JPH07303222A - Video signal amplitude restriction device - Google Patents

Abstract

PURPOSE:To sufficiently use the dynamic range of a cathode ray tube and to hold the contrast of an output video signal even on a signal where APL (average video level) is low. CONSTITUTION:The same video signals are given to a video signal processing circuit 30 and an APL detection circuit 32. An amplitude restriction control circuit 41 changes the amplitude restriction start voltage of a video signal amplitude restriction circuit 42 in accordance with the output of the APL detection circuit 32 and the output of an amplitude restriction start point adjustment circuit 43. In the case of a signal whose APL is high, amplitude restriction is not given so much in the video signal amplitude restriction circuit 42 and contrast/brightness are reduced in control by a beam current detection circuit 31 so as to prevent blooming. In the case of the signal whose APL is low, blooming at a white peak is prevented, and the contrast of the output video signal is held by holding amplitude restriction start voltage to prescribed one by the amplitude restriction start point adjustment circuit 43.

Description

Detailed Description of the Invention

[0001]

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.

[0002]

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 a cathode ray tube (cathode ray tube) has become important.

FIG. 4 shows a conventional video signal amplitude limiting device. In FIG. 4, reference numeral 30 denotes a video signal processing circuit for performing brightness control and contrast control on the video signal (video signal 2), and 31 detects the beam current of the cathode ray tube to control the brightness and contrast of the video signal 2. Is a beam current detection circuit for.

Reference numeral 32 is an APL detection circuit for outputting, as a voltage, an APL (average video level) of the same video signal (video signal 1) input to the input terminal 34. That is, from the case where the video signal 1 is a black signal (APL is 0%) to the case where the video signal 1 is a white signal (APL is 100%), the average level of the video signal is output as a DC voltage. The characteristics of the APL detection circuit 32 are as shown in FIG.

In the subsequent stage of the APL detection circuit 32, three-stage emitter followers composed of transistors 13, 15, 17 and constant current sources 14, 16, 18 are arranged. These circuit elements 13 to 18 have transistors 20, 21,
An amplitude limit control circuit 41 is configured by adding 23, resistors 19 and 24, and bias source 22. The output of the video signal processing circuit 30 is the transistor 28 and the constant current source 2
It is input to an emitter follower composed of 9 and 9. These circuit elements 28 and 29 have a transistor 25 and a resistor 2
The video signal amplitude limiting circuit 42 is formed by adding the signals 6 and 27.

Collector current IC21 of transistor 21
Since the transistors 20 and 21 have a mirror circuit configuration and are equal to the current flowing in the resistor 19, the forward base-emitter voltages of the transistors 13, 15, 17 and 20 are respectively VBE13, VBE15, VBE17, Letting VBE20, the resistance value of the resistor 19 be R19, and the output voltage of the APL detection circuit 32 being VAPL, IC21 = (VAPL + VBE13 + VBE15-VBE17-VBE20) / R19 (1). Here, VBE13 = VBE15 = VBE17 = VBE20 = V
If BE, then IC21 = VAPL / R19 (2). The base of the transistor 23 is the bias source 22.
Thus, the base voltage of the transistor 25 is biased to be equal to the base voltage of the transistor 28 during the pedestal period when the collector current of the transistor 21 is zero.

Therefore, the relationship between the APL of the video signal 1 and the base voltage of the transistor 25 is as shown in FIG.

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

The video signal 2 is input from the terminal 33 to the video signal processing circuit 30, where brightness control and contrast control are performed. In addition, the beam current detection circuit 31 detects the beam current of the cathode ray tube, and when a large amount of the beam current flows, the brightness control terminal 35 and the contrast control terminal 36 are controlled to reduce the beam current so that a beam current of a certain level or more flows. I try not to. This control is generally called ABL. This state is shown in FIG. That is, the gain from the input video signal to the output video signal is shown in FIG. However, with the control only by the beam current detection circuit 31, when a white peak signal is sent only to a part of the screen such as a telop in the video signal, a large amount of beam current flows only to that part on the cathode ray tube. The so-called blooming in which the image is blurred cannot be prevented.

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. According to the video signal amplitude limiting circuit 42, if the base voltage of the transistor 25 is V25, and the video signal drops to V25 below V25, the white signal output from the transistor 28 is divided by the resistors 27 and 26. The video signal is output in the form of a circle. This state is shown in FIG. As described above, the gain of the output on the white side of the amplitude limiting start voltage V25 with respect to the input of the base of the transistor 28 is lowered, and the amplitude of the video signal output is limited, thereby preventing blooming.

However, if the amplitude limit start voltage V25 is constant, if the amplitude limit is tried to be increased by a signal with a low APL, the amplitude limit becomes too high, and the white side shadow becomes conspicuous with a signal with a high APL. , If the amplitude limiting start voltage V25 is set so that there is no crushing on the white side in a signal with a high APL, there is a problem that blooming occurs without sufficient amplitude limitation if there is a white peak in a signal with a low APL.

In order to prevent the video signal from being crushed on the white side and to prevent blooming as much as possible, in the configuration of FIG. 4, the APL detection circuit 32 and the amplitude limit control circuit 41 are used.
By introducing the above, the amplitude limitation of the video signal is made APL interlocking type (see Japanese Patent Laid-Open No. 1-261087).

That is, in the case of a signal whose APL is 0%, the amplitude limit start voltage is made equal to the voltage of the pedestal period of the transistor 28, and the white side amplitude limit is often applied to prevent blooming. On the other hand, when the APL is high, the amplitude limiting start voltage is moved to the tip, and the separately provided beam current detection circuit 31 narrows the brightness and the contrast to prevent the white side crushing. This will be described with reference to FIGS. 10 and 11.

FIG. 10 shows the relationship between the input video signal amplitude and the output video signal amplitude when the beam current detection circuit 31 does not operate and the brightness and contrast are not controlled. The amplitude limit start voltage moves to the tip according to the APL of the input video signal. With this alone, the amplitude of the output signal becomes too large and blooming occurs. Therefore, as shown in FIG. 11, when the input signal exceeds a certain APL (in this case, it is set to 50%, for example), the beam current detection circuit 31 operates, and the brightness and contrast are reduced to output the output image. The signal amplitude does not exceed the blooming level.

[0015]

However, in such a configuration, as shown in FIG. 11, a signal with a low APL (for example, a signal with an APL of 50% or less) has a low video signal amplitude limit start voltage, and therefore, is sufficiently low. The dynamic range of CRT cannot be used. That is, there is a problem that the dynamic range of the output video signal cannot be used up to the full blooming level and the contrast of the output video signal is impaired.

In view of the above problems, the present invention provides a video signal amplitude limiting device capable of maintaining the contrast of an output video signal by fully utilizing the dynamic range of a cathode ray tube even for a signal having a low APL.

[0017]

In order to achieve this object, a video signal amplitude limiting device of the present invention is a video signal amplitude limiting circuit for limiting the amplitude of a video signal, and a video signal amplitude limiting circuit APL detection circuit for detecting APL, amplitude limit start point adjustment circuit for obtaining a constant output so as to adjust the amplitude limit start point, output of the APL detection circuit and output of the amplitude limit start point adjustment circuit And an amplitude limit control circuit for changing the amplitude limit start voltage of the video signal amplitude limit circuit according to the above.

[0018]

With the above-described structure, even in the case of a low APL signal, the video signal amplitude limiting start point adjusting circuit prevents the dynamic range of the cathode ray tube from being sufficiently used to impair the contrast of the output video signal. Set the starting voltage.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An 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, portions having the same functions as those in FIG. Reference numeral 12 is a transistor, and 11 is a voltage source for adjusting the amplitude limit starting point. Amplitude limiting start point adjusting circuit 4 including these circuit elements 11 and 12
3 sets the emitter voltage of the transistor 17 when APL is low.

FIG. 2 shows the relationship between the APL of the video signal 1 and the base voltage of the transistor 25. Here, the forward base-emitter voltages of the transistors 12, 13, 15, 17, 20 are equally set to VBE, and the voltage of the voltage source 11 is set to V11.
Then, when the base voltage of the transistor 17 is higher than V11 (when the APL of the video signal 1 is high), the emitter voltage V17 of the transistor 17 is V17 = VAPL + 2VBE− when the output voltage of the APL detection circuit 32 is VAPL. VBE = VAPL + VBE (3) and the base voltage of the transistor 17 is lower than V11 (when the APL of the video signal 1 is low), the transistor 1
Since the emitter voltage V17 of No. 7 is constant as V17 = V11-VBE (4), the relationship between the APL of the video signal 1 and the base voltage (amplitude limiting start voltage) of the transistor 25 is as shown in FIG. .

When the APL of the video signal 1 is low, the amplitude limit start voltage does not rise to the pedestal level and is clipped at a constant voltage according to the voltage set by the voltage source 11. In other words, by appropriately setting the voltage of the voltage source 11, when the APL of the video signal 1 is lower than an arbitrary APL (for example, APL1), the amplitude limit start voltage is set to a constant voltage and when it is higher than APL1. In addition, the amplitude limit start voltage can be changed according to the APL of the video signal 1. The relationship between the input video signal amplitude and the output video signal amplitude at this time is shown in FIG. In FIG. 3, as one specific example, the APL of the input video signal is 50%.
It is assumed that the beam current detection circuit 31 operates when the value exceeds, and the contrast and brightness are reduced, and the output video signal amplitude does not exceed the blooming level. At this time, if the APL of the input video signal is low (50% or less in this case), the amplitude limit start voltage is set to be constant when the APL of the input video signal is 50% or less. However, by preventing blooming at the white peak and maintaining the amplitude limit start voltage at a certain voltage, the dynamic range of the cathode ray tube can be fully used and the contrast of the output video signal can be maintained.

As described above, according to this embodiment, the APL
The detection circuit 32 detects the APL of the video signal, and in the case of the video signal having a high APL, the amplitude restriction start voltage is moved to the tip so that the amplitude is not restricted too much. At this time, the beam current detection circuit 31 In contrast, the contrast and brightness are reduced to prevent blooming. Further, in the case of a signal with a low APL, blooming at the white peak is prevented and the amplitude limit start point adjustment circuit 43 is provided.
Thus, by maintaining the amplitude limit start voltage at a constant voltage, the contrast of the output video signal can be maintained.

[0024]

According to the present invention, when the APL of the video signal is low, blooming at the white peak is prevented, and the amplitude limit start voltage adjusting circuit keeps the amplitude limit start voltage constant. The contrast of the output video signal can be maintained.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of a video signal amplitude limiting device of the present invention.

FIG. 2 is a diagram showing a relationship between an APL of an input video signal and an amplitude limiting start voltage in the video signal amplitude limiting device of FIG.

FIG. 3 is a diagram showing a relationship between an input video signal amplitude and an output video signal amplitude when a beam current detection circuit is operated in the video signal amplitude limiting device of FIG.

FIG. 4 is a circuit diagram of a conventional video signal amplitude limiting device.

5 is an explanatory diagram of the operation of the APL detection circuit in FIG.

6 is a diagram showing a relationship between an APL of an input video signal and an amplitude limiting start voltage in the video signal amplitude limiting device of FIG.

FIG. 7 is an operation explanatory diagram of the beam current detection circuit in FIG.

FIG. 8 is an operation explanatory diagram of another type of the beam current detection circuit in FIG.

9 is an operation explanatory diagram of the video signal amplitude limiting device in FIG. 4;

10 is a diagram showing a relationship between an input video signal amplitude and an output video signal amplitude when the beam current detection circuit is not operated in the video signal amplitude limiting device of FIG.

11 is a diagram showing the relationship between the input video signal amplitude and the output video signal amplitude when the beam current detection circuit is operated in the video signal amplitude limiting device of FIG.

[Explanation of symbols]

 30 video signal processing circuit 31 beam current detection circuit 32 APL detection circuit 41 amplitude limit control circuit 42 video signal amplitude limit circuit 43 amplitude limit start point adjustment circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhide Saeki 2-2-10, Kotobukicho, Takamatsu City, Kagawa Matsushita Electronic Industry Co., Ltd.

Claims (1)

[Claims] 1. A video signal amplitude limiting circuit for limiting the amplitude of a video signal, an APL detection circuit for detecting an APL (average video level) of the video signal from the video signal, and an amplitude limiting start point is adjusted. So as to obtain a constant output, and the amplitude limit start voltage of the video signal amplitude limit circuit is changed according to the output of the APL detection circuit and the output of the amplitude limit start point adjustment circuit. And a video signal amplitude limiting device.