JPH0230948Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a de-emphasis circuit, and particularly to a de-emphasis circuit that imparts de-emphasis characteristics to the vertical spatial frequency of a video signal.

Prior Art In recording and reproducing devices such as VTRs, a luminance signal and a carrier chrominance signal are separated from a color video signal to be recorded and reproduced, and the luminance signal is converted into a frequency-modulated luminance signal, and the carrier chrominance signal is a low frequency modulated luminance signal. These two signals are frequency-division multiplexed and recorded on a magnetic tape, and during reproduction, signal processing is performed in the opposite manner to that during recording to obtain a reproduced color video signal. In this case, the luminance signal is transmitted in the form of a frequency modulated wave (FM wave), but as is well known, the higher the modulation frequency component of the FM wave, the more susceptible it is to noise. For this reason, the conventional VTR luminance signal recording system is equipped with a pre-emphasis circuit, which emphasizes the high frequency components of the input luminance signal in advance, and supplies the output luminance signal to a frequency modulation circuit. It is well known that the S/N ratio is improved by increasing the modulation depth and reducing the influence of noise during FM demodulation. Further, the luminance signal reproduction system is provided with a de-emphasis circuit having characteristics complementary to the pre-emphasis characteristics of the pre-emphasis circuit, and restores the FM demodulated luminance signal to the original signal waveform.

Here, since the output luminance signal of the pre-emphasis circuit has high frequency components emphasized,
Sharp overshoots and undershoots occur in the rising and falling portions of the luminance signal, and if this luminance signal is supplied as is to the frequency modulation circuit, the frequency shift will be momentarily large at the overshoot portions. This may result in overmodulation.
During playback, this overmodulation occurs outside the slice range of the limiter at the input stage of the FM demodulation circuit, causing a lack of signal at the limiter output, which is FM demodulated as a low frequency in the FM demodulation circuit.
FM demodulation output luminance signal falls to black,
It is known that a so-called inversion phenomenon occurs.

For this reason, conventionally a circuit was installed at the input stage of the frequency modulation circuit to prevent the amplitude at the tip of overshoot or undershoot from exceeding a predetermined value, but since it was previously a horizontal emphasis, noise The reduction effect is not sufficient, and S/
In order to improve the N ratio, it is necessary to apply deep clipping, which poses the problem of some kind of image quality deterioration at the rising and falling edges of the waveform.

Therefore, the present applicant previously proposed in Japanese Patent Application No. 138873/1983 that a pre-emphasis characteristic was added to the vertical spatial frequency characteristic of the recorded luminance signal, and the above-mentioned pre-emphasis characteristic was added to the reproduced luminance signal. We have proposed a video signal recording and reproducing device that provides de-emphasis characteristics that are complementary to pre-emphasis characteristics. According to this proposed recording and reproducing device, recording and reproducing can be performed even at lower frequencies (horizontal spatial frequencies) compared to conventional devices that perform pre-emphasis and de-emphasis (horizontal emphasis) on horizontal spatial frequencies. It is possible to reduce the noise in the luminance signal, and in connection with this, it is no longer necessary to apply a very deep clip level to the clip circuit at the input stage of the frequency modulation circuit, which reduces the rise and fall of the waveform compared to the conventional method. It is possible to reduce image quality deterioration at the edges. In addition, when horizontal emphasis is applied to a luminance signal that is at the pedestal level during the vertical retrace period and at the sync chip level at the vertical sync signal position, a large overshoot occurs, and conventionally it is difficult to extract the vertical sync signal. However, the device proposed above has non-linear pre-emphasis characteristics and non-linear de-emphasis characteristics. Since the above-mentioned phenomenon can be prevented, and the video signal does not have to be extremely distorted, it can be compatible with conventional recording and reproducing devices, and furthermore, it can prevent extreme distortion in clip circuits, etc. component deletion or overmodulation,
Various defects caused by excessive emphasis such as inversion can be eliminated.

Problems to be Solved by the Invention However, whereas the de-emphasis circuit in the recording and reproducing device proposed above had a delay circuit whose delay time was fixed to one horizontal scanning period (1H), variable speed reproduction Sometimes, because the relative linear velocity between the tape and the head is different from that during recording or normal playback, the playback horizontal synchronizing signal frequency becomes a value that is slightly different from the regular horizontal scanning frequency. Accurate in 1H
delay could not be obtained. For this reason, the de-emphasis characteristics of the de-emphasis circuit are no longer complementary to the pre-emphasis characteristics of the pre-emphasis circuit of the recording system, and there is a problem in that smear occurs diagonally downward on the screen from the image change part where there is no line correlation on the screen. Ta. For example, when the upper half of the screen is dark and the lower half is bright, this smear appears as a black tail-like image diagonally downward from the boundary between the images.

Therefore, the present invention has developed a 1H delay circuit in a de-emphasis circuit that performs de-emphasis on vertical spatial frequencies for video signals such as luminance signals, which requires an external clock frequency such as a charge coupled device (CCD) or memory. It is an object of the present invention to provide a de-emphasis circuit which solves the above-mentioned problems by configuring the de-emphasis circuit using circuit elements and by variably controlling the external clock frequency during variable speed reproduction.

Means for Solving the Problems The de-emphasis circuit according to the present invention uses a delay circuit that adds a delay time of one horizontal scanning period to an external clock in order to obtain a low-pass filter characteristic of the vertical spatial frequency of a reproduced video signal. The clock frequency generating circuit includes a delay element whose delay time is controlled by frequency, a clock frequency generation circuit which applies the external clock frequency to the delay element, and means which switches the clock frequency according to the playback speed. .

Effect During variable speed playback, the relative linear velocity between the tape and the head is different from that during normal playback, so the horizontal scanning frequency of the played video signal is different from the normal value, and one horizontal scanning period (1H) has a different value than during normal playback. becomes. Therefore, the clock frequency switching means generates and outputs a clock frequency that makes the delay time of the delay element equal to the 1H period during the variable speed playback, so that an accurate 1H delay time can be given to the reproduced video signal. Can be done. Hereinafter, each embodiment of the present invention will be described with reference to the drawings.

Embodiment FIG. 1 shows a block system diagram of an embodiment of a 1H delay circuit which is a main part of the de-emphasis circuit according to the present invention. Further, FIGS. 2 and 3 respectively show block diagrams of examples of de-emphasis circuits to which the circuit of the present invention can be applied. Figure 2 shows a de-emphasis circuit that imparts a preset de-emphasis characteristic to the input video signal regardless of the amplitude of the input reproduced video signal, and Figure 3 shows a de-emphasis circuit that changes the de-emphasis characteristic according to the amplitude of the input reproduced video signal. 1 shows a de-emphasis circuit that imparts controlled non-linear de-emphasis characteristics to an input reproduced video signal. The block system of the de-emphasis circuit shown in FIGS. 2 and 3 is disclosed in the above-mentioned video signal recording and reproducing apparatus proposed by the present applicant. The present invention is characterized in that the 1H delay circuit for obtaining the low-pass filter characteristic of the vertical spatial frequency of the video signal is configured as shown in FIG. 1 or FIG. 5, which will be described later.

First, the de-emphasis circuit shown in FIG. 2 will be explained. In the figure, a video signal reproduced from a recording medium (hereinafter, a luminance signal will be explained as an example) is input to input terminal 1. . That is, for example, a luminance signal reproduced from a magnetic tape in the form of a frequency modulated wave by a rotating head of a VTR is FM demodulated by an FM demodulation circuit to become a reproduced luminance signal, and this reproduced luminance signal is sent via terminal 1 to an adder circuit. 2, are supplied to subtraction circuits 3 and 4, respectively. Note that this input reproduced luminance signal is given a pre-emphasis characteristic regarding the vertical spatial frequency that is complementary to the characteristic of this de-emphasis circuit at the time of recording, so that the high-frequency component of the vertical spatial frequency is emphasized.

The output luminance signal of the adder circuit 2 is delayed by one horizontal scanning period (1H) by a 1H delay circuit 5, which will be described later, and then supplied to the coefficient circuit 6, where the first value of the predetermined value is
The signal is multiplied by the coefficient of , weighted, and then supplied to the adder circuit 2. The input reproduced luminance signal added and combined by the adder circuit 2 and the output signal of the coefficient circuit 6 are supplied again to the 1H delay circuit 5, and are also supplied to the coefficient circuit, where they are multiplied by a second coefficient of a predetermined value. After that, it is supplied to the subtraction circuit 3. Subtraction circuit 3
performs a subtraction operation of subtracting the output signal of the coefficient circuit 7 from the reproduced luminance signal input from the input terminal 1, and supplies the resulting signal to the subtraction circuit 4 through the coefficient circuit 8. The subtracting circuit 4 subtracts the output signal of the coefficient circuit 8 from the input reproduced luminance signal, and outputs the reproduced luminance signal to which a de-emphasis characteristic has been applied to the spatial frequency in the vertical direction to an output terminal 9.
Output to.

This de-emphasis characteristic is unrelated to the amplitude of the input reproduced luminance signal, and if the horizontal scanning frequency is _H , it has an attenuation range at frequencies that are odd multiples of _H /2.
Figure 4 A with a passband at an even multiple of _H /2
The frequency characteristics are as shown in .

Next, the de-emphasis circuit shown in FIG. 3 will be described. In the figure, the same components as those in FIG. 2 are denoted by the same reference numerals, and the explanation thereof will be omitted. In FIG. 3, the input terminal 10, like the input terminal 1, is of course supplied with a signal obtained by reproducing a luminance signal to which a pre-emphasis characteristic has been given in advance for the spatial frequency in the vertical direction during recording. However, a reproduced signal of a luminance signal recorded by a current VTR and not provided with the above-mentioned pre-emphasis characteristic may also be supplied. This input reproduced luminance signal is subtracted from the output signal of the coefficient circuit 7 in the subtraction circuit 3 and then supplied to the non-linear circuit 11. The nonlinear circuit 11 is, for example, an amplitude limiter that limits the amplitude of an input signal to a constant value, and is configured so that input signals with an amplitude smaller than this constant value are passed through and output as they are without amplitude limiting. ing. The luminance signal taken out from the non-linear circuit 11 is supplied to the coefficient circuit 12, where it is multiplied by a coefficient of a predetermined value (for example, 0.57).

The subtraction circuit 4 performs a subtraction operation of subtracting the output signal of the coefficient circuit 12 from the input reproduced luminance signal, and outputs the reproduced luminance signal to which the de-emphasis characteristic has been added with respect to the spatial frequency in the vertical direction to the output terminal 1.
Output to 3.

FIG. 4A shows an example of the de-emphasis characteristic of this de-emphasis circuit, and _H represents the horizontal scanning frequency. This de-emphasis characteristic is similar to that of a comb filter, and the de-emphasis characteristic shown schematically in an enlarged manner in FIG. 4B in the frequency range below the horizontal scanning frequency _H is particularly used. As shown in FIG. 4B, when the amplitude of the input reproduced luminance signal is the minimum, a de-emphasis characteristic as shown by D ₁ is given, and as the amplitude of the input reproduced luminance signal becomes larger, the de-emphasis characteristic becomes (1/2). When the de-emphasis amount near _H gradually decreases and the amplitude is large enough to be limited by the non-linear circuit 11, as shown in Fig. 4B.
As shown by D ₂ , a de-emphasis characteristic in which the amount of de-emphasis is almost zero is imparted.

In the de-emphasis circuit shown in FIG. 2 or 3, the 1H delay circuit 5 of the present invention has a configuration as shown in FIG. 1, for example. In FIG. 1, the 1H delay element 15 is an element that delays the input signal of the input terminal 14 by 1H and outputs the delayed signal to the output terminal 16. Here, the delay time is controlled by the external clock frequency. Charge transfer devices such as brickade devices (BBD),
Alternatively, digital memory such as random access memory may be used.

On the other hand, the crystal oscillator circuits 17 to 20 each constitute a circuit that generates an external clock frequency, and their output signals are sent to terminals 21a to 21a of the switch circuit 21.
21d separately. The output oscillation frequency of the crystal oscillation circuit 17 is selected to be a clock frequency value that allows an accurate 1H delay time to be obtained during normal reproduction in the 1H delay element 15. Similarly, the oscillation frequencies of the crystal oscillator circuits 18, 19, and 20 control the reproduced luminance signal during still motion reproduction, forward high-speed search, and reverse high-speed search.
The clock frequency values are respectively selected to be delayed by 1H period during reproduction by the 1H delay element 15. Note that, of course, the above-mentioned high-speed search is performed at a constant playback speed, for example, about 10 times the speed.

Accordingly, the switch circuit 21 is connected to the terminal 21a during normal playback, and connected to the terminal 21b during still motion playback, and is connected to the terminal 21b during forward high-speed search according to a switching signal indicating the playback mode inputted from the terminal 22. is connected to terminal 21c,
Furthermore, it is connected to the terminal 21d during reverse high speed search. As a result, during normal playback, the output oscillation frequency of the crystal oscillation circuit 17 is controlled by the switch circuit 21.
The normally reproduced luminance signal is selectively outputted to the 1H delay element 15 as a clock frequency, and the normally reproduced luminance signal delayed by exactly 1H by the 1H delay element 15 is outputted to the output terminal 16. Similarly, the clock frequency supplied to the 1H delay element 15 through the switch circuit 21 becomes each output oscillation frequency of the crystal oscillator circuits 18, 19, and 20 during still motion reproduction, forward high speed search, and reverse high speed search. , even if the 1H period changes depending on the respective playback speed, the 1H at that time
A delay time of 1H is obtained by the 1H delay element 15.

As a result, even during variable-speed playback such as still motion playback and high-speed search, the de-emphasis circuit prevents smearing and is optimal.
The de-emphasis characteristic shown in is given to the reproduced luminance signal.

Next, another example of the 1H delay circuit 5 will be explained.
This will be explained with figures. In the figure, the same components as in FIG. 1 are denoted by the same reference numerals, and their explanations will be omitted. In FIG. 5, a noise bar detection signal is supplied to a response time constant control circuit 25 via an input terminal 24. In FIG. This noise bar detection signal is generated by detecting the fact that the envelope level of the frequency modulated video signal reproduced from the magnetic tape is greatly reduced during reverse track scanning during variable speed reproduction. As a result, the response time constant control circuit 25 changes the response time constant of the phase locked loop (PLL) 26 (for example, the cutoff frequency of the loop filter in the PLL) when detecting a noise bar, and transfers the signal from the PLL 26 to the 1H delay element 15. The clock frequency that is phase-synchronized with the reproduced horizontal synchronizing signal outputted from the input terminal 27 is controlled to an optimal value that does not cause effects such as skew. According to this embodiment, the PLL 26 constitutes a clock frequency generation circuit, and the response time constant control circuit 25 constitutes a clock frequency switching means.

The de-emphasis circuit devised by the present invention is
The present invention can be applied not only to the circuits shown in FIGS. 2 and 3, but also to all de-emphasis circuits in video signal recording and reproducing apparatuses proposed by the applicant.

Furthermore, in the above explanation, the de-emphasis circuit for the luminance signal was explained, but in principle, the present invention can also be applied to the de-emphasis circuit for the carrier color signal. (The present invention cannot be applied to VTRs in which playback time axis fluctuations are corrected even during variable speed playback.)

Effects of the invention As described above, according to the invention, the external clock frequency of the 1H delay element is variably controlled according to the playback speed, so even during variable speed playback, the 1H delay element can accurately control the 1H portion of the reproduced video signal. Therefore, the de-emphasis direction by the de-emphasis circuit is exactly vertically downward on the screen, and de-emphasis characteristics without smear can be provided.

[Brief explanation of drawings]

1 and 5 are block diagrams showing respective embodiments of the essential parts of the circuit of the present invention, and FIGS. 2 and 3 are diagrams of de-emphasis systems previously proposed by the applicant to which the circuit of the present invention can be applied. A block system diagram showing each example of the circuit, and FIG. 4 is a diagram showing an example of de-emphasis characteristics. 1, 10...video signal (luminance signal) input terminal,
2... Addition circuit, 3, 4... Subtraction circuit, 5...
1H delay circuit, 11...Non-linear circuit, 15...1H
Delay element, 17-20...Crystal oscillation circuit, 21...
...Switch circuit, 22...Switching signal input terminal, 26...Phase locked loop (PLL).

Claims (1)

[Claims for Utility Model Registration] (1) A video signal reproduced from a recording medium is supplied, and a delay time of one horizontal scanning period is required to obtain a low-pass filter characteristic of the vertical spatial frequency of the reproduced video signal. a de-emphasis circuit that provides a de-emphasis characteristic to the spatial frequency in the vertical direction of the reproduced video signal, the de-emphasis circuit having a delay circuit that provides a de-emphasis characteristic to the vertical spatial frequency of the reproduced video signal, the delay circuit having a delay time that is controlled by an external clock frequency; 1. A de-emphasis circuit comprising: a clock frequency generating circuit for applying the external clock frequency to the delay element; and means for switching the output clock frequency of the clock frequency generating circuit according to a playback speed. (2) The de-emphasis circuit according to claim 1, wherein the de-emphasis circuit imparts a preset de-emphasis characteristic to the input reproduced video signal, regardless of the amplitude of the input reproduced video signal. (3) The de-emphasis circuit provides a non-linear de-emphasis characteristic to the input reproduced video signal that allows the de-emphasis characteristic to be variably controlled according to the amplitude of the input reproduced video signal. Day emphasis circuit.