JPH0424701Y2 - - Google Patents

Description

[Detailed description of the invention] "Industrial Application Field" This invention relates to a video signal and test pattern signal switching circuit for a VTR (video tape recorder).

``Prior Art'' Some VTRs have a built-in test pattern generator that allows the user to easily adjust the various parts required. Such a VTR has a video signal, a test signal, which selects either a test pattern signal or a video signal and supplies it to the RF modulator, and also supplies operating power to the test pattern generator only during the period when the test pattern signal is selected. A pattern signal switching circuit is used.

The conventional circuit will be explained with reference to the circuit diagram of FIG. The video signal applied to the video input terminal 1 is applied to the fixed contact piece 2a of the switch 2;
A test pattern signal supplied from the test pattern generator 3 to the test pattern input terminal 4 is applied to the fixed contact piece 2b of the switch 2 through a series circuit including a DC blocking capacitor 5 and a level adjusting resistor 6. The movable contact piece 2c of the switch 2 is connected to a common potential point through a series circuit of a resistor 7 and a resistor 8, and the connection point between the resistors 7 and 8 is connected to a signal output terminal 10 through a DC blocking capacitor 9. connected to.

A power supply voltage is applied to the emitter of the transistor 12 via a power input terminal 11, and its collector is connected to the power terminal of the test pattern generator 3 via a power output terminal 13. The base of the transistor 12 is connected to the fixed contact piece 2b of the switch 2 via a resistor 14. A capacitor 14 is provided on the power line between the power supply input terminal 11 and the transistor 12 to bypass noise and ripples introduced from the power supply side.

When the movable contact piece 2c of the switch 2 is switched to the fixed contact piece 2a side, the video signal applied to the video input terminal 1 is selected, the level is adjusted by the voltage divider circuit consisting of resistors 7 and 8, and the DC blocking capacitor 9 is connected. The signal is supplied to the RF modulator from the signal output terminal 10 via the signal output terminal 10.

When the movable contact piece 2c of the switch 2 is switched to the fixed contact piece 2b side, the test pattern signal is selected and supplied to the RF modulator from the signal output terminal 10 as before. At the same time, transistor 1
Since the base current of 2 flows through the path of power supply input terminal 11 - emitter - base - resistor 14 - fixed contact piece 2b - movable contact piece 2c - resistor 7 - resistor 8 - common potential point, transistor 12 is turned on. , collector current is supplied to the test pattern generator 3 via the power supply output terminal 13. The test pattern generator 3 outputs a test pattern signal immediately upon being supplied with power.

``Problems with the Prior Art'' In the conventional circuit, a mechanical switch is used to select either the video signal or the test pattern signal. Video input signal is ^1V pp
The current flowing through the contact piece is also small. Therefore, the contact pieces 2a and 2c turn on,
The OFF operation has little effect on cleaning the contact surface of the contact piece by spark discharge. On the other hand, the test pattern signal or the base current of the transistor is interrupted by the contacts 2b and 2c, which are the paths for the test pattern signal, but since these currents are small, the cleaning effect of the contacts is as same as before. There are almost no Therefore, after a certain period of use, the contact surfaces of the contact pieces become corroded or chemically changed by the influence of harmful gases in the air, and become contaminated with dust, making the electrical contact between the contact pieces unstable. This often results in noise and eventually poor contact.

The purpose of this invention is to eliminate the above-mentioned drawbacks, and to improve the structure so that noise is less likely to occur and contact failures are less likely to occur.

``Means for solving the problem'' According to this invention, a FET is used that turns on when the gate voltage is zero volts between the video input terminal and the signal output terminal, and turns off when the voltage of the power input terminal is applied to the gate. is connected, a signal path is provided between the test pattern input terminal and the signal output terminal that allows signal transmission at all times, a switch is provided between the power input terminal and the power output terminal, and the power input terminal is connected to this switch. Connected to the gate of the above FET via. A test pattern signal and a video signal are respectively selected and output in accordance with whether this switch is turned on or off.

"Embodiment" An embodiment of this invention will be described with reference to the circuit diagram of FIG. Note that the same parts as in FIG. 2 are given the same reference numerals, and redundant explanation will be omitted. A FET 20 is provided between the video input terminal 1 and the signal output terminal 10, a switch 21 is provided between the power input terminal 11 and the power output terminal 13, and power is supplied to the gate of the FET 20 through the switch. . Its gate is connected via a resistor 22 to a common potential point. One end of the level adjustment resistor 6 is directly connected to one end of the resistor 8, forming a voltage dividing circuit.

When the switch 21 is turned on, operating power is applied to the test pattern generator 3, and a test pattern signal is applied from the generator 3 to the test pattern input terminal 4, and this signal is transmitted to the capacitor 5, resistor 6,
The signal is applied to the signal output terminal 10 through a signal path consisting of 8 capacitors 9 that can be transmitted at all times. On the other hand, the power supply voltage is applied to the gate of FET 20 via switch 21, and FET 20 is turned off.

When the switch 21 is turned off, the operating power of the test pattern generator 3 is cut off and the supply of output signals is stopped, while the resistor 2 is connected to the gate of the FET 20.
A common potential (zero volts) is applied through 2
The FET 20 is turned on, and the video signal applied to the video input terminal 1 is transmitted to the signal output terminal 10.

The FET 20 uses a so-called depletion type FET that allows drain current to flow when the gate voltage is zero volts. Depletion type
In the case of an N-channel type FET, the gate voltage at which the FET is cut off (cut-off voltage) is a negative voltage (for example, -3 volts), and in the case of a P-channel type, it is a positive voltage (for example, 3 volts). Therefore, a P channel type and an N channel type are used depending on whether the power supply voltage is positive or negative, respectively. Also, the absolute value of the power supply voltage is chosen to be greater than that of the cut-off voltage.

When the gate voltage is zero volts and the voltage applied between the source and drain is small, the source,
The resistance between the drains is small, on the order of several hundred ohms. This resistor and resistor 8 constitute a voltage dividing circuit for the video signal, and the voltage dividing resistor 7 of the circuit shown in FIG. 2 is omitted.

The switch 21 inserted into the power supply circuit intermittents a relatively large operating current of the test pattern generator 3, and the contact piece is cleaned by spark discharge, so that the electrical contact is kept stable for a long period of time. Noise generation and poor contact are prevented.

As is well known, FET20 differs from general transistors in that it has an extremely small gate leakage current and does not have a PN junction between the source and drain.
Does not generate offset voltage due to PN junction. Therefore, it can be used as a nearly ideal switch.

``Effect of the invention'' According to this invention, an FET and a switch are used to select the video signal and the test pattern signal, and the switch is configured to intermittent the operating power supply of the test pattern generator. Cleaning is carried out and stable electrical contact can be obtained over a long period of time. Additionally, FETs are less likely to generate noise or cause poor contact due to age-related deterioration of the contacts, unlike conventional mechanical switches that do not have a self-cleaning effect. Therefore, according to this invention, it is possible to prevent the generation of noise or poor contact in the signal switching circuit, which has been a problem in the past.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a video signal/test pattern signal switching circuit for a VTR according to the present invention, and FIG. 2 is a circuit diagram showing a conventional VTR video signal/test pattern signal switching circuit. 1...Video input terminal, 2...Test pattern input terminal, 3...Test pattern generator, 4...
DC blocking capacitor, 5, 6, 7, 13, 22
...Resistor, 8...DC blocking capacitor, 9...
...Signal output terminal, 10...Power input terminal, 11...
...Transistor, 12...Power output terminal, 14...
...Capacitor, 20...FET, 21...Switch.

Claims (1)

[Claims for Utility Model Registration] Select either the video signal applied to the video input terminal or the test pattern signal applied to the test pattern input terminal and supply it to the signal output terminal, and the voltage applied to the power supply input terminal. of,
Only during the period when the above test pattern signal is selected.
In the VTR video signal and test pattern signal switching circuit that supplies the test pattern generator via the power supply output terminal, the gate voltage is turned on at zero volts between the video input terminal and the signal output terminal, and the power supply input A FET that turns off when the terminal voltage is applied to the gate is connected, and a signal path is provided between the test pattern input terminal and the signal output terminal that can always transmit signals, and the power supply input terminal and the power input terminal are connected to each other. A switch is provided between the power input terminal and the power input terminal, and the power input terminal is connected to the
A video signal/test pattern signal switching circuit for a VTR, which is connected to a gate of a FET, and selects and outputs the test pattern signal and the video signal in response to on/off of the switch.