JPS63193777A - Television video modulator - Google Patents

Abstract

PURPOSE:To attain a highly stabilized action by setting a modulation degree and an output level at the stage of video signals preceding to a modulation circuit. CONSTITUTION:A video signal input 11 clamps the tip of a synchronizing signal in a prescribed voltage in a synchronous clamp circuit 9, and reproduces DC components required for the video signals. If the video signal 12 which has been synchronized and clamped is inputted to a carrier suppression modulation circuit 10 to which a DC bias voltage is not added, and a carrier 13 is amplified and modulated, carrier components appear in the output of the carrier suppression modulation circuit 10 with the operation of the DC components included in video signals 12a or 12b, and a prescribed modulated wave 14 can be obtained without damaging the DC balance of the carrier suppression modulator. In such a case, the carrier suppression modulation circuit 10 needs a circuit constitution in which a frequency component from a direct current can be passed.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to television video modulators.

(Prior Art) The configuration of a conventional video modulator is shown in FIG. 3, and the signal waveforms of each part are shown in FIG. 4. When the input video signal 4 is a black and white TV signal, as is well known, each level is 0 as the carrier wave level and 1 as the carrier wave level.
00% is the tip of the synchronization level, 12.5% is the reference white level, and 75% is the pedestal level (black level).

If the input video signal 4 is AC-coupled to the previous stage device (not shown), the DC component necessary for the video signal will be lost, so in order to regenerate this, the input video signal 4 must be connected to a pedestal clamp circuit l. Clamp the pedestal level to Ov. The pedestal-clamped video signal 5 undergoes amplitude modulation of a video carrier wave 6 in a carrier wave suppression modulation circuit 2 . but,
Since the pedestal level of the video signal is clamped to Ov, the carrier wave component of the modulated output is suppressed, and the modulated output waveform becomes like 7b and does not become the prescribed modulated waveform 7a.

For this reason, a DC bias 3 is applied to the carrier wave suppression modulation circuit 2 to disrupt the DC balance of the modulation circuit and cause the carrier wave component to leak into the modulated output, thereby obtaining a prescribed modulation waveform 7a.

Furthermore, in the conventional video modulator, the DC bias 3 is varied to adjust the carrier wave component appearing in the modulated output, thereby determining the degree of amplitude modulation.

(Problem to be Solved by the Invention) In this way, conventional video modulators perform amplitude modulation by applying a DC bias to the carrier wave suppression modulation circuit, so when the DC bias condition changes, the level of the modulated output and the modulation degree change. The disadvantage is that it changes significantly. Furthermore, since carrier wave suppression modulation circuits are generally constructed of semiconductors such as diodes or transistors, characteristics of the semiconductors change due to changes in ambient temperature, etc., and bias conditions fluctuate. This causes unstable phenomena such as fluctuations in the level of modulated output and the degree of modulation.

In addition, the differential phase, which is an important characteristic of video signals, may be due to the loss of the DC balance of the carrier suppression modulation circuit.
This causes deterioration of characteristics such as differential gain.

(Means for Solving the Problems) The video modulator of the present invention eliminates the DC bias of the carrier wave suppression modulation circuit, which is the cause of these fluctuations and characteristic deterioration, and synchronizes the carrier wave zero level of the video signal with OV. Stability and characteristics are improved by using a synchronous clamp circuit that clamps the level to a constant positive or negative voltage in place of the pedestal clamp circuit. This will be explained below with reference to the drawings.

(Embodiment) FIG. 1 is a basic configuration diagram showing an embodiment of the present invention, and FIG. 2 shows waveforms of each part thereof. For convenience of explanation, the video signal input 11 is assumed to be the same as that shown in FIG.

The video signal input 11 is converted into a waveform 12a by the synchronous clamp circuit 9.
Alternatively, as shown in 12b, the leading end of the synchronizing signal is clamped to a constant voltage to reproduce the DC component necessary for the video signal.

If the input video signal 11 has already been pedestally clamped by the pedestal clamp circuit l of FIG. 3 as shown in FIG.
A simple level shift circuit may be used to set the leading end of the synchronization signal to a constant voltage to obtain a waveform as shown at +2a or 12b.

As shown in waveform 12a or 12b, when the synchronously clamped video signal I2 is input to the carrier wave suppression modulation circuit 10 to which no DC bias voltage is applied and the carrier wave 13 is amplitude-modulated, the DC contained in the video signal 12a or 12b is Due to the function of the carrier wave component, a carrier wave component appears in the output of the carrier wave suppression modulation circuit 10, and a prescribed modulated wave 14 can be obtained without impairing the DC balance of the carrier wave suppression modulation circuit as in the conventional device. In this case, the carrier suppression modulation circuit 10 needs to have a circuit configuration that can pass frequency components from direct current.

(Effect of the invention) As described above, in this device, the modulation degree and output level are
It is set at the video signal stage before the modulation circuit, and unlike conventional devices, the carrier wave suppression modulation circuit itself does not affect characteristics such as the modulation degree, so highly stable operation can be expected.

Furthermore, since this device does not apply a DC bias to the carrier suppression modulation circuit, there is little deterioration in video characteristics such as differential phase and differential gain.

As described above, the device of the present invention has the advantage of improving the drawbacks seen in conventional devices and realizing a highly stable and high-performance television video modulator.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram of each part of FIG. 1, FIG. 3 is a configuration diagram of a conventional device, and FIG.
The figure is a waveform diagram of each part of FIG. 3. 9... Synchronous clamp circuit, 10... Carrier wave suppression modulation circuit, It-14... Waveforms of each part. Patent applicant: Japan Radio Co., Ltd.
Gu Soe - 豐

Claims (1)

[Claims] A synchronous clamp circuit that clamps the carrier wave zero level of a video signal to 0 volts and a synchronous level to a constant positive or negative voltage, and a circuit configuration that passes a DC component and uses the output signal of the synchronous clamp circuit as a modulation signal input. A television video modulator consisting of a carrier wave suppression modulation circuit.