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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Modern (and even not very modern) computers make it possible to simplify to the limit the equipment for conducting radio communications with digital modes of operation (RTTY-BAUDOT, RTTYASCII, AMTOR, PACKET), as well as for receiving and transmitting images (SSTV, WEFAX). For these purposes, even a computer with a 386th processor, VGA graphics, a color monitor and DOS, starting with version 3.0, is suitable. To get high-quality color images in SSTV, it is desirable, of course, to have more advanced equipment: an SVGA monitor (with a resolution of at least 800x600 and 256 colors), as well as a good video card. The computer takes on almost the entire signal processing load, leaving the hardware only to match the levels of the transceiver's input and output audio signals with the levels on the computer's serial port buses - COM1 or COM2 (RS-232).

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A diagram of the simplest set-top box that allows you to receive (if the appropriate software is available) signals from radio stations using the listed types of work is shown in the figure. The device is an amplifier-limiter on the op amp DA1. The operational amplifier is not covered by negative feedback, therefore (depending on the polarity of the input), its output signal will either be close to the supply voltage of the positive source or negative. The problem of power supply is solved in the set-top box in an extremely simple way - it is obtained by rectifying the pulse signals available on the port buses (DTR and RTS).

The set-top box is fed a signal from the output of the UHF receiver or transceiver. Connecting the output of the set-top box to the port depends on which of the two possible connector options is used on it - 25-pin or 9-pin. In the figure, in brackets after the designation of the signals, the corresponding pin numbers are given (the first digits are for the 25-pin connector).

In all the above types of work, frequency shift keying (modulation) is used. The computer analyzes the frequencies of the signal coming from the receiver and translates them into appropriate digital data. Signal processing is carried out by programs, of which the most widely used are HAMCOM 3.1 (used to receive BUADOT, ASCII, AMTOR, PACTOR and even regular CW) and JVFAX71 (SSTV, WEFAX). For packet communication, the PKTMON12 program is usually used. These programs are distributed as SHAREWARE.

See other articles Section Civil radio communications.

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