JPH0434555Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a two-frequency discrimination circuit for discriminating the horizontal frequency of an input signal in, for example, a display device.

[Summary of the idea]

This invention relates to a two-frequency discrimination circuit, and by changing the waveform with a monostable multivibrator, clamping the DC current, averaging it, and making a discrimination using a comparator, it is possible to stably discriminate even input signals with a small frequency difference. It is something to do.

[Conventional technology]

For example, in a display monitor device for a computer, the horizontal frequency of the input signal is not limited to just one type; for example, 15.7kHz and 31kHz, 15.7kHz and 24kHz, etc.
There are cases where video signals of two types of horizontal frequencies, such as Hz, are switched to enable image reception. In this case, if the horizontal frequency of the input signal could be determined and the operating mode of the device could be automatically switched accordingly, the user would no longer have to manually switch, which would be extremely advantageous in terms of usage.

Therefore, a circuit for determining the horizontal frequency was proposed. In FIG. 4, an input terminal 41 to which a horizontal synchronizing pulse is supplied is connected to an input of a monostable multivibrator (monomulti) 42 having a predetermined inversion time. The output of this monomulti 42 constitutes a resistor 43 and a capacitor 4 that constitute an averaging circuit.
This resistor 43 is grounded through a series circuit of 4
The midpoint between the capacitor 44 and the capacitor 44 is connected to the base of one transistor 45 constituting a comparator. Further, the power supply terminal 46 to which _Vcc is supplied is grounded through a series circuit of resistors 47 and 48, and the connection midpoint of the resistors 47 and 48 is connected to the capacitor 49.
The midpoint of this connection is connected to the base of the other transistor 50 constituting the above-mentioned comparator. And this transistor 45,
The emitters of transistor 50 are connected to each other and grounded, the collector of transistor 50 is connected to power supply terminal 46, and the collector of transistor 45 is connected to resistor 51.
The transistor 45 is connected to the power supply terminal 46 through the transistor 45, and an output terminal 52 is led out from the collector of the transistor 45.

In this circuit, when an input pulse signal S _x as shown in FIG. 5A is supplied, a signal S _y as shown in FIG. 5B is output from the monomulti 42, and this signal S _y is averaged. The signal S _z as shown in C of the same figure
is supplied to the base of a transistor 45 constituting a comparator. In this circuit, since the inversion time of the monomulti 42 is constant, the length of the non-inversion period (low potential period) of the signal S _y is changed according to the frequency of the input pulse signal S _x , and the length of the non-inversion period (low potential period) of the signal S _y is changed. Duty is changed. Therefore, the level of the signal _Sz , which is the average of this signal _Sy , is changed according to the frequency of the input pulse signal _Sx , and by determining the level of this signal _Sz with a comparator, the input pulse signal can be determined. It can be performed.

However, with this circuit, there is no problem if the horizontal frequency to be determined is a large difference, such as the above-mentioned 15.7kHz and 31kHz or 24kHz, but for example,
When the difference is small, such as between 15.7 kHz and 16.5 kHz or 17.2 kHz, the level change of the above-mentioned signal S _z becomes small, and the detection sensitivity of discrimination decreases. Furthermore, there was a risk of malfunction due to fluctuations in the power supply voltage, resulting in a lack of stability.

[Problem that the invention attempts to solve]

As mentioned above, with conventional technology, when the frequency difference to be determined is small, the detection sensitivity decreases,
There were problems such as the risk of malfunction due to fluctuations in the power supply voltage.

[Means for solving problems]

The present invention supplies an input pulse signal (input terminal 1) to a monostable multivibrator 2 whose inversion time is set so that there are no pulses at high frequencies above a predetermined value, and outputs the output of this monostable multivibrator using a DC clamp (capacitor). This is a two-frequency discriminating circuit in which means is provided to average the value (resistor 5, capacitor 6) after 3 diodes 4), and this average value is determined by a comparator (transistors 7, 12, etc.).

[Effect]

According to this, the output of the monostable multivibrator becomes DC at high frequencies above a certain level,
I decided to clamp this output to DC, so
When the frequency is high above a predetermined value, the output of the clamp circuit becomes 0 level, while when the frequency is low below a predetermined value, a pulse whose peak value is the power supply potential is output from the clamp circuit, and these can be discriminated extremely easily and stably. I can do it.

〔Example〕

In Fig. 1, the input terminal 1 to which the horizontal synchronizing pulse is supplied is a monostable multivibrator (mono-multi) having an inversion time longer than the period of the high frequency of the horizontal frequency to be determined and shorter than the period of the low frequency.
2 input. The output of this monomulti 2 is grounded through a series circuit of a capacitor 3 and a diode 4 that constitute a DC clamp circuit, and the midpoint of the connection between this capacitor 3 and diode 4 is connected to the series circuit of a resistor 5 and a capacitor 6 that constitute an averaging circuit. grounded through the circuit, this resistor 5 and capacitor 6
The midpoint of the connection is connected to the base of one transistor 7 constituting the comparator. Further, the power supply terminal 8 to which _Vcc is supplied is grounded through a series circuit of resistors 9 and 10, and the midpoint of the connection between the resistors 9 and 10 is grounded through the capacitor 11, and this midpoint of the connection constitutes the above-mentioned comparator. It is connected to the base of the other transistor 12. The emitters of transistors 7 and 12 are connected to each other and grounded, the collector of transistor 12 is connected to power supply terminal 8, the collector of transistor 7 is connected to power supply terminal 8 through resistor 13, and the collector of transistor 7 is connected to power supply terminal 8 through resistor 13. An output terminal 14 is led out from the collector.

In this circuit, when the input signal is low frequency, the input pulse signal S _a whose pulse period is longer than the inversion time of monomulti 2 is supplied as shown in Fig. 2A, and therefore the monomultiple input has the same frequency. As shown in FIG. B, a signal S _b having a low potential period is output after the inversion time has elapsed until the next pulse arrives. When this signal S _b is supplied to a DC clamp circuit consisting of a capacitor 3 and a diode 4, the low potential period of this signal S _b is clamped to the ground O potential, and the peak value of the pulse changes as shown in C in the same figure.
A signal S _c of V _cc is formed, and when this signal S _c is averaged, a signal at approximately the level of V _cc is extracted as shown in FIG.

On the other hand, when the input signal is high frequency, the third
As shown in Figure A, an input pulse signal _{S a} whose pulse period is shorter than the inversion time of the monomulti 2 is supplied,
Therefore, the monomulti 2 outputs a signal S _b having continuous high potential periods as shown in FIG. And when this signal is supplied to the DC clamp circuit,
The above-mentioned series of high potential periods are clamped to the ground potential, forming a signal S _c of the ground potential as shown in C of the same figure, and when this signal S _c is averaged, it becomes 0 level as shown in D of the same figure. signal is extracted.

Therefore, in this circuit, transistors 7, 1
A comparator consisting of 2 etc. can discriminate between the approximately V _cc level and the 0 level, and can perform the discrimination extremely easily and stably.

In other words, according to this circuit, the difference in the average signal level is large regardless of the size of the difference in frequency, so the detection sensitivity is extremely high, and there is little risk of being affected by fluctuations in the power supply potential, reducing the risk of malfunction etc. It is possible to perform stable discrimination without any problems.

[Effect of idea]

According to this invention, the output of the monostable multivibrator becomes DC at high frequencies above a certain level, and this output is clamped to DC, so that the output of the clamp circuit becomes 0 level at high frequencies above a certain level. On the other hand, when the frequency is low below a predetermined value, a pulse whose peak value is the power supply potential is output from the clamp circuit, and these can now be discriminated extremely easily and stably.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of an example of the present invention, Figures 2 and 3.
The figure is a diagram for explaining the same, and FIGS. 4 and 5 are diagrams for explaining the conventional technology. 1 is an input terminal, 2 is a monostable multivibrator, 3 and 4 are elements constituting a DC clamp circuit,
5 and 6 are elements constituting an averaging circuit; 7 and 9-
13 is an element constituting a comparator, 8 is a power supply terminal, and 14 is an output terminal.

Claims (1)

[Scope of Claim for Utility Model Registration] An input pulse signal is supplied to a monostable multivibrator whose inversion time is set so that there are no pulses at high frequencies above a certain level, and the output of this monostable multivibrator is DC-clamped and then averaged. A two-frequency discrimination circuit is provided with a means for converting the average value into a value, and uses a comparator to discriminate the average value.