JPH03173284A - On-screen circuit - Google Patents

Abstract

PURPOSE:To make the brightness of character display constant by providing a circuit means setting the level of a character display with a pedestal level as a reference since the brightness of a video signal is set based on the pedestal level. CONSTITUTION:A video input is subjected to SYNC tip clamp, a character is inserted by a switch circuit 3 and an output added is obtained at a terminal 5. A pedestal generating circuit 6 generates a pulse at a pedestal position by an inputted clamp pulse. The signal for the pedestal period of the video signal is extracted at a switch 7 by using the pulse a burst signal of a color signal is eliminated by a low pass filter circuit 8 to obtain a DC level for the pedestal period. The DC level is obtained at a point B through a buffer amplifier 9. A character display level setting circuit 4 consists of a constant current circuit I and a resistor R2 with respect to the pedestal DC level at a point B. This is because the voltage drop across the resistor R1 is made constant and the character display level at the point A is constant from the point B.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] This invention relates to an on-screen display that displays information such as text as well as images on a TV screen.

[Conventional technology]

FIG. 3 is a block diagram showing a conventional example. FIG. 4 shows a time chart of the operation.

In FIG. 3, a composite video input is input to the input terminal of (1).

FIG. 3 schematically shows a diagram of the 1 water period, and this input is a composite of a luminance signal representing brightness and a color signal. This signal is commonly found in TV textbooks and the like, so a detailed explanation will be omitted here.

Since the average DC level of the above video input signal changes depending on the level (brightness) of the luminance signal, the DC level of the sync tip is held constant by the sync tip clamp circuit (21), and the sync tip level is used as the reference for the DC voltage. This method is widely used when handling video signals.

According to the sync chip 101 path +2ji'q゛, the signal with the DC level matched is sent to the switch +31'jk street.If the switch 131 is set to the 1 side, it is directly sent to the terminal 16.
1 and 7'L, which becomes the video signal for the TV screen.

Here: 41C is a character display level setting circuit that sets the brightness of characters when displaying characters, and the voltage of the resistor dividing point hole corresponds to the brightness of the characters.

Normally, this DC voltage can be calculated from the input video signal level and set to a white level of 70% to 100%.

By applying a character switching input to the switch (3), the video signal is output to the switch (31) when it is on the L side, and the prisoner's character display DC' voltage is output to the 161 when it is switched to the 2 side, and the video signal is output. This means that the character display has been added to the number of characters displayed.Figure 4 shows this situation in a time chart.

[Problem to be solved by the invention]

In the conventional example described above, since the character display level is set as the sync chip voltage (!- reference), when the video human input signal level changes.

- In particular, the length of the sync for video signal output from a VTR etc. tends to change depending on the condition of the soft tape.

-There was a drawback that the reference voltage changed, and the brightness of the character display changed.

The present invention aims to solve the above-mentioned drawbacks and to obtain constant character display brightness without being affected by the video input signal (particularly the length of the sync).

[Means to solve the problem]

Extracts the pedestal level of the video input signal, rectifies it,
The circuit configuration is such that the character display level is set based on the pedestal level.

[Effect]

Since the brightness of the video signal is set based on the pedestal level, the brightness of the character display can be kept constant by circuit means that sets the level of the character display based on the pedestal level of the video signal.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention. FIG. 3 is a time chart showing its operation.

In Figure 1, the video input is sync-tip clamped by 121 sync-tip clamp circuits, (3)
As in the conventional example, a character is inserted by the switch MK of , and an output signal added to , 51 is obtained.

6) is a pedestal generating circuit which generates a nox at the input Klumph Hals and pedestal positions.

With this pulse, the signal of the pedestal period of the video signal is extracted by the switch (71), the nost signal of the color signal is removed by the low-pass filter circuit (8), and the DC level of the pedestal period is obtained. The level is added to the buffer amplifier i91 i d and +B1 point Km.The reason why the Bazza amplifier is used here is that during the pedestal period, the DC level is stored as a charge on the answer pad C1, but when the switch (71 is turned off) This is because it is necessary to hold this charge for a period of time (to increase the human power impedance of the amplifier 91).

41 is a character display level setting circuit, and the above +31
For the pedestal DC level at point, constant current circuit (I)
and resistor R2. This is to keep the voltage drop across the resistor R2 constant, and the character display level at the ^ point becomes a constant '11f value from the +31 point.

That is, even if the input video signal for setting the character display level from the pedestal level of the video signal changes, the character display level can always be kept constant.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to realize an on-screen display in which a constant brightness of character display can always be obtained even under conditions where the video signal level changes, such as the video playback output from a VTR.

[Brief explanation of the drawing]

11th! iii is a block diagram showing an on-screen circuit according to an embodiment of the present invention, FIG. 3 is a time chart of FIG. 1, and FIG. 3 is a conventional on-screen circuit? Figure 4 shows the time chart of Figure 8. In the figure, 111 is a video input terminal, 121 is a sync chip clamp circuit, (3) is a video and character display switch, and (41 is a character display level setting). Circuit, (6) is pedestal pulse generator, +71i switch circuit, +81
fl low pass filter, (9) is a buffer amplifier. +61 is video output Mizuko. FIG. 2 is a time chart of FIG. 1 showing the circuit operation of FIG. 1. Figure 8 shows a conventional example. FIG. 4 is a time chart showing the operation of the conventional example. In addition. In the figure. 14 - Is the code the same - or a corresponding part? show.

Claims (1)

[Claims] The pedestal period of the video signal is extracted by a switch circuit, and the DC level of the pedestal period is rectified and held by a low-pass filter.The voltage of the inserted signal is determined based on this DC level, and the video signal is An on-screen circuit characterized by switching between a signal and an insertion signal using a switch.