JPS6129583B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a horizontal synchronization signal detection circuit used in a television receiver having an automatic station searching function such as a frequency synthesizer system.

Some of the above-mentioned television receivers utilize a horizontal synchronization signal to determine whether a signal received by a tuner is a television signal. In that case, it is sufficient to detect the horizontal synchronization signal derived from the sync separation circuit inside the receiver, but since the normal sync separation circuit uses a relatively simple circuit with so-called amplitude separation operation, Even when the horizontal synchronization signal is not being received, the sync separation circuit generates noise at a level approximately the same as that of the horizontal sync signal, making it difficult to use the output of the sync separation circuit for the determination. .
Therefore, with these points in mind, the present applicant's application for a horizontal synchronization signal detection circuit that suppresses noise detection and can detect only horizontal synchronization signals has been proposed.
Although it was proposed in No. 53-154203, the future technology is to add a diode _D1 and a capacitor to the emitter of the first NPN transistor _TR1 , as shown in Figure 1.
C ₁ is connected in series, and a composite video signal from input terminal 1 is applied to the connection midpoint a of this diode D ₁ and capacitor C ₁ and the base of the first transistor TR ₁ via resistors R ₁ and R ₂ , respectively. The collector of the first transistor TR ₁ is connected to the power supply _voltage (+
B), the negative pulse derived from the collector is applied to the base of the PNP type second transistor TR ₂ via the base-emitter reverse protection diode D ₂ of the second transistor. Then, by applying the flyback pulse from terminal 2 to the emitter after dividing it into an appropriate voltage using resistors R ₄ and R ₅ , and connecting the collector to ground via resistor R ₆ , horizontal synchronization is obtained from the collector. It is designed to obtain pulses synchronized with the signal, and the time constant determined by the capacitor _C1 and resistor _R1 is selected to be sufficiently large compared to the horizontal frequency (15.75KHz), so the potential at point a is As shown by the broken line in Figure 2, the base potential of the first transistor _TR1 becomes greater than the sum of the potential at point a and the respective rising voltages between the base and emitter of the diode _D1 and the transistor _TR1 . Conductive only for the horizontal synchronizing signal in the composite video signal [Figure 2 A],
Therefore, the collector of the first transistor _TR1 has a second
The negative polarity pulse shown in Figure B is obtained. Now, if the voltage dividing ratio of the resistors R ₄ and R ₅ is selected so that the pulse LO and flypack pulse HA have the relationship shown in FIG. 2, the connection between the collector of the second transistor TR ₂ and its load resistor R ₆ is From point b, that is, terminal 3, a pulse synchronized with the horizontal synchronizing signal in the composite video signal A is obtained as shown in FIG. 2D. Therefore, if this pulse 2 is converted into a DC voltage using an integrating circuit or the like, it can be used as a signal for stopping the station searching operation of the station selection device. When noise arrives at terminal 1, the repetition frequency of such noise is considered to be sufficiently low compared to the horizontal frequency, so in this case, the difference between the potential generated at point a and the peak potential of the noise is is not large enough to exceed the sum of the respective rising voltages (approximately 1.2 V) between the diode D ₁ and the base-emitter of the first transistor TR ₁ , and therefore, such noise will cause the first transistor to
No pulse appears at the collector of TR ₁ ,
Even if such a pulse appears, the second
By selecting the pulse C so that the potential difference △V between B and C in the figure is as small as possible within the range that ensures reliable operation, the possibility that a pulse due to the noise will be generated at the collector of the second transistor _TR2 is reduced. Malfunctions in which noise is detected as a horizontal synchronization signal are almost eliminated.

By the way, with the prior art circuit shown in Figure 1, there is no problem with normal TV signals, but when the entire screen is black and uniform, the waveform of the composite video signal is A in Figure 3.
As a result, the level B of the integrated output increases compared to the normal case, so if the horizontal synchronization signal is small, and considering the voltage drop of diode _D1 ,
In the horizontal synchronization signal part, the first transistor TR ₁
This results in the disadvantage that a pulse cannot be obtained at the collector of the sensor, and the original horizontal synchronization signal cannot be detected. still,
Although there are few cases where the screen is black and uniform in real life, appropriate consideration should be given to such rare cases. Even when a grid pattern is projected during an adjustment process at a factory, the screen is perceived as being black and uniform.

For this reason, it is conceivable to lower the detection level of the horizontal synchronizing signal by removing the diode _D1 , for example, but noise will also be detected at the same time, resulting in malfunctions due to noise.

The present invention proposes a horizontal synchronizing signal detection circuit that is designed to avoid the above-mentioned drawbacks and to detect almost no noise.

In FIG. 4, in which the present invention is implemented, the same parts as in FIG. 1 are indicated by the same symbols, but the difference between the circuit in FIG. 4 and the circuit in _FIG. The diode D ₁ between the emitter and the capacitor C ₁ is removed, and
The collector output of the first transistor TR ₁ is applied to the base of the first differential pair transistor TR ₃ via the reverse protection prevention diode D 3, and the collector output of the first transistor TR 1 is applied to the base of the first differential pair transistor TR ₃ .
The collector output of TR ₂ is connected to the second differential pair transistor.
Applied to the base of TR ₄ , the first differential pair transistor
By connecting the collector of TR ₃ to the ground via resistor R ₈ , the output obtained at the collector of the first differential pair transistor TR ₃ is used as a horizontal synchronization signal detection output and is obtained at terminal 3. .

In this way, in the present invention, the output of the integrating circuit with a large time constant consisting of the resistor _R1 and the capacitor _C1 is directly applied to the emitter of the first transistor _TR1 , and the diode
Since _D1 is omitted, the input signal is easily led to the collector of the first transistor _TR1 ,
Therefore, even if the screen is black and uniform, there is no possibility that no output will be obtained in the horizontal synchronizing signal portion. And as a result of it becoming easier to obtain signals in this way,
The problem arises that noise is also easily derived from the collector output of the first transistor _TR1 as shown in FIG. 5;
However, the output obtained by gating this output with the second transistor _TR2 by a flyback pulse [6th]
In the special case, that is, the base power of the first differential pair transistor TR3 is higher than that of the second differential pair transistor _TR3 .
By ensuring that a pulse is obtained at output terminal 3 only when it is smaller than the base input of TR ₄ , a considerable amount of noise is suppressed, and the noise that appears as an output is small, so even if this is integrated, it is only a small amount. Since the difference from the case where pulses synchronized with the original horizontal synchronizing signal are integrated and converted to DC voltage is significant, malfunctions due to noise will not occur. Incidentally, among the noises shown in FIG. 6, the noise derived to the output terminal 3 side is of positive polarity and the noise input to the base of the first differential pair transistor _TR3 is of negative polarity. 1 transistor
The collector output of TR ₁ is the first differential pair transistor
The path applied to the base of TR ₃ and the output
Comparing the path applied to the base of the differential pair transistor TR ₄ , since the second transistor TR ₂ is inserted in the latter path, a signal delay occurs, and this delay causes the phase of high-frequency noise on one side to change. By shifting, the probability that the above-mentioned situation will occur will be greatly reduced. Therefore, the noise led to the output terminal 3 side is extremely reduced. On the other hand, in the case of television signal reception, the time delay due to the difference in the transmission path can be ignored when detecting the horizontal synchronizing signal, and as shown in _FIG.
In the portion where the output E of TR ₂ is smaller than the output F of TR 2, a pulse synchronized with the horizontal synchronizing signal is obtained as shown in FIG.

As described above, the present invention applies a composite video signal to the first electrode of a transistor, and connects the transistor so that the output generated by applying the composite video signal to an integrating circuit with a time constant sufficiently larger than that of horizontal synchronization is applied to the second electrode. However, by making the output generated at the third electrode of the transistor and the output generated by gating this output with a flyback pulse in an opposite phase relationship, and applying both to each first electrode of the differential pair transistor, Due to the time difference caused by the difference in the transmission paths between the two, high-frequency noise components are greatly reduced, and the horizontal synchronization signal can be detected reliably, making it possible to avoid malfunctions due to noise, and to ensure that no noise is displayed on any screen. This circuit has the effect of reliably detecting the horizontal synchronization signal, and is extremely useful as a circuit for providing a search stop signal in a television receiver having an automatic station search function.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the prior art, and FIGS. 2 and 3 are explanatory diagrams thereof. FIG. 4 is a circuit diagram showing a horizontal synchronizing signal detection circuit embodying the present invention, and FIGS. 5, 6, 7, and 8 are explanatory diagrams thereof. TR ₁ ...first transistor, _C1 , _R1 ...capacitor and resistor forming an integrating circuit, 4 ...differential amplifier.

Claims (1)

[Claims] 1. A composite video signal is applied to the first electrode of the first transistor, and the output generated by applying the composite video signal to an integrating circuit with a time constant sufficiently larger than the horizontal period is connected to the second electrode,
an output produced at the third electrode of the first transistor and a period of a flyback pulse produced at the third electrode by applying this output to the first electrode of the two transistors and a flyback pulse to the second electrode; The phase-inverted outputs are respectively applied to the first electrodes of the differential pair transistors, and the output generated at the third electrode of one of the differential pair transistors is used as the horizontal synchronization signal detection output. Horizontal sync signal detection circuit.