JPH0746439A - Terminal circuit for video signal - Google Patents

Abstract

PURPOSE:To attain automatic impedance matching by providing a means detecting a falling time of an edge with respect to a received synchronizing signal and a means selecting a termination resistor based on the result of detection of the detection means to the circuit. CONSTITUTION:A level of a synchronizing signal 4 is compared with a level Va deciding an upper limit designated in advance by a microcomputer 13 at a comparator 5 and when the level is lower than the Va, an output of the comparator 5 goes to 1. When the level of the signal 4 is going to be less than a level Vb deciding a lower limit of a comparator 19 as lapse of time, an output of the comparator 19 goes to 0, an analog switch 6 is turned off and a charging voltage is held. Whether or not reflection is produced is discriminated by the microcomputer 13 based on the transition from the Va to the Vb, that is, of a falling time T of the signal 4. The microcomputer 13 outputs a selection signal 16 to an analog switch 2 when the falling time T is longer, a termination resistor 3 is sequentially selected to control an ON/OFF control signal 14 so as to minimize the falling time T.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display device for displaying a video signal sent from a video signal output device, and to a control circuit for surely receiving a synchronizing signal of the video signal.

[0002]

2. Description of the Related Art A conventional method of receiving a sync signal of a video signal will be described with reference to FIGS. Conventionally, a video signal is output from a video signal output device represented by a computer, and a video can be viewed on a monitor (display device). As the video signal, a horizontal sync signal, a vertical sync signal, or a composite video signal in which the horizontal and vertical sync signals are present in the same line is sent together with the R, G, B video signals. However, there are various types of signal source impedances from the video signal output device on the video signal sending side, and there are devices that send at TTL level and devices that send at 75Ω impedance, and the signal source impedance is displayed as unknown. On the device side, the circuit of FIG. 3 received the synchronizing signal.

In the circuit shown in FIG. 3, the synchronizing signal input 19 has a termination resistance 20 for impedance matching. Further, after the waveform is slightly blunted by the capacitor 21, it is received by the Schmitt inverter 22 having a hysteresis characteristic and the waveform-shaped output signal 23 is used as a reference synchronization signal. The RGB video signal was captured and displayed in synchronization with the video signal.

[0004]

However, when synchronizing signals having different signal source impedances come in, there may be a case where the matching with the resistor 20 cannot be obtained, as shown in FIG.
Reflection occurred as shown in (b). In FIG. 4B, the waveform is considerably blunted by the capacitor 21 as shown in FIG. 4C, and temporal variations occur in the timing of the repeatedly generated synchronizing signal. Further, since the reference signal varies, flicker occurs in the display reproduced by the display device. Above all, flicker was particularly noticeable in a display device using liquid crystal.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems. Means for detecting a falling time of an edge with respect to an input synchronizing signal, and a termination resistance value based on a detection result of the detecting means are provided. The present invention proposes a termination circuit for video signals having a means for switching between.

[0006]

According to the present invention, when the synchronizing signal of the video signal is inputted, the termination resistance matched with the impedance is automatically selected to select the optimum impedance.

[0007]

Embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is an electrical block diagram of the present invention. The horizontal synchronizing signal of the video signal from the outside is input to the input terminal 1,
The-input terminal of the comparator 5 and the + of the comparator 19
It is input to the input terminal. The termination resistance 3 of the synchronizing signal can be selected from four resistance values by switching the analog switch 2. The selection method is selected by the selection signal 16 for switching the switch from the microcomputer 13, and the termination resistance matched with the signal source impedance of the synchronization signal 1 can be selected. Further, the comparison reference levels Va and Vb are input to the synchronization signals input to the comparators 5 and 19, respectively, and they are compared with each other. Va and Vb are variable, and the D / A converter 1 is operated by the reference level change signal from the microcomputer 13.
The output of 5 can be changed.

The output of the comparator 5 is connected to the input of the analog switch 6, and the output of the comparator 19 is ON /
ON / OFF of the analog switch 6 as the OFF control signal 14
The microcomputer 13 can be turned off and monitored by the microcomputer 13. The output of the analog switch 6 is a resistor 7 that controls the charging current of the charging capacitor 10.
Connected to a charging capacitor 10 and an amplifier 9 at one end.
Connected to the positive terminal of. The negative terminal of the amplifier 9 is connected to the output of the amplifier 9 to form a voltage follow, and the output signal is input to the A / D converter 11.

The elements 6 to 10 form a sample and hold circuit. The digital output data 12 of the A / D converter 11 is sent to the microcomputer 13 and the sampled and held voltage value can be detected.

The operation will now be described. When the impedance matching does not match when the horizontal synchronizing signal 1 in the video signal is input from the video signal output device, reflection occurs in the waveform as shown in FIG. 2 (a). Will end up. This synchronizing signal 6 is compared with a level Va that determines the upper limit designated by the microcomputer 13 to the D / A converter 15 in advance by the comparator 5, and when it falls below Va, the output of the comparator 5 becomes 1. (See FIG. 2B) At this time, the output of the comparator 19 is 1 and the analog switch 6 is ON, so that the output voltage of the comparator 5 is charged in the capacitor 10 via the resistor 7. See FIG. 2 (d). When the sync signal 1 falls below Vb which determines the lower limit of the comparator 19 as time passes, the output of the comparator 19 becomes 0 (see FIG. 2c), the analog switch 6 is turned off, and the charging voltage is held. Fall time T of sync signal 1
Is determined by the microcomputer 13 from the transition from Va to Vb. If the falling time T is long, the microcomputer 13 outputs the selection signal 16 to the analog switch to sequentially switch the termination resistor 3, and the falling time T
Can be controlled by reading the output of the A / D converter 11 at the timing of the ON / OFF control signal 14 so as to minimize.

[0011]

As described above, the present invention can automatically control the impedance matching of the synchronizing signal with respect to the video signal output devices having different signal source impedances, so that a stable synchronizing signal can be input and the display quality can be improved. is there.

[Brief description of drawings]

FIG. 1 is an electrical block diagram of the present invention.

FIG. 2 is a timing diagram of the present invention.

FIG. 3 A conventional synchronizing signal interface circuit

FIG. 4 Conventional timing diagram

[Explanation of symbols]

 1 Horizontal sync signal input terminal 2 Analog switch 3 Termination resistance 4 Horizontal sync signal 5 Comparator 6 Analog switch 7 Charging current control resistor 8 Analog switch output 9 Operational amplifier 10 Charging capacitor 11 A / D converter 12 A / D converter digital output 13 Microcomputer 14 ON / OFF control signal 15 D / A converter 16 Analog switch selection signal 17 Reference level Vb 18 Reference level Va 19 Synchronous signal input terminal 20 Termination resistor 21 Capacitor 22 Schmitt inverter 23 Waveform shaped output signal

Claims (1)

[Claims] 1. A termination circuit for a video signal, comprising: a means for detecting a falling time of an edge with respect to an input synchronizing signal; and a means for switching a termination resistance value according to a detection result of the detection means.