JP2537959B2 - Video signal amplitude limiter - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplitude limiting device used in a video amplifier circuit of a television receiver.

2. Description of the Related Art In recent years, with the increase in screen size of television receivers, circuit technology for maximizing the dynamic range of cathode ray tubes has come to be emphasized.

The conventional video signal amplitude limiting device as described above will be described below with reference to the drawings.

FIG. 3 shows a conventional video signal amplitude limiting device. In FIG. 3, 22 is a DC bias source, 25 is an NPN transistor, 26 and 27 are resistors, 28 is an NPN transistor, 29 is a constant current source for supplying a constant current 1, 30 is brightness control and contrast control for video signals. Video signal processing circuit,
A beam current detection circuit 31 detects the beam current of a cathode ray tube (hereinafter abbreviated as CRT) and controls the brightness and contrast of the video signal.

The operation of the video signal amplitude limiting device configured as described above will be described below.

First, the video signal 2 is input to the video signal processing circuit 30 from the terminal 33, where the brightness and contrast are controlled. Further, in the beam current detection circuit 31, the beam current of the CRT is detected, and when a large amount of beam current flows, the contrast control terminal 35 and the brightness control terminal 34 are controlled,
The beam current is reduced so that a certain level of beam current does not flow in the CRT (generally called ABL). This state is shown in FIG. As shown in FIG. 5, when the APL (average video level) of the input video signal becomes high, the beam current detection circuit 31 detects the beam current,
In order to reduce the contrast, the amplitude of the output video signal becomes constant when the APL exceeds a certain level.

Next, the output of the video signal processing circuit 30 is input to the base of the NPN transistor 28 which constitutes an emitter follower. A current source 29 for flowing a constant current 1 is connected to the emitter of the transistor 28. The video signal applied to the base of the transistor 28 has a polarity such that the white side is lower and the black side is upper. Next, a bias of V _A is applied to the base of the transistor 25 by the DC bias source 22. V _A is a transistor
Biased below the pedestal level at 28 bases (white side is bottom polarity). Therefore, when the video signal falls below V _A to the white side, the signal on the white side of the output of the transistor 28 becomes a video signal output in the form of being resistance-divided by the resistors 27 and 26. This is shown in FIG. As shown in FIG. 4, the gain of the white side output is lower than the amplitude limit start voltage V _{A with} respect to the input of the base of the transistor 29, and finally the white side signal is V _A − (resistor 26) × 1. It does not fall and the amplitude is completely limited. By setting the characteristics as shown in FIG. 4 in this way, when a white peak signal such as a telop is sent in the video signal, the beam current partially flows and the image is blurred on the CRT. So-called blooming is prevented.

However, in the above configuration, the amplitude limit starting voltage
Since V _A is constant, if the amplitude is limited by a signal with a low APL, the amplitude will be too limited and the AP
When the L becomes a little higher, the crush on the white side becomes conspicuous. Conversely, when the amplitude limit start voltage V _A is set so that there is no crush on the white side when the APL is high to some extent, the APL
If there is a white peak in the low signal, the amplitude is not sufficiently limited and blooming occurs.

In view of the above problems, the present invention provides a video signal amplitude limiting device capable of reducing blooming at a white peak in a signal having a low APL and preventing whiteout at a high APL.

Means for Solving the Problems In order to achieve this object, a video signal amplitude limiting device of the present invention is an APL that detects the APL of the video signal from the video signal.
From a detection circuit, an amplitude limiting circuit that limits the amplitude of the video signal that has passed through the video signal processing circuit, and an amplitude limiting control circuit that can change the amplitude limiting start voltage of the amplitude limiting circuit by the output of the APL detection circuit. It is configured.

Function With this configuration, for signals with low APL, the amplitude limit start voltage is used as the root, blooming is prevented by making the amplitude limit on the white side well, and at high APL, the amplitude limit start voltage is used as the tip and provided separately. By narrowing the brightness and contrast by the beam current detection circuit, it is possible to prevent the white side from being crushed.

Embodiment One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a video signal amplitude limiting device according to an embodiment of the present invention.

In FIG. 1, 1 and 2 are resistors, 3 is an NPN transistor,
Reference numeral 4 is a resistor, which constitutes an amplifier. Reference numerals 5 and 6 are resistors that give a bias to the base of the transistor 3. 7 is
PNP transistor, 8 and 9 are resistors, 10 is a capacitor,
An amplifier is configured by these elements 7 to 9, and the output of the amplifier is integrated by the capacitor 10 and the average value is output. 11 is an NPN transistor and 12 and 13 are resistors to form an amplifier. 14 is a PNP transistor, and 15 and 16 are resistors to form an amplifier. Reference numeral 17 is an NPN transistor, and 18 is a resistor, which constitutes an emitter follower.
Reference numeral 19 is a resistor, and 20 and 21 are NPN transistors, which form a current mirror. 22 is a DC bias source, 23 is an NPN transistor, 2
4 is a resistor, 25 is an NPN transistor, 26 is a resistor, and the amplitude of the video signal is limited by 22 to 26. Reference numeral 27 is a resistor, 28 is an NPN transistor, and 29 is a constant current source for supplying a constant current 1. Reference numeral 30 is a video signal processing circuit for controlling the brightness and contrast of the video signal. 31 detects the beam current of CRT and controls brightness, contrast, etc., and CR
A beam current detection circuit for limiting the beam of T.

The operation of the video signal amplitude limiting device configured as described above will be described below.

First, the APL detection circuit will be described. A negative video signal whose white side is below the terminal 32 of the video signal 1 is input. Since a bias corresponding to the pedestal level of the video signal is applied to the base of the transistor 3 by the resistors 5 and 6, only the video portion is amplified and output to the collector of the transistor 3. Next, the video signal inverted by the transistor 7 has a so-called positive polarity in which the white side is directed upward at the collector of the transistor 7. Here, it is integrated by the capacitor 10 and the average value of the level of the video signal appears. That is, when APL is high, the voltage of the capacitor 10 rises. Since this voltage is passed through the two-stage inverting amplifier of the transistor 11 and the transistor 14, the potential of the collector of the transistor 14 rises when APL is high. Next, the transistor 17 constitutes an emitter follower, and the potential of the emitter of the transistor 17 rises when APL is high. The above is the APL detection circuit.

Next, a limiter circuit that limits the amplitude of the video signal will be described. As described in the conventional example, the same video signal is applied from the terminal 33 of the video signal 2, and the video signal processing circuit 30
Brightness and contrast are controlled at, and it is added to the base of the transistor 28 and output from the emitter. An amplitude limit start voltage of V _A is applied to the base of the transistor 25, and as explained in the conventional example, the signal on the white side of V _A at the base of the transistor 28 is the resistance 27 and the resistance.
It is output in the form of being divided by 26, and finally the amplitude is completely limited when it falls below V _A by (resistor 26) × 1 (where 1 is the constant current source 29).

Next, an amplitude limit control circuit that changes the amplitude limit start voltage V _A by the output of the APL detection circuit will be described.

The potential of the emitter of the transistor 17 rises when APL is high. Then, a large amount of current flows in the transistor 20 through the resistor 19, this current is mirrored by the transistor 21, and the collector current of the transistor 21 increases. On the other hand, since the base of the transistor 23 is biased by the bias source 22, a constant voltage is applied to the emitter of the transistor 23. So the base of transistor 25 is a transistor
A potential lower than the emitter potential of 23 by (resistance 24) × (collector current of transistor 21) is added as V _A. AP now
When L is high, the current of the transistor 21 increases as described above, so that the base potential of the transistor 25 decreases and the amplitude limit start voltage V _A decreases.

Now, a video signal whose white side is lower is applied to the base of the transistor 28. As shown in FIG. 2, when APL is high, the amplitude limit start voltage is lowered and the characteristic becomes as shown in FIG. The amplitude limit start voltage also becomes lower, the white side of the output starts to bend, and the white side of the output is also moved toward the white side. At this time, since the beam current increases, the beam current detection circuit 31 reduces the contrast and brightness. On the contrary, when APL is low, the base potential V _A of the transistor 25 becomes high and the amplitude limit start voltage rises, contrary to the above.
The characteristics are as shown in FIG. 2, the amplitude on the white side begins to bend quickly, and the amplitude is limited when the amplitude on the white side is also small. At this time, the beam current detection circuit
Since 31 does not work at all, increasing the amplitude limit level is effective in preventing blooming.

EFFECTS OF THE INVENTION According to the invention described above, the APL of a video signal is detected, and the detection output is used to control an amplitude limiting circuit that limits the amplitude of the video signal. The voltage is used as the root, and the white side amplitude is well controlled to prevent blooming. In the case of a video signal with a high APL, the amplitude limit start voltage is set at the tip, and the video signal amplitude limiter does not apply too much amplitude limit.At this time, the beam current detection circuit provided separately lowers the contrast and brightness, Prevent crushing on the white side.

[Brief description of drawings]

FIG. 1 is a circuit diagram of one embodiment of a video signal amplitude limiting device of the present invention, FIG. 2 is an explanatory diagram of an amplitude limiting operation of one embodiment of the present invention, and FIG. 3 is a conventional video signal amplitude limiting circuit. FIG. 4 is a circuit diagram, FIG. 4 is an operation explanatory diagram of a conventional video signal amplitude limiting circuit, and FIG. 5 is an operation explanatory diagram of a beam current detection circuit. 1-18 ... APL detection circuit, 19-24 ... amplitude limit control circuit, 25-29 ... video signal amplitude limit circuit.

Claims (1)

(57) [Claims] 1. An APL detection circuit for detecting the APL (average video level) of the video signal from the video signal, a video signal amplitude limiting circuit for limiting the amplitude of the video signal on the white side, and the APL.
A video signal amplitude limiting device characterized by comprising an amplitude limiting control circuit capable of changing the white side amplitude limiting starting voltage of the video signal of the video signal amplitude limiting circuit by the output of the detection circuit.