JP2624191B2 - Video black level reproducing method and circuit - Google Patents

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 In general, a video signal is sent with a 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 a setup, and is determined by the convenience of a broadcasting station, a TV camera or a home VD.
It varies depending on the manufacturer and model of the TR, etc., and varies considerably widely. For this reason, it is necessary to correctly reproduce the black level on the reproduction side (monitor TV) (matching the black level of the signal with the cutoff level of the CRT).

As a conventional black level reproducing method, a method of lowering the DC transmission rate and a method of lowering the luminance level for the setup are used. In the former method, the level of the setup (for DC) is lowered, so that the floating of black is inconspicuous. However, for a video signal whose setup is originally low, the black level is collapsed and the resolution of the black portion is deteriorated. Also, in an image in which the beam current of a CRT is large, the brightness level is reduced by lowering the brightness level to limit the beam amount.
itter) is often used in combination, so that 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 is set to the maximum black level (pedestal level). ), And is also referred to as a dynamic picture system. This method enables stable black level reproduction 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 having a large CRT beam current level.

[0005]

However, since the luminance level is changed, the signal level (luminance) always fluctuates over the whole in accordance with the black peak level in the signal, and the signal level (luminance) between the luminance signal Y and the chroma signal C is changed. The level ratio Y / C constantly changes. Therefore, the color density changes depending on the size of the black level. For example, human skin color (50 to 8 for white peak)
(0% luminance) and the like, when the black level is high in the white direction, the luminance signal is reduced, and as a result, C is increased with respect to Y, resulting in an overcolored skin color. On the other hand, when the black peak is low, the color becomes lighter.

Further, when black reproduction is being performed at a certain time point t ₁ as shown in FIG. 2A, a further black side signal P (for example, with a black border) is obtained at a time point t ₂ as shown in FIG. 2B. When the telop signal is inserted, black reproduction is performed so that the peak of the signal P becomes a pedestal, so that the luminance of the screen sharply increases. Conversely, when the black peak suddenly rises at time t3 as shown in FIG. 2 (C), the luminance suddenly falls. This transient phenomenon is generally called "black paka"
The screen becomes very hard to see. The present invention proposes a black level reproduction method that solves the above-mentioned disadvantages.

[0007]

According to the present invention, there is provided a video black level reproducing method for extracting a black signal having a predetermined level or less from an input video signal, controlling the amplitude of the extracted black signal, and combining the input video signal with the input video signal. The amplitude-controlled black signal is added, and a black peak level of the added signal is detected. When the detected black peak level is different from the pedestal level,
If the black peak level matches the above pedestal level
In this case, the amplitude of the extracted black signal is controlled.

The video black level reproducing circuit of the present invention comprises:
A black detection circuit 2 for extracting a black signal of a predetermined level or less of the input video signal, a gain control amplifier 3 for controlling the amplitude of the extracted black signal, and adding the input video signal and the output of the gain control amplifier 3 It includes an adder 8, and a black peak detection circuit 5 for detecting a black peak level of the signal obtained from the output of the adder 8, the black peak detection
The output of the output circuit is different from the pedestal level of the video signal.
Output from the black peak detection circuit 5
The gain of the gain control amplifier 3 is controlled so that the peak level coincides with the pedestal level (FIG. 5).

[0009]

According to the video black level reproducing method of the present invention, the amplitude operation is not performed for a white signal having a certain level or higher in the video signal, so that the luminance level does not fluctuate due to the black level reproduction. Stable color reproduction is possible. In addition, since level control for black reproduction occurs only in the vicinity of black, a stable received image with a constant brightness can be obtained without the entire screen becoming bright or dark due to a sudden change in the black peak.

According to the video black level reproducing circuit of the present invention, amplitude control is performed only on a black signal of a predetermined level or less of an input video signal, and black is reproduced by adding the original input signal and the amplitude-controlled black signal. Since the amplitude of the entire video signal is controlled by a control loop acting locally and a process of adding a part of the entire amplitude, the loop gain may be relatively small, and the operation may be reduced. An extremely stable black reproduction circuit can be configured.

[0011]

The present invention will be described below with reference to examples. FIG.
FIGS. 4 and 4 are waveform diagrams of video signals 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 (black side) below this level is extracted. To control Fig.3 (B)
, The black peak coincides with the pedestal level. That is, the operation on the amplitude axis is not performed at all with respect to the video signal above the threshold level (white side) at the time of reproducing the black level. Therefore, for a relatively bright signal such as a flesh color, no matter what kind of black signal is present 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.

Also, as shown in FIGS. 4A, 4B and 4C, a level control for black reproduction performed following a rapid change of the black peak (t ₁ → t ₂ → t ₃ ) is performed. Since it occurs only in the vicinity of black, the entire brightness level of the screen does not move due to 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-described black reproduction method.
5 is supplied with a video signal obtained through tuning, IF, and detection, and is supplied to the 1 × amplifier 1 via a clamp capacitor 12. The output of the 1 × amplifier 1 is led to an unshown luminance / chroma processing circuit via an addition point 8 (adder) and a DC transmission rate correction circuit 9.

The output of the adder 8 forming the adder is also supplied to a pedestal clamp circuit 7, and a reference voltage Ep is applied 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, thereby changing the DC charge of the clamp capacitor 12 and converging the error feedback. In the stable state, the pedestal level of the video signal output from the addition point 8 is clamped to Ep.

The input signal of the 1 × amplifier 1 is also supplied to a black detection circuit 2, and as shown in FIG. 3A, a black signal having a predetermined threshold level TH or less based on the pedestal is detected. . Although the black detection circuit 2 is a clip circuit, a reference voltage E ₁ (ground reference) is used as a clip 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, the black detection circuit 2 clips the video signal pedestal-clamped to E ₁ -TH at the clip level E ₁ , thereby obtaining the signal shown in FIG.
A black signal shown at the oblique edge of (A) is detected. A blanking pulse is applied to the detection circuit 2 so that the detected black signal does not include a synchronization signal portion.

The detected black signal is led to an addition point 8 after its amplitude is controlled by the gain control amplifier 3, and is added to the output of the 1 × amplifier 1. The gain of the gain control amplifier 3 is varied between 0 and 1, and the variable control is performed based on the peak detection value of the black signal. Therefore, the black signal having the threshold level TH or less included in the video signal output from the addition point 8 is at most twice (1+
Until 1), the amplitude is extended in the blackest direction (pedestal direction).

This expansion operation is performed by loop control.
Control by error feedback 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 supplied to the blanking circuit 4 for the synchronization signal portion, and a video signal not including the synchronization signal portion is extracted. This video signal is sent to a black peak hold circuit 5 as a black peak detection circuit, and a 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 a loop operates so as to eliminate the difference between the black peak and the pedestal. As a result, as shown in FIG. 3B, black reproduction is performed in the monitor receiver in a state where the black peak and the pedestal coincide with each other.

When the black peak hold circuit 5 detects a peak extending to the black side such as a large amplitude inter-station noise generated at the time of channel switching, it takes a long time to return to the normal black peak detection, and during that time, it is very whitish. Since a 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 be significantly lower than that of the pedestal.

FIG. 6 is an input / output characteristic diagram and an input / output waveform diagram of the black extension operation in FIG. FIG. 7 shows examples of various video signals to be reproduced in black. As shown in FIG. 6, in the portion above the threshold level TH of the input video signal, the input / output characteristic is a straight line having a slope of 1, and therefore, no operation is performed on the amplitude 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, and as shown in FIG. The input / output characteristics for signals below TH are 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 having a black peak floating above the pedestal, black expansion is performed in accordance with the amount of floating. The maximum value of the elongation factor 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 of the signal below TH is a straight line q _max with a slope 2 shown in FIG.

The black peak is TH / as _in the input S _in of FIG.
If the output S _out extends to 2, the output S _out
Comes to coincide with the black peak pedestal. FIG. 7
As shown in (A), the input is also double-extended so that the amount of protrusion of the black peak does not reach TH / 2, but the extension of the black peak does not extend to the pedestal. That is, it is restricted so that the elongation does not occur twice or more.

For an input having a black peak deeper than TH / 2 as shown in FIG. 7 (B), the black extension is performed at a gain within a range of 1 to 2 times according to the amount of floating from the pedestal. Done. That is, one of the innumerable straight lines in the range of the straight lines q _{0 to} q _max in FIG. 6 is the input / output characteristic in this case, and the slope is determined by the difference between the black peak and the pedestal. In this case, the gain control amplifier 3 shown in 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 twice as described above is to prevent the screen from becoming unnatural due to the non-linear characteristic exhibited by the input / output characteristics of black expansion. The maximum of the expansion gain can be set to 2 or more or 2 or less. If the value is 2 or more, the "dominantness" of black reproduction is improved, but the nonlinear characteristic is stronger. If the value is 2 or less, the black reproduction is less effective, but the nonlinear characteristic is reduced.

FIG. 8 is a graph showing the input / output characteristics of the conventional black reproduction. As described above, the signal level (entire luminance) is reduced in accordance with the floating amount of the black peak level from the pedestal. Output characteristic varies from the straight line p ₀ ~p _max according to the black level as shown in Figure 8. Straight p ₀ in the case that match the black peak and pedestal level correction is not performed. The straight line p _max is the case where the maximum level correction has been performed. If the black peak of the signal changes rapidly, energy change (light amount change) of the screen corresponding to the hatched region of p ₀ ~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 clear from the figure, black reproduction can be performed without noticeable luminance change.

FIG. 9 is a detailed circuit diagram of the video signal processing circuit of FIG. Note that 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.
The IC has pin terminals with diagonal lines. The terminals T1 and T3 are connected to a power supply V _CC and a ground potential, respectively.

The video signal is input from the terminal T 2 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 supplied to one transistor Q06 via an emitter follower transistor Q01. The reference voltage E ₁ (FIG. 3A) is applied to the base of the other transistor Q07 from the bias circuit unit 15 via the conducting wire 16.

The emitters of the transistor pairs Q06 and Q07 are connected to constant current transistors Q03 and Q04, respectively, and are supplied with constant currents I ₁ and I ₂ . Accordingly, the 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.
07 and appears as a change in the collector current of Q07. The collector current of Q07 flows through the load resistor R09, and a signal voltage is extracted from the collector of Q07. The amplification gain is about 1.

The signal at the collector of transistor Q07 is
The signal is applied to one (Q13) of a pair of transistors Q13 and Q14 constituting 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 has emitter follower transistors Q18, Q1
5 through, is given clamp potential E _p shown in FIG. 10 (A). This potential _Ep is supplied from the bias circuit 15 to the lead 1
The reference voltage E _{1 described} above, which is given via
This is a constant voltage formed based on (B)). That is,
Conductor 16 is coupled to the base of transistor Q08,
A constant current transistor Q05 is connected to the emitter of Q08.
(Current I ₃₎ is coupled, E _p = V to the collector of Q08
_CC -I ₃ R10 (R10 are collector load resistance of Q08)
Clamp potential E _p is given consisting. The potential E _p is applied to emitter follower Q18 described above via a line 18.

The comparator 17 of the clamping circuit 7, the active state by the clamping pulse pedestal portion provided via lead 20 (current I _p) from the pulse shaping circuit 19, the clamp potential E _p and video signals (collector of Q07) a pedestal level e _p of are compared. Note that the emitter followers Q09, Q1
2, the base-emitter voltages of Q15 and Q18 are canceled by the symmetrical arrangement of these transistors. The difference voltage of the comparison result is determined by transistors Q19 and Q19.
20 and Q21, the current is converted into a current by an active load having a current mirror configuration, and the current is converted into a current through a conducting wire 21.
To the input side of the clamp capacitor 12.

For example, if e _p <E _p , the comparator 1
7, transistor Q13 is turned on, Q14 is turned off,
The ON current of the transistor Q13 is charged in the capacitor 12 through the conductor 21. As a result, the transistor Q0
The DC component (pedestal level) of the base input of No. 1 rises. Conversely, if e _p <E _p , transistor Q13 turns off and Q14 turns on, and discharge current flows from capacitor 12 through conductor 21 into Q19.

[0033] In this way until the pedestal level e _p a clamp potential E _p of the video signal is equal to, the pedestal level correction of the video signal by the feedback of the detection error is performed. In a state in which the error is gone in comparator 17, the video signal pedestal clamp to the clamp potential E _p at the output of the 1-fold amplifier 1 (collector of Q07) is obtained.

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

The output of the emitter follower Q01 on the input side of the 1 × amplifier 1 is also led out to the black detection circuit 2 through the conductor 24. The 1 × amplifier 1 adjusts the pedestal level of the signal on the conductor 24 as shown in FIG.
Amount corresponding function of the offset of the reference voltage threshold drain downy Le TH from E ₁ to as the (C) also has.

The transistors Q06 and Q0 of the 1 × amplifier 1
7, a current source transistor Q03 connected to the emitter of
Current value I _1, I ₂ and the current value I ₃ of the current source transistor is connected to the emitter of the transistor Q08 for generating a clamp potential E _p of Q04 is mutually set equal _{(I 1 = I 2 = I} 3 ). That is, each transistor Q03-
The emitter resistor R0 is connected so that the base of Q05 is commonly connected and the same current flows as the bias transistor Q02.
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
The pedestal section of the signal is obtained by the feedback clamp described above.
The potential is the same between them. Transistors Q07 and Q0
8, the collector resistances R09 and R10 are set equal.
And each base is also connected in common, and the reference voltage E ₁
Q07 and Q08 are in the pedestal zone
Operate under the same condition (DC operating point). Therefore Q0
7 is equal to the emitter current and I_Two(=
I_Three), And the emitter coupling resistance R0 of Q06 and Q07.
No current flows through 8. For this reason, the pedestal section
The base potential of transistor Q06 is E as in Q07.₁When
Become. That is, the video signal at the base of Q06 is shown in FIG.
0 (B) is clamped to the reference voltage E1.
You.

Here, the offset current α is supplied from the bias circuit 15 to the emitter of the transistor Q07 through the conducting wire 25.
Flows through the emitter coupling resistor R08 and flows into the current source of the emitter of the transistor Q06. Note Since the emitter current of Q06 is a constant value I _1, so that the collector current is reduced by alpha. This offset current causes a potential difference of R08 × α between the emitters of Q07 and Q06. R08
When × α is set to the above-described threshold level TH, the video signal at the base of Q06
The pedestal is offset to E ₁ -TH as in (C).

The offset current α flowing through the resistor R08 is
The adjustment can be made by changing the value of the resistor R75 in the bias circuit 15 in various ways. As a result, FIG.
, The threshold level TH of black detection in the black detection circuit 2 can be varied.

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

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 falls below E ₁ (e _V <
E ₁ ), that is, a signal protruding to the black side from E _{1 as} shown by the hatching in FIG.
6 conducts. The emitter of the transistor Q45 than the signal voltage e _V resulting, e _V corresponding to the black signal of the hatched portion
/ R46 of the signal current flows through resistor R46, which the Ho Isuzu same signal current i _B flows to 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, whereby the current addition between the output of the 1 × amplifier and the black signal is applied to the resistor R 09. Done. By this addition, black expansion is performed with a maximum gain of about twice as described above.

The gain control amplifier 3 is constituted by an operational amplifier comprising a pair of transistors Q47 and Q48, and shunts the black signal current i _B flowing out of the common emitter at a ratio controlled by Q47 and Q48. 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 the conductors 28 and 29.

In the state controlled to the maximum gain, the transistor Q48 of the gain control amplifier 3 becomes substantially conductive, and almost all of the black detection current i _B of the black detection circuit 2 becomes Q4.
8, the conductor 22, the addition point 8, and flows through the load resistor R09 of the 1 × amplifier 1. Therefore, the voltage gain of the added black signal at one end of R09 in this state is given by R09 / R46, which is approximately 1. The gain of the 1 × amplifier 1 is such that the signal current is equal to the emitter coupling resistance R08 and the load resistance R08.
Since only 09 flows, 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 take place.

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

It should be noted that the clipper 27 is disabled at the synchronizing signal portion so that the synchronizing signal portion extending below the pedestal level by the black detecting circuit 2 is not detected as a black signal. That is, a blanking pulse BLK is supplied to the control transistor Q42 from T4, and the transistor Q4 is supplied in a synchronization signal section (return blanking section).
2 is turned on, and the current source transistor Q41 is turned off. As a result, the transistor pair Q43 and Q44 constituting the clipper 27 are turned off, and the black detection operation is prohibited.

The emitters of the transistor pairs Q45 and Q46 forming the clipper 27 are diodes Q43 and Q43.
It is connected by a resistor R46 through the resistor 44. As is well known, these diodes Q43 and Q44 do not change from off to on (or from on to off) at a sharp rise and fall, and have an exponential transition region.
Therefore, sharp clipping does not occur above the threshold level TH, and soft clipping is performed in a region having a certain width near the threshold level. As a result, the break point of the non-linear characteristic of black extension is eliminated, a non-linear characteristic of a smooth curve is obtained as shown by a dotted line r in FIG. 11, and the image is obtained by performing non-linear processing on the amplitude axis of the video signal. The adverse effects on

The black decompressed video signal, as already described, with the addition point 8 is pedestal clamp from the collector to a potential E _p of 1 times the amplifier 1 of a transistor Q07 provided, through the emitter follower Q12, It is led to the black peak hold circuit 5 through the conductor 23. The peak hold circuit 5 includes a pair of emitter-coupled transistors Q25 and Q26, one of which (Q25)
Is supplied with a video signal that is black-extended as indicated by the dotted line in FIG. On the other hand, a terminal T8 derived from the base of (Q26) has a peak hold capacitor 3
0 is coupled between the power supply V _CC, a hold voltage corresponding to the black peak value is made to occur in the base of the transistor Q26 as the charging voltage of the capacitor 30.

If the black peak protrudes further to the black side, the charge amount of the capacitor 30 increases, and the base potential of Q26 becomes closer to the ground side. Further, since the black peak value of the video signal changes every moment according to the video content, a discharge resistor 31 is connected in parallel with the capacitor 30 in order to update the detected black peak value as time elapses. The discharge time constant (recovery time) by the capacitor 30 and the resistor 31 is set to several seconds.

If the black peak of the video signal at the base of transistor Q25 is lower than the black peak hold value at the base of transistor Q26, Q25 is off and Q26 is on. Then, a current for turning on the transistor Q29 flows through a 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 the turning on of the transistor Q29 causes a small resistance R29. Through 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 sufficiently small. When the input video level becomes higher than the detected black peak hold value, the transistor Q25 is turned on and Q2
6 is turned off, and a peak hold state is set.

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

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

[0053] The higher the black peak-hold value than E _p is the white side, the collector output of the transistor Q32 of the differential amplifier 35 becomes high. This output voltage is applied to the base of the transistor Q48 of the gain control amplifier 3 via the conducting wire 29, and the impedance of the transistor Q48 is reduced. 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 the base-emitter voltage V _{BE by} 9 is applied through the conductor 28, and Q47 is turned off.

As a result, as described above, the black signal current i _B flowing 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 black peak is performed until reaching the pedestal level E _p. When black peak reaches pedestal E _p, the differential amplifier 35 ho Isuzu balanced, black extension operation stops. In this state, the transistor Q of the gain control amplifier 3
47, the small potential difference between the base of Q48 have definite current ratio of the collector of the respective black at a summing point 8 is shunted black signal current i _B of the collector of the transistor Q46 in response to the current ratio at a predetermined ratio The signal will be superimposed.

The collectors of the transistors Q31 and Q32 forming the differential amplifier 35 are connected to resistors R37 and R3.
The bases of the transistors Q36 and Q38 are maintained at a constant voltage supplied from the bias circuit 15 via the lead 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 is
6, flows through the resistors R39 and R41 through the collector of Q37. The bases of a pair of transistors Q38 and Q39 constituting 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 large and the base voltage of Q38 is small, and Q39 is on and Q38 is off.

At the time of switching the channel of the TV receiver or the like,
Large amplitude noise may be input to the terminal T1. Since the peak of this noise is 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 −
ΔE), 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. As a result, the transistor Q30 is turned on and the constant current becomes Q
30 to resistor R30, resistor R29, transistor Q29
And the peak hold value occurring in the conductor 33 is pushed up. That is, the peak hold value E _p -Δ
It is limited so that it does not drop below E. Limit level may be a level that fell to about the sync tip level than the pedestal level E _p (Ep-ΔE). 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 black-extended in this manner is supplied from the addition point 8 (collector of Q07) of the 1 × amplifier 1 to the emitter follower transistors Q09 and Q10, the conducting wire 38 and the output transistor Q of the DC transmission rate correction circuit 9.
It is derived from terminal T7 via 60. The 1-fold Ep pedestal clamp signal in (FIG. 12 (A)) and the clamp potential E _p in the addition point 8 of the amplifier 1, a pair of transistors Q11 provided in the clamp circuit 7, Q
Supplied to each of the 16 bases. The emitters of these transistors Q11 and Q16 are commonly coupled, and a resistor R1
3, only the video signal exceeding the pedestal potential Ep is extracted from the emitter. That is, the transistors Q11 and Q16 are provided with a clipper (or N) for removing the synchronization signal portion as indicated by a dotted line in FIG.
AM circuit).

The clipped video signal is converted to an APL composed of a resistor R56 and a capacitor 41 connected in series to a resistor R56 and a terminal T6 of the transmission rate correction circuit 9 via a conductor 39.
(Average Picture Level) It is derived to the detection circuit. In this APL detection circuit, a 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 in a state where the voltage is divided to an appropriate value by the resistors R56, R40. The base of the other transistor Q56, the clamp voltage E _p in the clamping circuit 7, an emitter follower transistor Q17, Q18 and conductor 4
2, provided via a resistor R61. The emitters of these transistors Q55 and Q56 are commonly connected via resistors R57 and R58, and a pulse current corresponding to the pedestal section is supplied from the pulse shaping circuit 19 via the conductor 42.

[0060] Thus transistor Q55, Q56 operates only pedestal interval, AP than the potential E _p in this interval
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 applied 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, correction is performed so as to lower the pedestal level in a portion where the APL is low. That is, reverse correction is performed when the DC transmission rate at 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 supplied by a blanking pulse BLK applied to the terminals T4 and T5 and a clamp pulse in the pedestal section.
53 and Q54 are turned on, and in the pedestal section within the blanking section, the current sources Q49 and Q50 are turned on, and the clamp pulse is led to the clamp circuit 7 and the DC rate transmission circuit 9 through the transistors Q51 and Q52. I have.

[0062]

According to the video black level reproducing method of the present invention, a black signal having a predetermined level or less of an input video signal can be reproduced .
The amplitude expansion control in the pedestal level direction is performed, and the amplitude operation is not performed at all for the white signal that is higher than a certain level in the video signal. Can be reproduced. In addition, since level control for black reproduction occurs only in the vicinity of black, a stable received image with a constant brightness can be obtained without the entire screen becoming bright or dark due to a sudden change in the black peak.

According to the video black level reproducing circuit of the present invention, amplitude expansion control in the pedestal level direction is performed only for a black signal of a predetermined level or less of the input video signal, and the original input signal and the amplitude-controlled black signal are converted. Since the circuit configuration performs black reproduction by adding, the amplitude control is performed for the entire video signal by an amplitude control loop that acts locally and a process of adding a part (amplitude controlled local) to the entire amplitude. Therefore, a black reproduction circuit having a very small loop gain and a very stable operation can be constructed.

[Brief description of the drawings]

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

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

3 and 2 show the principle of black reproduction according to the present invention.
FIG. 4 is a waveform diagram of a video signal corresponding to FIG.

FIGS. 1 and 2 show the principle of black reproduction according to the present invention.
FIG. 4 is a waveform diagram of a video signal corresponding to FIG.

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 extension circuit.

FIG. 7 is a waveform chart showing various modes of black extension.

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

FIG. 9 is a circuit diagram showing details of blocks in FIG. 5;

FIG. 10 is a waveform diagram of a video signal for explaining the operation of FIG. 9;

FIG. 11 is an enlarged view showing details of the input / output characteristic diagram of FIG. 6;

FIG. 12 is a waveform diagram of a video signal for explaining the operation of FIG. 9;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1 time 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 forming circuit 27 Clipper 30 Peak hold capacitor 35 Differential amplifier 37 Detector 43 Correction pulse

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-53978 (JP, A) JP-A-51-87911 (JP, A) JP-A-56-75780 (JP, A)

Claims (2)

(57) [Claims] A black signal of a predetermined level or less of an input video signal is extracted, an amplitude of the extracted black signal is controlled, an input video signal is added to the amplitude-controlled black signal, and the added The black peak level of the signal is detected, and the detected black peak level is the pedestal level.
When the black peak level is different from the pedestal level
A method for reproducing a black level of an image, comprising controlling the amplitude of the extracted black signal in a direction that coincides with a bell . 2. A black detection circuit for extracting a black signal of a predetermined level or less of an input video signal, a gain control amplifier for controlling an amplitude of the extracted black signal, and an input video signal and an output of the gain control amplifier. An adder for adding, and a black peak detection circuit for detecting a black peak level of a signal obtained from an output of the adder, wherein an output of the black peak detection circuit is a pedestal of the video signal.
Output level of the black peak detection circuit
Therefore, the black peak level is equal to the pedestal level.
A video black level reproducing circuit for controlling a gain of the gain control amplifier in a direction in which the video signal is matched.