JPS5920312B2 - Scan speed modulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scanning speed modulation circuit that performs contour compensation by modulating the horizontal scanning speed of a television receiver. purpose.

It is generally known to perform contour compensation by modulating the scanning speed.

This principle will be briefly explained using the waveform diagram in FIG. When a video signal as shown in figure a is applied, it is first differentiated to create the waveform shown in figure b. This linearly differentiated signal current is added to the main deflection current A to create a scanning signal as shown in c in the figure. Although this adding means may actually send this first-order differential signal to the main deflection yoke, it is more practical to provide a separate auxiliary deflection yoke. This is because a large flyback pulse is generated in the main deflection yoke, and this pulse flows back into the scanning speed modulation circuit, requiring a transistor with a very high withstand voltage in the circuit, and the main deflection yoke is subject to high frequency current. This is due to the fact that it is difficult to flow. In this way, when the main deflection current is modulated by the first-order differential waveform of the video signal, the steepest part of the deflection radio waveform, i.e., cC7)B in the same figure.
In the part, the scanning speed becomes faster and the image becomes darker.
The part with a gentle slope of the polarized current waveform, i.e., c (
7) In the part C, the scanning speed becomes slower, so the image becomes brighter, and the image is displayed as an image with emphasized outlines as shown in d of the same figure.

Generally, when contour compensation is performed by scanning speed modulation, the S/N on the screen does not deteriorate much even if scanning speed modulation is performed in a strong electric field, that is, when screen noise is not noticeable, but in a weak electric field, that is, when the screen noise is not noticeable. If scanning speed modulation is performed in a state where noise is noticeable, the S/N ratio on the screen will deteriorate considerably. As a means to solve the above drawbacks, the amount of contour compensation by scanning speed modulation is controlled according to the electric field strength, and the amount of contour compensation by scanning speed modulation is reduced when the electric field is weak, that is, when screen noise is noticeable. A method has been proposed to prevent deterioration of the S/N ratio.

This method will be explained in detail using FIG. In the figure, 1 is a video signal input terminal, 2 is an N-force terminal, 3 is a DC amplifier, 4 is a gate circuit, 5 is a scanning speed modulation circuit, and 6 is an auxiliary deflection yoke.

The AGC voltage is applied to the input terminal 2, and the DC amplifier 3 compares it with the reference voltage Eo and amplifies it. The video signal from the input terminal 1 is added to the gate circuit 4, and an amplifier 3 is obtained according to the AGC voltage.
The gate circuit 4 is controlled by opening and closing using the output voltage.

Therefore, when the electric field is strong, that is, when the AGC voltage is high,
The gate circuit 4 is opened, and the video signal from the input terminal 1 is input to the scanning speed modulation circuit 5, where contour compensation is performed by scanning speed modulation. Furthermore, when the electric field is weak, that is, when the AGC voltage is low, the gate circuit 4 is closed, so the video signal from the input terminal 1 is not input to the scanning speed modulation circuit 5, and contour compensation by scanning speed modulation is not performed.

As described above, by detecting the change in electric field strength using the AGC voltage and reducing the amount of contour compensation due to scanning speed modulation in the case of a weak electric field, deterioration of S/N on the screen can be prevented.

When controlling the amount of contour compensation by scanning speed modulation in accordance with the AGC voltage in this manner, the AGC voltage varies depending on the receiver, so that the amount of contour compensation shown in FIG. 2E. It was necessary to adjust the reference voltage for each set. FIG. 3 shows the relationship between AGC voltage and antenna input voltage. A is the RF, AGC voltage, and ( is the IF, AGC voltage. As shown in Figure 3, the AG
The ideal way to apply the C voltage is to apply the AGC voltage only to the 1F circuit in a relatively weak electric field and do not apply the AGC voltage to the tuner, so use it in a hidden state with less noise as the maximum gain, and on the other hand, when the input voltage is above a certain level (E2 or higher) only when RF. Apply AGC voltage. Therefore, when there is a large input signal, AGC is applied to the tuner sufficiently to prevent the 1F input from becoming too large and saturating the 1F circuit. By the way, as the AGC voltage applied to the input terminal 2 in FIG. 2, the RF. When trying to perform contour compensation by scanning velocity modulation using AGC voltage, if the antenna input voltage is lower than E2, RF. Since the AGC voltage is constant, it is impossible to control the contour compensation amount by changing the strength of the electric field. Moreover, the electric field at E2 is generally 80
It is a strong electric field of about dBμ, and the S/N is reduced by performing contour compensation.
at such a level that it deteriorates. There isn't. Therefore, if
．． It is practical to use the AGC voltage to control the gain of the scan rate modulation circuit. However, 1F. AGC voltage is 10
Since the value of m is very small and is easily affected by variations in the receiver, it is not easy to control.

The present invention provides a scanning speed modulation circuit that eliminates the above-mentioned drawbacks.

FIG. 4 shows an embodiment of the scanning speed modulation circuit of the present invention.

In the figure, 1 is an input terminal to which a video signal is applied, 12 is an input terminal to which a synchronization signal from a synchronization signal separation circuit is applied, and 13
14 is a gate circuit, 14 is a gain control circuit, 5 is a scanning speed modulation circuit, and 6 is an auxiliary deflection yoke. Here, a video signal is applied to the input terminal 1 and guided to the gate circuit 13.

A synchronization signal from a synchronization separation circuit is applied to an input terminal 12 and guided to a gate circuit 13, and the image signal is gated with the synchronization signal. That is, noise superimposed on the synchronization signal of the video signal during a weak electric field is detected only during the period of the synchronization signal. The detection signal is rectified by a diode D1 and a capacitor C1 to a DC voltage, which is used as a gain control voltage for the gain control circuit 14. Therefore, the video signal from the input terminal 1 is input to the gain control circuit 4.
In addition, if the gain of the video signal is controlled according to the DC voltage (according to the electric field strength) and added to the scanning speed modulation circuit 5, the amount of contour compensation by scanning speed modulation can be controlled according to the electric field strength. can do. The operation of the circuit of this embodiment will be explained in detail below with reference to FIGS. 5 and 6. In the case of a strong electric field, a video signal as shown in Fig. 6a is applied to the input terminal 1, guided to the base of the gate transistor Trl of the gate circuit 13, and a synchronizing signal shown in Fig. 6b from the input terminal 12 is applied to the emitter. , the video signal is gated with a synchronization signal. That is, noise superimposed on the video signal during a strong electric field is detected only during the synchronization signal period. The above detection signal is rectified by a diode D1 and a capacitor C1 to a DC voltage such as e. When the electric field is relatively strong, there is little noise, so this rectified voltage is low and becomes E1 []. Gain control circuit 14
When comparing the DC voltage E1 that can be applied to the base of the gain control transistor Tr at the emitter with the voltage obtained by dividing the resistor R1 and the voltage at the emitter (hereinafter referred to as reference voltage E), Since there is almost no difference between the DC voltages E1 and EO, there is a high impedance between the gain control transistors T and CE of the gain control transistors 2, so the video signal from the input terminal 1 is guided to the scanning speed modulation circuit 5 without being attenuated. It will be destroyed. In other words, the amount of the signal component bypassed through the capacitor C2 is very small. Therefore, in the case of a strong electric field, a relatively large amount of contour compensation can be obtained by scanning speed modulation. On the other hand, in the case of a weak electric field, a video signal with superimposed noise as shown in Fig. 6C is applied to the input terminal 1, and is guided to the base of the gate transistor Trl, and a synchronization signal shown in Fig. 6B from the input terminal 12 is sent to the emitter. The video signal is gated with the synchronization signal.

Therefore, as shown in FIG. 3A, noise superimposed on the video signal when the electric field is weak is detected only during the period of the synchronization signal. When the detection signal shown in the figure d is rectified by the diode D1 and the capacitor C1, it becomes a DC voltage of EOE2 (V) in the figure.The DC voltage E2 is applied to the base of the gain controlling transistor Tr2.
and the reference voltage E obtained by dividing the voltage between resistors R1 and R2 at the emitter. Since the DC voltage E2 is quite high, the impedance between C and E of the gain control transistor Tr2 becomes low, so a considerable part of the video signal from the input terminal 1 passes through the converter C2 and T,2. The attenuated video signal is bypassed through the scanning speed modulation circuit 5 and guided to the scanning speed modulation circuit 5. Therefore, in the case of a weak electric field, only a relatively small amount of contour compensation can be obtained by scanning speed modulation. That is, in the case of a weak electric field when screen noise is high, the amount of contour compensation by scanning speed modulation can be attenuated to prevent deterioration of the S/N ratio on the screen. As described above, the present invention detects the amount of noise superimposed on a video signal according to the electric field strength only during the synchronization signal period, rectifies the detection signal to obtain a DC voltage,
Adding the video signal to a gain control circuit and controlling the amount of the video signal according to the DC voltage (according to electric field strength),
By adding it to the scanning speed modulation circuit, the amount of contour compensation due to scanning speed modulation can be controlled in accordance with the electric field strength.

Therefore, it is possible to reduce the amount of contour compensation by scanning speed modulation when the electric field is weak, that is, when noise on the screen is noticeable, thereby preventing deterioration of the S/N ratio on the screen.

In addition, as a means of detecting electric field strength, the gain control circuit is operated with a DC voltage obtained by rectifying a signal that detects the amount of noise superimposed on the video signal, rather than using an AGC voltage, which eliminates variations in the receiver. Since there is no need to take this into consideration, there is no need for adjustment.

[Brief explanation of drawings]

Figure 1 A, b, c, d (are waveform diagrams showing the principle of scanning speed modulation, Figure 2 is a block diagram showing the configuration of a conventional scanning speed modulation circuit, Figure 3 is a characteristic diagram showing AGC voltage, FIG. 4 is a block diagram showing the configuration of the scanning speed modulation circuit in one embodiment of the present invention, FIG. 5 is an electrical wiring diagram showing the specific configuration of the same scanning speed modulation circuit, and FIGS. 6 a to 6e show its operation. 1... Video signal input terminal, 12... Synchronization signal input terminal, 13... Gate circuit, 14.
...Gain control circuit, 5...Scanning speed modulation circuit.

Claims (1)

[Claims] 1 means for detecting a synchronization signal of a video signal, means for gating the video signal with the synchronization signal, means for rectifying noise components contained in the signal obtained by said gating, and scanning speed modulation for the video signal. A scanning velocity modulation circuit comprising means for supplying the scanning velocity to the circuit, scanning velocity modulation means including an auxiliary deflection yoke for performing contour compensation by scanning velocity modulation, and means for controlling the gain of the scanning velocity modulation means by the rectified signal.