JPH0588033B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal determining circuit that determines the presence or absence of a television video signal (hereinafter simply referred to as a video signal).

For example, when a signal other than a regular video signal, that is, a non-video signal, arrives at the television receiver, the video signal discrimination circuit discriminates and detects the signal, and uses its output signal to block the audio signal, It is used to block out unpleasant noises.

One of the simplest ways to determine the presence or absence of a video signal is to detect the peak value of the video signal, but this is not practical because it is difficult to distinguish between the video signal and noise.

The present invention provides a video signal discrimination circuit that has reliable discrimination performance and utilizes a horizontal AFC circuit section and a horizontal oscillator that are incorporated as functional circuit elements in a television receiver.

Hereinafter, the operation of the video signal discrimination circuit of the present invention will be explained using an embodiment.

FIG. 1 shows a video signal discrimination circuit according to an embodiment of the present invention, the functional circuit elements of which are shown in a block diagram, and FIG. 2 shows signal waveforms of each main part in the circuit.

In FIG. 1, when a horizontal synchronizing signal is applied to an input terminal 1, a horizontal AFC circuit section 2 operates using the horizontal synchronizing signal as a reference input signal. the horizontal
A comparison input signal is required for the AFC circuit unit 2 to operate normally, and a rectangular wave is used for the comparison input signal. This rectangular wave is a sawtooth wave shown in FIG. 2a formed by the horizontal oscillator 3, which is converted into a shape as shown in FIG. 2b by the waveform conversion circuit 4. The formation of the sawtooth wave is achieved by charging the horizontal oscillation capacitor 6 from the power supply voltage supply terminal 15 through the horizontal hold resistor 5, and supplying the charged charge to the discharging circuit built in the horizontal oscillator 3. It is carried out by. At this time, the upper limit of the sawtooth wave shown in Figure 2a
V _H is the voltage source 7 provided in the horizontal oscillator 3
The lower limit value V _L is set by the voltage source 8 . That is, the horizontal oscillation capacitor 6 is charged to the voltage of the voltage source 7, and when the voltage value V _H is reached, the horizontal oscillation capacitor 6 is charged by the level switching function provided in the horizontal oscillator 3. By repeating the operation of discharging the charged charges until they reach the voltage value V _L of the other voltage source 8, a sawtooth wave is generated at point A in the circuit of FIG.

The sawtooth wave obtained as described above is supplied to the waveform conversion circuit 4, and the output section of the waveform conversion circuit 4, that is, the point B in the circuit in FIG.
Obtain the square wave shown in . This rectangular wave is generated by the voltage of the slice voltage source 9 provided in the waveform conversion circuit 4.
In a circuit that conducts at a level higher or lower than V _S , for example, in a differential amplifier consisting of a pair of transistors, the sawtooth wave is applied to the base of one transistor, and the sawtooth wave is applied to the base of the other transistor. By applying the voltage V _S of the slice voltage source 9, it is possible to derive it relatively easily. As mentioned above, the rectangular wave shown in FIG. 2b obtained in this manner is applied to the point B as a comparison input to the horizontal AFC circuit section 2.

Under the above conditions, the horizontal AFC circuit unit 2 compares the horizontal sync signal with the horizontal oscillation frequency of the horizontal oscillator 3, and matches the horizontal oscillation frequency with the frequency of the horizontal sync signal. The current or voltage for controlling the horizontal oscillator 3 is obtained by smoothing (direct current) the signal output to the output terminal 10 of the horizontal AFC circuit section 2 using a smoothing circuit, and applying the current or voltage to the horizontal oscillator 3 via a resistor 11. It is applied to the charging end of the coupled horizontal oscillation capacitor 6, thereby controlling the horizontal oscillation frequency. Here, capacitors 12 and 13 and resistor 14 are elements constituting a smoothing circuit, and 15 is a power supply voltage supply terminal.

When the horizontal AFC circuit section 2 is operating normally, AFC (automatic frequency control) is performed at the rectangular wave pulse edge portion of the comparison input signal to the circuit section shown in FIG. The edge part of the signal waveform (Fig. 2b), for example, the falling edge part X point is the R of the horizontal synchronizing signal in Fig. 2d.
-S, or another rising edge Y point is located between R-S. FIG. 2 is a diagram showing that the aforementioned point X is located between R and S. And the second
A horizontal oscillation pulse shown in FIG. 2c having a pulse width corresponding to the time axis between P and Q that is automatically generated when obtaining the sawtooth wave shown in FIG. It is formed by selecting a time constant and is derived from this horizontal oscillator 3 to point C in the circuit of FIG. As shown in FIG. 2, this horizontal oscillation pulse and the horizontal synchronizing signal in FIG.
Preferably, the phase relationship is set such that the time width P-Q of the horizontal oscillation pulse exists within the time period of the pulse width R-S of the horizontal synchronization signal.

In the phase detection circuit 16 in the circuit of FIG. 1, the horizontal oscillation pulse shown in FIG. 2 c is input to point C, and the horizontal synchronization signal shown in FIG. 2 d is input to point D as the other input. and their phases are compared.

FIG. 3 shows an example of the operation of the phase detection circuit 16. This will be explained in detail based on FIG.

In each of FIGS. 3A, 3B, and 3C, c indicates the same horizontal oscillation pulse as in FIG. 2C, and d indicates the signal at input terminal 1, that is, point D.

Figure 3A shows a state in which a regular video signal is present and the horizontal oscillation pulse c (same as Figure 2c) and horizontal synchronization signal d (same as Figure 2d) are in phase. This is when the horizontal AFC circuit section 2 operates normally and the horizontal oscillation frequency is adjusted within the horizontal pull-in range. Under the above conditions, if the phase detection circuit 16 is constituted by an AND circuit, the output of the AND circuit, that is, the output terminal 17 of this phase detection circuit 16 has a voltage as shown in V ₁₇ in FIG. 3A. Zero voltage is output.

Next, as shown in d of FIG. As shown at _V17 in FIG. 3B, a pulse synchronized with the horizontal oscillation pulse c in FIG. 3B is output.

Furthermore, when there is no horizontal synchronizing signal and only noise is input to the input terminal 1, as shown in d in FIG. 3C, the voltage V ₁₇ at the output terminal 17 is The horizontal oscillation pulse of c is output as a pulse containing noise on the time axis.

As you can understand from the above, input terminal 1
No output appears at the output terminal 17 of the phase detection circuit 16 only when there is a video signal in the phase detection circuit 16, that is, when the horizontal synchronization signal d is normally input as shown in FIG. 3A. Therefore, to this output terminal 17,
For example, if a suitable capacitor circuit is provided to hold the peak value of its output, the detected potential at this time is zero potential. However, in the cases shown in FIG. 3 (b) and (c), it is possible to obtain a DC voltage value corresponding to the output V ₁₇ , and thereby it can be determined from the zero voltage in the case of FIG. 3 (a). This means that it is possible to determine the presence or absence of a video signal.

As is clear from FIGS. 2 and 3, the video signal discrimination circuit shown in the embodiment of the present invention was able to achieve its purpose under the condition that the horizontal oscillation pulse was narrower than the pulse width of the horizontal synchronization signal. However, let's consider other conditions.

FIG. 4 shows a case where the horizontal oscillation pulse is wider than the pulse width of the horizontal synchronizing signal.

The pulse width of the horizontal oscillation pulse shown in Fig. 4a is
When the pulse width is wider than the pulse width τ _syoc of the horizontal synchronizing signal shown in b, the pulse shown in c is output even when there is a horizontal synchronizing signal, that is, even when there is a video signal. Also, if the pulse width τ _syoc of the horizontal synchronization signal is zero,
In other words, even when there is no video signal, the pulse shown in d is output. Therefore, in this case, pulses such as those shown in FIG. 4c and d will be output regardless of the presence or absence of a video signal, and the function of the video signal discrimination circuit cannot be achieved.

What is shown in FIG. 5 is the horizontal synchronizing signal of FIG. 4 with the polarity reversed. As mentioned above, if the phase detection circuit 16 is configured with an AND circuit and the horizontal oscillation pulse shown in a is applied to point C in FIG. 1 and the horizontal synchronization signal shown in b is applied to point D, a video signal is obtained. Sometimes, the pulse shown in FIG. 5c is not outputted when there is no video signal, as shown in FIG. 5d. Therefore, there is no problem like that shown in FIG. 4, and it is possible to determine the presence or absence of a video signal. However, there was no video signal and the fifth
When only the noise shown in FIG. e is input, the output terminal 17 of the video signal discriminating circuit is in a state where noise is included in the time axis of the horizontal oscillation pulse, as shown in FIG. In the above case, when there is a video signal,
It becomes difficult to distinguish the state from state c, and in practical terms, the circuit becomes incomplete as a video signal discrimination circuit.

As described above, the video signal discrimination circuit of the present invention is useful when obtaining a sawtooth wave generated by the resistor 5, capacitor 6, and voltage sources 7 and 8 coupled to the horizontal oscillator 3, as shown in FIG. , so that the horizontal oscillation pulse automatically derived from point C as the output of the horizontal oscillator 3 matches the position of the horizontal synchronization signal which is the reference input of the horizontal AFC circuit section 2.
The rectangular wave period of the horizontal AFC comparison input signal applied to the AFC circuit section 2 can be freely set by controlling the period of the sawtooth wave generated by being coupled to the horizontal oscillator 2 through the output feedback of the horizontal AFC circuit section 2 itself. , the phases of both the horizontal oscillation pulse and the horizontal synchronization signal are detected by a phase detection circuit 16. In addition,
When the video signal discrimination circuit of the present invention shown in FIG. 1 is integrated into an IC, it becomes possible to easily determine the voltage relationship between the voltage sources 7 and 8 and the slice voltage source 9 using a resistance ratio. The horizontal AFC circuit section 2, the horizontal oscillator 3, the waveform converter 4, and the phase detection circuit 16 can be easily connected, and the industrial value thereof is high.

[Brief explanation of the drawing]

FIG. 1 is a functional element block diagram of a video signal discrimination circuit according to an embodiment of the present invention, FIG. 2 a to d, and FIG. 3 a to i.
C. FIGS. 4a to 4d and 5a to 5f are signal waveform diagrams of each part of the circuit. 1...Horizontal synchronization signal input terminal, 2...Horizontal
AFC circuit section, 3...Horizontal oscillator, 4...Waveform conversion circuit, 5...Horizontal hold resistor, 6...Horizontal oscillation capacitor, 7, 8, 9...Voltage source, 11...
...Resistor, 12, 13...Smoothing capacitor, 14
... Smoothing resistor, 15 ... Power supply voltage supply terminal, 1
6... Phase detection circuit, 17... Output terminal.

Claims (1)

[Claims] 1 A sawtooth wave is generated by a charging/discharging circuit including a resistor and a capacitor, and the width is within the pulse width of the horizontal synchronization signal in the video signal in synchronization with the discharge period from the upper limit value to the lower limit value of the sawtooth wave. a horizontal oscillator that derives the pulse of the signal; a waveform conversion circuit that compares the sawtooth wave with a slice power source and converts it into a rectangular wave; the rectangular wave is used as a comparison input signal, and the horizontal synchronization signal of the video signal is used as a reference input signal. Horizontal AFC activated
a feedback circuit that smooths the output of the horizontal AFC circuit and returns it to the charge/discharge circuit to match the period of the sawtooth wave with the period of the horizontal synchronization signal; and a pulse from the horizontal oscillator. and a phase detection circuit that compares the phase of the signal and the horizontal synchronization signal.