JPS6043971A - Black level reproducing circuit - Google Patents

Abstract

PURPOSE:To exclude the influence of noise by providing a level limiting circuit that limits the output of a black peak holding circuit to above a fixed value. CONSTITUTION:The black level reproducing circuit consists of a black peak holding circuit 1, black level correcting section 3 and a detected level limiter 4. The black peak holding circuit 1 is provided with a pair of transistors Q1, Q2 and a hold condenser C. A pedestal clamped video signal is applied to the base of the transistor Q1, and black peak holding voltage is obtained in the holding condenser. This voltage is fed to the black level correcting section 3, and at the same time, supplied to the detected level limiter 4. The detected level limiter 4 consists of a differential amplifier made up of a couple of transistors Q5, Q6, and a detecting section made up of a couple of transistors Q5, Q6. When the holding value goes to lower than a fixed value, transistors Q7, Q9 turn on and the holding value is fixed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a black level reproduction circuit for stably maintaining a black level in a TV receiver or the like.

BACKGROUND ART AND PROBLEMS Generally, video signals are sent with the actual illumination level floating toward the white side with respect to the pedestal level, which is the hottest reference. This difference between the black level and the pedestal level is called a set-up, and it varies widely depending on the circumstances of the broadcasting station and the TV left camera, depending on the manufacturer and model of the home VT. Therefore, on the playback side (monitor TV), it is necessary to reproduce the black level correctly (
signal) black level (!: % must match the cant-off level of the CRT).

As a conventional black level reproduction method, a method is known in which, for example, a black peak in one vertical scanning section is detected and the luminance level of the entire signal is lowered so that the black peak level matches the pedestal level. This method detects when the CRT beam current increases and lowers the contrast.
When used in combination with atic beam limitter), it is possible to achieve good black reproduction.

Generally, a TV receiver equipped with such a black reproduction system is provided with a black peak detection circuit. This black peak detection circuit usually consists of a peak hold circuit,
A peak hold capacitor, a charging circuit that charges this capacitor to the black peak value with a very fast time constant (attack time), and a slow time constant that updates the peak hold value to follow the black peak that changes depending on the screen. A constant (recovery IJ - time) discharge circuit is provided.

When noise with a large amplitude [1] is input to this black peak detection circuit, such as when switching the door channel, the peak value of this noise is bolded, and this state (i4) indicates that the hold capacitor has not been sufficiently discharged. No Yannerri J
Control for a few seconds after 4A. During this period, the noise peak is considered to be a black peak, so the black reproduction circuit becomes whitish on the screen.

OBJECTS OF THE INVENTION 1. An object of the invention is to provide a black reproduction system that solves these problems.

Invention, peace of mind! The first invented black level reproducing circuit is based on a black peak hold circuit that has an attack frequency of H> (reh/7) and a slow capacitor charging/discharging circuit, and a black peak hold circuit that has a slow recovery time. a black level correction circuit for controlling the level of the video signal to reproduce black on the screen; and a black peak hold circuit for preventing the black peak hold circuit from holding the -IN signal below a predetermined black level. A level limiting circuit that limits the output to a constant value is provided, and by the combination of the two, black 1'f generation is performed based on black peak detection which is stable against noise.

Embodiment FIG. 1 is a circuit diagram of a below-black level circuit to which the present invention is applied, and FIG. 2 is a waveform diagram of the input video of g1 (g).

In FIG. 1, the block (1) surrounded by dotted lines is a black peak hold circuit, and the transistor pair QI, C2 (
comparator) and a hold capacitor C. As shown in Figure 2, the potential of one (Ql) of the transistor pair Q1 and C2 constituting the comparator is 1g91.
Your pedestal is given a clamped video signal.

The other transistor Q2 is connected to one end of the hold capacitor C, and the black peak hold voltage is applied in the form of the charging voltage of the capacitor.

Note that the closer the black peak level shown in FIG. 2 is to the hottest side (pedestal level), the more the hold capacitor C is charged, and the lower the peak hold value is to the ground side.

In addition, a discharge resistor R is connected in parallel with the hold capacitor C, and the capacitor is discharged with a relatively slow time constant (recovery time), and the hold value is further increased by following the black beak of the image that changes from time to time. , li is now available.

The emitters of transistors Q1 and C2 constituting the comparator are commonly connected, and the blanking pulse source (2
), and in the synchronization signal section, the blanking pulse source (2) turns off the transistors Q1 and C2, so that the synchronization tip level is not mistakenly detected as a black peak.

Video of the black peak hold level at the pace of transistor Q2 being IJ at the pace of remote transistor Ql (If the peak of p44 of No. 8 is low, C2 is on and Q
l is turned off. Then, a current flows through the current mirror formed by the diode D1 and the transistor Q5 in the collector of the transistor Q2, and this current turns on the transistor Q4. This transistor Q4 is
Since a small resistor r is inserted in series between one end of capacitor C and the ground, when C4 is turned on, the charging current becomes v
From ce line to capacitor C1 resistor r,) transistor Q
4, capacitor C is charged until the pace of transistor Q2 reaches the black peak of the input signal, turning off C2, C5, and C4. The charging time constant (attack time) is set very small.

The peak hold value at one end of the capacitor C thus obtained is derived to the black level correction section (3). For example, when the black peak value rises with respect to the pedestal level Ep, this black level correction circuit (3) performs level correction so that the black level of the input video signal ρ7 matches the pedestal according to the amount of rise. It may be a circuit like this.

The output of the black peak hold circuit (1) is also sent to the detection level limiter (4). This limiter (4) is a transistor pair Q5, Q6 that forms a differential amplifier.
(+ii+), one of them (Q5) is given a peak hold value, and the other (Q6) is given a pedestal clamp potential Ep.The output of this differential amplifier is given to a detector consisting of a transistor pair Q7 and Q8. and its output level is discriminated.

Collector load resistance R1, R of left-handed amplifier (Q5, Q6)
2 is R1<iko2, and when Q5 and Q6 are balanced, the transistor Q that constitutes the gay detector
8 is on and Q7 is off.

If large amplitude noise is input instead of the video signal when switching channels, etc., black peak hold circuit 1 (1) will erroneously detect the peak of the noise as a black signal, but if the peak hold value is below a certain level. , that is, when E in FIG. 2 drops below -ΔE, the detector transistor Q7 is on and Q
8 flips off. A current flows through the collector of the transistor Q7 and the collector of the transistor Q7, turning on the transistor Q9.

As a result, a current flows through the transistor Q9, the resistor R3, the resistor r, and the transistor Q4, and the hold voltage at one end of the capacitor C is fixed at a constant value so as not to fall below Ep-ΔE. Detection level limiter compatible with ΔE
The threshold level of the detector (Q7, Q8) in (11) can be determined by the ratio of the resistors R1, R2 and the gain of the differential amplifier (Q5, Q6).

When the noise disappears and a regular video signal is input, the black peak hold circuit (1) immediately holds the new black peak. For this reason, as in the past,
Black peak hold is no longer disturbed by noise, and a stable screen can be obtained.

FIG. 6 is a circuit diagram showing an example of a black level correction circuit (3) for non-reproduction. The input video signal is passed through a 1x amplifier (
6) and an adder (7), and is also supplied to a black detection circuit (8) to detect a black signal below a certain threshold. The detected black signal is subjected to gain control and / (0) -Q gain control, and is then applied to an adder (7). As a result, 9 works of ``Black Signal Expansion'' were performed. The black extension is determined by the black peak hold value (su(51)) in the comparator (10).
The output of the black peak hold circuit (1) shown in the figure) is compared with the Bedeskull clamp 'i44th position Ep, and the calculation is performed according to one accent (error) of the comparison result. That is, black expansion is performed such that the black peak hold value matches the pedestal level Ep. By using the black-expanded video signal, an image is obtained which is rendered black so that the cut-off of the CRT and the black level match.

Figure 4 shows the detection level limiter for e+', 1 ill.
A modification of the circuit 1'A shown in FIG. When the output of the black peak hold circuit (1), which is the same as 1.), attempts to fall below Ep-VF (■1. is the on-voltage of the diode (13)), the diode (13)
turns on, and the peak hold '1d pressure becomes Ep -VF
It is designed to be held (restricted) to

Effects of the Invention 14.6 As mentioned in L, since a hold level limiting circuit is provided in the black peak hold circuit in the black level reproducing circuit, the black peak hold value abnormally decreases due to noise and the hold level is reduced. Due to the long recovery time of the circuit, there is no possibility that an abnormal hold state will continue for a long time, and even if noise is input, the hold level will not fall below a certain level, so even if the hold recovery time is short, It can easily return to normal hold circuits and has little effect on the image.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of the black level reproduction circuit D& according to the present invention, FIG. 2 is a waveform diagram of the input video signal in FIG. 1, and FIG. Fig. 4 shows a circuit 1 that shows a modification of Fig. 1. In the symbols used in the drawing, (1) - Black peak hold j (!l E valley (2)...
・・・・ Bununki/Gupurusu source (3)・ ・ ・
Black level FIiii main part (4) ・・・, sharp level limiter (6) ・・1× amplifier (power) Adder (8) ・−・Black detection circuit (9) ・・・ Gain control amplifier +1i
1) - Comparator (rule: diode - Q). Agent: Katsutsune Ueya, Yoshi Kamo, Toshiki Sugiura

Claims (1)

[Claims] A black peak hold circuit with a capacitor charging/discharging circuit that has a fast activation time and a slow recovery time, and a black level correction circuit that controls the level of the video signal based on the detected black peak value and reproduces black on the screen. and a level limiting circuit that limits the output of the black peak bold circuit to a constant value so that the black peak hold circuit does not hold a signal below a predetermined black level.