JPH0377531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a signal processing device used in a color image printer that inputs video signals used for hard copies of CRTs, etc. It concerns level control.

Conventional configurations and their problems In general, the dynamic range of printing density of an image printer is smaller than the dynamic range of brightness of a CRT, so hard copies of video signals under the same conditions tend to result in images with low contrast. It is.

Therefore, in order to effectively utilize the limited dynamic range of printing density, a method is used to obtain a printed image with high contrast by adjusting the gain of the video signal amplifier.

Figure 1 shows the conventional monochrome video signal or
FIG. 2 is a block diagram of a circuit that automatically adjusts an NTSC color video signal to the optimum gain.

1 is a VCA (voltage control amplifier) that amplifies video signal a with a gain corresponding to control signal e, 2 is VCA1
A peak detector 3 detects the peak of the output b of the peak detector 2, and an error amplifier 3 amplifies the difference between the output C of the peak detector 2 and the reference voltage d to control the gain of the VCA 1.

When the amplitude of the video signal output b is too small, the output c of the peak detector 2 becomes smaller than the reference voltage d, and the output e of the error amplifier 3 acts in the direction of increasing the gain of VCA2, and the output b of VCA1 The peak of is always equal to the reference voltage. Even if the amplitude of the video signal output b is too large, gain control is applied as in the previous period, and the output b is stabilized at a constant amplitude.

The conventional configuration is for monochrome video signals or
It can be used for gain control of NTSC color video signals, etc., but RGB video signals are divided into three sets of signals and there is a correlation between each signal, so if you perform conventional gain control for each signal, each signal The gains will be different. In other words, if such a method is adopted in a color image printer, an image with incorrect color balance will be printed, so it cannot be used for gain control of RGB video signals.

Purpose of the Invention The present invention provides RGB color balance without disturbing color balance.
It is an object of the present invention to provide a signal processing device that can control the signal level of a video signal and obtain a high contrast image.

Structure of the Invention The present invention includes three sets of variable amplification amplifiers or variable attenuation attenuators that amplify or attenuate three sets of RGB video signals, and a maximum output of the amplifier or attenuator that outputs the maximum of the outputs of the three sets of RGB. It has a value detection circuit, a peak detection circuit that holds the peak value of the maximum value detection output, and an error amplifier that amplifies the difference between the peak detection output and the reference voltage.
By equally controlling the amplification or attenuation of the three RGB sets of variable amplification amplifiers or variable attenuation attenuators, the gain control of the RGB video signal is performed without destroying the color balance.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 is a block diagram of the present invention, and FIG. 3 is a signal waveform diagram.

In Figures 2 and 3, 4, 5, and 6 are VCAs (voltage control amplifiers) that amplify the RGB video signals f, g, and h with a gain corresponding to the control signal n, and 7 is a VCA.
8 is a peak detection circuit that obtains the maximum value of the output l of the maximum value detection circuit in the time axis domain; 9 is a peak detection circuit This is an error amplifier that amplifies the difference between the output m of the VCA and the reference voltage p to obtain the control voltage output n of the VCA.

When the video signal outputs i, j, and k of VCAs 4, 5, and 6 have the signal waveforms shown in , in Fig. 3,
The output l of the maximum value detection circuit 7 becomes the signal shown in FIG. 3, and since the peak in this case is the peak of l, the output m of the peak detection circuit 8 becomes the signal shown in FIG.

When the peak detection output m is smaller than the reference voltage p, the error amplifier 9 increases m to p.
The output n of is varied and applied equally to VCAs 4, 5, and 6. Therefore, the VCAs 4, 5, and 6 can control the gains while keeping the gain ratio constant.

FIG. 4a shows a specific example of the aforementioned maximum value detection circuit, and FIG. 4b shows a specific example of the peak detection circuit.

i, j, k are diodes D ₁ with common cathodes,
Only the one with the maximum potential is selected by D ₂ , D ₃ and resistor R ₁ and added to the base of transistor Q ₁ , and Q ₁ ,
The buffer consisting of the resistor _R2 provides the maximum output value l. The voltage drop due to D ₁ , D ₂ , and D ₃ is compensated by Q ₁ .

l maintains the peak voltage at C ₁ by a peak rectifier circuit consisting of diode D ₄ and capacitor C ₁ ,
It is gradually discharged by resistor R ₃ . The held peak voltage is compensated for the voltage drop due to _D4 by a buffer consisting of transistor _Q2 and resistor _R4 , and becomes a peak detection output m.

FIG. 5 shows a combination of the maximum value detection circuit and the peak detection circuit of FIG. 4, and is a practical embodiment.

The above specific examples are for illustrative purposes only.
It is not intended to limit the scope of the invention.

Effects of the Invention According to the present invention, it is possible to accurately perform automatic gain control on RGB video signals without destroying the color balance, and to reduce variations in the level of the video signal.
A printed image with high contrast can be obtained regardless of the content of the image.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional automatic gain control circuit, FIG. 2 is a block diagram of a signal processing device according to an embodiment of the present invention, FIG. 3 is a signal waveform diagram for explaining the device, and FIG. 4 is a block diagram of a conventional automatic gain control circuit. , FIG. 5 is a circuit diagram showing a specific circuit example of the maximum value detection circuit and the peak detection circuit of the same device. 1, 4, 5, 6...VCA (voltage control amplifier),
2, 8...Peak detector, 3,9...Error amplifier, 7...Maximum value detection circuit, _D1 to _D4 ...Diode, _C1 ...Capacitor, _R1 to _R3 ...Resistance, Q ₁ ,
Q ₂ ...Transistor.

Claims (1)

[Claims] 1. Three sets of variable amplification amplifiers or variable attenuation attenuators that amplify or attenuate three sets of video signals;
a maximum value detection circuit that selects and outputs the maximum value of the three sets of outputs of the variable amplification amplifier or the variable attenuation attenuator; and a peak detection circuit that holds the peak value of the output of the maximum value detection circuit for a predetermined period of time. and an error amplifier that amplifies the difference between the output of this peak detection circuit and a reference voltage, and the output of this error amplifier is used to determine the amplification degree of the three sets of variable amplification amplifiers or the attenuation degree of the variable attenuation degree attenuator. A signal processing device configured to control equally.