Reception of CW (Morse) and radio teletype signals
In the sixties, for the reception of Radio teletype (RTTY) signals, not only a powerful shortwave receiver was needed, but also a FSK converter connected to a noisy rattling electric teletypewriter was needed.
In the eighties, automatic decoders became available: These could decode the text of a morse code or radio teletype transmission and display it on a monitor or a simple domestic television set.
A contemporary radioteletype receiving system consists of a shortwave receiver with sufficient stability and capability for SSB reception, a decoder and a monitor, and possibly a printer for printing out the received message.
For radioteletype (RTTY), usually the Baudot code (CCITT No. 2) is used. With this two-tone mode (signal level „1“ or „Mark“ and signal level „0“ or „Space“), several paramters have to be set correctly on the decoder. The speed measured in baud (bit/sec.), the frequency shift (distance between the two tone frequencies, usually 170, 425 or 850 Hz) and the correct phase (whether „Mark“ or „Space“ is assigned to the lower of the two frequencies) has be set on the decoder.
ASCII transmissions were only rarely used on shortwaves. The ASCII code includes letters in upper and lower case, numbers and some special characters. This code is mainly known from its use in electronic data processing.
Further developments to the usual Baudot transmission mode were the ARQ procedures: Error-correcting transmission modes in which acknowledgement characters or repeat requests (RQ) are returned from the receiving station to the transmitter site. With the SITOR-A method, an acknowledgement signal is expected from the remote receiving station only after the transmission of a block of three characters, which significantly increases the transmission speed while maintaining the same reliability; in amateur radio, AMTOR-A („Amateur Microprocessor Teleprinter Over Radio“) is used as an equivalent.
Technically, the FEC („Forward Error Correction“) mode differs significantly. Here, characters are transmitted twice and a check bit allows error correction without the need for feedback to the transmitting station and thus a two-way communication / duplex connection. FEC-COL - Transmissions are directed „to all“, FEC-SEL is directed to a single remote station equipped with a selective call system. SITOR-B is an FEC method for transmission to different receivers.
In the 1980s, a large number of press agencies were still using radio teletype transmissions, but today radio amateurs and reports from weather radio services account for the majority of radio teletype transmissions. The sending of text messages in commercial shortwave traffic became almost obsolete after the establishment of email and SMS traffic.
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