JPH0365716B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a technique for improving the contrast of a television receiver.

BACKGROUND TECHNOLOGY AND PROBLEMS Generally, when a television screen is viewed in the presence of external light, the contrast decreases.

Figure 1 is a graph showing the results of an experiment regarding the reduction in contrast.The horizontal axis shows the level (instantaneous level) of the luminance signal, and the vertical axis shows the amount of light that enters the eye from the picture tube surface, i.e. , indicates the amount of light that is the sum of the light emitted from the picture tube and the light due to reflection of external light on the tube surface. And the curves L ₀ , L ₅ , L ₁₀ , L ₂₀
The amount of outside light L is 0 (dark room), 500, 1000, and
This is the characteristic at 2000lux.

Therefore, when L=L ₀ (when there is no external light), there is a one-to-one relationship between the luminance signal and the amount of light from the tube surface, resulting in a screen with good contrast. However, when L = L ₅ to _{L 20} (when there is external light), the higher the luminance signal becomes at the black level, the worse the contrast becomes, causing the phenomenon of blackout. The bigger it gets, the stronger it gets.

Therefore, it is conceivable to provide the luminance signal system with a correction circuit (nonlinear circuit) for input/output characteristics as shown as curve _C10 in FIG. 2.

That is, if you do that, when L=L ₁₀ ,
Due to the nonlinearity of curve C ₁₀ , curve L ₁₀ becomes the curve in Figure 3.
The contrast is improved because it is corrected to the characteristics of D ₁₀ . Similarly, curves L ₅ ,
When L ₂₀ , if corrected by curves C ₅ and C ₂₀ ,
Contrast is also improved when these amounts of external light are L ₅ and L ₂₀ . In other words, if the luminance signal before correction (horizontal axis) is x and the luminance signal after correction (vertical axis) is y, then y
=x〓(1/2≦γ≦1), and the multiplier is set according to the amount of external light L so that γ=1 when L=L ₀ and γ=1/2 when L=L ₂₀ . By controlling γ, good contrast can be obtained even if there is external light or the amount L of external light changes.

However, when such correction is actually performed,
It was very effective in scenes with a dark screen, but
In bright scenes, it looks whitish,
In fact, it turned out to have the opposite effect.

This is because non-linear correction is performed uniquely from the black level to the white level, even though the presence or absence of external light does not have a great effect on the brightness of the screen at the white level. It has also been found that the causes are that the level becomes larger than necessary and the contrast becomes smaller.

Purpose of the Invention In view of the above-mentioned points, the present invention aims to provide good contrast not only when the screen is a dark scene but also when the screen is a bright scene.

SUMMARY OF THE INVENTION Therefore, in the present invention, the contrast is corrected as explained in FIGS. 1 to 3 in accordance with the amount of external light L, and the brightness signal is
It detects APL and uses this detection signal to reduce the amount of correction required in bright scenes.

Therefore, appropriate contrast can always be obtained regardless of the brightness of the screen.

Embodiment In FIG. 4, a luminance signal Sy from a video detection circuit 11 is supplied to a clamp circuit 12, subjected to pedestal clamping, and then supplied to a correction circuit 13. This correction circuit 13 has correction characteristics shown in FIG. 2, and a specific example thereof will be described later.

Further, a CdS 16 is provided as a detection element for the amount of external light L, and is connected to a detection circuit 17 to extract a detection signal S ₁₇ for the amount of external light L having characteristics as shown in FIG. 5, for example, and this signal S ₁₇ is multiplied. circuit 18
is supplied to Further, the luminance signal Sy from the clamp circuit 12 is supplied to the APL detection circuit 19, and an APL detection signal _S19 having characteristics as shown in FIG.
is taken out and this signal S ₁₉ is supplied to the multiplier circuit 18 .

Then, a multiplication signal S 18 of the signal S ₁₇ and the signal S ₁₉ is taken out from the multiplication circuit 18, and this signal _{S 18 is} supplied to the correction circuit 13 as a control signal for its correction characteristic. In this case, the correction characteristic of the correction circuit 13 is controlled by the signal _S18 as shown in FIG. That is, FIG. 7 shows the correction characteristics of the correction circuit 13 using a 10-step method, where "0" corresponds to the straight line C ₀ and is the characteristic of y=x (γ=1, no correction); Ten"
corresponds to the curve C ₂₀ and has the characteristic of y=√ (γ=1/2,
maximum correction), and "1" to "9" are intermediate y
=x〓(1/2<γ<1). The amount of correction is increased as the amount of external light L increases, but the amount of correction is decreased as the APL increases, that is, the brighter the screen becomes.

Then, the brightness signal Sy corrected in this way is
The image is supplied to the picture tube 15 through the video output circuit 14.

According to such a configuration, the brightness signal Sy is corrected according to the characteristics C ₀ to C ₂₀ shown in FIG. 2 in accordance with the amount L of outside light, so that good contrast can be obtained regardless of the amount L of outside light. At this time,
When the APL is high, the amount of correction is reduced, so when the screen is bright, the screen will not look washed out.

Thus, according to the present invention, a screen with optimal contrast can be reproduced under any external light conditions and video content.

FIG. 8 shows an example of the correction circuit 13. That is, in this correction circuit 13, the clamp circuit 1
The signal Sy from 2 is made into a constant current (constant current signal) Ii by constant current sources Q ₀ and Q ₁ . And constant current source
The current of Q ₆ is I ₆ , and the base current of transistors Q ₂ to _{Q 5} is
Assuming that the emitter-to-emitter forward voltage is V _BE2 to V _BE5 and the collector current of transistors Q ₂ to Q ₅ is I ₂ to I ₅ , V _BE2 + V _BE3 = V _BE4 + V _BE5 (i). In general, the base of the transistor
The relationship between the emitter forward voltage V _BE and the collector current I _C and saturation current I _S is V _BE = KT/qlnI _C /I _S ...(ii) K: Boltzmann's constant T: Absolute temperature q: Electron charge Since there is a relationship, I ₂ · I ₃ = I ₄ · I ₅ ... (iii) from equations (i) and (ii).

And the current I ₂ is equal to the input current I _i and the current I ₃
is equal to the current I ₆ of the constant current source Q ₆ , and in addition, the current
Since I ₄ and I ₅ are equal to each other, formula (iii) becomes I _i・I ₆ =I ₄ ² , and I ₄ =√ ₆・√ _i …(iv). If ₆ = 1, equation (iv) becomes I ₄ =√ _i .

Therefore, if the constant determined by the magnitude of the signal _S18 is k (0≦k≦1), the collector current of the transistor _Q12 is k√, and the transistor
The collector current of _Q14 is (1-k)I _i , so
The sum of the currents _{I 0} , that is, the output current I 0 of I ₀ = _k√i + (4−k)I _i flows through the load resistor R ₁₁ . When the signal S ₁₈ is at its minimum, transistors Q ₁₁ and Q ₁₄ are turned off, k=1, and I ₀ = _√i . Also, signal S ₁₈
When is maximum, transistors Q ₁₂ and Q ₁₃ are turned off, k=0, and I ₀ =I _i .

Therefore, correction is performed in the range of γ=1/2 to 1 using the signal _S18 .

Note that in the above description, the signal is generated by the element 16.
Although the change in _S17 was made automatic, it may be changed manually by using the element 16 as a variable resistor. Further, when the output of the detection circuit 11 is a composite color video signal, the output circuit 1
4, the color picture tube 15 undergoes color processing.
It should be supplied to

Effects of the Invention A screen with optimal contrast can be reproduced under any external light conditions and video content.

[Brief explanation of drawings]

1 to 3 and 5 to 8 are diagrams for explaining this invention, and FIG. 4 is a system diagram of an example of this invention. 12 is a clamp circuit, 13 is a correction circuit, 17,
19 is a detection circuit.

Claims (1)

[Claims] 1 means for forming a first signal that changes automatically or manually in response to the amount of external light; a correction circuit for correcting the signal gradation of the video signal;
a detection circuit for detecting APL, the first signal is supplied to the correction circuit as a control signal for its correction characteristic, and as the amount of external light increases, the correction amount in the correction circuit increases, and the detection circuit A television receiver, wherein a detection signal of the circuit is supplied to the correction circuit as a control signal for the correction characteristic, so that the higher the APL, the smaller the correction amount in the correction circuit.