Automatic LF - TV video output. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Automatic LF - TV video output. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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When watching video cassettes, the VCR can be connected to the TV via the antenna input, or you can use the low-frequency (LF) video input on the TV. In the first case, a double conversion of the video signal occurs. In the tape recorder, the video signal is converted to high-frequency, and in the TV the reverse conversion takes place. As a result, distortions are introduced into the signal and the level of noise increases, which is noticeable in the quality of the image on the TV when watching video cassettes.

There are other inconveniences and problems, especially when connecting imported video recorders.

There is a second way - to connect a VCR using the low-frequency video input (video adapter) of the TV. Unfortunately, most domestic TVs, especially early releases, do not have such a device, although there is a place for installation.

On fig. 1.1 shows a diagram of a simple video adapter for a TV. The circuit does not need to be turned on and off, as it will turn on automatically when a signal from the connected VCR appears. The device consists of a video signal switch on a D1.1 chip and a transistor VT1, a key for changing the sweep time constant on D1.2, and a sync pulse selector on VT2 and VT3 (used to automatically turn on the VCR operation mode). The signal from the output of the tape recorder through the isolation capacitor C3 and the emitter follower enters the radio channel module of the TV. The constant component of the voltage at the emitter VT1 disables the operation of the radio channel of the TV, and only the signal from the VCR is received at the input of the video amplifier of the TV.

Rice. 1.1 (click to enlarge)

The sound signal from the VCR through the capacitor C1 and trimmer resistor R4 is fed to the sound input of the TV. Resistor R4, when the VCR is running, you can set the sound volume to the same as when receiving television programs. It is convenient to carry out a preliminary check of the assembled circuit by applying pulses (T \u64d 58 μs, ti \u0,5d 1 μs) with an amplitude of 1 V from the generator to the input of the X1 / 470 socket, and control their appearance at the VT6,5 emitter without distortion. To do this, it is temporarily necessary to connect a resistor R with a resistance of XNUMX ohms to the emitter (shown by a dotted line in the diagram). In this case, the constant component of the voltage across the resistor should be about XNUMX V (control with an oscilloscope). This voltage ensures that the radio channel module of the TV is locked when working with a VCR.

The video adapter circuit is designed for installation in ZUSCT models in TVs. Structurally, the entire device is placed on one printed circuit board, which is installed in the TV on the A1 board of the radio channel module (MRK-2-5) in the X1 connector, if it is on the board, or soldered to the corresponding contacts of the board in place of this connector. Socket X40 (any type) is fixed on the back of the TV and connected to the adapter board with two shielded wires about XNUMX cm long.

In conclusion, we note that this video adapter can also be used to connect a simple household computer to a TV. To do this, a sync mixture of signals from the computer output is fed to the input of the video adapter through a limiting resistor of 0.1 ... 1 kOhm (select when connected).

Publication: cxem.net

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