JPH057805Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Technical Field] This invention relates to a video signal compression circuit that compresses a video signal having a wide dynamic range into a limited dynamic range.

[Technical background of the invention and its problems] As is well known, in a color television camera device (hereinafter referred to as a color camera), it is necessary to increase the camera output signal in accordance with the amount of incident light from the subject. However, unlimited increase in this relationship is not possible. The main causes of these limitations are the dynamic range of the transmission system and the dynamic range of the electrical output relative to the optical input of the image pickup tube (or solid-state image sensor). The former is a limit due to, for example, regulations on the degree of modulation in broadcast waves, and as a result, the video level is generally set at a dynamic range of 110% IRE. On the other hand, in the case of an image pickup tube, for example, the limit in the latter is determined by how many times the beam current can be passed through the signal current under specified operating conditions. It is now possible to secure a dynamic range of %.

When transmitting a signal with such a wide dynamic range, it is necessary to compress it into a narrow dynamic range. A compression circuit (generally called a knee compression circuit or knee circuit) that performs this signal compression has conventionally been implemented based on a circuit as shown in FIG. That is, in FIG. 3, transistors Q1, Q2, resistors R1, R
2 and constant current sources 11 and 12 constitute a differential amplifier circuit, and the base of the transistor Q1 receives a video signal from a signal source S (here, it is assumed to be a sawtooth wave voltage signal for ease of explanation) e _io is supplied via input terminal IN. The base input of transistor Q2 is set to the ground level, and its collector output is level-compressed by a level compression circuit (knee circuit) composed of a diode D, a variable resistor VR, and a variable voltage source E, and then compressed. The video signal e _put is output from the output terminal OUT to the subsequent circuit.

In other words, the above circuit uses a fixed DC video signal.
e _io above a predetermined level is compressed by the switching action of the diode D1, and this threshold is generally called the knee level (in the circuit shown in Figure 3, the output level E of the variable voltage source E is approximately the knee level). ). Here, if there is no knee circuit and the circuit has a wide dynamic range, the output increase due to an increase in the input of the above circuit will increase as shown by the broken line in Figure 4, but if the knee circuit does not have a knee circuit and the circuit has a wide dynamic range, If there is a circuit, the collector resistance of transistor Q2 will be connected to resistance R2 and variable resistance at the output above knee level E due to the switching action of diode D.
Since it is a parallel value with VR, it is compressed as shown by the solid line in the figure.

The above is the general principle of current knee circuits, but if you try to remotely control the knee compression by this knee circuit from outside the circuit (for example, from the shelf board that houses the circuit), you will be able to use the remote control of the variable voltage source that sets the knee level. Although it is easy to control, it is not easy to remotely control the variable resistor VR1 that sets the degree of compression (hereinafter referred to as knee slope). This is because the amplification degree of the video signal (0 to 7 [MHz]) is set here, and for example, it is impossible to install the variable resistor VR away from the circuit. Therefore, a method can be considered to remotely control the knee slope by replacing the variable resistor VR with a field effect transistor (hereinafter referred to as FET) and controlling the gate voltage. But in this case
Even if the gate voltage is kept constant, the impedance between the FET's source and drain fluctuates with temperature, resulting in a knee-slope temperature drift. This drift especially occurs in a three-tube color camera for broadcasting (which has an image pickup tube for the three primary colors of red, green, and blue, and an image processing circuit corresponding to the primary colors), which causes uneven knee slopes between the primary colors. This results in deterioration of color reproducibility.

There are other gamma correction circuits for compressing video signals, but these circuits perform inverse correction of the gamma characteristics of the picture tube. Separately, the knee circuit has the function of compressing signals above a predetermined level to prevent signals from being clipped when the amplitude is limited in dynamic range settings.

[Purpose of the invention] This invention was made to improve the above problems, and the purpose is to provide a video signal compression circuit that can stably remote control the knee slope as well as the knee level remote control. do.

[Summary of the invention] That is, the video signal compression circuit according to the present invention generates a knee level signal from a reference voltage based on a knee level control signal, and also generates a reference level signal proportional to the knee level signal from the reference voltage,
A reference level signal is inserted for a certain period of time within the retrace period of the input video signal, a reference pulse is placed on the video signal, and the signal is amplified by an amplifier circuit. On the other hand, a knee slope signal is generated from the reference voltage based on the knee slope control signal, and the level compression circuit compares the video signal output from the amplifier circuit with the knee level signal and outputs it as is until it reaches the knee level, and then outputs it as is until it reaches the knee level. When , compression is performed using the compression degree based on the knee slope signal. Furthermore, the peak level of the additional pulse component is detected from the output signal of this level compression circuit, the difference component between this peak level signal and the knee level signal is determined, and the knee slope signal is increased or decreased so that this difference component remains constant. It is characterized by the following.

[Embodiment of the invention] Hereinafter, an embodiment of the invention will be described in detail with reference to FIGS. 1 and 2.

Figure 1 shows its configuration.
The video signal _eio from is supplied to the clamp circuit 11 via the first input terminal IN1. This clamp circuit 11 includes a transistor Q3, a first switching circuit Sa1, a capacitor C1, and a buffer circuit A.
1 and a differential amplifier circuit A2, and a feedback loop is formed for the input and output of the transistor Q3. The switching circuit Sa1 receives the sample pulse P1 from the second input terminal IN2.
When is at a high level, it is switched and connected to the capacitor C1 side. It is assumed here that the sample pulse P1 is generated within the vertical retrace period of the video signal. The output of this clamp circuit is led out to the second switching circuit 12.

This second switching circuit 12 derives the output e _io ' of the clamp circuit 11 when the reference pulse P2 from the third input terminal IN3 is at a low level, and the signal e _io ' derived here is fed back. The signal is supplied to the amplifier circuit 13. This feedback amplification circuit 13 is composed of a pair of transistors Q4 and Q5, an output transistor Q6, feedback level setting resistors R3 and R4, etc., and amplifies and outputs an input signal with a gain set by the resistors R3 and R4. The output of this feedback amplifier circuit 13 is supplied to a level compression circuit 14.

This level compression circuit 14 is composed of a pair of transistors Q71, Q72, FETQ8, and resistors R5, R6, and includes a base input of transistor Q72 and
The level of the input signal is compressed according to the gate input of FETQ8. This level compression circuit 1
The output en of No. 4 is led out by the transistor Q9 constituting the output circuit 15 to the subsequent circuit via the output terminal OUT.

The control circuit that controls the operation of the level compression circuit 14 includes a first operational amplifier circuit 16 that operationally amplifies the reference voltage _ER in accordance with the knee level control signal e _k 1 from the fourth input terminal IN4; a second operational amplifier circuit 17 that operationally amplifies the reference voltage E _R according to the output e1 of the first operational amplifier circuit 16;
The output e2 of the operational amplifier circuit 17 is limited in level.
ep and a voltage dividing circuit 18 that leads to the second switching circuit 12, and a sample hold circuit (S/
H circuit) 19 and this sample hold circuit 19
The first operational amplifier circuit 16
A third operational amplifier circuit 20 operationally amplifies the output e1 of
and a fourth operational amplifier circuit 21 which operationally amplifies the summed voltage of the output e3 of the third operational amplifier circuit 20 and the reference voltage E _R in accordance with the knee slope control signal e _ks from the fifth input terminal IN5. and a third switching circuit 22 for switching and deriving the output e4 of the fourth operational amplifier circuit 21 and the -V _CC voltage.

The first operational amplifier circuit 16 includes operational amplifiers A3, A
4, consists of resistors R7 to R12 and capacitor C2, and its output e1 is connected to the second operational amplifier circuit 1.
7 and 20, and also to the base of the transistor Q72 of the level compression circuit 14. The second operational amplifier circuit 17 includes an operational amplifier A5,
A6, resistors R13 to R17, and a capacitor C3, the output e2 is level-limited by a voltage dividing circuit 18 consisting of resistors R18 and R19, and then output to the second switching circuit 12. Note that the resistor R18 is set to the same resistance value as the resistor R3 of the feedback amplifier circuit 13, and the resistor R19 is similarly set to the same resistance value as the resistor R4 of the feedback amplifier circuit 13. The sample hold circuit 19 has a resistor R20
and a capacitor C4, the level compression circuit 14
The output en is sampled and held using the reference pulse P, and the output ep' is supplied to the third operational amplifier circuit 20. This third operational amplifier circuit 20 is composed of an operational amplifier A7 and resistors R21 to R24, and its output e3 is transmitted to the fourth operational amplifier circuit 21.
is supplied to This fourth operational amplifier circuit 21 is composed of an operational amplifier A8, resistors R25 to R28, and a capacitor C5, and its output e4 is sent to the level compression circuit 14 via a third switching circuit 22.
Selectively supplied to the gate of FETQ8.

The operation of the above configuration will be described below with reference to FIG. 2.

First, if a video signal e _io having a DC component as shown in FIG. 2a is supplied to the first input terminal IN1, this video signal e _io is outputted from the transistor Q3 as shown in FIG. 2b. Sample pulse P1 in the vertical retrace feedback samples and holds onto capacitor C1. The voltage held in the capacitor C1 is led out to the differential amplifier circuit A2 via the buffer circuit A1 and compared with the ground potential. If the gain of the differential amplifier circuit A2 is large, the output of the transistor Q3 is fixed to the ground potential during the vertical retrace period. Since this operation is a general feedback clamp operation, the details will be omitted.

The output of this feedback clamp circuit 11
e _io ' is led out to the second switching circuit 12. This second switching circuit 12 is shown in FIG.
When the reference pulse P2 is at a high level, the output ep of the voltage dividing circuit 18 is derived, and when the reference pulse P2 is at a low level, the output e _io ' of the clamp circuit 11 is derived. Therefore, the signal derived through the second switching circuit 12 is transmitted to the feedback amplifier circuit 1 as shown in FIG. 2d.
After being amplified by a gain Av=(R3+R4)/R4, the signal is supplied to the level compression circuit 14. In this level compression circuit 14, the knee level is determined by the base potential of the transistor Q72, and signals above the knee level are compressed by the switching operation of the transistor Q71. The slope of the knee slope at this time is determined by the resistance value between the emitters of transistors Q71 and Q72, and this resistance value is
is determined by the drain-source resistance of
In other words, this level compression circuit 14 sets the output e1 of the first operational amplifier circuit 16 to the knee level, and outputs the output e4 of the fourth operational amplifier circuit 21 derived from the third switching circuit 22 or the power supply voltage -V _CC. The knee slope is thus determined and compressed as shown in Figure 2e. The signal e _o compressed by this level compression circuit 14 is transmitted to the transistor Q9 of the output circuit 15.
is inputted to the base of the output terminal OUT, and is output from its emitter to the output terminal OUT as a knee-compressed signal e _put .

On the other hand, the output e _o of the level compression circuit 14 is supplied to the sample hold circuit 19 via the resistor R20, and the signal during the period when the reference pulse P2 is at a high level, that is, the amplified signal ep' of the output ep of the voltage dividing circuit 18 is sampled. will be held. The held signal ep' is supplied to the third operational amplifier circuit 20.

Furthermore, the first to fourth operational amplifier circuits 1
6, 17, 20, and 21. First, in the first operational amplifier circuit 16, the output of the operational amplifier A3 is e1-E _R , and the output of the operational amplifier A4 is
Since the sum of the currents flowing into the (-) input terminal of is zero, it is balanced in the following state.

(e1−E _R )/R8+e _k1 /R7=0 ∴e1=E _R −(R8/R ₇ )e _k1 … (1) Next, the second operational amplifier circuit uses the operational amplifier A5
Since the output of is e2−e1, it is balanced in the following state.

e2−e1=E _R ∴e2=E _R +e1… (2) Therefore, the voltage signal ep output from the voltage divider circuit 18 is: ep=R19/R18+R19 (E _R +e1) Here, R18=R3, R19=R4, and R4/
Since (R3+R4)=R19/(R18+R19)=Av, it can be expressed as ep=(E _R +e1)/Av. Therefore, the Av·ep component of the output of the feedback amplifier circuit 13 is Av·ep=E _R +e1 (3).

On the other hand, the transistor Q7 of the level compression circuit 14
Since the output e1 of the first arithmetic circuit 16 is supplied to the base of the transistor Q71, the signal input level at which the emitter current of the transistor Q71 starts to flow is equal to e1. In other words, the signal in the region where Av・ep≧e1 (E _R in equation (3)
components) are compressed. In other words, the knee level is given by e1, and the E _R component is compressed. Therefore, the first operational amplifier circuit 1
By changing the output e1 of No. 6 using the knee level control signal e _k 1, the knee level can be remotely controlled.

In addition, the Av/V compressed by the level compression circuit 14
The peak value ep' of the component ep is sampled and held by the sample and hold circuit 19, as described above, and is supplied to the third operational amplifier circuit 20. Therefore, the output e3 of the third operational amplifier circuit 20 becomes e3=ep'-e1... (4) and is supplied to the fourth operational amplifier circuit 21.

This fourth operational amplifier circuit 21 operates with the output e3 of the third operational amplifier circuit 20, the reference voltage E _R , and the knee slope control signal e _ks as input, and its output e4 is sent via the third switching circuit 22. It is supplied to the gate of FETQ8 and controls its drain-source resistance value. i.e. determine knee compression
The FETQ8 exists in a feedback loop consisting of a sample hold circuit 19 and third and fourth operational amplifier circuits 20 and 21. The equilibrium state of this feedback loop is determined by finding e3, which provides an equilibrium condition for the fourth operational amplifier circuit 21.
That is, the fourth operational amplifier circuit 21 receives e _ks ·R28/ which is applied to the (+) input terminal of the operational amplifier A8.
e3 and E _R for (R27+R28) (=eR…(5))
The following equilibrium conditions can be found by

E _R −eR/R26+e3−eR/R25=0 ∴e3=eR−R25/R26(E _R −eR)… (6) On the other hand, since e3 is given by equation (4), (4) and (6) From the equation, we can find the following relationship: ep′−e1′=eR=R25/R26=(E _R −eR) ∴ep′=eR(1+R25/R26) −R25/R26E _R +e1… (7)

From the above explanation of the operation, the knee level is given by equation (1) and determined only by the voltage e _kl . In other words, the peak value of the pulse ep inserted into the video signal during the high level period of the reference pulse P2 is the knee level plus a constant value E _R at the input of the level compression circuit 14 (before compression), and this E _R The component is subject to knee compression. And after knee compression
ep' is considered to be the sum of the el component that is not compressed below the knee level and the output component after compression of E _R that is compressed with the component above the knee level. On the other hand, e3 is obtained by subtracting el from ep' (formula (4)), and it can be said that only the component subjected to knee compression is extracted. Therefore, e4 is unrelated to knee level e1 and is not affected by e _kl .

As mentioned above, e4 is generated based on the reference voltage E _R , the knee slope control signal e _ks and the knee compression component e3, so if E _R is constant and the circuit operation is stable, the knee slope is It is determined only by e _ks , and the value of e3 is also constant. Here, if a drift occurs in the drain-source resistance of FETQ8 due to a temperature change or the like, the peak level ep' changes, and a level change occurs in e3. However, in the fourth operational amplifier circuit 21, the level of e4 is changed opposite to the change in the level of e3, and the gate voltage of FETQ8 is controlled in the direction opposite to the drift.
Therefore, no drift appears in ep′, and e3
becomes stabilized at its initial value.

To put the above in simpler terms, the knee level can be controlled by e _kl , and at this time the inclination of the knee slope after compression is not affected at all. Also, the compression of the knee slope determines the knee level.
determined only by e _ks without being affected by e _kl , and
Since the drift in the drain-source resistance of FETQ8 is absorbed into the feedback loop by the fourth operational amplifier circuit 21, no drift occurs in the slope. Furthermore, if the third switching circuit 22 is connected to the -V _CC side, FETQ8 of the level compression circuit 14 is cut off,
This prevents the compression operation from occurring.

Therefore, in the video signal compression circuit configured as described above, both the knee level and the knee slope can be externally controlled by varying the DC voltage.
In addition, since a feedback loop is formed to control the slope, the stability of the slope can ignore the influence of the drift of the FET, which functions as a variable resistor, and is not affected by conventional diodes and variable resistors (especially Considering the occurrence of diode drift, it is highly stable. In addition, since the knee slope is determined only by controlling the reference voltage of the feedback loop, in a three-tube color camera there is very little variation among the primary color circuit tubes, which allows stable and high-quality images to be obtained. be able to.

In the above embodiment, the reference pulse P2 is inserted in the vertical blanking period, but it may be inserted in the vertical blanking period or in both blanking periods.

[Effects of the Invention] As detailed above, according to the invention, it is possible to provide a video signal compression circuit that can stably remotely control the knee slope as well as the knee level.

[Brief explanation of the drawing]

Fig. 1 is a circuit configuration diagram showing an embodiment of the video signal compression circuit according to this invention, Fig. 2 is an input/output waveform diagram for explaining the operation of the same embodiment, and Fig. 3 is a conventional video signal compression circuit. FIG. 4 is a diagram for explaining the operation of the conventional circuit. 11...Feedback clamp circuit, 12...
...Switching circuit, 13...Period amplification circuit, 1
4... Level compression circuit, 15... Output circuit, 1
6, 17, 20, 21... operational amplifier circuit, 18...
...Voltage divider circuit, 19...Sample hold circuit, 2
2... Switching circuit, P1... Sample pulse, P2... Reference pulse, e _kl knee level control signal, e _ks ... knee slope control signal, E _R ... reference voltage.

Claims (1)

[Scope of utility model registration request] knee level signal generation means for generating a knee level signal from a reference voltage based on the knee level control signal;
a reference level signal generating means for generating a reference level signal proportional to the knee level signal from the reference voltage; and a reference level signal generating means for inputting a video signal and inserting the reference level signal for a certain period within the retrace period to generate a reference pulse to the video signal. an amplifier circuit for amplifying the video signal to which the pulses have been added by the means; a knee slope signal generating means for generating a knee slope signal from the reference voltage based on the knee slope control signal; and the amplifier circuit. A level compression unit that compares the video signal output from the knee level signal with the knee level signal, outputs the video signal as it is until the knee level is reached, and when the video signal reaches the knee level or higher, compresses and outputs the video signal with a degree of compression based on the knee slope signal. a circuit, a peak level detecting means for detecting the peak level of the pulse portion added by the pulse adding means from the output signal of the level compression circuit, and a difference component between the peak level signal detected by the means and the knee level signal. and feedback control means for increasing or decreasing the knee slope signal so that the difference component becomes constant.