JPS60257674A - Broad band gamma correction circuit - Google Patents

Abstract

PURPOSE:To realize a gamma correction circuit having a broad band characteristic to be reproduced at a correct gradation by using a current switch comprising a differential amplifier provided with a required limiter characteristic to form the gamma characteristic. CONSTITUTION:A couple of transistors Qi, Qi' (i=1, 2, 3) formed in the circuit constitution of a differential amplifier require arranged characteristics in current switches 12A, 12B, 12C and arranged resistance value and temperature coefficient are used for a couple of emitter resistors REi, REi'. In setting properly amplification factors a1, a2, a3 of the current switches 12A, 12B, 12C, the input characteristic is similar to the desired gamma characteristic curve.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention 1] The present invention provides a method for faithfully reproducing an image of a subject
This relates to a range gamma correction H circuit.

[Technical background of the invention and its problems] =・General (J, In order to correctly reproduce the gradation of the subject)
- The gamma of the overall characteristics of the system body from the camera to the final display device such as a color cathode ray tube is set to 1.
It is necessary to do so. Now, change the brightness of the subject horizontally! +
For I11, the brightness of the reproduced image is plotted along the vertical axis, and the respective corresponding points are plotted on the logarithmic scale 'c1'. The case is gamma γ) 1's contemplative system d4.

However, the photoelectric conversion characteristics (gamma characteristics) of the image tube widely used in color cameras are, for example, 0.6-0.7 for Visin C. Approximately 1 for Reachcon and Karini
As you can see, each one is different. On the other hand, the luminance of the phosphor screen of a color cathode ray tube is not linear, and its gamma characteristic is +! l is about 2.2. Therefore, in order to reduce the overall gamma characteristic of the entire system to 1, it is necessary to perform gamma correction using a circuit. In this case, the permissiveness is fJ
It is specified that the correction and gamma correction should be performed on the camera side.

The overall gamma of the system 7 system is γ, the gamma of the color cathode ray tube is γl, the gamma of the visual tube is 72°, and the gamma of the correction circuit is T3. A gamma correction circuit is used to compensate iI so that γ1 and γ2.

The conventional gamma correction circuit has a diode D1 and a resistor r1 connected in series to different bias power supplies [-1], respectively, which are installed on the output sides of transistors 1 to 1R, as shown in FIG.
Connect (1-・1, 2.3) and each Taioh 1 to D1
is sequentially turned on according to the output signal level (-), and the combined resistance value ([ is lowered, and the amplification degree is varied in a polygonal manner accordingly to perform gamma correction so that the total gamma characteristic becomes 1.

1. The conventional example uses a diode, so when a pulse with a width tP is input to the diode D shown in FIG. 2, tJ is shown in FIG. , the output pulse is shown in the same figure (C), and the manual pulse
C Reverse recovery time (recovery time) until the charge is neutralized even when reverse biased 11. or the response is lost, and the response time +1 for rapidly changing signals worsens.

Also, regarding Shiranjisku, when the manual pulse of width IP shown in Fig. 3 (a) is applied to the transistor T shown in Fig. 3 (b),
When it is input to the base of R, its collector output waveform becomes as shown in FIG. 3(C), and even if the input pulse disappears, the charge near the junction spreads, resulting in a slow accumulation time Lstg and a slow response.

Therefore, in the conventional example described above, since the response time fj1 is low, it is impossible to reproduce the color tone of minute parts of the subject or the gray scale of the subject with correct gradation.

Furthermore, in recent years, color cameras and endoscopes using solid-state image carriers as image carriers have been proposed. Especially in endoscopy,
As disclosed in Japanese Patent Application No. 58-18'I683, a line transfer type solid-state image pickup device that can be made most compact is suitable. However, this line transfer type solid-state image sensor
Since the photosensitive section (light receiving section) and the transfer section are shared, it is necessary to read out the charge signal corresponding to each pixel.
] It is necessary to prevent the smear phenomenon when applying a pulse pulse and transmitting data to its output terminal.
I can't do it. In other words, the signal transferred to the signal processing circuit side must be read out within a short period of time between illumination periods (light reception and accumulation periods). (31, 1-2 When the illumination period is shortened, the image becomes dark and low -C]n1~last, so this illumination 1g1 interval is crowded and 11 to obtain an image with less time delay. (The signal readout interval is
This is because J, Cr < , it is necessary to read out the data within such a short period of time. In this case, the video signal t)
Of course, along with this, wideband (0-10M1-12) and 41
In order to faithfully reproduce the subject, a gamma compensation circuit with good frequency characteristics is required.

[Objective of the Invention 1] The present invention has been made in view of the above-mentioned points, and provides a wide-band comma correction circuit that has good response QLR characteristics and is capable of reproducing the subject with precise gradation down to minute parts. The purpose is to provide the following.

[Summary of the Invention] The present invention is capable of realizing a response characteristic of 1 that sufficiently responds to a wide band of frequencies by configuring a gamma correction circuit using a plurality of differential amplifiers and current switches. , it is possible to reproduce an image with the correct gradation. Furthermore, by using a differential amplifier, it is possible to realize a characteristic f1 that is almost unaffected by temperature.

[Embodiment 1 of the invention The present invention will now be described in detail with reference to the drawings.

4 to 7 relate to the first embodiment of the present invention, and FIG.
The figure shows the basic configuration of the first embodiment, FIG. 5 shows the differential amplifier forming the first embodiment of FIG. 4, and FIG. 6(a) shows each differential amplifier forming the first embodiment. The input/output characteristics of the amplifier are shown, and FIG. 7(b) shows the input/output characteristics of the first embodiment, and FIG. 7 shows the specific circuit configuration of the first embodiment.1゜Comma correction circuit of the first embodiment 11! ;L, current switch 12Δ, 12B formed by differential 11 width switch;

12C, these current switches V-12A, 12B,
It consists of an adder 13 that adds the outputs of 12C.

The above Nojren 1 to Switch 12△, 12B, 12G are as follows:
Each of them is composed of a differential amplifier as shown in FIG.

Therefore, a pair of NPN types 1 to Rungis Q with uniform characteristics,
Q- is connected to constant current source I through its respective resistors Rc and R[-.

One side of each base is connected to the input terminal V1 and the other is grounded (-) through resistors r<, r<-, respectively. Also, one side of each collector is connected to the positive power supply terminal -IV, and the other The output terminal V o is connected to the positive power supply Da -1-V via the load resistor 1-. Note that both transistors Q and Q''' are connected to the load resistor 1 or 1 is set only on one side (
J3 is asymmetrical, so it is sometimes called a semi-differential amplifier, but in the present invention, J3 is called a differential amplifier.

] - When the signal applied to the input terminal 1 becomes 6 higher than the other base potential, the differential amplifier generates a signal between the collector and emitter of transistor Q and transistor Q according to the level. A current flows, and one core is connected to the other transistor Q'.
As the current flowing through the output terminal VO decreases, the output level at the output terminal VO increases. Therefore, 7f flowing between the collector and J-mitter
The current defined by the constant current source I reaches its upper limit, and the output level is held at a state where no further flow occurs, and the transistor Q between A and C becomes saturated.
Since it operates in the active state, the storage time is short and it responds well to ultra-high-speed operation.This input/output characteristic is determined by the resistances RE, RE', and RL. Accordingly, 1l is limited as shown in Figure 6(a)!
It increases approximately near to ft and has a curved line-like characteristic.

In addition, by using a pair of 1-transistors Q and Q' with the same characteristics as described above, the humidity dependence of each 1-transistor Q and Q- is canceled out, so that it is almost unaffected by current current.
It is a stable current sweep that works well. Also, Karen 1~Suitsuji C is Kiru.

In the specific circuit of the first embodiment shown in FIG. 7, each of the currents 1 to 12Δ, 12B, and 42C is formed with the circuit configuration of a differential amplifier shown in FIG. One transistor Qi forming each differential amplifier
, Qi ” (!=1.2.3)1;
Fi, R1, i - By using I-components with the same values and temperature coefficients, etc., it is almost unaffected by temperature. Ip -12Δ, 128.12G load resistance Rl-L;L is shared, current is added by this resistor R1-, and the added value I1-18 is output from its output terminal -)-
Yes, there is.

As shown in FIG. 4 or FIG. 6(a), the amplification degrees of the switches 1 to 12A, 12B, and 120 are as shown in FIG. a3, and these are added, and the input/output characteristics of the first embodiment with this configuration are as shown in FIG. 6(b), so the coordinates (XI, Y+), (X2.Y>>
, (X3, Y3) to approximate the desired gamma characteristic curve, the following relational expression % formula % holds true. Solving this simultaneous equation gives a+ =
(XI Y2-X; + Y+ )/ (XI(XI-X
2)) a7 = ((X2-X3)Y+ -(XI-X:
l) Y2 (XI --X2) Y3)
/ ((XI -X2) (X2 -X3))a3
- (Y2 - Y3)/(X2 - You can achieve anything.

Incidentally, the amplification degree al, al, 83 of each differential amplifier is approximately 8 + =RL/Re + , a2 = RL 11'
<e2. a:+-Iku1. /Re 3, and the limiter level a1 ・×1. a2 ・×2. a3 ・×3
(If the current source of each differential amplifier is It, 12.13, al 'X+ = RL 01+ , a2 °X2 =
Rt・+2, a3・X3-RL-13 are intertwined.

The above resistance ROi is (from REi-R-Ei) R
e' i -2R[E i .

FIG. 8 shows a second embodiment of the invention.

This second embodiment is based on FIG.
3, and its output terminal is -12△ in each current switch,
128. Configure it to be a parallel connection point with the output end of 120 [For base-grounded Balanoa amplification] ・Lanister 0
4 are connected in cascade.

This ] helangistor Q4 is connected in cascade to prevent the frequency characteristics from deteriorating due to the Missy effect in which the diffusion capacitance thereof equivalently increases depending on the amplification degree of each Karen 1-switch 12A, 12B, 12C, Wider1)
11i U.

In addition, the current switches 12△, 12B, 1 shown in FIG.
A common-base 1H transistor is connected to each differential load side of 2C to form a differential type buffer, and the mirror effect of each current switch 12△, 12B, and 12C is reduced, and a common load resistance is It is also possible to have a structure in which the current is added at RL.

In addition, the child series current switches 12Δ, 12B.

By increasing the number of 12Cs, the number of broken lines can be increased, and the desired gun 7 characteristics can be approached. Also, the dynamic range can be widened.

In addition, in each of the above-mentioned examples, each Karen 1
- The outputs of switches 12Δ, 12B, and 12G are added, but the paired 1 to transistors (e.g. Q2, Q2-)
Replace the collector (or replace the base side)
By connecting, subtraction can also be performed to obtain the desired gun 7 characteristics with t+-:> instead of addition.

Note that a C-current 1-switch such as a field effect type 1-transistor may be formed instead of a bipolar 1-transistor.

Furthermore, input No. 111 (C level shift 1 to
Form a constant current source (by applying it to the base of the transistor or the control end of the resistor, etc., and apply a variable current to it). It is also possible to achieve a desired gamma characteristic by giving a characteristic (or subtractive) characteristic.

[Effect of the Invention 1] As described above, if the present invention is applied, each gamma characteristic has the required limiter characteristic! Since -C is formed using a switch to Karen 1, which is made up of a differential amplifier,
It is possible to realize a gamma correction circuit that has a wide band characteristic that has a fast response and can reproduce the correct gradation. In addition, it is possible to realize a gamma correction circuit with a wider dynamic range without being affected by temperature.

[Brief explanation of drawings]

Figures 1 to 3 relate to conventional examples, Figure 1 is a circuit diagram based on the conventional example, and Figure 2 is an explanatory diagram showing the switching characteristic f1 of the diode used in Figure 1. ,
Figure 3 shows the switching characteristics f! of the transistor used in Figure 1. ] Characteristic diagrams, Figures 4 to 7
The figure relates to the '1st embodiment of the present invention, and is shown in FIG. -L 1st
A circuit diagram showing the basic configuration of the embodiment, Fig. 5 (31) A circuit diagram showing the configuration of each current sweep, Fig. 6 <a>
Figure 6(b) 4 is a characteristic diagram showing the input/output characteristics of the switch to each current 1, Figure 6(b) is a characteristic diagram showing the input/output characteristics of the first embodiment, and Figure 7 is a specific diagram of the first embodiment. Circuit diagram showing the configuration,
FIG. 8 is a circuit diagram showing a second embodiment of the present invention. 11...Gun 7?1ti positive circuit 12△, 1211, 12C...Karen l-switch 1;
3...Q adder 1...constant current source Q)"...1
~Ran resistor 1<1-...f+load resistance \ゝ-1//

Claims (1)

[Claims] In order to reproduce the subject with the correct gradation, the gamma complement circuit is used to set the required limiter characteristics, and the outputs of these current switches, such as multiple current switches made up of differential amplifiers, are 9 means for calculating and outputting (
) wideband gamma correction circuit, 6