JPH01254996A - Signal selecting circuit - Google Patents

Abstract

PURPOSE:To enable automatic switching of the synchronizing signal separated from a composite video signal and a separate horizontal synchronizing signal by providing a logical circuit which outputs the synchronizing signal inverted by a polarization inverting circuit at the time of detecting the separate horizontal synchronizing signal and outputs the synchronizing signal separated in a synchronizing sepn. circuit at the time of non-detection. CONSTITUTION:A horizontal synchronizing signal input terminal Bb is connected to the base of a transistor 7 via a resistor R6, a capacitor 4, a diode 6, and a resistor R7, and the TR 8 forms a detection circuit of the separate horizontal synchronizing signal. The detection circuit turns off the TR 8 at the time of detecting the separate horizontal synchronizing signal. The output of the TR 8 and the polarity-inverted separate horizontal synchronizing signal are ANDed by the logical circuit 10 and the polarity-inverted separate horizontal synchronizing signal is outputted to the next stage. The TR 8 turns on to output the synchronizing signal separated from the composite video signal in the synchronizing sepn. circuit 2 at the time of the non-detection of the separate horizontal synchronizing signal. Two kinds of the synchronizing signals which are different in specifications are thereby automatically switched.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a display device whose purpose is to receive signals from a personal computer, by automatically detecting the presence or absence of a separate horizontal synchronization signal. The present invention relates to a signal switching circuit capable of switching between a synchronization signal separated from a composite video signal and a separate horizontal synchronization signal.

[Conventional technology]

There are various types of display devices that serve as display devices for personal computers, depending on the type of personal computer. Among these, recently, multi-input display devices that can be connected to various types of personal computers have become mainstream.

In this display device, the input format of the synchronization signal is that horizontal synchronization and vertical synchronization signals are input separately in addition to the R, G, and B video signals, but some No synchronization signal is input, R, G, B
There is a personal computer that outputs only a composite video signal that includes a synchronization signal within the blanking period of any of the video signals as a synchronization signal.

In the case of a display device that inputs and receives two types of synchronization signals, in the former case, it is possible to receive images by inputting the separate synchronization signals as they are to the synchronization circuit, but in the case of the latter, it is possible to receive images by manually inputting the separate synchronization signals to the synchronization circuit. In the case of a video signal, it is necessary to separate only the synchronization signal part from the composite video signal and input the separated synchronization signal to a synchronization circuit.

In this case, if two synchronization signals are connected in parallel to the inputs of the synchronization circuit, they may disturb their synchronization, so it is necessary to switch them somehow and connect them to the synchronization circuit.

Conventionally, as shown in FIG. 2, a composite video signal 12 is input from a composite video input terminal A, and is input to a video circuit 1 and a synchronization separation circuit 2. As shown in FIG. A synchronization separation circuit 2 separates a synchronization signal from a composite video signal 12, while a separate horizontal synchronization signal 13 is input to a horizontal synchronization signal input terminal B and then input to a polarity inversion circuit 3, where its polarity is inverted. .

Sync signal separated by sync separation circuit 2 and polarity inversion circuit 3
The separate horizontal synchronization signal whose polarity has been inverted is input to the synchronization circuit 14 by switching between two synchronization signals using a manual switch 11 or an analog switch.

Therefore, in this case, the user must operate the changeover switch of the display device according to the signal connected, and if the operation is incorrect, the synchronization signal will not be input,
Loss of synchronization.

[Problem to be solved by the invention]

The signal switching circuit of a conventional display device is configured as described above, so in this case, the user has to operate a switch to switch the display device depending on the signal connected, and if the user makes an incorrect operation, , there was a problem that the synchronization signal was not input and the synchronization was lost.

Further, extra manual switches 11, analog switches, etc. are required, which causes many problems in terms of space and cost.

This invention was made to solve the above problems, and provides a signal switching circuit that can automatically and inexpensively switch between a synchronization signal separated from a composite video signal and a separate horizontal synchronization signal. The purpose is to obtain.

[Means to solve the problem]

The signal switching circuit according to the present invention includes a detection circuit that detects the presence or absence of a separate horizontal synchronization signal, a transistor that is turned on or off depending on whether or not the separate horizontal synchronization signal is detected by the detection circuit, and an output and polarity of the transistor. A logic circuit that performs an AND operation with the inverted separate horizontal synchronizing signal is provided.

[For production]

The detection circuit in this invention turns off the transistor when a separate horizontal synchronizing signal is detected, and uses a logic circuit to AND the output of this transistor and the separate horizontal synchronizing signal whose polarity has been inverted. Outputs a synchronization signal, and when a separate horizontal synchronization signal is not detected, the transistor turns on and outputs a synchronization signal separated from the composite video signal by the synchronization separation circuit, and outputs it to the next stage synchronization circuit through this synchronization signal logic circuit. do.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, reference numeral 4 denotes a composite video signal input terminal, which inputs the composite video signal 12, is grounded via a resistor R1, and is connected to the input terminal of the video circuit 1 via a capacitor CI. It is connected to the input end of the synchronous separation circuit 2 via a resistor R2 and a capacitor C2.

The output terminal of the synchronous separation circuit 2 is connected to the collector of the transistor 8 and the base of the transistor 9. A resistor R3 is connected between the base and emitter of the transistor 9,
The emitter is grounded, the collector is connected to the power supply Vcc via a resistor R4, and as a logic circuit,
It is connected to the first input terminal of the input NAND circuit 10.

On the other hand, a separate horizontal synchronizing signal 13 is introduced into the horizontal synchronizing signal input terminal B. This horizontal synchronizing signal input terminal B is grounded via a resistor R5, and is connected to a second input terminal of a NAND circuit 10 via a polarity inversion circuit 3.

The output of the NAND circuit 10 is sent to a synchronous circuit (not shown).

Furthermore, the horizontal synchronizing signal input terminal B is connected to the base of the transistor 8 via a resistor R6, a capacitor 4, a diode 6, and a resistor R7.

A resistor R8 is connected between the base and emitter of the transistor 8, and the emitter is grounded.

Transistor 8 forms a separate horizontal synchronization signal detection circuit.

The connection point between the capacitor 4 and the diode 6 is grounded via the diode 5, and the connection point between the diode 6 and the resistor R
The connection point with 7 is grounded via electrolytic capacitor 7.

Next, the operation will be explained. First, when the separate horizontal synchronizing signal 13 is input to the horizontal synchronizing signal input terminal B, the DC level of this separate horizontal synchronizing signal 13 is clamped by the capacitor 4 and the diode 4 via the resistor R6, and the DC level is clamped by the diode 4 and the diode 6. The voltage detected and smoothed by the electrolytic capacitor 7 turns on the transistor 8.

By turning on transistor 8, transistor 9
turn off. Therefore, the collector of the transistor 9 becomes high level, and the high level becomes the NAND circuit 1.
1 is manually inputted to the first input terminal of No. 1. .

On the other hand, the separate horizontal synchronizing signal 13 introduced into the horizontal synchronizing signal input terminal B has its polarity inverted by the polarity inverting circuit 3 and is applied to the second input terminal of the NAND circuit 10. Thereby, the NAND circuit 10 outputs this separate horizontal synchronization signal 13 to the synchronization circuit.

Next, a case where the composite video signal 12 is introduced into the composite video signal input terminal A and the separate horizontal synchronization signal is not input into the horizontal synchronization signal input terminal B will be described. In this case, the base of transistor 8 is at low level, so
Transistor 8 is turned off. Therefore, the composite video signal 12 at the composite video signal input terminal A passes through the synchronization separation circuit 2 to separate the synchronization signal, and this separated synchronization signal is input to the base of the transistor 9, and from the collector to the NAND
It is input to the first input terminal of the circuit 10.

At this time, since the output of the polarity inversion circuit 3 is at a high level, the periodic signal from the composite video signal is transferred to the NAND circuit 1.
0 and is input to the synchronous circuit.

This allows automatic selection when the synchronization signal is switched to a synchronization signal separated from the composite video signal.

〔Effect of the invention〕

As described above, according to the present invention, when the detection circuit detects a separate horizontal synchronizing signal, the logic circuit performs the logical product of the signal obtained by turning off the transistor and the inverted separate horizontal synchronizing signal, and synchronizes. When the separate horizontal synchronization signal is not input, the synchronization signal separated from the composite video signal by the synchronization separation circuit is output as a synchronization signal through a transistor and a NAND circuit. This has the advantage that it is possible to automatically switch between two types of synchronization signals with different specifications.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a signal switching circuit according to an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional signal switching circuit. 2...Synchronization separation circuit, 3...Polarity inversion circuit, 8...
・Transistor (detection circuit), 9...transistor,
10...NAND circuit. In addition, in the figures, the same reference numerals indicate the same or equivalent parts. Agent Masuo Oiwa

Claims (1)

[Claims] a detection circuit that detects the presence or absence of a separate horizontal synchronization signal;
a sync separation circuit that separates a sync signal from a composite video signal;
a polarity inversion circuit that inverts the polarity of the separate horizontal synchronization signal; and a polarity inversion circuit that turns off when detecting the separate horizontal synchronization signal to prevent output of the synchronization signal separated by the synchronization separation circuit; is a transistor that outputs the synchronization signal separated by the synchronization separation circuit when the separate horizontal synchronization signal is not detected, and the output of the transistor and the output of the polarity inversion circuit are ANDed to generate the separate horizontal synchronization signal. When detected, the separate horizontal sync signal inverted by the polarity inversion circuit is output to the next-stage sync circuit, and when the separate horizontal sync signal is not detected, the sync signal separated by the sync separation circuit is output to the next-stage sync circuit. A signal switching circuit equipped with a logic circuit that outputs to the circuit.