JPH07212619A - Method and circuit for recovering black level of video image - Google Patents

Abstract

PURPOSE:To avoid a luminance level or a color tone of a video image from being changed attended with black level recovery by applying amplitude control only to a black level signal whose level is a prescribed level or below in an input video signal and implementing the black level recovery by adding the original input signal and the black level signal subjected to amplitude control. CONSTITUTION:A black level detection circuit 2 extracts a black Level signal of an input video signal whose level is a prescribed level or below, a gain control amplifier 3 controls an amplitude of the extracted black level signal, and the input video signal and an output of a gain control amplifier 3 are added by an adder 8. A black level peak detection circuit 5 detects a black peak level obtained from an output of the adder 8 and the gain of a gain control amplifier 3 is controlled by a detected black peak level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a black level reproducing method and circuit for stably reproducing a black level in a TV receiver or the like.

[0002]

2. Description of the Related Art Generally, a video signal is sent with the pedestal level, which is the reference for the blackest, floating on the white side. The difference between the black level and the pedestal level is called the setup, and it depends on the convenience of the broadcasting station, TV camera, or home V
It varies depending on the manufacturer and model of TR, etc., and varies widely. Therefore, it is necessary to correctly reproduce the black level on the reproducing side (monitor TV) (match the black level of the signal with the cutoff level of the CRT).

As a conventional black level reproducing method, a method of decreasing the DC transmission rate and a method of decreasing the brightness level by the setup are used. In the former method, the level of the setup (DC component) is lowered, so black floating is not noticeable, but for a video signal whose setup is originally low, the black level is destroyed and the resolution of the black portion is deteriorated. In the case of an image with a large CRT beam current, the brightness level is lowered to limit the beam amount.
itter) is often used together, and therefore blackening is promoted, and accurate black reproduction cannot be expected.

In the latter method, for example, a black peak in one vertical scanning section is detected in a video signal as shown in FIG. 1A, and the black peak level becomes the blackest (pedestal level). ), The brightness level is controlled, and is also called a dynamic picture system. In this method, stable black level reproduction is possible even when used in combination with a contrast ABL (or picture ABL) configured to reduce the contrast and limit the beam amount in an image in which the beam current level of the CRT is large.

[0005]

However, since the brightness level is changed, the signal level (brightness) always fluctuates over the whole according to the black peak level in the signal, and the brightness signal Y and the chroma signal C are changed. The level ratio Y / C constantly fluctuates. Therefore, the color density changes depending on the magnitude of the black level. For example, human skin color (50 to 8 with respect to white peak)
(0% luminance) and the like, when the black level is high in the white direction, the luminance signal is lowered, and as a result, C increases with respect to Y, resulting in an over-colored skin color. On the contrary, when the black peak is low, the color becomes lighter.

Further, when black reproduction is performed at a certain time point t ₁ as shown in FIG. 2 (A), a signal P on the black side (for example, with a black border is added at a time point t ₂ as shown in FIG. 2B). When a telop signal) is inserted, black reproduction is performed so that the peak of this signal P becomes a pedestal, so that the brightness of the screen sharply increases. On the contrary, as shown in FIG. 2C, when the black peak sharply rises at time t3, the luminance sharply drops accordingly. This transient phenomenon is generally called "black paka",
The screen is very difficult to see. The present invention proposes a black level reproduction system that solves the above-mentioned drawbacks.

[0007]

According to a method of reproducing a black level of an image of the present invention, a black signal having a level lower than a predetermined level of an input video signal is extracted, the amplitude of the extracted black signal is controlled, and An amplitude-controlled black signal is added, the black peak level of the added signal is detected, and the amplitude of the extracted black signal is controlled by the detected black peak level.

The video black level reproducing circuit of the present invention is
A black detection circuit 2 that extracts a black signal of a predetermined level or lower of an input video signal, a gain control amplifier 3 that controls the amplitude of the extracted black signal, and an input video signal and the output of the gain control amplifier 3 are added. An adder 8 and a black peak detection circuit 5 for detecting the black peak level of the signal obtained from the output of the adder 8 are provided, and the output of the black peak detection circuit 5 controls the gain of the gain control amplifier 3. (FIG. 5).

[0009]

According to the image black level reproducing method of the present invention, since the amplitude operation is not performed for the signal in the white direction above a certain level in the image signal, the brightness level does not change due to the black level reproduction. Stable color reproduction is possible. Further, since the level control for reproducing black occurs only in the vicinity of black, the entire screen is not brightened or darkened due to a sudden change in black peak, and a stable received image with constant brightness can be obtained.

According to the video black level reproducing circuit of the present invention, the amplitude is controlled only for the black signal of the input video signal having the predetermined level or less, and the black signal is reproduced by adding the original input signal and the amplitude-controlled black signal. Since the circuit configuration is such that the amplitude control is performed for the entire video signal by the control loop that locally acts and the addition processing of a part of the entire amplitude, the loop gain may be relatively small, and the operation is It is possible to construct a very stable black reproduction circuit.

[0011]

EXAMPLES The present invention will be described below based on examples. Figure 3
4 and FIG. 4 are waveform diagrams of a video signal for explaining the principle operation of black reproduction according to the present invention, and correspond to FIGS. 1 and 2 of the prior art, respectively. In the black reproduction of the present invention, FIG.
As shown in (A), a predetermined threshold level TH based on the pedestal level is provided on the black side of the video signal, and a black signal protruding below this level (black side) is taken out and its level is extracted. 3B by controlling the
The black peak is made to coincide with the pedestal level as shown in. That is, when reproducing the black level, no operation is performed on the amplitude axis for the video signal above the white level (white side). Therefore, for a relatively bright signal such as a skin color, regardless of what black signal is in the screen, the luminance / chroma level ratio does not change due to the reproduction of the chroma signal, and stable color reproduction can be performed.

Further, as shown in FIGS. 4A, 4B, and 4C, the level control for black reproduction performed following the sudden change (t ₁ → t ₂ → t ₃ ) of the black peak is performed. Since it occurs only in the vicinity of black, the entire brightness level of the screen does not move with the sudden change of the black peak, and a stable image with constant brightness can be obtained.

FIG. 5 is a block diagram of a video signal processing circuit of a TV receiver for realizing the above black reproduction system.
The input signal 11 shown in FIG. 5 is given a video signal obtained through tuning, IF and detection, and is given to the 1 × amplifier 1 via the clamp capacitor 12. The output of the 1 × amplifier 1 is led to a luminance / chroma processing circuit (not shown) via an addition point 8 (adder) and a DC transmission rate correction circuit 9.

The output of the addition point 8 which constitutes the adder is also given to the pedestal clamp circuit 7, and the reference voltage Ep is supplied to the section of the clamp pulse supplied in the pedestal portion.
(Clamp potential). If there is a difference between the pedestal level of the video signal and the reference voltage Ep, the error is fed back to the input of the 1 × amplifier 1, and the DC charge amount of the clamp capacitor 12 is changed by this, and the error feedback is converged. In the stable state, the pedestal level of the video signal output from the addition point 8 is clamped at Ep.

The input signal of the 1 × amplifier 1 is also given to the black detection circuit 2, and as shown in FIG. 3A, a black signal below a predetermined threshold level TH based on the pedestal is detected. . Although this black detection circuit 2 is a clipping circuit, the reference voltage E ₁ (ground reference) is used as a clipping level.
Is given. On the other hand, the 1 × amplifier 1 is basically composed of a single stage differential amplifier, and by changing its DC balance with an offset adjustment signal from the outside,
The pedestal level of the input video signal can be offset to E ₁ -TH. Therefore, in the black detection circuit 2, the video signal pedestal clamped to E ₁ -TH is clipped at the clip level E ₁ so that the video signal shown in FIG.
The black signal shown in the oblique edge portion of (A) is detected. A blanking pulse is applied to the detection circuit 2 so that the detected black signal does not include the sync signal portion.

The detected black signal has its amplitude controlled by the gain control amplifier 3 and is then led to the addition point 8 where it is added to the output of the 1 × amplifier 1. The gain of the gain control amplifier 3 is variable between 0 and 1, and the variable control is performed based on the peak detection value of the black signal. Therefore, the black signal below the threshold level TH included in the video signal output from the addition point 8 is doubled at maximum (1+
Up to 1), the amplitude is extended in the blackest direction (pedestal direction).

This decompression operation is loop control,
The error feedback control is performed so that the peak of the expanded black signal matches the pedestal level. That is, the output of the addition point 8 is given to the blanking circuit 4 for the sync signal portion, and the video signal not including the sync signal portion is taken out. This video signal is sent to the black peak hold circuit 5 as a black peak detection circuit, and the black peak level is detected. The detected black peak level is given to the black-pedestal comparison circuit 6, and the difference between the black peak level and the pedestal level Ep is obtained. The detected difference is supplied to the gain control amplifier 3 as a gain control signal, and the loop operates so that there is no difference between the black peak and the pedestal. As a result, black reproduction is performed in the monitor receiver in the state where the black peak and the pedestal match as shown in FIG.

When the black peak hold circuit 5 detects a peak extending to the black side due to a large amplitude inter-station noise or the like generated at the time of channel switching, it takes time to return to normal black peak detection, and during that time, it is very whitish. Since the screen is displayed, a peak detection limiter 10 that regulates the detection level of the black peak hold circuit 5 is provided. The peak value detected by the limiter 10 is limited so as not to fall much below the pedestal.

FIG. 6 is an input / output characteristic diagram and an input / output waveform diagram of the black expansion operation in FIG. Further, FIG. 7 shows examples of various video signals to be reproduced in black. As shown in FIG. 6, the input / output characteristic is a straight line with a slope of 1 in the portion above the threshold level TH of the input video signal, and therefore no operation is performed on the swing axis for this signal portion. That is, in FIG. 5, the output of the 1 × amplifier 1 is derived as it is through the addition point 8.

Although the black signal below the threshold level is expanded, the black expansion is not performed for the input whose black peak coincides with the pedestal as shown in FIG. 7C, as shown in FIG. The input / output characteristic for signals below TH is a straight line q ₀ with a slope of 1. At this time, the gain of the gain control amplifier 3 in FIG. 5 is zero.

For an input whose black peak floats above the pedestal, black expansion is performed according to the amount of floating. The maximum value of the extension ratio is 2. That is, the gain of the gain control amplifier 3 is 1 in FIG.
The output of the double amplifier 1 and the output of the gain control amplifier 3 are added, and the amplitude of the black signal is doubled. In this state, the input / output characteristic for a signal of TH or less becomes a straight line q _max with a slope 2 shown in FIG.

As shown in the input S _{in in} FIG. 6, the black peak is TH /
If it is extended to 2, output S _out due to the extension of gain 2.
Will match the black peak pedestal. Note that FIG.
As in (A), with respect to the input so that the amount of protrusion of the black peak does not reach TH / 2, the black extension is also doubled, but the extension does not extend the black peak to the pedestal. That is, the extension is limited so as not to be doubled or more.

For an input having a black peak deeper than TH / 2 as shown in FIG. 7B, black expansion is performed with a gain in the range of 1 to 2 depending on the amount of floating from the pedestal. Done. That is, one of the countless straight lines within the range of straight lines q _{0 to} q _max in FIG. 6 becomes the input / output characteristic in this case, and the slope thereof is determined by the difference between the black peak and the pedestal. In this case, the gain control amplifier 3 of FIG.
Is controlled by the output of the black-pedestal comparison circuit 6 so that its gain takes a value of 0 to 1.

The reason why the maximum expansion gain is limited to double in this way is to prevent the screen from being unnatural due to the non-linear characteristics exhibited by the input / output characteristics of black expansion. The maximum extension gain can be set to 2 or more or 2 or less. When it is 2 or more, the "difficulty" of black reproduction is good, but the non-linear characteristic is stronger. When it is 2 or less, the black reproduction is poor, but the non-linear characteristic is alleviated.

FIG. 8 is an input / output characteristic diagram of the conventional black reproduction. As described above, the signal level (entire luminance) is lowered according to the amount of lift of the black peak level from the pedestal. The input / output characteristics change within the range of straight lines p ₀ to p _max according to the black level as shown in FIG. The straight line p ₀ is the case where the black peak and the pedestal match, and no level correction is performed. The straight line p _max is the case where the maximum level correction is performed. When the black peak of the signal changes abruptly, an energy change (light amount change) of the screen corresponding to the shaded area of p ₀ to p _{max in} FIG. 8 occurs. On the other hand, the corresponding energy change in FIG. 6 is within the shaded area below the threshold level TH, and as is apparent from the figure, black reproduction can be performed without any noticeable change in luminance.

FIG. 9 is a detailed circuit diagram of the video signal processing circuit of FIG. The blocks divided by broken lines in FIG. 9 correspond to the blocks in FIG. The processing circuit of FIG. 9 is an integrated circuit (IC) formed on one silicon chip.
However, this IC is provided with the hatched pin terminals. The terminals T1 and T3 are connected to the power supply V _CC and the ground potential, respectively.

The video signal is input from the terminal T2 to the 1 × amplifier 1 via the clamp capacitor 12. The 1 × amplifier 1 includes a pair of input transistors Q06 and Q07 whose emitters are coupled by a resistor R08, and an input video signal is given to one transistor Q06 through an emitter follower transistor Q01. The reference voltage E ₁ (FIG. 3A) is applied from the bias circuit section 15 to the base of the other transistor Q07 through the conductor 16.

Constant current transistors Q03 and Q04 are connected to the respective emitters of the transistor pair Q06 and Q07, and constant currents I ₁ and I ₂ are passed through them. Therefore, the amount of change in the collector current of one transistor Q06 due to the input video signal is transmitted through the resistor R08 to the other transistor Q06.
It is transmitted to the emitter side of 07 and appears as a change in the collector current of Q07. The collector current of Q07 flows through the load resistor R09, and the signal voltage is taken out from the collector of Q07. The amplification gain is about 1.

The signal at the collector of the transistor Q07 is
It is given to one of the pair of transistors Q13 and Q14 (Q13) forming the comparator 17 of the clamp circuit 7 via the emitter follower transistors Q09 and Q12.

The other transistor Q of the comparator 17
14 is an emitter follower transistor Q18, Q1
The clamp potential E _p shown in FIG. This potential E _p is supplied from the bias circuit 15 to the conductor 1
6 and the reference voltage E ₁ (see FIG. 10).
It is a constant voltage formed based on (B). That is,
The base of the transistor Q08 is coupled to the lead wire 16,
The constant current transistor Q05 is used as the emitter of Q08.
(Current I ₃ ) is coupled to the collector of Q08 at E _p = V
_CC- I ₃ R10 (R10 is collector load resistance of Q08)
Clamping potential E _p is given. This potential E _p is given to the above-mentioned emitter follower Q18 via the lead wire 18.

The comparator 17 of the clamp circuit 7 is activated by the clamp pulse (current I _p ) of the pedestal portion given from the pulse shaping circuit 19 through the conductor 20, and the clamp potential E _p and the video signal (collector of Q07). Pedestal level e _p is compared. For level comparison, the emitter followers Q09 and Q1
The base-emitter voltages of 2, Q15 and Q18 are canceled by the symmetrical arrangement of these transistors. The difference voltage of the comparison result is the transistors Q19 and Q.
The current is converted into a current by the active load of the current mirror configuration composed of 20, Q21, and is passed through the conductor 21 to obtain the 1 × amplifier 1
Is injected into the clamp capacitor 12 on the input side of.

For example, when e _p <E _p , the comparator 1
7 transistor Q13 is on, Q14 is off,
The on-current of the transistor Q13 is charged in the capacitor 12 through the lead wire 21. As a result, the transistor Q0
The DC component (pedestal level) of the base input of 1 rises. On the other hand, if e _p <E _p , the transistor Q13 is turned off and the transistor Q14 is turned on, and the discharge current flows from the capacitor 12 through the lead wire 21 into Q19.

In this way, the pedestal level of the video signal is corrected by the feedback of the detection error until the pedestal level e _p of the video signal becomes equal to the clamp potential E _p . When the error is eliminated in the comparator 17, a video signal pedestal clamped to the clamp potential E _p is obtained at the output of the 1 × amplifier 1 (collector of Q07).

The collector of the transistor Q07 is the addition point 8 in FIG. 5, and the addition signal for black expansion from the gain control amplifier 3 is added to this addition point 8 via the lead wire 22. The black-expanded video signal by addition is led to the black peak hold circuit 5 through the conductor follower 23 through the emitter follower transistor Q12.

The output of the emitter follower Q01 on the input side of the 1 × amplifier 1 is also led to the black detection circuit 2 through the lead wire 24. The 1 × amplifier 1 determines the pedestal level of the signal on the lead wire 24 as shown in FIG.
As in (C), it also has a function of offsetting the reference voltage E ₁ by the threshold level TH.

Transistors Q06 and Q0 of the 1 × amplifier 1
Current source transistor Q03 connected to the emitter of 7.
The current values I ₁ and I _{2 of} Q04 and the current value I ₃ of the current source transistor connected to the emitter of the transistor Q08 that generates the clamp potential E _p are set equal to each other (I ₁ = I ₂ = I _3). ). That is, each transistor Q03-
The bases of Q05 are commonly connected so that the same current as the bias transistor Q02 flows so that the emitter resistance R0
5, R06 and R07 are set to the same value.

One transistor Q07 of the amplifier section and
E_pEach collector of the setting transistor Q08 is
The pedestal section of the signal by the above-mentioned feedback clamp
They have the same potential. Transistors Q07 and Q0
8 collector resistors R09 and R10 are set equal
In addition, each base is also connected in common and the reference voltage E ₁
Is given, so Q07 and Q08 are pedestal
Between them, they operate under the same conditions (DC operating point). Therefore Q0
The collector current of 7 is equal to the emitter current I₂(=
I₃), And the emitter coupling resistance R0 of Q06 and Q07
No current flows through 8. Therefore, the pedestal section
The base potential of the transistor Q06 is E as in Q07.₁When
Become. That is, the video signal at the base of Q06 is as shown in FIG.
It is clamped to the reference voltage E1 as shown in 0 (B).
It

Here, the offset current α is supplied from the bias circuit 15 to the emitter of the transistor Q07 through the lead wire 25.
Current flows through the emitter coupling resistor R08 and flows into the current source of the emitter of the transistor Q06. Since the emitter current of Q06 has a constant value I ₁ , its collector current is reduced by α. The flow of this offset current causes a potential difference of R08 × α between the emitters of Q07 and Q06. R08
When xα is set to the above-mentioned threshold level TH, the video signal at the base of Q06 is as shown in FIG.
The pedestal is offset to E ₁ -TH as in (C).

The offset current α flowing through the resistor R08 is
The bias circuit 15 can be adjusted by variously changing the value of the resistor R75. As a result,
It is possible to change the black detection threshold level TH in the black detection circuit 2.

The video signal of FIG. 10 (C) pedestal clamped to E ₁ -TH obtained at the output of the emitter follower Q01 (base of Q06) of the 1 × amplifier 1 is:
Transistor pair Q4 of black detection circuit 2 via conductor 24
5, the base of one of Q46 (Q45). These transistors Q45 and Q46 form the clipper 27, and the base of the other transistor Q46 is connected to the reference voltage E ₁ described above via the conductor 16 from the bias circuit 15.
Is given as the clip level. The emitters of the transistors Q45 and Q46 are diode Q4.
3, Q44 and resistor R46, and Q4
The emitter (cathode) of 3 is a current source transistor Q41.
And a constant current I ₄ is supplied.

When the signal voltage e _{V at} the base of the transistor Q45 is higher than E ₁ (e _V ≧ E ₁ ), no current flows through the resistor R46 and the transistor Q46 is cut off. When e _V is lower than E ₁ (e _V <
E ₁ ), that is, the signal protruding to the black side from E _{1 as} shown by the slanted line in FIG.
6 is conducting. Since the signal voltage e _V is generated at the emitter of the transistor Q45, e _V corresponding to the black signal in the shaded area
The signal current of / R46 flows through the resistor R46, and almost the same signal current i _B flows through the collector of Q46.

This black signal current i _B is derived from the addition point 8 of the 1 × amplifier 1 through the gain control circuit 3 and the conductor 22, and the current addition of the output of the 1 × amplifier and the black signal is made at the resistor R09. Done. By this addition, black expansion is performed with a maximum gain of about twice as described above.

The gain control amplifier 3 is composed of an operational amplifier composed of a pair of transistors Q47 and Q48, and the black signal current i _B flowing out from the common emitter thereof is shunted by Q47 and Q48 at a controlled ratio. This shunt ratio corresponds to the variable gain of the gain control amplifier 3. The gain control signal is provided from the black pedestal comparison circuit 6 via leads 28 and 29.

In the state of being controlled to the maximum gain, the transistor Q48 of the gain control amplifier 3 becomes almost conductive, and almost all the black detection current i _B of the black detection circuit 2 becomes Q4.
The load resistance R09 of the 1-time amplifier 1 flows through 8, the lead wire 22, and the addition point 8. Therefore, the voltage gain of the added black signal at one end of R09 in this state is given by R09 / R46, which is about 1. The gain of the 1 × amplifier 1 is such that the signal current is equal to the emitter coupling resistor R08 and the load resistor R
Since it only flows through 09, it is determined by R09 / R08, which is also set to about 1. Therefore, the voltage gain of the superimposed signal at one end of the load resistor R09 (collector of Q07) is R09 / R49 + R09 / R08, and as shown by the straight line q _max in FIG. Decompression will be performed.

When the gain control amplifier 3 is controlled to the minimum gain, the transistor Q47 becomes conductive,
Q48 becomes non-conductive. Therefore, the black detection signal current i _B is shunted toward Q47, and the gain of the black signal applied to the addition point 8 becomes zero. That is, black expansion is not performed at all as indicated by the straight line q ₀ in FIG.

The clipper 27 is made inoperative at the sync signal portion so that the sync signal portion extending below the pedestal level is not detected as a black signal by the black detection circuit 2. That is, the blanking pulse BLK is given to the control transistor Q42 from T4, and this transistor Q4 is supplied in the synchronizing signal section (retrace line erasing section).
2 is turned on and the current source transistor Q41 is turned off. As a result, the transistor pair Q43 and Q44 forming the clipper 27 are turned off, and the black detection operation is prohibited.

The emitters of the pair of transistors Q45 and Q46 forming the clipper 27 are diodes Q43 and Q.
They are connected via a resistor R46 via 44. As well known, these diodes Q43 and Q44 do not change from off to on (or on to off) at a steep rise and fall, and have a transition region of an exponential function.
Therefore, abrupt clipping does not occur above the threshold level TH, and soft clipping is performed in a region having a certain width in the vicinity of the threshold level. As a result, there is no break point that the non-linear characteristic of black expansion has, and the non-linear characteristic of a smooth curve is obtained as shown by the dotted line r in FIG. 11, and the amplitude axis of the video signal is non-linearly processed to obtain an image. The adverse effect on is reduced.

As described above, the black-expanded video signal is pedestal clamped to the potential E _p from the collector of the transistor Q07 of the 1 × amplifier 1 provided with the addition point 8 and passes through the emitter follower Q12. It is led out to the black peak hold circuit 5 through the lead wire 23. The peak hold circuit 5 includes a pair of emitter-coupled transistors Q25 and Q26, one of which (Q25).
A black-expanded video signal as shown by the dotted line in FIG. At the terminal T8 derived from the other base (Q26), the peak hold capacitor 3
0 is coupled to the power supply V _{CC so} that a hold voltage corresponding to the black peak value is generated at the base of the transistor Q26 as the charging voltage of the capacitor 30.

If the black peak projects further toward the black side, the amount of charge in the capacitor 30 increases and the base potential of Q26 becomes closer to the ground side. Since the black peak value of the video signal changes momentarily according to the image content, a discharge resistor 31 is connected in parallel with the capacitor 30 in order to update the black peak detection value over time. The discharge time constant (recovery time) of the capacitor 30 and the resistor 31 is set to several seconds.

If the black peak of the video signal at the base of the transistor Q25 is lower than the black peak hold value at the base of the transistor Q26, Q25 is turned off and Q26 is turned on. Then, a current for turning on the transistor Q29 flows through the current mirror composed of the transistors Q27 and Q28 coupled to the collector of the transistor Q26, a current for turning on the transistor Q29 flows, and a small resistance R29 is turned on by turning on the transistor Q29. Through which the capacitor 30 is charged to the black peak value. The charging time constant (attack time) determined by the capacitor 30 and the resistor R29 is set to be sufficiently small. When the input video level becomes higher than the detected black peak hold value, the transistor Q25 turns on and Q2 turns on.
6 is turned off and the peak hold state is set.

The blanking circuit 4 operates in the sync signal portion so that the black peak hold circuit 5 does not erroneously detect the tip level of the sync signal as a black peak. That is, the blanking pulse BLK is supplied from the terminal T4 to the conductor 32.
Is supplied to the control transistor Q24 via the transistor Q24, the transistor Q24 is turned on in the synchronization signal section, and the transistor Q23
Turns off. The transistor Q25 serves as a current source of the transistor pair Q25 and Q26 of the black peak hold circuit 5, and these transistors are turned off in the sync signal section, and the black peak hold is interrupted.

The output of the black peak hold circuit 5 is the conductor 33.
To the black-pedestal comparison circuit 6 via. The comparison circuit 6 includes a differential amplifier 35 including transistors Q31 and Q32. Each transistor Q31, Q3
The two emitters are coupled via resistors R35 and R36,
A constant current is supplied to the connection point from a current mirror composed of transistors Q33 and Q34. The conductor 33 is connected to the base of one transistor Q31 of the differential amplifier 35.
To the black peak hold level, and the clamp potential E _p is applied to the base of the other transistor Q32 through the emitter followers Q18 and Q15 and the conductor 34, and the two are compared.

If the black peak hold value is higher than E _p on the white side, the collector output of the transistor Q32 of the differential amplifier 35 becomes higher. This output voltage is given to the base of the transistor Q48 of the gain control amplifier 3 via the lead wire 29, and the impedance of this transistor Q48 is lowered. The base of the other transistor Q47 of the gain control amplifier 3 is connected to the conductor 2 from the emitter of the transistor Q35 of the black-pedestal comparison circuit 6.
A voltage lower than 9 by the base-emitter voltage V _BE is applied via the conductor 28, and Q47 is turned off.

As a result, as described above, the black signal current i _B flowing out from the addition point 8 of the 1 × amplifier 1 increases according to the difference between the black peak and the pedestal, and is obtained as the output of the addition point 8. The black signal of the video signal is expanded. This black extension is performed until the black peak reaches the pedestal level E _p . When the black peak reaches the pedestal E _p , the differential amplifier 35 is almost balanced and the black extension operation is stopped. In this state, the transistor Q of the gain control amplifier 3
The current ratios of the respective collectors are determined by the minute potential difference between the bases of 47 and Q48, and the black signal current i _B of the collector of the transistor Q46 is shunted according to this current ratio, and the black signal current i _B at the addition point 8 is blacked at a predetermined ratio. The signals will be superimposed.

The collectors of the transistors Q31 and Q32 forming the differential amplifier 35 are resistors R37 and R3.
8 are coupled to the emitters of the transistors Q36 and Q37, and the bases of these transistors Q36 and Q38 are held at a constant voltage supplied from the bias circuit 15 through the conductor 36. Therefore, when the differential amplifier 35 is in a balanced state, almost the same current flows through the collectors of the transistors Q31 and Q32, and this current flows in the transistor Q3.
6, through the collector of Q37 and through resistors R39 and R41. The bases of a pair of transistors Q38 and Q39 that form the detector 37 of the peak detection limiter 10 are coupled to these resistors R39 and R41. And R3
When 9 <R41 is set and the differential amplifier 35 is balanced, the base voltage of Q39 is high and the base voltage of Q38 is low, so that Q39 is on and Q38 is off.

When switching channels of the TV receiver, etc.
Large amplitude noise may be input to the terminal T1. Since the peak of this noise becomes abnormally lower than the pedestal level, in such a case, the peak detection limiter 10 operates so that the black peak hold circuit 5 does not erroneously detect the noise peak as the peak of the black signal.

That is, when the black peak hold value given to the base of one transistor Q31 of the differential amplifier 35 becomes abnormally lower than the pedestal level (E _p −
(When ΔE is exceeded), the collector current of Q31 increases,
The base of one transistor Q38 of the detector 37 rises, Q38 is turned on and Q39 is turned off. Therefore, the transistor Q30 is turned on and the constant current is Q
30 to resistor R30, resistor R29, transistor Q29
The peak hold value generated in the conductor 33 is pushed up. That is, the peak hold value is E _p −Δ
It is restricted so that it is not lower than E. The limit level may be a level (Ep-ΔE) that is lower than the pedestal level E _p by about a sync tip level. The magnitude of ΔE can be set by the ratio of the resistors 39 and R41 and the gain of the differential amplifier 35.

The video signal thus black-stretched in this manner is output from the addition point 8 (collector of Q07) of the 1 × amplifier 1 to the emitter follower transistors Q09 and Q10, the lead wire 38, and the output transistor Q of the DC transmission rate correction circuit 9.
It is led from the terminal T7 via 60. The signal pedestal clamped to Ep (FIG. 12 (A)) at the addition point 8 of the 1 × amplifier 1 and the clamp potential E _p are a pair of transistors Q11, Q provided in the clamp circuit 7.
Supplied to each of the 16 bases. The emitters of these transistors Q11 and Q16 are commonly coupled to form a resistor R1.
Since it is grounded through 3, only the video signal exceeding the pedestal potential Ep is taken out from the emitter. That is, the transistors Q11 and Q16 are clippers (or N) for eliminating the synchronizing signal portion as shown by the dotted line in FIG.
It operates as an AM circuit).

The clipped video signal is an APL composed of a resistor R56 of the transmission rate correction circuit 9 and a resistor 40 and a capacitor 41 which are connected in series to a terminal T6 via a conductor 39.
(Average Picture Level) is derived to the detection circuit. In this APL detection circuit, the resistor R56,
The signal is smoothed with a time constant determined by 40 and the capacitor 41, and the average value of the signal is detected. The detected average value is applied to the base of one (Q55) of the transistor pair Q55, Q56 while being divided into appropriate values by the resistors R56, 40. At the base of the other transistor Q56, the clamp voltage E _p in the clamp circuit 7 is connected to the emitter follower transistors Q17 and Q18 and the conducting wire 4.
2, given through the resistor R61. The emitters of these transistors Q55, Q56 are commonly connected via resistors R57, R58, and the pulse current corresponding to the pedestal section is supplied from the pulse shaping circuit 19 via the lead wire 42.

Therefore, the transistors Q55 and Q56 operate only in the pedestal section, and in this section, the potential AP is higher than the potential E _p.
If the L detection level is high, the current i flows through the active load including the resistor R62, the transistor Q57, and the current mirror transistors Q58 and Q59 according to the difference. As a result, as shown in FIG. 12B, the transmission rate correction pulse 4 is added to the pedestal section of the video signal derived from the terminal T7.
3 is superimposed. Since the level of the correction pulse is proportional to the difference between the APL detection level and the reference level, the correction is performed so that the pedestal level is lowered accordingly in the portion where the APL is low. That is, reverse correction is performed when the DC transmission rate in the subsequent stage is 100% or less, the DC transmission rate is corrected to 100% at the cathode of the CRT, and stable black reproduction is performed.

In the pulse shaping circuit 19, the transistor Q is driven by the blanking pulse BLK applied to the terminals T4 and T5 and the clamp pulse in the pedestal section.
53 and Q54 are turned on, the current sources Q49 and Q50 are turned on in the pedestal section in the blanking section, and the clamp pulse is led to the clamp circuit 7 and the direct current rate transmission circuit 9 through the transistors Q51 and Q52. There is.

[0062]

According to the video black level reproducing method of the present invention, since the amplitude operation is not performed for a signal in the white direction above a certain level in the video signal, the luminance level may fluctuate due to the black level reproduction. Without, stable color reproduction is possible. Further, since the level control for reproducing black occurs only in the vicinity of black, the entire screen is not brightened or darkened due to a sudden change in black peak, and a stable received image with constant brightness can be obtained.

Further, according to the video black level reproducing circuit of the present invention, the amplitude is controlled only for the black signal of the input video signal which is equal to or lower than the predetermined level, and the black signal is reproduced by adding the original input signal and the amplitude-controlled black signal. Since the circuit configuration is such that the amplitude control is performed for the entire video signal by the control loop that locally acts and the addition processing of a part of the entire amplitude, the loop gain may be relatively small, and the operation is It is possible to construct a very stable black reproduction circuit.

[Brief description of drawings]

FIG. 1 is a waveform diagram of a video signal showing a signal processing of black reproduction by a conventional brightness control.

FIG. 2 is a waveform diagram of a video signal showing the signal processing of black reproduction by the conventional brightness control.

FIG. 3 is a diagram showing the principle of black reproduction according to the present invention.
It is a waveform diagram of a video signal corresponding to.

FIG. 4 shows the principle of black reproduction according to the present invention.
It is a waveform diagram of a video signal corresponding to.

FIG. 5 is a block diagram of a video signal processing circuit to which the black reproduction method of the present invention is applied.

FIG. 6 is an input / output characteristic diagram and an input / output waveform diagram of the black expansion circuit.

FIG. 7 is a waveform diagram showing various aspects of black expansion.

FIG. 8 is an input / output characteristic diagram showing signal processing of conventional black level reproduction.

9 is a circuit diagram showing details of the block of FIG.

10 is a waveform diagram of a video signal for explaining the operation of FIG.

11 is an enlarged view showing details of the input / output characteristic diagram of FIG.

FIG. 12 is a waveform diagram of a video signal for explaining the operation of FIG.

[Explanation of symbols]

 1 1 × amplifier 2 Black detection circuit 3 Gain control amplifier 4 Blanking circuit 5 Black peak hold circuit 6 Black-pedestal comparison circuit 7 Pedestal clamp circuit 8 Addition point 9 DC transmission rate correction circuit 10 Peak detection limiter 15 Bias circuit 17 Comparator 19 Pulse shaping circuit 27 Clipper 30 Peak hold capacitor 35 Differential amplifier 37 Detector 43 Correction pulse

Claims (2)

[Claims] 1. A black signal below a predetermined level of an input video signal is extracted, the amplitude of the extracted black signal is controlled, the input video signal and the amplitude-controlled black signal are added, and the added signal is added. A method of reproducing a black level of an image, which comprises detecting a black peak level of a signal and controlling the amplitude of the extracted black signal according to the detected black peak level. 2. A black detection circuit for extracting a black signal below a predetermined level of an input video signal, a gain control amplifier for controlling the amplitude of the extracted black signal, an input video signal and an output of the gain control amplifier. It is characterized by comprising an adder for adding and a black peak detection circuit for detecting a black peak level of a signal obtained from the output of the adder, and controlling the gain of the gain control amplifier by the output of the black peak detection circuit. Black level reproduction circuit for the video to be.