JPH02261283A - Clamp circuit - Google Patents

Abstract

PURPOSE:To improve the elimination ratio of an energy spread signal without deteriorating the elimination ratio of low frequency noise by providing plural stage clamp means whose time constant differs and setting the elimination ratio of the low frequency noise and a dispersal signal independently. CONSTITUTION:The time constant represented by a capacitance of a 1st clamp capacitor 31 and a resistance of a resistor 32 is set to a value sufficiently eliminating low frequency noise and the time constant represented by a capacitance of a 2nd clamp capacitor 33 and a resistance of a resistor 34 is set to a value sufficiently eliminating a dispersal signal. Through the constitution above, the low frequency noise included in a MUSE signal is eliminated by the actin of the capacitor 31 and the resistor 32 and the dispersal signal is eliminated by the action of the capacitor 33 and the resistor 34 thereby obtaining the MUSE signal, from which both the low frequency noise and the dispersal signal are sufficiently eliminated.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention provides a method for receiving a television signal subjected to band compression, frequency modulation, and addition of an energy spread signal using a multiplex subsampling method, The present invention relates to a decoder that demodulates the received television signal into the original wideband television signal, and particularly to a clamp circuit that removes the energy spread signal by clamping the received television signal to a predetermined level.

(Prior Art) As a method for transmitting a wideband high-definition television signal by compressing it to a practical band for transmission using a multiplex subsampling method, and demodulating the original wideband television signal on the receiving side, for example, , MUSE (MultlpI
eSub-NyqulstSampullngEnco
ding) method.

The transmission signal format of this MUSE method is shown in FIG. Further, the synchronization signal waveform is shown in FIG.

FIG. 3 shows the frame pulse and horizontal synchronization signal HD. Here, CK is the transmission lock, and its frequency is 1
It is set to 6.2MHz.

By the way, when broadcasting satellites are used to transmit television signals after changing the frequency (A) (FM), interference with terrestrial fixed services becomes a problem. In order to prevent this interference, it has been decided to reduce the power density per 4 KHz by 22 dB for the broadcasting satellite service and superimpose the 600 KHz r-p energy spread signal.

In a decoder that receives a television signal on which such an energy spread signal is superimposed and demodulates it,
Generally, a clamp circuit is provided that clamps the pedestal level of the television signal in synchronization with a synchronization signal, and the energy diffusion signal is removed by regenerating the direct current of the television signal using this clamp circuit.

According to an evaluation experiment on flicker interference conducted using a general NTSC television signal using a triangular wave with a repetition frequency of 307 as an energy spread signal, it was found that the flicker interference was approximately 0.0. It has been found that flicker can be detected by adding an OIV energy spread signal.

FIG. 4 shows an energy diffusion signal (hereinafter referred to as a disversal signal) when a MUSE television signal (hereinafter referred to as MUSE signal) is transmitted by a broadcasting satellite. This disversal signal has a period of 1/30 s and a frequency deviation of 600K for the maximum frequency deviation (frequency difference between white level and black level) of 10.2 MH2 during FM transmission.
It is a triangular wave of Hz.

In a MUSE decoder that demodulates a MUSE signal to which such a disversal signal has been added, the disversal signal can be recovered by clamping the MUSE signal to a predetermined level using a clamp circuit, similar to the current NTSC decoder. It is designed to be removed. In this case, the clamp processing (DC regeneration processing Pl) takes advantage of the fact that the average DC level of the horizontal synchronizing signal HD is equal to the DC level during the clamp level period of line 563 in Figure 2. This is done by clamping the average DC level of the synchronization signal HD to the DC level of the clamp level period.

Here, a conventional clamp circuit in a MUSE type decoder will be explained using FIG. 5.

In FIG. 5, an input terminal 11 is supplied with a MUSE signal. This MUSE signal is amplified by an amplifier circuit 12 to a predetermined signal amplitude, and then supplied to a clamping capacitor 14 via a band-limiting low-pass filter (hereinafter referred to as LPF) 13 to a predetermined level. be clamped. This clamp output is current-amplified by an amplifier circuit 15, and then converted into a digital signal by an analog/digital conversion circuit (hereinafter referred to as an A/D conversion circuit). This digital signal is supplied to a MUSE signal processing section (not shown).

The MUSE signal digitized by the A/D conversion circuit 16 is further supplied to a level detection circuit 17. This level detection circuit 17 detects the DC level based on the digital code of the human input signal, and supplies the detection result to the DC regeneration circuit 18. This DC regeneration circuit 18 includes a level detection circuit 18
A signal having a level corresponding to the difference between the detection level of the clamp level period (reference signal level period) of the 563rd line and the 1125th line (reference signal level period) shown in FIG. 2 and the reference DC level is generated among the detection levels of . This difference signal is converted into an analog signal by one digital/analog conversion circuit (hereinafter referred to as a D/A conversion circuit), and then supplied as a clamp control signal to a circuit consisting of a resistor 21 and a capacitor 14 via a switch 20. Ru. Here, the switch 20 is a terminal 22
The clamp gate pulse CP supplied to MUSE
It is turned on during the horizontal synchronization signal period. As a result, the MUSE signal appearing at the output terminal of the capacitor 14 is
During the horizontal synchronization signal period, the average DC level of the horizontal synchronization signal HD is clamped to match the reference DC level.

The clamp characteristic of the above-mentioned clamp circuit, that is, the dispersal signal removal characteristic is determined by the time constant τ of the clamp circuit. This time constant τ is expressed by the following equation, and the smaller the value, the higher the disversal signal removal function as a DC regeneration function.

τ-RCX (Tl /T2) However, R: resistance value of the resistor 21 C: capacitance value of the capacitor 14 T1: 1 horizontal period T2: 1 clamp period in the horizontal period In the MUSE method, as shown in FIG. Horizontal period T
The number of samples is 480 (16,2Mk), of which 11 samples are allocated to the horizontal synchronization signal period. Therefore, the clamp period T2 per horizontal period T is a period corresponding to 11 samples at most. The values of T and /T2 found from this are about 10 times larger than those of the NTSC system. Therefore, in the conventional MUSE type clamp circuit, by reducing the time constant CR due to the capacitance value C of the capacitor 14 and the resistance value R of the resistor 21, a disversal signal rejection rate of approximately the same level as the NTSC type can be achieved. I was trying to get it.

However, by reducing the time constant CR in this way, conventional methods have solved the problem that when the S/N (signal-to-noise ratio) of the received signal is poor, the clamp level fluctuates due to low-frequency noise and the image quality deteriorates. was occurring.

That is, the circuit consisting of the capacitor 14 and the resistor 21 is
During the clamp period T2, it is a low-frequency removal circuit and serves to reduce low-frequency noise during the clamp period T2. In this case, the low-frequency noise removal rate is determined only by the time constant CR, and the larger this is, the higher it becomes. Therefore, as described above, the configuration in which the time constant CR is reduced in order to increase the disversal signal removal rate cannot sufficiently remove low-frequency noise. This poses no problem if the S/N of the received signal is good, but the S/N of the received signal
If N deteriorates, the clamp level will fluctuate due to low-frequency noise, and the image quality will deteriorate.

(Problem to be Solved by the Invention) As described above, in the conventional clamp circuit that removes the disversal signal from the MUSE signal by DC regeneration, in order to increase the removal rate of the disversal signal, the time constant of the time constant circuit is Because of this, it was not possible to remove low-frequency noise sufficiently, and the S/
When N deteriorates, there is a problem in that the clamp level fluctuates and the image quality deteriorates.

Therefore, an object of the present invention is to provide a clamp circuit that can sufficiently remove not only dispersal signals but also low-frequency noise.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides two stages of clamping means with different time constants, the first stage clamping means removes low frequency noise, and the second stage clamping means removes low frequency noise. The energy diffusion signal is removed using a clamp circuit.

(Function) According to the above configuration, since the low-frequency noise removal rate and the dispersal signal removal rate can be set independently, both the low-frequency noise and the dispersal signal can be sufficiently removed.

(Embodiments) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention. In FIG. 1, the same parts as those in FIG. 5 are given the same reference numerals, and detailed explanations will be omitted.

In FIG. 1, the output terminal of the LPF 13 is connected to one end of a first clamp capacitor 31. The other end of this capacitor 31 is connected to one end of the first clamping resistor 32, and the second clamping capacitor 33
connected to one end of the The other end of the second clamping capacitor 33 is connected to one end of the second clamping resistor 34 and also to the input terminal of the amplifier circuit 15 . 1st. The other ends of the second clamping resistors 32 and 34 are connected to the output terminal of the D/A conversion circuit via switches 35 and 36, respectively. The switches 35 and 36 are turned on during the horizontal synchronization signal period for each line according to the clamp gate pulse CP.

Time constants C and R, represented by the capacitance value C1 of the first clamping capacitor 31 and the resistance value R1 of the first clamping resistor 32, are set to values sufficient to remove low-frequency noise. . On the other hand, the second clamp capacitor 3
3 and the resistance value R of the second clamping resistor 34.
The time constant C2R2, denoted by 2, is set to a value sufficient to eliminate the dispersal signal. That is, the time constant ClR1 is set to approximately the same value as the time constant CR in the NTSC system clamp circuit, and the time constant C2
R2 is set to approximately the same value as the time constant CR in a conventional MUSE type clamp circuit. In other words, C2R2 is set to have a value ten times or more greater than C1R1.

In the above configuration, the switches 35 and 36 are turned on during the horizontal synchronization signal period of the MUSE signal, so that the MUSE signal band-limited by the LPF 13 is first affected by the clamping action of the capacitor 31 and the resistor 32. After the average DC level of the horizontal synchronizing signal HD is clamped to the reference DC level, it is again clamped to the reference DC level by the clamping action of the capacitor 33 and the resistor 34.

In this case, the low frequency noise contained in the MUSE signal is removed by the action of the capacitor 31 and the resistor 32, and the disversal signal is removed by the action of the capacitor 33 and the resistor 34. As a result, a MUSE signal from which both low-frequency noise and disversal signals are sufficiently removed can be obtained.

As described above, in this embodiment, by providing two stages of clamping means with different time constants CR, it is possible to independently set the low frequency noise rejection rate and the disversal signal rejection rate. be.

Therefore, according to this embodiment, not only the disversal signal but also low-frequency noise can be sufficiently removed. As a result, even if the S/N of the received MUSE signal is poor, fluctuations in the clamp level due to low-frequency noise can be suppressed, and deterioration in image quality due to fluctuations in the clamp level can be prevented. I can do it.

Although one embodiment of the present invention has been described above in detail, the present invention is not limited to this embodiment.

For example, in the previous embodiment, a case has been described in which the DC level is generated by detecting the DC level during the clamp level period of the clamp control signal, but other methods may be used.

Further, in the previous embodiment, the case where the clamping process is performed every horizontal period has been described, but it may be performed every two horizontal periods, every three horizontal periods, and so on.

Furthermore, in the previous embodiment, the case where the present invention was applied to a MUSU type clamp circuit was explained, but the present invention
The present invention can also be applied to clamp circuits using multiple subsampling methods other than the MUSE method.

It goes without saying that this invention can be modified in many other ways without departing from the spirit thereof.

[Effects of the Invention] As described above, according to the present invention, by providing a two-stage clamping means, it is possible to independently set the low frequency noise removal rate and the energy diffusion signal removal rate. Therefore, the rejection rate of energy spread signals can be increased without reducing the rejection rate of low-frequency noise, and even if the S/N of the received signal is poor, the image quality will not deteriorate due to fluctuations in the clamp level. You can avoid it.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention, and FIG.
The figure shows the transmission signal format of the MUSU signal, and Figure 3 shows the MUSU signal transmission signal format.
FIG. 4 is a diagram showing the synchronous signal waveform of the USE signal, FIG. 4 is a diagram showing the standard of the disversal signal, and FIG. 5 is a circuit diagram showing the configuration of a conventional clamp circuit. 11...Input terminal, 12.15...Amplification circuit, 13
...LPF, 16...A/D conversion circuit, 17...
Level detection circuit, 18... DC regeneration circuit, 19...
D/A conversion circuit, 22... terminal, 31.33... capacitor, 32.34... resistor, 35.36... switch.

Claims (1)

[Scope of Claims] Provided in a decoder that receives a television signal subjected to band compression, frequency modulation, and addition of an energy spread signal using a multiplex subsampling method, and demodulates this into the original wideband television signal, A clamp circuit that removes the energy spread signal by clamping the received television signal to a predetermined level has a time constant capable of removing low-frequency noise contained in the received television signal, a first clamping means for clamping the signal to a predetermined level at a predetermined period; and a time constant capable of removing the energy spread signal contained in the received television signal, and a clamp output of the first clamping means. and second clamping means for clamping the voltage at a predetermined level at a predetermined period.