JPS6314530Y2 - - Google Patents

Description

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

Some television receivers as mentioned above 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. Therefore, it is difficult to use the output of the sync separation circuit for the determination. It was hot.

Therefore, the present invention takes these points into consideration and proposes a horizontal synchronization signal detection circuit that can reliably detect only the horizontal synchronization signal with an extremely simple circuit.The details thereof will be explained below with reference to the drawings.

FIG. 1 shows an embodiment of the circuit according to the present invention. In the figure, reference numeral 1 denotes a decoded video signal (second At the application terminal in Figure a), the decoded video signal applied to this terminal is passed through resistor 2.
It is applied to the base of the first transistor 4 of the NPN type, and is also applied to the midpoint A of the connection between the diode 5 and the capacitor 6, which are connected in series to the emitter of the transistor 4 via the other low resistor 3. At this time, the time constant determined by the capacitor 6 and resistor 3 is selected to be sufficiently large compared to the horizontal frequency (15.75 KHz), so the potential at point A is as shown in Figure 2 a.
It will look like the dashed line. Therefore, the base potential of the first transistor 4 is greater than the sum of the voltage at the point A and the respective rising voltages between the base and emitter of the diode 5 and the transistor 4. Therefore, the negative polarity pulse shown in FIG. 2b is obtained at the collector of this transistor 4. Note that 7 is the collector load resistance of the first transistor 4.

The pulse b taken out from the collector of the first transistor 4 is applied to the current limiting resistor 8 and the base.
The voltage is applied to the base of a PNP type second transistor 10 via a diode 9 for preventing reverse bias between emitters. On the other hand, the horizontal flyback pulse shown in FIG. applied.

Therefore, if the voltage dividing ratio of the resistors 12 and 13 is selected so that the pulses b and c have the relationship shown in FIG. B, that is, from the terminal 15, a pulse synchronized with the horizontal synchronizing signal in the composite video signal a is obtained as shown in FIG. 2b. Therefore, if this pulse d is converted into a DC voltage using a low-pass filter such as an integrating circuit, the horizontal synchronizing signal can be detected.

By the way, in the circuit shown in Figure 1, even if noise arrives at terminal 1, the repetition frequency of such noise is considered to be sufficiently lower than the horizontal frequency, so the potential generated at point A in this case The difference between the peak potential of the noise and the peak potential of the noise is not large enough to exceed the sum of the respective rising voltages (approximately 1.2V) between the base and emitter of the diode 5 and the first transistor 4. No pulse will ever appear at the collector of transistor 4, and even if such a pulse does appear, the pulse should be set so that the potential difference Δv between b and c in FIG. 2 is as small as possible within the range of reliable operation. If c is selected, the possibility that a pulse due to the noise will be generated at the collector of the second transistor 10 will be extremely small, and therefore, it can be said that there will be almost no malfunction of detecting such noise as a horizontal synchronizing signal. .

Furthermore, since the magnitude of the composite video signal applied to the terminal 1 changes depending on each television signal, the emitter potential of the first transistor 4 also changes accordingly. The signal separation operation can always be performed reliably.

In the embodiment shown in FIG. 1, the configuration as shown is used to utilize a horizontal flyback pulse of positive polarity, but when using a horizontal flyback pulse of negative polarity, the output pulse of the first transistor is inverted. Of course, the same can be realized by using an NPN type transistor for the second transistor, or by making other changes.

Since the horizontal sync signal detection circuit of the present invention is as described above, it can reliably detect only the horizontal sync signal without malfunctioning due to noise, etc., and does not require adjustment of the operating level for each television signal. Therefore, it is suitable for use in the television receiver described at the beginning.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the horizontal synchronizing signal detection circuit of the present invention, and FIG. 2 is a signal waveform diagram of each part thereof. 2, 3...Resistor, 4...First transistor, 5
...Diode, 6...Capacitor, 10...Second transistor.

Claims (1)

[Scope of utility model registration request] A diode and a capacitor are connected in series between the emitter of the first transistor and the ground point, and a resistor is connected between the midpoint of the connection between the diode and the capacitor and the input terminal of the composite video signal taken out from the video detection circuit. The time constant determined by this resistor and the capacitor is selected to be sufficiently large compared to the horizontal period, and the input terminal is connected to the base of the first transistor, and the collector of the first transistor is connected to the base of the second transistor. Alternatively, the emitter is connected to one side of the emitter, and the other of the emitter or base is connected to the input terminal of the horizontal flyback pulse having the opposite polarity to the collector output pulse of the first transistor, and the collector output pulse is applied to the collector of the second transistor. and the flyback pulse, and detects the presence or absence of the AND pulse.