JPH0732466B2 - Clamp device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a clamp device used in a MUSE decoder.

2. Description of the Related Art Satellite broadcasting of high-definition television signals capable of obtaining high-definition images compared with the current standard television system 1
MU that performs band compression to enable transmission on a channel
The SE method has been proposed.

Reference: Yuichi Ninomiya, et al., "Satellite 1-Channel Transmission System (MUSE) for High-Definition Television", Technical Report System, Television Society of Japan, TEBS 95-2, March 22, 1984, Television Engineering Report VOL. 7, No.44.

This MUSE signal is FM-transmitted by satellite broadcasting.
Emphasis and dispersal (energy spread signal) are applied to improve N and to prevent interference with terrestrial communication systems. Therefore, the MUSE decoder removes dispersal when receiving the transmission signal from satellite broadcasting.
After performing de-emphasis, clamp and apply A / D conversion for signal processing. The level of the clamp voltage is detected after A / D conversion and fed back to the clamp circuit through the LPF and amplifier. The speed of the clamp response at that time is determined by the LPF time constant. In satellite broadcasting, the fluctuation of DC level is slow, the fast response of the clamp is not necessary, and the response speed of the clamp is slow because of the stability in the weak electric field.

Problems to be Solved by the Invention As described above, the MUSE signal compresses the frequency band of a high-definition television signal to about 1/3, so recording on a home VTR or video disc, which is a package system, is advantageous. Is. When these signals are reproduced by the MUSE decoder, for example, in a VTR, there is a change in DC level due to program editing, dropout, and device switching, and a fast clamp response is required. However, as described above, in the clamp circuit of the decoder suitable for the conventional satellite broadcasting, the response of the clamp is insufficient for reproducing the package system such as the VTR or the video disk.

In view of such a point, the present invention has an object to provide a clamp device capable of obtaining a response speed of a clamp suitable for an input signal.

Means for Solving the Problems The present invention includes an input circuit that performs dispersal removal and de-emphasis on an input signal, and a first switch that selects one of an output signal of this input circuit and another signal. ,
A clamp circuit that clamps the signal selected by the first switch, a DC level detection circuit that detects the DC level of the clamped signal, and a DC voltage detection circuit that integrates the detected DC level to control the clamp voltage of the clamp circuit. A clamp device for a MUSE decoder, which has an LPF output as a signal for switching and a second switch for changing the time constant of the LPF, and switches the first switch and the second switch in conjunction with each other.

Operation With the above-described configuration, the present invention increases the LPF time constant and delays the response of the clamp when a signal passes through the dispersal removal and de-emphasis circuit, that is, when receiving satellite broadcasting. Next, when a signal does not pass through the dispersal removal de-emphasis circuit, that is, a signal from a package system that interfaces with a signal having a flat frequency characteristic, the time constant of the clamp circuit is reduced to speed up the clamp response.

Embodiment 1 FIG. 1 shows a clamp circuit of a MUSE decoder in the first embodiment of the present invention.

The satellite broadcast signal is FM demodulated by the BS tuner 1, dispersal removed and de-emphasized, and input to the input terminal A of the switch 9. Input terminal B of switch 9 has VT
The signal from R2 is input. The signal selected by the switch 9 is clamped by the clamp circuit 4 and converted into a digital signal by the A / D converter 5. The DC level of the signal clamped by the clamp circuit 4 is A / D converted, detected by the DC level detector, integrated by the LPF 7, amplified by the amplifier 8, and then controlled by the clamp voltage of the clamp circuit. LPF7 is
It is composed of a time constant circuit of a resistor and a capacitor. The time constant of the LPF 7 is changed by controlling with the switch 10 whether or not the capacitor C ₂ is connected in parallel with the capacitor C ₁ . Switch 9 is A
The switch 9 and the switch 10 are interlocked so that the switch 10 is turned on when the switch 9 is tilted to the side, and the switch 10 is opened when the switch 9 is tilted to the position B. R (C ₁ + C ₂ ) = 0.5 to 1.0 sec and RC ₁ = 0.1 to 0.5 sec are suitable as specific time constant values.

As described above, when the input signal passes through the dispersal removal and de-emphasis circuit, the time constant of the clamp response is delayed, and when the input signal does not pass through the dispersal removal and the de-emphasis circuit, the time constant of the clamp response is increased. By doing so, an appropriate clamp response speed can be obtained according to the input signal.

Next, a second embodiment will be described with reference to FIG.
In FIG. 2, the same blocks as those in FIG. 1 will be described with the same numbers. It may be unclear whether the BS tuner 1 is separated from the decoder and the input signal of the decoder is emphasized. Therefore, the decoder detects whether the input signal is emphasized or not, and determines whether to pass the dispersal removal and de-emphasis circuit 3. At the same time, set the response speed of the clamp as described above. As a method of detecting whether or not the input signal is emphasized, attention is paid to the frame pulse added to the MUSE signal. The frame pulse has the waveform shown in FIG. 3 (a), but when emphasis is applied to this frame pulse, it becomes the waveform shown in FIG. 3 (b). Therefore, if the frame pulse has a value equal to or higher than the voltage level A, it can be detected that the input signal is emphasized.

In Figure 2, set level A with Vref14
Vref is compared by the comparator 12. FP detection circuit to extract only the frame pulse period from the output of comparator 12
The frame pulse period is detected at 16 and the output of the comparator 12 is gated at the gate 13. The output of the gate operates the monomulti 15, and the output controls the time constants of the switch 17 and the LPF 7. The mono-multi 15 operates at a frame cycle and controls for each frame pulse.

As described above, according to the second embodiment, the clamp response speed can be automatically set to the optimum value for the input signal depending on whether the input signal is emphasized or not.

As described above, according to the present invention, even when the MUSE decoder receives a signal from the VTR or a satellite broadcast as an input signal, the MUSE decoder has a response speed suitable for the characteristics of the DC level fluctuation of each input signal. It can be clamped, and the unsightlyness of the screen due to DC level fluctuations of the signal can be reduced. Moreover, its circuit configuration is simple, and its practical effect is very large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a clamp device according to a first embodiment of the present invention, FIG. 2 is a block diagram of a clamp device according to a second embodiment of the present invention, and FIG. 3 is a frame. It is a waveform diagram of a pulse. 3 ... Dispersal removal de-emphasis circuit, 4 ...
… Clamp circuit, 5 …… A / D converter, 6 …… DC level detector, 7 …… LPF, 8 …… Amplifier.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshimichi Otsuka 1-10-11 Kinuta, Setagaya-ku, Tokyo Inside the broadcasting technology research institute of the Japan Broadcasting Corporation (72) Inventor Izumi ▲ Yoshi ▼ Nori Kinuta, Setagaya-ku, Tokyo Broadcasting Technical Research Institute, Japan Broadcasting Corporation (72) Inventor Seiichi Koshi 1-10-11 Kinuta, Setagaya-ku, Tokyo Inside Broadcasting Technical Research Institute, Japan Broadcasting Association

Claims (2)

[Claims] 1. An input circuit for performing dispersal removal and de-emphasis on an input signal, a first switch for selecting one of an output signal of this input circuit and another signal, and this first switch A clamp circuit that clamps the selected signal, a DC level detection circuit that detects the DC level of the clamped signal, and a low level that integrates the detected DC level and outputs a signal that controls the clamp voltage of the clamp circuit. A clamping device comprising a bandpass filter and a second switch for changing a time constant of the lowpass filter, wherein the first switch and the second switch are interlocked to switch. 2. A patent characterized in that it has an emphasis detection circuit for detecting whether an input signal is emphasized or not, and controls the first switch and the second switch by the output of the emphasis detection circuit. The clamp device according to claim 1.