4D.
In this way, in both the first and second embodiments of the present invention of Figs. 1 and 3, a reproduced low frequency chrominance signal having suppressed or reduced cross modulation distortion components is obtained at the output terminal 16 or 20. It is to be noted that since the comb filter 13 outputs noise components having no line correlation, which noise is included in the reproduced low frequency chrominance signal, signal-to-noise ratio is also improved through the same principle as that in conventional noise reduction circuit utilizing line correlation.
Generally speaking in a video tape recorder, since chrominance signal is separated from a composite color video signal, which includes multiplexed luminance components and chrominance components in a common frequency band, the low frequency chrominance signal sometimes includes luminance signal components which causes undesirable influence appearing on a reproduced picture called cross-color interference. According to the present invention the occurrence of cross-color interference is suppressed because the abovementioned components 2fs are reduced.
The present invention is not limited to the abovedescribed embodiments, and for instance, the delay time To of the delay circuit 11 may be n/(2fs) (wherein n is a positive integer). In other words, the comb filter 13 is just required to remove a frequency component 2fs. Although the embodiments have been described in connection with a video tape recorder, the present invention may be applicable to other apparatus, such as a reproducing system in a video disc player in which low frequency chrominance signal is processed, or a recording system in any system handing lowfrequency chrominance signal.
The above-described embodiments are just examples of the present invention, and therefore, it will be apparentforthoseskilled in the artthat many modifications and variations may be made without departing from the scope of the present invention.
Claims (5)
1. A circuit arrangement for reducing cross modulation distortion in a low frequency chrominance signal, comprising:
(a) filter means responsive to said low frequency chrominance signal for passing a frequency component 2fs wherein fs is a frequency of chrominance subcarrier in said low frequency chrominance signal, so that said frequency component2fs is emphasized;
(b) amplitude limiting means responsive to an output signal from said filter means for passing only a small-amplitude component; and
(c) a subtracting means responsive to an output signal from said amplitude limiting means and to said low frequency chrominance signal for producing an output signal by subtracting the former from the latter.
2. A circuit arrangement as claimed in Claim 1, wherein said filter means is a comb filter having pass bands at even harmonics of said frequency fs and reject bands at odd harmonics of said frequency fs.
3. A circuit arrangement as claimed in Claim 1 or 2, wherein said filter means comprises:
(a) a.delay circuit responsive to said low frequency chrominance signal for retarding the same by a period of time given by n/(2fs) wherein n is a positive integer; and
(b) an adder responsive to an output signal from said delay circuit and to said low frequency chrominance signal.
4. A circuit arrangement for reducing cross modulation distortion in a low frequency chrominance signal, comprising:
(a) filter means responsive to said low frequency chrominance signal for passing a frequency component 2fs wherein fs is a frequency of chrominance subcarrier in said low frequency chrominance signal, so that said frequency component 2fs is emphasized, said filter means having a delay circuit responsive to said low frequency chrominance signal for retarding the same by a period of time given by n/(2fs) wherein n is a positive integer, and an adder responsive to an output signal from said delay circuit and to said low frequency chrominance signal,
(b) subtracting means responsive to an output signal from said delay circuit and to said low frequency chrominance signal for producing an output signal by subtracting the former from the latter; ;
(c) slicing means responsive to an output signal from said filter means for passing only large amplitude components; and
(d) adding means responsive to an output signal from said slicing means and to an output signalfrom said subtracting means for producing an output signal by subtracting the former from the latter.
5. A circuit arrangement for reducing cross modulation distortion substantially as hereinbefore described with reference to and as illustrated in
Figures 1 to 4 of the accompanying drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP60108773A JPS61265991A (en) | 1985-05-21 | 1985-05-21 | Cross modulation distortion reducing device for low frequency conversion carrier chrominance signal |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8612343D0 GB8612343D0 (en) | 1986-06-25 |
| GB2176367A true GB2176367A (en) | 1986-12-17 |
| GB2176367B GB2176367B (en) | 1988-08-03 |
Family
ID=14493116
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08612343A Expired GB2176367B (en) | 1985-05-21 | 1986-05-21 | Circuit arrangement for reducing cross modulation distortion in low frequency chrominance signal |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS61265991A (en) |
| GB (1) | GB2176367B (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0300801A2 (en) * | 1987-07-21 | 1989-01-25 | Matsushita Electric Industrial Co., Ltd. | Frequency demodulation apparatus |
| EP0314270A2 (en) * | 1987-10-30 | 1989-05-03 | Pioneer Electronic Corporation | Apparatus for suppressing noises in a video signal |
| US4908581A (en) * | 1988-07-21 | 1990-03-13 | Matsushita Electric Industrial Co., Ltd. | Frequency demodulator having circuit cancelling undesired signal components |
| EP0429926A2 (en) * | 1989-11-30 | 1991-06-05 | Nokia (Deutschland) GmbH | Circuit for regaining the chrominance signal |
-
1985
- 1985-05-21 JP JP60108773A patent/JPS61265991A/en active Granted
-
1986
- 1986-05-21 GB GB08612343A patent/GB2176367B/en not_active Expired
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0300801A2 (en) * | 1987-07-21 | 1989-01-25 | Matsushita Electric Industrial Co., Ltd. | Frequency demodulation apparatus |
| EP0300801A3 (en) * | 1987-07-21 | 1989-03-29 | Matsushita Electric Industrial Co. Ltd. | Frequency demodulation apparatus |
| EP0314270A2 (en) * | 1987-10-30 | 1989-05-03 | Pioneer Electronic Corporation | Apparatus for suppressing noises in a video signal |
| EP0314270A3 (en) * | 1987-10-30 | 1990-07-25 | Pioneer Electronic Corporation | Apparatus for suppressing noises in a video signal |
| US4908581A (en) * | 1988-07-21 | 1990-03-13 | Matsushita Electric Industrial Co., Ltd. | Frequency demodulator having circuit cancelling undesired signal components |
| EP0429926A2 (en) * | 1989-11-30 | 1991-06-05 | Nokia (Deutschland) GmbH | Circuit for regaining the chrominance signal |
| EP0429926A3 (en) * | 1989-11-30 | 1992-08-05 | Nokia Unterhaltungselektronik (Deutschland) Gmbh | Circuit for regaining the chrominance signal |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2176367B (en) | 1988-08-03 |
| JPH051679B2 (en) | 1993-01-08 |
| JPS61265991A (en) | 1986-11-25 |
| GB8612343D0 (en) | 1986-06-25 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4302768A (en) | System for reducing or suppressing noise components in television signal | |
| CA1201804A (en) | Chroma signal recording and reproducing circuit | |
| JPH0154915B2 (en) | ||
| EP0186514B1 (en) | Comb filter | |
| JPS6119198B2 (en) | ||
| GB2176367A (en) | Circuit arrangement for reducing cross modulation distortion in low frequency chrominance signal | |
| KR950012330B1 (en) | Chrominance and luminance singal separating circuit for modifying the leuel-frequency characteristic of the luminance signal | |
| US4809085A (en) | Recording and reproducing apparatus for reducing cross-talk in color television signal | |
| US5822490A (en) | Apparatus and method for color-under chroma channel encoded with a high frequency luminance signal | |
| JPH0135558B2 (en) | ||
| KR890007510Y1 (en) | Picture reproducers | |
| JP2916737B2 (en) | Crosstalk removing device for reproduced carrier color signal such as VTR and delay device used therefor | |
| KR910009884B1 (en) | Recording and playback apparatus of picture image signal | |
| JP2995446B2 (en) | Delay device and crosstalk removing device for reproduced carrier color signal such as VTR using the same | |
| JP2635846B2 (en) | Magnetic recording device | |
| JPH0657069B2 (en) | Color video signal playback device | |
| JPH0545116B2 (en) | ||
| GB2252005A (en) | Video signal processing for recording and playback systems | |
| JPH046317B2 (en) | ||
| JPS608677B2 (en) | Color video signal converter | |
| JPS59101993A (en) | Video signal recording and reproducing device | |
| JPS63146591A (en) | Noise eliminating circuit for video signal | |
| JPH01220594A (en) | Magnetic recorder for television signal | |
| JPS6340396B2 (en) | ||
| JPH0481094A (en) | Video signal processor |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19990521 |