JPS5887993A - Discriminating circuit for black-and-white broadcast - Google Patents

Abstract

PURPOSE:To avoid the circuit from being affected with noise at a weak electric field, by holding a phase detection output of a burst signal and a continuous carrier wave with one capacitor and applying it to a Schmitt trigger section. CONSTITUTION:A phase detection circuit section X introduces a phase detection voltage of a burst signal BR and a continuous carrier CW at the gate pulse period CP. The phase detection voltage is held in a capacitor C of a filter section Y. The holding voltage of the capacitor C is applied to a base of a transistor (TR) Q19 forming a Schmitt trigger section Z. TRs Q19 and Q20 from a differential amplifier.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to a black and white broadcast discrimination circuit used in color television receivers and the like.

Technical Background of the Invention A color television receiver is provided with a discrimination circuit for discriminating between monochrome broadcasting and color broadcasting.

This discrimination circuit conventionally utilizes a phase detection circuit including transistors Q+-Qa as shown in FIG. A continuous carrier wave of 158 MHz (used for color signal demodulation) is input between input terminals 1 and 2, and a burst signal is input to input terminal 3. 4 is a bias terminal. The burst signal is extracted by the burst gate circuit.

Further, when receiving color broadcasting, the continuous carrier wave is phase-synchronized with the burst signal by an automatic phase control circuit. In this case, the normal burst signal and continuous carrier wave are set so that a phase difference of ζ90 occurs between the two signals.

When a burst signal and a continuous carrier wave having a phase difference of 90 as described above are applied to the discriminating circuit, the burst signal and the continuous carrier wave having a phase difference of 90 are set to be added so that a phase difference of 90 is further generated between the two signals. Therefore, there is a phase difference of 0 or 180 between the Nost signal input to the discrimination circuit using phase detection and the continuous carrier wave. As a result, when receiving color broadcasting, the output of this discrimination circuit is the minimum value or maximum value of the detected voltage, as shown in FIG. Furthermore, when receiving monochrome broadcasting, the burst signal disappears, so no detection voltage is output.

As shown in FIG. 1, the discrimination circuit described above is connected with capacitors 5 and 6 for holding the detected voltage. For example, the hold voltage of the capacitor 5 becomes high during color broadcasting and becomes low during black and white broadcasting. The hole toy pressure of the capacitor 6 is low during color broadcasting and high during black and white broadcasting. This circuit can discriminate between black and white broadcasting and color broadcasting by comparing the magnitudes of the hold voltages of capacitors 5 and 6. However, in this way, capacitor 5,
Providing two bins of 6 has the disadvantage that the number of bins increases when integrated. In order to reduce the number of bins, it may be possible to leave only one capacitor and determine whether the voltage of this capacitor is high or low, but a comparator circuit that compares the voltage of one capacitor with the reference voltage is necessary. Separately required. In this case, the reference voltage is required to be highly accurate, and the comparator circuit is required to be an expensive one that is not affected by drift or the like.

Furthermore, in conventional circuits, the detection voltage may be affected by noise in the case of a weak electric field, resulting in erroneous judgments, and improvements in this point are desired.

Purpose of the Invention The present invention was made in view of the above circumstances, and is suitable for integrated circuits with a reduced number of bins, and is capable of providing black and white broadcasting that provides accurate operation and is less affected by noise such as in the case of weak electric fields. The purpose is to provide a discrimination circuit.

Embodiments of the Invention Examples of the invention will now be described with reference to the drawings. Third
In the illustrated phase detection circuit section X, a continuous carrier wave CW of 3.58 MHz is introduced into the input section 21, and a chroma signal including the burst signal BR is introduced into the input section 22.

Continuous carrier wave CW of input section 2 in phase detection circuit section
is input to the base of the transistor Ql□. Transistor Q II + Q +□ constitutes a differential amplifier,
The bases are connected to the bias bowl VB+ through resistors Rl + R2, respectively, and the common emitter is connected to the constant current source 25
It is connected to the. The burst signal BR of the input s22
is input to the common pace of transistors QCs + Q+e. There is a difference N! between the set of transistors QIllQ14 and the set of transistors QCs and Q+a. It constitutes a J amplifier. Transistor Q+s, Q+a
The common emitter of transistor Q+ is connected to the collector of transistor Q+, and the common emitter of transistor Q++ + Q+a is connected to the collector of transistor Q+t. A resistor R is connected to the common base of the transistors Qss and Qlll and the common base of the transistors Q+4 + Q+6, respectively.
13, valve R13 and bias 'lif source VB, 7'l
) are given the biases. Transistor Chs,
The collector of Q15 is connected to the reference potential (ground) terminal,
The collectors of transistors Q14, Q+a are connected to the common emitter of transistors Q+v, Q+a. Transistor Ql? The bias power supply V is used for the pace of
B is connected, and the gate pulse GP input section 23 is connected to the pace of the transistor QCs. Further, the collector of the transistor Q+v is connected to a reference potential terminal, and the collector of the transistor Q+a is connected to the reference potential terminal via a load resistor R6 and to the pin 24. The bin 24 is grounded via the capacitor C of the filter section Y. The phase detection circuit section X is connected to the burst signal BR and the continuous carrier C.
Derive the phase detection voltage with W during the Kate pulse GP period,
The phase detection voltage is held in capacitor C.

The hold voltage of capacitor C is applied to the base of transistor QIQ, which constitutes part Z of the Schmitt trigger. Transistor Ql[1lQ20 also forms a differential amplifier, and their emitters are common? Connected to current source 26.

A bias resistor R7 is connected to the base of the transistor Q2°. The collector of the transistor Q2G is connected to the reference potential terminal, and the collector of the transistor QIQ is connected to the reference potential terminal by connecting a resistor R6 and a reverse diode D in parallel. Further, the collector of transistor Ql11 is connected to the base of transistor Q21. The collector of this transistor Q21 is connected to the reference potential terminal, and the emitter is connected to the transistor Q21 via a diode D.
It is connected to the base of to.

Now, if the phase difference between the burst signal BR and the continuous carrier wave CW is O, the current flowing through the valley is f, as shown in Fig. 5.
(Figure 5 (at is the continuous carrier wave CW1 (bl is the burst signal BR). The current flow to the constant current source 25 is 0.
For, transistor Qr+ + Qt2 )Qt4
+ The electric current aIII2+"3+I4 flowing to Qta is shown in FIGS. 5(d), (C), and (e), respectively.

(fl #C as shown. Transistors Q1□, Q
The combined emitter current l511 of 18 is I, -I.+I, as shown in FIG.
When Cs is turned on, the current ■ flowing through this transistor Q+a is 1. =1. = Io. As described above, when a color broadcast is damaged, the current I0 reaches its maximum during the burst period. On the other hand, when receiving black and white broadcasting, there is no burst signal, so the differential amplifier is in a balanced state, and I6-EO/2
Become a male. In the filter part Y, the voltage R,
A voltage rise and fall of X I o /2 M is obtained. In Fig. 6, period T1 is defined as Hakuen Tokomiuke 1g1 period T
2 when a color broadcast is damaged, the base voltage of the transistor Q19 becomes a waveform 6a shown in FIG.

The following operation can be obtained by using a Schmitt trigger (ZIC76). When the hold voltage of capacitor C gradually decreases from 0 volts in the negative direction, transistor Q+o turns on, and its threshold level (J1 diode 1) +, transistor Q21,
Tyode D21 is 3VHB. ■BF+
is the drop EE of the diode and transistor 2.

Conversely, the hold-up pressure of capacitor C is -3vBEil.
When the voltage gradually increases from Ill to Ill, the transistor Q+o is off, so it is inverted by the transistor Q20 and the diode Ill) to the threshold level -VBB.

By operating as described above, the hold voltage decreases when receiving color broadcasting, and when the base voltage of transistor Q20 is 12 VBw, the hold 'pressure increases due to noise in the weak electric field or phase shift of the gate pulse. However, the state will not be reversed until -2vng. In other words, it has a noise margin.

Conversely, the same is true when receiving black-and-white broadcasts;
The state of transistors Q+e+Qto will not be reversed as long as hold 1 (a force that does not lower the field) up to BFL. The base voltage of transistor Q2o is 12VB when receiving color broadcasting.
B. When receiving a black and white broadcast, it is 13VB IN, and a determination output 6b shown in FIG. 6 is obtained. 6C shown in FIG. 6 is a Kate pulse. When controlling a 1' circuit using the judgment output, use -2.5VHB as the reference voltage, for example.
You can easily control it by creating and comparing it with this.

Effects of the Invention According to the above-mentioned determination circuit, the phase transverse wave output obtained in the phase imitation circuit section The point where Z has a river 1 has a characteristic λ. In particular, the Schmitt Trika part Z differential amplifier is a transistor Q
119 I Qt.

As a condition for reversing the state, since the threshold 1/pel when reversing the state from one side to the other and from the other side to the other is different from each other, the judgment output -9VBI? .

-3vBg1 river (with a width of -2.5Vba used in the subsequent stage), a margin of ±0.4VBR8g is obtained, and high accuracy is no longer required. By setting the above threshold level to Sarah,
Especially when the electric field is weak, even if the hold voltage fluctuates due to noise or a phase shift of the gate pulse, the singing operation will not occur immediately. This is effective in increasing the reliability of the receiver and the like. Of course, as mentioned above, Schmitt trigger part 2
By using this, only one capacitor is required for the phase detection circuit section X, and when the circuit is converted into a custom circuit, only one pin is required for this section, which further contributes to miniaturization.

The phase detection circuit section X only needs to be of the two-y balanced connection type, and it is also possible to add the extracted burst signal directly to the input section 22. Q +a (7) The '7' toe part is uncomfortable.

As mentioned above, the present invention is a black-and-white broadcast discriminating circuit that can reduce the number of pins, is suitable for integrated circuits, provides accurate determination operations, and is less susceptible to noise such as when there is a weak edge. can be provided.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional black-and-white broadcast determination circuit, FIG. 2 is an explanatory diagram showing the phase detection characteristics of the circuit in FIG. 1, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. Figure (al ~ (g
1 is a signal waveform diagram of each part of the circuit of FIG. 3, and FIG. 5 is an explanatory diagram of the operation of the circuit of FIG. 3. X...phase detection circuit section, Y...filter part, 2.
...Part of Schmidt Trika, Q ++ ~Q 21
”・l□Transistor, C...Capacitor, D7.D2
···diode. Applicant's Representative Patent Attorney Takehiko Suzue No. 1 - Figure 2 Figure 3 Y Figure 4 Figure 5 1110 and 1C

Claims (1)

[Claims] A two-car balanced connection type phase detection circuit section that obtains the phase detection output of the burst signal and continuous carrier wave, one capacitor that holds the detection output of this phase detection circuit section, and the terminal voltage of this capacitor constitute a differential amplifier. A bias is set to the base of the first transistor and the base of the other second transistor, and the bias is set to the base of the first transistor. A black-and-white Schmitt transistor including a circuit section in which the inversion threshold level is set to be different from one state to the other and from the other to one state inversion condition of the second transistor. Broadcast discrimination circuit.