JPS5827480A - Video level correction circuit for television receiver - Google Patents

Abstract

PURPOSE:To eliminate floated black on a screen, by detecting a state with low average video level and compulsively lowering a DC level of a video signal with this detection signal. CONSTITUTION:A beam current detection circuit 9 detects excessive state in beam current of a picture tube 8 and executes automatic contrast limit operation and automatic luminance limit. A beam current detection circuit 10 detects too little state of beam current and this circuit detects lower average video level APL of the video signal and compulsively lowers the DC clamp level of a DC clamp and luminance control circuit 3. As a result, the blackmost level of the video signal applied to the picture tube 8 is made coincident with a cut-off level of the picture tube 8 at the lower level APL, allowing to eliminate bloated black on screen.

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 according to an AFL (average video level) to a picture tube. The purpose of this invention is to prevent the brightness from floating when the APL is low.

In general television receivers, when a video signal with a very high AFL is input, in order to prevent the tube from becoming saturated and the three primary color phosphors from deteriorating, automatic brightness limiting (BL) is used. The circuit is used and this ARL
As is well known, the circuit detects when the tube beam current is excessive, and uses the detection output to control the DC level of the video signal, that is, the brightness. In order to faithfully reproduce the video signal on the machine side, it is necessary to correctly reproduce the DC level of the Eirin signal, and this circuit uses a so-called DC branch main circuit. However, if the above-mentioned ABL control is performed, the following drawbacks arise. That is, the II stream reproducing circuit clamps the video signal so that its blackest level (usually the pedestal level) always matches the cutoff level of the picture tube, regardless of the level of the AFL. If ABI is applied to the video signal with high AFL that has been Niklanfed in this way, the blackest level of the video signal will be below the cutoff level of the picture tube and the black signal will not be reproduced. ).

Therefore, as an alternative method, the reproduction rate of the DC level of the video signal is made to be 100- or less (that is, the blackest level changes slightly depending on the APL), and the ABL111J is controlled for this video signal. A method has been proposed.

Figure 2 is a diagram explaining this method.
In FIG. 10(a) showing the Muden R%, the broken line represents the case where the DC regeneration rate is 10056, and the solid line represents the case where the DC regeneration rate is set to less than too. The P point and the P point each indicate the APL starting point. That is, when the DC reproduction rate is 100 inches or less and the APL is, for example, 40%, the video gI! If you select an appropriate DC reproduction rate so that the blackest level of the signal matches the cutoff level (Ll) of the picture tube, the APLK
The change in the maximum black level for the second v! J(bl) as shown in the lower solid line, and the change in the whitest level as shown in the upper solid line. Therefore, the same figure (b) shows the case where ABL is applied to a video signal with a DC reproduction rate of 100ch. Compared to the dashed line, there is a black sink (hatched part A) when the APL is high.
It can be seen that there has been some improvement.

However, although this method improves the darkening when the AFL is high, on the other hand, when the APL is low, the white level exceeds the saturation level (Lt) of the picture tube ([
Hatching B in Figure 2), i/kll! There is a drawback that the level exceeds the picture tube cutoff level (Ll) (hunting part C), and these are respectively white cast and
Is it called black floating? This will reduce the quality of the video.

However, the present invention reduces the brightness by 1% in such a low AFL state! ”, and the following θ
The details will be explained with reference to the drawings.

FIG. 1 shows a schematic diagram of the essential parts of a television receiver using the video level correction circuit according to the present invention. In the same figure, the +1+Fi video signal input terminal (2) is a contrast control circuit into which the video signal from this terminal is input, and the contrast control circuit can be controlled manually and the automatic contrast limit (AOL) can be operated when the AFL is high. This is what we do. (3) is a DC clamp/brightness control circuit, which applies a DC clamp to the Eirin signal and controls the brightness by manually changing the clamp level.
When the PL is high, an automatic brightness control a (APL) operation is performed to forcibly lower the crank level. (4) is a DC transmission rate setting circuit, which mixes the video signal output from the DC clamp and brightness control circuit (3) to the next stage with a DC transmission rate of about 4[)-60ch. This leads to circuit +51, and image 1 from this circuit (5)! After the signal passes through a video amplification circuit (6), it is mixed with a color difference signal in a color matrix circuit (7) and 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 picture tube (8) is excessive.
A high state of I is detected, and the above-mentioned ACL operation and ABL
It is now possible to make WIJ works.

Further, αa 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 state of the AFL of the video signal, and the DC clamp and brightness control circuit (3 ) is forcibly lowered in the DC clamp level.

According to such 11f design, DC clamp and brightness control circuit (
Since the video signal output from 3) is set below 100% and then passed through the transmission rate setting circuit (4), the blackest level of the video signal applied to the picture tube (8) is It will change depending on the AFL. That is, the circuit (
Even if the blackest level of the video signal is always clamped to match the cutoff level of the picture tube (8) in 3), the hottest and whitest levels of the video signal applied to the picture tube (8) tends to change according to the APL as indicated by the broken line in the above-mentioned 3N(1)). However, when the AFL is relatively low, that is, when the picture tube beam current is below a predetermined value, the second
The beam current detection circuit (IQ) detects this state, and thereby forcibly lowers the DC clamp level of the DC clamp and bright full JIIE1 circuit (3) to a predetermined constant level.As a result, the 9 Kenkan (8) The worst level of the Eirin signal applied to is Fi Fig. 3 (b+
As shown by the solid line, it matches the cutoff level of the upper distribution channel, and the black floating mentioned above is eliminated. FIG. 3(a) Id shows the characteristics of APL versus beam current in this case. At this time, the operation start point of the second beam current detection circuit αG needs to be set in consideration of the above-mentioned DC transmission rate.

In addition, in Fig. 3 (1+), the darkening when the APL is high cannot be improved from the state shown in Fig. 2 (1)) only by the above-mentioned operation of the DC flang and brightness control circuit (3). Not done. However, here, AaL is applied at the same time as ABL, so the black sink described above can be improved slightly from the situation shown in Figure 2 (b), and if AFL is high, it is better to slightly sink the black signal. Since the image quality is good, there is no problem even if such darkening cannot be completely corrected.

Furthermore, in the circuit shown in Fig. 1, the white peak is clipped by the peak clipping/blanking pulse mixing circuit (5), so the operation is similar to that of the DC clamping/blanking pulse mixing circuit (3) described above. Combined with this, white cast when the APL is low is also improved.

Next, a description will be given of FIG. 4, which shows an example of the main part of the circuit surrounded by the broken line in FIG. 1. The circuit in the figure can be roughly divided into an IC circuit (IC) comprising a DC clamp/brightness control circuit (3) and a DC transmission rate setting circuit (4), and an external circuit including the 1st fig. 2 beam current detection circuit (9) αω. It is made up of departments.

In the above construction C1N21, basically, the negative polarity applied video signal (A) introduced into the terminal (T1) is sent to the emitter follower (A).
Tr, ) and is amplified by a differential pair (Trl) (Trx), and the video signal taken out from the collector of one of the pair <'rrt) is transmitted to transistors (Try) to (Tr
+y) is amplified and inverted and output as a negative polarity signal to the terminal (
T6), and at that time, DC clamp (brightness control) and DC transmission rate setting are performed as follows.

First, the operation of the brightness control circuit (3) will be explained.
The positive junction signal taken out from the collector of one (Try) of the differential pair (Tr, ) (Tr, ) to the emitter C2 of the transistor (Tr, 4) is connected to the differential pair (Trtt) on the other hand. (Trtt) is applied to the pace of one ('rr,,). The pace of the other differential pair (Trl2) is connected to the terminal (Tr, ), and constant external devices are connected to the variable resistor (VR) and resistors (R1) (Rt) in the circuit housing, and the ICC circuit. A DC voltage determined by the transistors (Trs) to (Tr, 1), etc. is applied, and the gate pulse corresponding to the front porch of the pedestal portion of the video signal is applied to the transistor (TrlJ) for the leg current source of the differential pair. P
9 is applied via the terminal (Ti). For this reason,
The collector (Q point) of the other (Tru) of the differential pair has a predetermined amplitude at the timing of the front porch)
) appears in the pulse of the transistor (T
rs) (Try) After being amplified and inverted by (Tr5), it is connected to the terminal (T, ) and the next capacitor (C2)
The voltage is smoothed by a resistor (horse), converted into a DC voltage, and applied to Tr3 of the input stage differential pair (Tr, ) (Trl).

Therefore, the collector potential of Try of this differential pair, that is, the DC level of the composite video signal (a), is the DC voltage, that is, the pulse (
It will be determined by the extent of the grudge against (e).

Therefore, when the DC potential of the terminal (T4) is increased by the variable resistor (VR), the transistor (T4) is increased.
Since the pace potential of rJ increases and the collector current of this transistor decreases, the pace potential of the transistor (Try) decreases. During this ninth gate pulse period, the collector potential of the transistor (Tr, 2) rises, and the amplitude of the pulse decreases.

Therefore, the amplitude of the pulse (E) applied to Trl of the differential pair (Tr,) (Tr,) also becomes small, and the
)'s @ current potential decreases. As a result, Tr2 of the differential pair
This means that the collector potential of , that is, the DC level of the video signal (a) falls, and the brightness decreases. Also, the terminal (T
When the direct current potential in step 4) is lowered, the complete opposite operation is performed and the brightness increases.

On the other hand, the DC transmission rate setting circuit (4) is configured so that the DC transmission rate setting circuit (4) is connected to the DC-clamped (brightness side m>
) is amplified by transistors (Tr+1) to (Tr+6), the capacitor (C1) connected to the terminal (T, ) is
The DC transmission rate is set by the action of the resistor (R4).

That is, when the above terminal (T) is opened, the DC transmission is
00chi state, and the transistor at this time (Tr, @
) The voltage amplification degree A0 due to (Tr, , ) is A. Medium Rb7
become employed.

On the other hand, the above capacitor (C2) and resistor (R4)
The voltage amplification degree A1 for the AC when connected is as follows, and the DC transmission rate can be found by D"" VA6, so 4 D = m (X 100%) Ra + R.

It will be expressed as Therefore, this DC transmission rate is set in the range of 40 to 60% as mentioned above.

Next, the external part is the first beam current detection circuit +
91Fi, a beam current detection resistor (R, ) (R6) connected 1 pin between the low potential side terminal of the high voltage winding of the flyback transformer (FT) and the DC power supply (+VTI), and both of these resistors. It is mainly composed of a first switching transistor (Tr+e) for ABL, which performs switching operation in response to the change in potential of the connection midpoint (R), and a second switching transistor (Tr+e) for ACL. ing. In other words, when the beam current of the Shaowei tube increases, the potential at point R decreases, so that the first and second switching transistors (Tr
,,) (Try.) are both turned on. When the first switching transistor (Tr, , ) is turned on, it bridges the circuit including the variable resistor (VR) for brightness adjustment, so the DC potential at the terminal (T4) increases, resulting in the above-mentioned This operation reduces the brightness and limits the brightness. Also, at -1 o'clock, the second switching transistor (Tr
□) is turned on, the contrast is limited by a contrast control circuit (not shown).

In addition, the second beam current detection circuit αG, which is a feature of the present invention,
is a Zener diode (Dθ) that becomes conductive when the potential at the connection midpoint P of the detection resistor (us) Dta) becomes higher than a predetermined value for more than a second, and a third and fourth switch that is turned on when this diode is conductive. Transistor (Tryl) (
It is structured with Tr*)轡 as the main vote. In other words, the beam current of the tube decreases and the potential at point R becomes diode (Dl).
), this diode (
DI) becomes conductive, which turns on the third switching transistor (Trtt). Then, the fourth switching transistor (Tr,) also turns on and lowers the pace potential of the first switching transistor (Tr, @) for ABL, so this first switching transistor (Tr +e) turns on. Therefore, the direct transmission level of the Eirin signal is forcibly lowered by the same operation as described above. Note that the diode (D,) is connected to A CT when the fourth switching transistor (Tr□) is turned on.
This is for backflow prevention to prevent the second switching transistor (Tr2°) from turning on.

On the ridge, the present invention is particularly applicable to ABL and AC! : Although the case where the present invention is applied to a television receiver equipped with the L function has been described, since the purpose of the present invention is to prevent the brightness from floating when the APL is low, the maximum black level is adjusted depending on the AFL. The present invention can be applied to any television receiver in which a changing video signal is applied to a receiver tube.

Also, in order to apply the video (i) whose maximum black level changes according to t%AFL to the receiver tube, the video signal was subjected to DC cranking and then passed through the DC transmission rate setting circuit. However, it is also possible to adopt other methods.

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

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a schematic configuration of a friend 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; It is a figure which shows the example of an implementation circuit of lj1 part of. Figure 2 Figure 3

Claims (3)

[Claims] (1) A television receiver that applies an Eirin signal whose maximum black level changes depending on the average Eirin level to the injury tube *
A video level correction circuit for a television receiver, which detects a low state of the average video level in si, and forcibly adjusts the DC level of the video signal based on the detection output. (2) The input video signal I8 is temporarily clamped,
The video level correction circuit 0 according to claim 1, characterized in that the blackest level is changed by controlling the DC transmission rate of the clamped video signal. (3) The video level correction circuit 0 according to claim 1, wherein the video level correction circuit 0 is configured to detect a state where the average video level is low by detecting a state where the picture tube beam current is low.