GB2089166A - Dropout compensation circuit for a reproduced video signal in a reproducing apparatus - Google Patents

Abstract

A dropout compensation circuit for reproducing a composite color video signal recorded in a recording medium 10, wherein said signal consists of a luminance signal occupying a predetermined frequency band and a carrier chrominance signal at a high frequency part of the luminance signal, comprises an amplitude modulator 24 for amplitude-modulating a carrier by said signal, a delay circuit 25 having a frequency band for passing the central frequency range of an output signal from the amplitude modulator, and delaying the signal by a predetermined delay time, a demodulating circuit 26 for demodulating the output of the delay circuit, a detection circuit 17 for detecting dropout within the said reproduced video signal, a switching circuit 18 operated by an output of the detection circuit, for supplying the said reproduced signal when dropout is not detected, and supplying the delayed signal when dropout is detected. <IMAGE>

Description

SPECIFICATION Dropout compensation circuit for a reproduced video signal in a reproducing apparatus The present invention generally relates to dropout compensation circuits for reproduced video signals in reproducing apparatuses, and more particularly to a dropout compensation circuit capable of effectively compensating for dropouts in a luminance signal within a color video signal reproduced from a recording medium in a reproducing apparatus.

Generally, when reproducing a recorded signal from a rotary recording medium by a reproducing stylus, dropouts are instantaneously introduced in the reproduced signal in a case where the reproducing stylus skips due to dust particles, scratches, and the like on the surface of the rotary recording medium. These signal dropouts are also generated when reproducing a recording medium such as a magnetic tape. Accordingly, as a conventional cicuit for compensating for the dropouts, there was a circuit consisting of a detecting circuit for detecting the dropout, a delay circuit for delaying the reproduced composite color video signal, and a circuit for using the output of the delay circuit instead of the dropout part of the reproduced composite color video signal when the dropout is detected by the detecting circuit.

However, a delay circuit having a wide frequency band must be used in order to delay the whole reproduced composite color video signal. Hence, there was a disadvantage in that the conventional dropout compensation circuit could not be constructed at low cost. Further, there was a problem in that when a delay operation is performed with a wide frequency band, it is difficult to match the phase of the color signal.

On the other hand, a delay line having glass as the medium for delaying a signal by an interval of 1 H (H indicates a horizontal scanning period), that is, a so-called 1 H glass delay line, is low in cost. However, the operational frequency band of the above 1H glass delay line is narrow, and cannot be used independently for the above described conventional dropout compensation circuit.

Accordingly, it is a general object of the present invention to provide a novel and useful dropout compensation circuit for a reproduced video signal in a reproducing apparatus, in which the above described disadvantages have been overcome.

The present invention provides a dropout compensation circuit for a reproduced video signal in a reproducing apparatus for reproducing a composite color video signal recorded in a recording medium, said composite color video signal consisting of a luminance signal occupying a predetermined frequency band and a carrier chrominance signal multiplexed with a common frequency band at a high frequency part of said luminance signal, said dropout compensation circuit comprising, an amplitude modulator for amplitude-modulating a carrier by a reproduced composite color video signal, a delay circuit having a frequency band for passing a part excluding a high frequency part of an upper side band and a low frequency part of a lower side band of an output amplitude-modulated signal of said amplitude modulator, and delaying the signal by a predetermined delay time, a demodulating circuit for demodulating an output amplitude-modulated signal of said delay circuit, a detection circuit for detecting dropout within said reproduced composite color video signal, a switching circuit switched over by an output of said detection circuit, for passing and supplying said reproduced composite color video signal when dropout is not detected, and passing and supplying said reproduced composite color video signal when dropout is not detected, and passing and supplying an output signal of said demodulating circuit when dropout is detected, and a circuit for obtaining a luminance signal compensated of the dropout from the signal passed through said switching circuit.

Another and more specific object of the present invention is to provide a dropout compensation circuit for a reproduced video signal, in which a reproduced composite color video signal is amplitude-modulated and then passed through a delay circuit to obtain a delayed luminance signal, and compensate for dropouts in the luminance signal by use of the delayed luminance signal in a reproducing apparatus.

Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompaning drawings.

Figure 1 is a systematic block diagram showing a rotary recording medium reproducing apparatus applied with an embodiment of a dropout compensation circuit for a reproduced video signal according to the present invention; and Figures 2A, 2B, and 2C are diagrams respectively showing frequency spectrums of signals at each part of the block system shown in Figure 1.

In Figure 1, a rotary recording medium (hereinafter simply referred to as a disc) 10 is recorded with a composite color video signal on a spiral track, as variations in geometrical configuration. The disc 10 is placed onto a turntable 11 and rotated by a motor 12.

As indicated in Figure 2A, for example, the above composite color video signal consists of a luminance signal indicated by land a carrier chrominance signal indicated by II, obtained from the NTSC system color video signal, respectively. The luminance signal has a frequency band (zero to 3 MHz) wherein the upper limit frequency is bandlimited to approximately 3 MHz. On the other hand, the carrier chrominance signal has a frequency band (2.56 MHz 1 500 kHz) which is frequency-converted into a frequency band at a high frequency range part ofthe frequency band / of the above luminance signal and multiplexed with a common frequency band by frequency-interleaving. The composite color video signal is recorded in a state where the above signals as a whole is frequency-modulated.A signal pickup device 13 has a reproducing stylus 14, and repro ducts the recorded signal according to variations in electrostatic capacitance between the disc surface of the disc 10 and the reproducing stylus 14.

The signal reproduced by the signal pickup device 13 is supplied through a preamplifier 15to a demodulator 16 and a dropout detection circuit 17.

The above signal reproduced by the signal pickup device 13 is a frequency-modulated signal, and is demodulated at the demodulator 16. Hence, a demodulated composite color video signal indicated in Figure 2A is obtained from the demodulator 16. This demodulated signal is supplied to a contact point 18a of a switching circuit 18, and also to a color signal processing circuit 19.

The dropout detection circuit 17 consists of a conventional dropout detection circuit using a retriggerable multivibrator or an envelope detection circuit and the like, and detects the dropout within the frequency-modulated reproduced signal. A contact member 18c of the switching circuit 18 is normally connected to the contact point 18a. When a detected output is obtained from the dropout detection circuit 17, the contact member 18c is switched over and connected to a contact point 18b of the switching circuit 18. In Figure 1,theswitching circuit 18 is diagrammatized and shown as a mechanical switch, however, in reality, an analog switch which is swiched over electronically, is used for the switching circuit 18.

The color signal processing circuit 19 performs a dropout compensation operation with respect to the carrier chrominance signal of the demodulated signal supplied thereto. Moreover, the color signal processing circuit 19 performs color signal processing such as returning the carrier chrominance signal which is frequency-converted by the above recording system to the carrier chrominance signal having the original centerfrequency of 3.58 MHz, by use of a frequency signal of 3.58 MHz supplied from an oscillator 20. The carrier chrominance signal subjected to the above signal processing, is supplied to an adding circuit 21.

On the other hand, the output signal of the demodulator 16 indicated in Figure 2A which has passed through the switching circuit 18, is supplied to a comb filter 23 and an amplitude modulator 24.

The carrier chrominance signal indicated by the frequency band ll of the signal indicated in Figure 2A is eliminated at the comb filter 23. Hence, only the luminance signal having the frequency band /which passes through the comb filter 23, is supplied to the adding circuit 21. The carrier chrominance signal from the above color signal processing circuit 19 and the luminance signal from the comb filter 23 are added at the adding circuit 21. Accordingly, the color video signal thus obtained is supplied to a television receiver through an output terminal 22, and reproduced as a color picture.

In a state where no detected output is obtained from the dropout detection circuit 17 and the contact member 1Bc of the switching circuit 18 is connected to the contact point 18a, the signal from the demodulator 16 indicated in Figure 2A is supplied to the above comb filter 23 and the amplitude modulator 24, through the switching circuit 18. The amplitude modulator 24 performs amplitude-modulation by the signal from the demodulator 16, using the frequency signal of 3.58 MHz obtained from the oscillator 20 as the carrier. Accordingly, the frequency spectrum of the output amplitude-modulated signal of the amplitude modulator 24 becomes as indicated by Ill in Figure 2B. This frequency spectrum has a lower side band Illa and an upper side band Illb about a center frequency which is the carrier frequency of 3.58 MHz.The carrier chrominance signal II appears as indicated by IVa at the lower frequency part of the lower side band Illa and as indicated by IVb at the higher frequency part of the upper side band Illb, in the amplitude-modulated signal.

The above output amplitude-modulated signal is supplied to an 1H glass delay line 25, and delayed by an interval of 1 H. This 1 H glass delay line 25 is low in cost, however, the 1 H delay line 25 has a relatively narrowfrequency band of approximately 1.6 MHzto 5.6 MHz. Hence, when the output amplitidemodulated signal of the amplitude modulator 24 passes through the 1 H glass delay line 25, the output amplitude modulated signal is not only delayed, but also limited of the frequency band. Accordingly, the above output amplitude-modulated signal thus passed through the 1 H glass delay line 25 is supplied to a demodulator 26 as a signal having a frequency band indicated by Vin Figure 2B, and demodulated thereat. The signal thus demodulated at the demodulator 26 is applied to the contact point 18b of the switching circuit 18.The output demodulated signal of the demodulator 26, is a signal obtained by demodulating the amplitude-modulated signal limited of the frequency band and having the frequency band V. As indicated in Figure 2C, the above output demodulated signal of the demodulator 26 is a luminance signal having a frequency band of zero to 2 MHz.

When the dropout detection circuit 17 detects a dropout within the reproduced signal, the contact member 1 8c of the switching circuit 18 is switched over from the contact point 18a and connected to the contact point 1 8b by the output signal of the dropout detection circuit 17. Accordingly, the signal from the demodulator 26 which is delayed by the interval of 1 H, that is, the signal of an interval of 1 H before the point in time when the dropout is generated, having no dropout, is supplied to the comb filter 23 and the amplitude modulator 24, through the switching circuit 18. The above delayed luminance signal indicated in Figure 2C passes through the comb filter 23 as it is, and is then supplied to the adding circuit 21. Therefore, a color video signal compensated of the dropout, is obtained from the output terminal 22.

In a case where the dropout continues throughout an interval of over 1 H, the contact member 1Bc of the switching circuit 18 remains in a state connected to the contact point 18b during that interval. Hence, the already delayed signal is further delayed, and used as a compensated signal having no dropout. Therefore, during the interval in which compensation is performed with respect to the dropout, the frequency band of the luminance signal which is added with the carrier chrominance signal at the adding circuit 21 is zero to 2 MHz, and is narrower than the original frequency band of zero to 3 MHz. However, the interval of the dropout introduced upon reproduction of the disc 10 is generally in the range of several H at the most. Thus, even when the frequency band of the luminance signal becomes narrow to the above described extent, the visual effect due to the dropout having an interval of the above range is exceedingly small, and is practically negligible.

Moreover, the 1 H delay line 25 is not limited to the above glass delay line, and may be a charge transfer element having a narrow frequency band such as a charge-coupled device (CCD).

Further, this invention is not limited to these embodiments but various variations and modifications may be made without departing from the scope of the invention.

Claims (6)

1. A dropout compensation circuit for a reproduced video signal in a reproducing apparatus for reproducing a composite color video signal recorded in a recording medium, said composite color video signal consisting of a luminance signal occupying a predetermined frequency band and a carrier chrominance signal multiplexed with a common frequency band at a high frequency part of said luminance signal, said dropout compensation circuit comprising:: an amplitude modulatorforamplitude-modulating a carrier by a reproduced composite color video signal; a delay circuit having a frequency band for passing a part excluding a high frequency part of an upper side band and a low frequency part of a lower side band of an output amplitude-modulated signal of said amplitude modulator, and delaying the signal by a predetermined delay time; a demodulating circuitfordemodulating an output amplitude-modulated signal of said delay circuit; a detection circuit for detecting dropout within said reproduced composite color video signal;; a switching circuit switched over by an output of said detection circuit, for passing and supplying said reproduced composite color video signal when dropout is not detected, and passing and supplying an output signal of said demodulating circuitwhen dropout is detected; and a circuit for obtaining a luminance signal compensated of the dropout from the signal passed through said switching circuit.

2. A dropout compensation circuit as claimed in claim 1 in which said delay circuit has a frequency band for passing only a signal in a frequency band part between frequency band parts of a carrier chrominance signal component existing at the high frequency part of the upper side band and the low frequency part of the lower side band of said amplitude-modulated signal.

3. A dropout compensation circuit as claimed in claim 2 in which said delay circuit consists of a glass delay line having glass as a medium, for delaying a signal by an interval of one horizontal scanning period.

4. A dropout compensation circuit as claimed in claim 1 further comprising a signal processing circuit for performing a signal processing operation with respect to the carrier chrominance signal within said reproduced composite color video signal, and an adding circuit for adding the carrier chrominance signal subjected to the signal processing and said luminance signal thus obtained.

5. A dropout compensation circuit as claimed in claim 4 further comprising an oscillator for supplying a signal of a reference carrier chrominance signal frequency to said signal processing circuit, in which said amplitude modulator is supplied with an output signal of said oscillator as the carrier to perform amplitude-modulation.

6. A dropout compensation circuit as claimed in claim 1 in which the high frequency part of the luminance signal of said composite color video signal is band-limited, the carrier chrominance signal of said composite color video signal is frequency-converted into a low frequency band so as to exist in the high frequency part of said band-limited luminance signal, and the frequency of the carrier used for said amplitude modulator is equal to the chrominance sub-carrier frequency of said carrier chrominance signal before said carrier chrominance signal is frequency-converted into said low frequency band.