JPS615341A - Signal processor - Google Patents

Abstract

PURPOSE:To obtain a print picture of high contrast by giving accurately automatic gain control to video signals of R, G and B with no loss of color balance. CONSTITUTION:The R, G and B video signals (f), (g) and (h) are amplified by VCA4-6 which perform amplification with the gain corresponding to the control signal (n) and turned into output (i), (j) and (k) respectively. These outputs are supplied to a maximum value detecting circuit 7, and an output (l) is obtained. This output (l) is supplied to a peak detecting circuit 8 to obtain a peak output (m). This output (m) is supplied to an error amplifier 9 and compared with the reference voltage (p). When the output (m) is smaller than the voltage (p), the output (n) of the amplifier 9 is varied to increase the (m) up to the (p). Then the output (m) is applied equally to the VCA4-6. Thus the VCA4-6 can control the gain while keeping the gain ratio at a constant level.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a signal processing device used in a color image printer which inputs a video signal used for a CRT hard copy or the like. This relates to signal 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. .

Therefore, in order to effectively utilize the limited dynamic range of printing density, methods are being debated 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 NTSC
FIG. 2 is a block diagram of a circuit that automatically adjusts a color video signal to an optimal gain.

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

If the amplitude of the video signal output is too small, the output C of the peak detector 2 becomes smaller than the reference voltage d, and the error amplifier 3
The output e of VCA2 acts in the direction of increasing the gain of VCA2, and the peak of the output of vCAl is always equal to the reference voltage. Even if the amplitude of the video signal output is too large, gain control is applied in the same manner as described above, and the output is stabilized at a constant amplitude.

The conventional configuration can be used for gain control of monochrome video signals or NTSC color video signals, but RGB video signals are divided into three sets and there is a correlation between each signal, so it is necessary to control the gain of each signal separately. If conventional gain control is performed, the gain of each signal will be different. That is, if such a method is adopted in a color image printer, an image with incorrect color balance will be printed, and therefore it cannot be used for gain control of RGB video signals.

OBJECTS OF THE INVENTION An object of the present invention is to provide a signal processing device that can control the signal levels of RGB video signals and obtain a high contrast image without destroying the color balance.

Structure of the Invention The present invention provides three sets of variable amplification amplifiers or variable attenuation attenuators for amplifying or attenuating RGBs sets of video signals, an RGBa set of amplifiers or attenuators, a peak detection output and a reference voltage. It has an error amplifier that amplifies the difference between ! and the output of the error amplifier! By controlling the amplification or attenuation of the JRGBs group's variable amplification amplifier or attenuation variable attenuator to be equal to the RGBa group, RC can be achieved without disturbing the color balance.
This is to perform gain control of the iB video signal.

Description of examples Embodiments of the present invention will be described below 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 RGB video signals f
+q, ht”V amplified with a gain corresponding to the control signal n
CA (voltage controlled amplifier), A is the maximum value detection circuit that obtains the maximum value of VCA4.5.6 output '+] rk, and 8 is the peak that obtains the maximum value of the output λ of the maximum value detection circuit in the time axis domain. The detection circuit 9 is an error amplifier that amplifies the difference between the output m of the peak detection circuit 8 and the reference voltage p to obtain the control voltage output n of the VCA.
In the case of the signal waveforms shown in i, ii, and iii of Fig. 3, the output 2 of the maximum value detection circuit 7 is as shown in Fig. 3 iV.
In this case, the peak of i becomes the peak of L.

Therefore, the output m of the peak detection circuit 8 becomes 1v in FIG. 3.0 When the peak detection output m is smaller than the reference voltage p,
The output n of the error amplifier 9 is changed so as to increase m to p and is applied equally to the VCA 4.5.6. For this reason, V
CA4, 5.6 can control the gain while keeping the gain ratio constant at 4.

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

'+]+ has a common diode D1゜D2. D
Only the one with the maximum potential is selected by 3 and the resistor R1 and added to the base of the transistor Q1, and the maximum value output 2 is obtained by the buffer consisting of Q1 and the resistor R2. Dl, D2
．． The voltage drop due to D3 is compensated by Ql.

! The peak voltage is maintained at 01 by a peak rectifier circuit consisting of a diode D4 and a capacitor C1, and is gradually discharged by a resistor R3. The peak voltage held is transferred to D4 by a buffer consisting of transistor Q2 and resistor R4.
The voltage drop caused by this is compensated for, resulting in a peak detection output m.

FIG. 6 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 illustration only and do not limit the scope of the invention.

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

[Brief explanation of the drawing]

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 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~
D4...Diode, C...Capacitor, R-R...Resistor, Ql, o2...Transistor. Name of agent: Patent attorney Toshio Nakao and 1 other professional 1
Map area – 3rd m

Claims (1)

[Claims] 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 outputs the maximum value of the three outputs of the amplifiers or attenuators, and this maximum value detection circuit. , 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 amplify the three sets of variable amplification amplifiers. A signal processing device characterized in that the degree of attenuation or the degree of attenuation of a variable attenuator is equally controlled.