JPS63152290A - White level inverting compensation circuit - Google Patents

Abstract

PURPOSE:To obtain a video image without white inversion by outputting a luminance signal before one horizontal scanning period stored in a storage means in place of an output luminance signal of a demodulation circuit when the luminance signal level changes suddenly from black to white level. CONSTITUTION:A delayed luminance signal YD is inputted to other terminal of a switch 17 of a dropout compensation circuit 16, after the luminance signal Y through a de-emphasis circuit 15 is delayed by a CCD element 18 having a storage time of 1H (H=horizontal scanning period) and suppressed a high frequency component by a low pass filter 19. On the other hand, a detection circuit 22 to detect the sudden change from the black to white level of luminance signal Y is connected to a de-emphasis circuit 15 and when the sudden change from black to white level is detected by the circuit 22, the detection pulse is fed via an OR gate 20 as a switching signal of the switch 17 of a dropout compensation circuit 16. Thus, at the sudden change from the black to the white level, the signal YD before 1H in place of the luminance signal Y is outputted from the output terminal 21. Thus, the video image without white level inversion is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a white reversal compensation circuit in a magnetic recording/reproducing apparatus (VTR) for recording and reproducing video signals.

(Prior art) In a VTR, when there is a sudden change in the video signal from black to white, the frequency of the FM carrier wave increases and the lower sideband component of the FM is superimposed, so when this is played back from the video head. , there appear peaks and troughs in the reproduced FM video signal that do not cross the center line on the time axis, as shown in Figure 6 (a). Therefore, such a reproduced FM video signal cannot be directly applied to the limiter. When passing through, the part that does not cross the center line is ignored, so the output waveform of the limiter is as shown in Figure 6 (b)
It will look like the one shown below. When the output signal of this limiter is input to the FM demodulation circuit, the portions that do not cross the center line on the time axis are demodulated as a black level as shown in FIG. 6(C).

Such a phenomenon is generally called a white reversal phenomenon.

Conventionally, in order to prevent such a white reversal phenomenon, a white reversal prevention circuit as shown in FIGS. 7 to 9 has been used.

FIG. 7 shows that the reproduced FM signal reproduced by the video head 1 is input to the reproduction equalizer 4 via the rotary transformer 2 and the preamplifier 3, and the equalizer 4 restores it to a reproduced FM video signal with balanced upper and lower sidebands. This signal is input to the post-FM demodulation circuit 5.

Fig. 8 shows a non-linear emphasis system in which the gain of the amplifier 6 is changed according to the frequency characteristics shown in Fig. 8 (b), and the upper and lower sidebands of the reproduced FM video signal are balanced and output to the FMI harmonic harmonic path. It is a circuit.

In FIG. 9, a reproduced FM video signal is input to a high-pass filter 7 and a low-pass filter 8, and after sufficiently attenuating the lower side band components by the high-pass filter 7, the level is adjusted by a first limiter 9. The signal is input to the mixer 10, mixed with the case band wave extracted by the rough reduction filter 8, and finally the level is adjusted by the second limiter 11 to extract a signal with balanced upper and lower sidebands.
This is a double limiter type circuit that inputs to the FM demodulation circuit.

However, since the circuits shown in FIGS. 7 to 9 described above are intended to prevent white reversal, there is a problem in that white reversal cannot be prevented if the change from black to white is steep.

(Problems to be Solved by the Invention) The present invention has been devised in view of the fact that conventional circuits may not be able to prevent white reversal. It is about providing.

[Configuration of the Invention (Means for Solving Problems)] The present invention provides a storage means for storing a demodulated luminance signal for at least one horizontal scanning period, and a storage means for storing a demodulated luminance signal from a black level to a white level. and a switch means that is switched by the detection output of the detection means and outputs the luminance signal stored in the storage means instead of the output luminance signal of the demodulation circuit. .

(Function) When the luminance signal suddenly changes from black to white, a luminance signal stored in the storage means that is one horizontal scanning period earlier is output instead of the output luminance signal of the Vi tone circuit.

(Embodiment) FIG. 1 is a circuit diagram showing an embodiment of a VTR playback circuit incorporating the white inversion compensation circuit of the present invention. After being amplified to a predetermined level, the signal is input to an FM demodulation circuit 13 and a dropout detection circuit 14 via an AGC circuit 12. The FM demodulation circuit 13 demodulates the luminance signal Y from the reproduced FM video signal, and de-emphasis circuit 15
After suppressing high-frequency components, the dropout compensation circuit 16
input to one end of the switch 17.

The other end of the switch 17 of the dropout compensation circuit 16 receives the luminance signal Y that has passed through the de-emphasis circuit 15 as an IH (
COD element 18 having a storage time of H=horizontal scanning period)
A one-time delayed signal YD is input which is delayed by 1 and whose high-frequency components are roughly suppressed by a low-pass filter 19.

On the other hand, when the reproduced video signal is smaller than a predetermined amplitude value, the dropout detection circuit 14 outputs a dropout detection pulse indicating this, and sends this detection pulse to the switch 17 of the dropout compensation circuit 16 via the OR gate 20. Supplied as a switching signal. As a result, for the FM video signal Y smaller than the predetermined amplitude value, the switch 17 of the dropout compensation circuit 16 is switched to the signal YD side, so that the signal YD is output from the output terminal 21 instead of the signal Y.

On the other hand, a detection circuit 22 that detects a sudden change in the luminance signal Y from black to white is also connected to the de-emphasis circuit 15, and when this detection circuit 22 detects a sudden change from black to white, it is detected. The pulse is supplied as a switching signal to the switch 17 of the dropout compensation circuit 16 via the OR gate 20. Therefore, when there is a sudden change from black to white, the signal YD from one day earlier is output from the output terminal 21 instead of the luminance signal Y.

In this way, by the same principle as dropout compensation, 1
By outputting the luminance signal YD before H in place of the luminance signal Y in which the sudden change from black to white has occurred, an image free from the white inversion phenomenon can be obtained.

By the way, the detection circuit 22 can be configured similarly to the synchronous separation circuit. That is, if the output signal Y of the de-emphasis circuit 15 is as shown in FIG. If input into a similar configuration circuit,
A signal as shown in FIG. 2(b) is obtained. Therefore, if you mask the signal part of the synchronization signal,
As shown in FIG. 4C, a detection pulse with a pulse width S can be obtained at a timing corresponding to the sudden change part a from black to white.

In this case, a response delay of the dropout compensation circuit 16 may occur depending on how the threshold value for detecting a sudden change in the luminance signal Y is set.

FIG. 3 is a circuit diagram showing a main part of another embodiment of the present invention designed to prevent such response delay.

Here, if we enlarge the part a where a sudden change from black to white occurs in the luminance signal Y in FIG. 2(a) and its detection pulse, we get the following:
As shown in FIGS. 4(a) and 4(b), the detection circuit 22
If the threshold value vth is as shown by the broken line, a 1D width detection pulse is output.6However, since the width of the sudden change from black to white is tw, the tp width detection pulse is After 1x, 1. (1,=11)+1x+1. ,)
A time difference of 1x, 1. In this section, the dropout compensation circuit 16 may not respond. Figure 3 is 1
This is designed to obtain a detection pulse with a width of w, and the detection circuit 2
2, the luminance signal Y as shown enlarged in Fig. 5 (a)
is input, this is identified by the threshold value yth, and it is shown in Fig. 5 (b
) is output.

Therefore, this detection pulse is inverted by the inverter 23 to obtain a signal as shown in FIG. 5(C), and this signal is roughly integrated by the integrating circuit 24 to obtain a signal as shown in FIG. 5(d). Form. Then, the integrated signal is shaped by a waveform shaping circuit 25 to form a signal as shown in FIG. 5(e).

As a result, a signal whose fall timing is delayed by t×time from the trailing edge of the abrupt change portion of the luminance signal Y is obtained. Therefore, when the detection pulse of the detection circuit 22 (FIG. 5(b)) and the output signal of the waveform shaping circuit 25 are combined by the OR gate 26, a signal with a width of (1, +1.) is obtained as shown in FIG. Therefore, this (1, +1.) width signal is input as the switching signal 9 of the switch 17 of the dropout compensation circuit 16, and the luminance signal @Y of the de-emphasis circuit 15 is input to the delay circuit 27. As shown in FIG. 5(g), the dropout compensation circuit 1 is delayed by tx time.
Enter 6. This is equivalent to moving the output signal of the OR gate 26 forward by the time tx with respect to the output signal Y of the delay circuit 27. That is, the time width and timing of the sudden change portion of the output signal Y of the de-emphasis circuit 15 match the time width and timing of the output signal of the OR gate 26. As a result, the brightness signal Y of the sudden change part a is changed to 1H.
It is possible to completely replace the previous luminance signal YD and output it.

In the above embodiment, the dropout compensation circuit and the white inversion compensation circuit are used as storage means for the luminance signal from one day ago, but they may be provided separately. When used in combination, there is an advantage that costs can be saved.

[Effects of the Invention] As explained above, according to the present invention, when the reproduced video signal suddenly changes from black to white, the video signal of the previous horizontal scanning period is embedded and output, so that the video without the white inversion phenomenon can be produced. can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a VTR reproducing circuit to which the present invention is applied, FIG. 2 is a waveform diagram for explaining the operation of the circuit in FIG. 1, and FIG. Figures 4 and 5 are waveform diagrams for explaining the operation of the circuit in Figure 3;
FIG. 6 is a waveform diagram for explaining the reason why the white reversal phenomenon occurs, and FIGS. 7 to 9 are circuit diagrams showing examples of conventional white reversal prevention circuits for preventing the white reversal phenomenon. 1... Video head 13... FM demodulation circuit 16... Dropout compensation circuit 18... COD element 22... Detection circuit Agent Patent attorney Nori Chika Yudo Hiroshi Uji Figure 2 Figure 3

Claims (1)

[Claims] a demodulation circuit for demodulating a luminance signal from a reproduced video signal reproduced by a video head; a storage means for storing the demodulated luminance signal for at least one horizontal scanning period; and a switch means that is switched by the detection output of the detection means and outputs a luminance signal stored in the storage means in place of the output luminance signal of the demodulation circuit. Inversion compensation circuit.