JPH0247155B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video level correction circuit for a television receiver, and relates to a video level correction circuit for a television receiver, which supplies a video signal whose maximum black level changes depending on the APL (average video level) to a picture tube. In the machine, the above
The purpose is to prevent brightness from floating when the APL is low.

In general television receivers, APL
An automatic brightness limiting (ABL) circuit is used to prevent the picture tube from saturating and deteriorating the primary color phosphors when very high video signals are input. As is well known, this ABL circuit is
Excessive picture tube beam current is detected, and the detected output is used to control the DC level of the video signal, that is, the brightness, to be lowered.

By the way, in order to faithfully reproduce the brightness and darkness of the original image (subject) on the receiver side, it is necessary to correctly reproduce the DC level of the video signal, and for this purpose, a so-called DC component regeneration circuit is used. Therefore, for a DC-clamped video signal, the above-mentioned
ABL control has the following drawbacks. In other words, the above DC component regeneration circuit clamps the video signal so that its blackest level (usually the pedestal level) always matches the cut-off level of the picture tube, regardless of the level of the APL. If ABL is applied to a clamped video signal with a high APL as in be called).

Therefore, as an alternative method, the reproduction rate of the DC level of the video signal is less than 100% (i.e.,
A method has been proposed in which the maximum black level changes slightly depending on the APL, and ABL control is applied to this video signal.

Figure 2 is a diagram explaining this method, and APL
In Figure a, which shows the beam current (I _B ) characteristics,
The broken line represents the case when the DC regeneration rate is 100%,
The solid line represents the case where the DC regeneration rate is 100% or less. Point P and point P' respectively indicate the ABL starting point. In other words, direct reproduction rate
When the APL is 100% or less, select an appropriate DC reproduction rate so that the blackest level of the video signal matches the cut-off level (L ₁ ) of the picture tube when the APL is, for example, 40%. 2b, the change in the blackest level with respect to APL will be as shown by the lower solid line in FIG. 2b, and the change in the whitest level will be as shown in the upper solid line in FIG. 2b. Therefore, a video signal with a DC reproduction rate of 100%
Comparing with the broken line in Figure b, which shows the case where ABL is applied, it can be seen that the darkening (hatched area A) when APL is high has been somewhat improved.

However, although this method improves the darkening when the APL is high, on the other hand,
When the APL is low, the white level may exceed the saturation level (L ₂ ) of the picture tube (hatching B in Figure 2),
There is a drawback that the maximum black level exceeds the picture tube cutoff level (L ₁ ) (hatched area C), and these are called white floating and black floating, respectively, and degrade the image quality.

However, the present invention is designed to eliminate such "floating" in brightness when the APL is low, and the details thereof will be explained below with reference to the drawings.

FIG. 1 shows a schematic configuration of the main parts of a television receiver using a video level correction circuit according to the present invention. In the figure, 1 is a video signal input terminal;
Reference numeral 2 denotes a contrast control circuit to which the video signal from this terminal is input, and it performs contrast control and automatic contrast limiting (ACL) operation when the APL is high by manual operation. 3 is a DC clamp/brightness control circuit, which applies a DC clamp to the video signal and also controls brightness and changes the clamp level by manual operation.
When the APL is high, an automatic brightness limit (ABL) operation is performed that forcibly lowers the clamp level. 4 is a DC transmission rate setting circuit, which guides the video signal output from the DC clamp/brightness control circuit 3 to the next stage peak clip/blanking pulse mixing circuit 5 at a DC transmission rate of about 40 to 60%. The video signal from this circuit 5 passes through a video amplification circuit 6, is mixed with a color difference signal in a color matrix circuit 7, and is applied to a picture tube 8.

On the other hand, 9 is a first beam current detection circuit that detects when the beam current of the receiver 8 is excessive, and this circuit detects a high APL state of the video signal introduced to the input terminal 1, The above-mentioned ACL operation and ABL operation are performed.
Further, 10 is a second beam current detection circuit that detects when the beam current is too low. In the present invention, this circuit detects a low APL state of the video signal, and the DC clamp and brightness control circuit 3 The DC clamp level is forcibly lowered.

According to this configuration, the video signal output from the DC clamp/brightness control circuit 3 is passed through the DC transmission rate setting circuit 4 which is set to 100% or less, so that the video signal is applied to the picture tube 8. The blackest level of the video signal changes depending on the APL. That is,
Even if the circuit 3 clamps the blackest level of the video signal so that it always matches the cut-off level of the picture tube 8, the blackest and whitest levels of the video signal applied to the picture tube 8 will depend on the APL. The above-mentioned third
It tends to change as shown by the broken line in Figure b. but,
When the APL is relatively low, that is, when the picture tube beam current is below a predetermined value, the second beam current detection circuit 10 detects this state, and thereby sets the DC clamp level of the DC clamp/brightness control circuit 3 to a predetermined value. is forced down to a certain level. As a result, the blackest level of the video signal applied to the picture tube 8 coincides with the cutoff level of the picture tube when the APL is low, as shown by the solid line in Figure 3b. This eliminates the black floating. FIG. 3a shows the APL vs. beam current I _B characteristic in this case. At this time, the operation start point of the second beam current detection circuit 10 needs to be set in consideration of the above-mentioned DC transmission rate.

In addition, in FIG. 3b, the darkening when the APL is high cannot be improved from the state shown in FIG. 2b only by the above-described operation of the DC clamp/brightness control circuit 3. However, here ACL is used at the same time as ABL.
Since the black signal is multiplied, the black sinking described above can be improved slightly from the state shown in Figure 2b.Also, when the APL is high, it is better in terms of image quality to slightly sink the black signal, so such There is no problem even if it is not possible to completely correct dark spots.

Furthermore, in the circuit shown in FIG. 1, since the white peak is clipped by the peak clip/blanking pulse mixing circuit 5, in conjunction with the operation of the DC clamp/brightness control circuit 3 described above,
Whitening when the APL is low will also be improved.

Next, FIG. 4, which shows an example of an implementation circuit of the main part surrounded by the broken line in FIG. 1, will be described. The circuit shown in the figure is roughly divided into an IC 11 that includes a DC clamp/brightness control circuit 3 and a DC transmission rate setting circuit 4, and an external circuit section 12 that includes first and second beam current detection circuits 9 and 10. ing.

Basically, the IC 11 has a negative polarity composite video signal (a) introduced into the terminal _T1 as an emitter follower.
The video signal that passes through Tr ₁ and is amplified by differential elements Tr _{2 and} Tr ₃ , and taken out from the collector of Tr ₂ , one of the differential pairs, is amplified and inverted by transistors Tr ₁₄ to _{Tr 17} , and becomes a negative polarity signal. It is configured to be taken out to the terminal _T6 , and in this case, DC clamp (brightness control) and DC transmission rate setting are performed as follows.

First, to explain the operation of the brightness control circuit 3, the positive composite video signal taken out from the collector of one Tr ₂ of the differential pair Tr ₂ and Tr ₃ to the emitter P point of the transistor Tr ₁₄ is transmitted to the differential pair Tr 2 and Tr 3. versus
One of Tr ₁₂ and Tr ₁₃ is applied to the base of Tr ₁₃ .
The base of the other Tr ₁₂ of this differential pair is connected to a variable resistor VR in the external circuit section 12 connected to the terminal Tr ₄ .
and resistors R ₁ , R ₂ etc. and transistor Tr ₉ in IC 11
~ A DC voltage determined by Tr ₁₁ , etc. is applied, and a gate pulse (C) corresponding to the front porch of the pedestal portion of the video signal is applied to the base of the constant current source transistor Tr ₁₈ of the differential pair _. is applied via. Therefore, the other of the above differential pair
A pulse (d) of a predetermined magnitude (amplitude) appears at the collector Q point of TR ₁₂ at the timing of the front porch, and this pulse is transmitted to the transistors Tr ₈ , Tr ₇ ,
After being amplified and inverted by Tr ₅ , it is smoothed by capacitor C ₁ and resistor R ₃ connected to terminal T ₃ , converted to a DC voltage, and applied to Tr ₃ of the input stage differential pair Tr ₂ and Tr ₃ . be done. Therefore, the collector potential of Tr ₂ of this differential pair, that is, the DC level of the composite video signal (A), is determined by the DC voltage, that is, the amplitude of the pulse (E).

Therefore, if the DC potential of the terminal _T4 is increased by the variable resistor VR, the base potential of the transistor _Tr9 will increase, and the collector current of this transistor will decrease.
The base potential of Tr ₁₂ decreases. Therefore, the collector potential of the transistor _Tr12 rises during the gate pulse period, and the amplitude of the pulse (d) becomes smaller. Therefore, the amplitude of the pulse (e) applied to Tr ₅ of the differential pair Tr ₂ and Tr ₃ also becomes smaller, and the DC potential of the terminal T ₃ decreases. As a result, the collector potential of Tr ₂ of the differential pair, that is, the DC level of the video signal (a) falls, and the brightness decreases. Also, terminal T ₄
When the DC potential is lowered, the complete opposite operation occurs and the brightness increases.

On the other hand, the DC transmission rate setting circuit 4 is connected to the terminal _T8 when the composite video signal (b) subjected to DC clamping (brightness control) as described above is amplified by the transistors _Tr15 to _Tr16 . capacitor C ₂ and resistor
The DC transmission rate is set by the action of _R4 . In other words, when the above terminal _T8 is opened, the DC transmission rate is 100.
%, and at this time the transistor Tr ₁₅ ,
The voltage amplification degree A ₀ by Tr ₁₆ is A ₀ ≈Rb/Ra.
On the other hand, the voltage amplification degree A ₁ for the AC component when the capacitor C ₂ and resistor R ₄ are connected is A ₁ ≒ Rb/Ra・R ₄ /Ra+R ₄ =Rb (Ra+R ₄ )/Ra・R ₄ , and since the DC transmission rate D can be found as D=A ₁ /A ₀ , it is expressed as D=R ₄ /Ra+R ₄ (×100%). Therefore, this DC transmission rate D is set in the range of 40 to 60% as described above.

Next, the first beam current detection circuit 9 of the external circuit section 12 is a beam current detection circuit connected between the low potential side terminal of the high voltage winding of the flyback transformer FT and the DC power supply (+V _H ). Resistors R ₅ , R ₆ , a first switching transistor Tr ₁₉ for ABL, a second switching transistor Tr ₂₀ for ACL, etc., which perform a switching operation in response to a change in the potential at a midpoint R where these two resistors are connected. It is configured as a main element. That is, when the picture tube beam current increases, the potential at point R decreases, so both the first and second switching transistors Tr ₁₉ and Tr ₂₀ are turned on. When the first switching transistor Tr ₁₉ is turned on, it bridges the circuit including the brightness adjustment variable resistor VR, etc., so the DC potential of the terminal T ₄ increases, and as a result, the brightness decreases due to the above operation. It is forced and restricted. At the same time, the second
By turning on the switching transistor Tr ₂₀ , the contrast is limited by a contrast control circuit (not shown).

Further, the second beam current detection circuit 10, which is a feature of the present invention, has a connection midpoint R of the detection resistors _R5 and _R6 .
The main elements include a Zener diode D ₁ that becomes conductive when the potential of the diode becomes higher than a predetermined value, and third and fourth switching transistors Tr ₂₁ and Tr ₂₂ that are turned on when this diode becomes conductive. . That is, when the picture tube beam current decreases and the potential at point R rises above the Zener voltage of the diode _D1 , the diode _D1 becomes conductive, thereby turning on the third switching transistor _Tr21 . Then, the fourth switching transistor Tr ₂₂ is also turned on and lowers _the base potential of the first switching transistor Tr ₁₉ for ABL.
turns on. Therefore, the DC level of the video signal is forcibly lowered by the same operation as described above. Note that the diode _D2 is connected to the second switching transistor for ACL when the fourth switching transistor _Tr22 is turned on.
This is for backflow prevention to prevent the switching transistor Tr ₂₀ from turning on.

In the above, the present invention was specifically applied to a television receiver equipped with ABL and ACL functions, but since the present invention is intended to prevent brightness from floating when the APL is low, The present invention can be applied to any television receiver that applies a video signal whose maximum black level changes depending on the APL to the picture tube.

In addition, in order to apply a video signal whose maximum black level changes according to the APL to the picture tube, the video signal was once DC clamped and then passed through a DC transmission rate setting circuit, but other methods were used. It is also possible to adopt

As described above, according to the video level correction circuit of the present invention, it is possible to prevent brightness floating, especially black floating when the APL is low, and to improve image quality.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the schematic configuration of a television receiver to which the present invention is applied, FIGS. 2 and 3 are diagrams showing the characteristics of the conventional example and the present invention, respectively, and FIG. 4 is the same as that of FIG. FIG. 3 is a diagram illustrating an example of an implementation circuit of a main part.

Claims (1)

[Claims] 1. In a television receiver equipped with an automatic brightness limiting circuit that detects when the picture tube beam current is excessive and reduces the brightness based on the detected output, a state where the picture tube beam current is low is detected. Detecting a state in which the average video level is low, and using this detection output, forcibly lowering the DC clamp level of the video signal and lowering the DC transmission rate of the clamped video signal. A video level correction circuit that changes the maximum black level according to the average video level.