Splitters of video and audio signals. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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The problem of distributing signals from their sources to several consumers has recently often faced users of video and audio equipment. To improve the quality of playback, the author of the published article suggests doing this at low frequencies, separately for video and audio signals.

There are more and more sources and consumers of video and audio signals in Russian apartments. These are TVs, VCRs, satellite receivers, video cameras, players, etc. Some of them are traditionally connected to each other via high frequency (radio frequency). Signals from VCRs and receivers (usually in the UHF range) are supplied to the TV via a coaxial cable. If there are two or three TVs in an apartment, the signals have to be divided and transmitted over distances of up to several tens of meters, which leads to their noticeable weakening. In addition, it is necessary to use additional RF amplifiers. In addition, not every cable is suitable for transmitting signals in the UHF range.

Another problem that arises is the supply of several signals to the TV if it has one antenna input, and therefore you need to use adders. Moreover, mutual interference often appears between the radio signals of terrestrial television stations and the radio signal from a VCR or receiver, which deteriorates the quality of reception of both.

In such cases, it is advisable to transmit video and audio signals separately at low frequencies, which will provide higher quality playback, although this will require two shielded cables. However, they can be the cheapest because the signals are transmitted at relatively low frequencies. In addition, one conductor (for an audio signal) can be low-frequency; a regular microphone cable is quite sufficient.

When there is only one source of separate video and audio signals, and there are several consumers, a splitter is needed, which is what radio amateurs themselves are asked to assemble. Its circuit diagram is shown in Fig. 1. It is made on two microcircuits. One of them (DA1) is a high-speed (1600 V/µs) two-channel powerful op-amp AD812A. Its distinctive parameters are a wide band of amplified frequencies (up to 100 MHz), linear phase-frequency characteristic, output current up to 50 mA and relatively low cost ($3...4). Such parameters make it possible to implement a video signal amplifier on it, which fits well with a coaxial cable and is capable of simultaneously working for two or three consumers. The total number of consumers can reach six (three for each channel).

From the source, the video signal is sent to the “Video Input”, to which resistor R1 is connected for matching (its value is equal to the characteristic impedance of the connecting coaxial cable). The signal arrives simultaneously on two channels of the video amplifier, and in each of them, using a trimming resistor (R3 or R4), you can obtain the required transmission coefficient. This construction allows you to individually set the most suitable video signal level for each consumer. The frequency response of both channels of the video amplifier is linear up to a frequency of 10 MHz.

The audio signal amplification path is assembled according to a similar circuit on a two-channel op-amp KR140UD20. The transmission coefficient in each channel is also set with its own trimming resistor (R5 or R6). The frequency response of both channels of the AF amplifier is linear up to 20 kHz.

All parts of the device are placed on a printed circuit board made of single-sided foil fiberglass. A drawing of printed conductors on the board is shown in Fig. 2. The appearance of the splitter is shown in Fig. 3.

In the device, instead of AD812A, it is possible to use AD817A, AD818A microcircuits (with a change in the printed circuit board). However, they are less fast and also single-channel. It is advisable to use them where the number of consumers does not exceed three. The AD813A chip, which has three op-amps in the package, is also suitable (but the printed circuit board also needs to be changed).

The tuning resistors in the device are small-sized SP3-19a, and when the board is changed, others of a larger size are also used. The remaining resistors are MLT, C2-33 (all of them, except R9 and R10, are installed perpendicular to the board). Capacitors C1, C3 - KM, KLS, K10-77, and C2, C4 - oxide K50-35 or similar.

To power the device, we use a bipolar stabilized power supply with an output voltage of 5 to 15 V. The current consumed when there is no load is approximately 15 mA, and with load it increases to 70...100 mA.

When starting to set up a splitter, you must first set the sliders of all trimming resistors to the leftmost position in the diagram, and then, with consumers connected, using resistors, achieve the most suitable signal level.

The number of device outputs can be increased to six. To do this, a resistor of the appropriate value is additionally connected to the output of each op-amp: 56 Ohm for a video amplifier, 1 kOhm for an audio signal amplifier.

Author: I.Nechaev

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