JPS6357995B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for removing energy spread signals superimposed on television signals.

As is well known, frequency modulation is used in satellite television broadcasting. If the video signal component of a frequency-modulated TV signal becomes almost unmodulated, the sideband energy will be concentrated around a certain frequency due to the synchronization signal pulse, potentially affecting other communication systems. . For this reason, satellite TV signals are synchronized with the vertical synchronization signal.
A 30Hz triangular wave is superimposed as an energy spread signal.

Therefore, when receiving this broadcast radio wave with a television receiver, it is necessary to remove this energy spread signal from the demodulated video signal.

FIG. 1 shows a television signal on which a spread signal is superimposed. This spread signal 1 is a 30 Hz triangular wave and is superimposed in synchronization with the vertical synchronization signal 2 of the television signal. A diode peak clamp circuit has conventionally been used as this spread signal removal circuit.

Figure 2 shows a diode peak clamp circuit. The peak clamp circuit consists of a capacitor 4 in series and a diode 5 in parallel, as shown.
It consists of , and operates as follows. The signal input from the terminal 3 has its tip potential fixed at the voltage V ₀ supplied from the diode 5 by charging and discharging the capacitor 4 and switching operation of the diode 5, and an output signal is taken out from the terminal 6.

In this peak clamp circuit, if the capacitance of the capacitor 4 is selected to be large, the television signal on which the spread signal is superimposed is directly applied to the diode 5, and the spread signal cannot be removed sufficiently.

On the other hand, if the capacitance of capacitor 4 is selected to be small, the amplitude of the spread signal can be made smaller by capacitor 4,
The diode 5 allows the voltage at the tip of the synchronizing signal of the television signal to be maintained at a substantially constant voltage. Then, a signal from which the energy spread signal has been removed can be obtained from the output terminal 6. However, capacitor 4
If the value is selected so that the low frequency components of the video signal, such as the energy spread signal, can be removed, the low frequency components of the vertical synchronization signal, which has many low frequency components, will also be removed at the same time, as shown in Figure 3. Moreover, distortion occurred in the vertical synchronizing signal waveform, resulting in sag 2', making the synchronization of television images unstable.

That is, in the conventional diode clamp circuit, if the capacitance of the capacitor is increased, the energy spread signal cannot be removed, and if the capacitance is reduced, the energy spread signal is removed, but a sag occurs in the vertical synchronization signal.

SUMMARY OF THE INVENTION An object of the present invention is to provide a circuit capable of eliminating the above-mentioned drawbacks of the prior art, faithfully reproducing an output waveform, and sufficiently removing energy spread signals.

The present invention will be explained below by way of examples. Fourth
The figure is a block diagram showing one embodiment of the present invention.
In the figure, 7 is an input terminal, and 8 is a distribution circuit that branches the signal into two. 9 is a peak clamp circuit composed of a diode and a relatively large coupling capacitance.
Most of the energy spread signal superimposed on the video signal is removed to reduce the level fluctuation of the peak value of the synchronization signal. Reference numeral 10 denotes a synchronization separation circuit, which functions to extract a synchronization signal from the video signal in which the energy spread signal has been attenuated. Reference numeral 11 denotes a differentiating circuit, which is composed of a capacitor in series with the line and a parallel resistor, and removes low frequency components of the synchronizing signal. Then, the energy spread signal is removed by narrowing the pulse width of the vertical synchronization signal and making the pulse widths of the horizontal and vertical synchronization pulses substantially equal and applying them to the pulse clamp circuit as a gate signal.

FIG. 5 is a circuit diagram showing a specific example of this embodiment. In the figure, a video signal transmitted from an input terminal 7 is branched into two by a distribution circuit 8.

The first branch output is amplified by an amplifying transistor 19 and transmitted to a pulse clamp circuit 12 constituted by a coupling capacitor 20, a transistor 21, and a resistor and capacitor (not labeled). A pulse waveform-shaped by a differentiating circuit 11 is applied to the base of the transistor 21 via a synchronous separation circuit 10.

Further, the second branch output is applied to the peak clamp circuit 9 via the transistor 14. The peak clamp circuit 9 includes a coupling capacitor 15 and a clamping diode 16 to which a bias voltage is applied by a DC voltage source 17, and reduces the amplitude of the energy spread signal superimposed on the video signal.

As described above, reducing the value of the coupling capacitor 15 reduces the amplitude of the energy spread signal, but causes waveform distortion of the synchronizing signal. On the other hand, when the value of the coupling capacitor 15 is increased, the residual amount of the energy spread signal becomes larger, but the waveform distortion of the synchronizing signal becomes smaller. In this specific example, a capacitor with a relatively large value, for example, about 2 μF, is used as the coupling capacitor 15. Although some residual energy spread signals remain, synchronization separation can be ensured.
Reduce the waveform distortion (sag) of the synchronization signal.

In this way, when a capacitor with a relatively large capacity is used as the coupling capacitor 15, some energy diffusion signal remains, but according to this embodiment, it is possible to prevent the adverse effects of this residual amount.
The reason for this will be explained below with reference to FIGS. 6 and 7.

As shown in FIG. 6a, a television video signal is composed of a horizontal synchronizing signal 24 having an amplitude of about 20% and a video signal component 23 having an amplitude of 80%. Further, the vertical synchronization signal 26 has a wider pulse width than the horizontal synchronization signal 25, as shown in the figure.

FIG. 6b shows the waveform of the synchronization signal separated from the video signal a. The polarity of the synchronizing pulse applied to the base of the transistor 21 of the pulse clamp circuit 12 needs to be opposite to the waveform shown in the video signal a, as shown in b. Therefore, vertical synchronization signal 2
6, the part where the pulse level is high is wide and flat.

The output wave b of this synchronous separation circuit 10 is differentiated by a differentiating circuit
1, the pulse width at the upper end of the vertical synchronizing signal 26 becomes approximately the same width as the horizontal synchronizing signal 25, like the differentiated vertical synchronizing signal 27, as shown in FIG. 6c.

Now, the energy spread signal superimposed on the video signal is attenuated after passing through the pulse clamp circuit 12, and the minute amplitude ΔE becomes the residual portion of the energy spread signal as shown in FIG. 7a. Unless the value of this residual ΔE is attenuated below the detection limit with respect to the amplitude of the entire video signal, flickering of brightness and darkness on the screen, so-called flicker, will occur. Note that point P is the position of the vertical synchronization signal.

Now, if we were to apply the synchronization pulse without differential shaping, that is, the synchronization pulse shown in FIG. Therefore, the level at the tip of the vertical synchronization signal is fixed at a constant value. Therefore, even if the amplitude of the residual portion of the spread signal is △E, if the P point is fixed as P ₀ in Figure 7b, if a synchronization error occurs, the vertical synchronization signal P ₀ ，
At central points Q ₁ and Q ₂ far from P ₀ ', the amplitude of the level fluctuation is larger than the residual ΔE,
Its maximum amplitude is 2ΔE. When such a level fluctuation occurs, the points Q ₁ and Q ₂ are located almost at the center of the screen, which causes screen flickering and is easily noticeable.

On the other hand, if the voltage applied to the pulse clamp circuit 12 is shaped into a waveform by a differentiating circuit as shown in FIG. It turns out. For this reason, the level at the tip of the vertical synchronization signal is not fixed at a constant value, and the level of the vertical synchronization signal changes △ in response to the phase fluctuation of the superimposed voltage, such as P ₁ , P ₂ , and P ₃ . This results in fluctuations in the amount of E. However, since the level fluctuation of the entire screen has a value of ΔE, flickering does not occur. In the pulse clamp circuit 12, the leading edges of the vertical and horizontal synchronizing signals always correspond to the pulses 25 and 26 shown in FIG. 6c.
Since the vertical synchronization signal is clamped, the sag that occurs particularly in the vertical synchronization signal is reduced. Sag occurring near the vertical synchronization period is reduced, and a video signal faithful to the original signal as shown in FIG. 6a can be reproduced.

As described above, according to the present invention, the entire period of the video signal can be clamped with uniform pulses, so level fluctuations due to the residual energy spread signal in the video signal are reduced, resulting in a stable signal with less flicker interference. You can get the image. Furthermore, since no sag occurs in the synchronization signal, a synchronization signal with less waveform distortion can be obtained within the television receiver. Therefore, the stability of image synchronization is improved, and even when a multiplexed signal is superimposed on a synchronization signal, the multiplexed signal can be faithfully reproduced.

[Brief explanation of drawings]

Figure 1 is a waveform diagram of a television video signal superimposed with an energy spread signal, Figure 2 is a circuit diagram of an energy spread signal removal circuit configured with a conventional diode peak clamp circuit, and Figure 3 is a diagram of a conventional circuit. An output waveform diagram when the energy spread signal is removed, FIG. 4 is a block diagram of an embodiment of the present invention,
FIG. 5 is a circuit diagram showing a specific example of one embodiment of the present invention, and FIGS. 6 and 7 are signal waveform diagrams for explaining the circuit operation of the present invention. 9...Peak clamp circuit, 10...Synchronization separation circuit, 11...Differential circuit, 12...Pulse clamp circuit.

Claims (1)

[Claims] 1 A distribution circuit receives a television signal on which an energy spread signal is superimposed and branches the input television signal into two, and a distribution circuit receives a first branch output signal and pulses the first branch output signal. A pulse clamp circuit that clamps the second branch output signal; a peak clamp circuit that receives the branched second branch output signal, has a coupling capacitor of relatively large capacity, and peak-clamps the second branch output signal; and the peak clamp circuit. A sync signal separation circuit receives an output video signal of the output signal, and includes a sync signal separation circuit that separates and outputs a sync signal included in the video signal, and a differentiation circuit that receives the sync signal separated by the sync signal separation circuit. By applying a synchronizing pulse whose pulse width is narrowly shaped by the differentiating circuit to the gate signal input terminal of the pulse clamp circuit, discontinuous fluctuations in the level at the tip of the vertical synchronizing pulse are eliminated, and the pulse width of the vertical synchronizing signal is narrowly shaped by the differentiating circuit. An energy spread signal removal circuit characterized in that a video signal having a small variation in the leading edge level of vertical and horizontal synchronizing signals is extracted from an output end of a pulse clamp circuit.