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Decoding CW (Morse code) and RTTY (radio teletype) signals

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überarbeitet am 17.10.2010

In the sixties, You needed a FSK converter for the reception and decoding of RTTY / radio teleprinter signals. In addition to this converter You need an efficient stable short wave receiver, and a loud rattling mechanical teletypewriter was attached. In the eighties, electronic decoders that were able to make the decoded text visible on a monitor or domestic television set, made their appearence.

A radio teleteletype reception system consists thus of a short wave receiver suited to single sideband reception with sufficient frequency stability, a electronic decoder and a monitor, a printer can be added to print out the received text on paper. Nowadays, the decoder and monitor can be replaced by a personal computer that can process the signal feed to the line input of the sound board.

The electronic decoders have the ability to decode CW / dot-and-dash signals, so it is helpful to decode some radio amateur's messages if You cannot read Morse code.

For radio teletyping of messages, usually the Baudot code (CCITT No. 2) is used. It is a dual tone code (signal "1" called "Mark" and signal "0" so called "space"). You should know about the speed measured in Baud (bit/second), the frequency shift (distance between the carrier signals for the two tones, usually 170, 525 or 850 cycles per second) for correct settings if the decoder. You also have to switch the correct phase position (whether the lower or higher frequency carrier in frequency modulation corresponds to "Mark" or "space").

ASCII code was used very rarely on short waves. The ASCII code with its upper and lower case letters, numbers and some special characters was particularly used in early electronic data processing systems, those ancient personal computers with information stored on cassette recorders.

A further improvement to the traditional radioteletype transmission protocols using the Baudot code were the ARQ procedures providing error correction. After every 7 impulses, the receiving station sends back a acknowledgement signal, signalig correct reception of the group.
In the SITOR-A protocol, the acknowledgement signal is sent after transmission of a three characters, this technique did clearly increase the transmission speed with cutting off on the side of reliability. On the amateur radio side, the corresponding protocol was called AMTOR ("Amateur Microprocessor Teleprinter Over Radio").
Technically the SITOR-B / FEC ("Forward Error Correction") protocol differs significantly. In this protocol, characters are sent twice and a parity bit permit error correction, without waiting for feedback from the receiving station to the transmitter and thus making two-way communication necessary. FEC-COL - transmission are addreed "to everybody", the FEC-SEL only to receiving stations equipped with a selective call system.

In the eighties, still quite a large number of press agencies and maritime stations were on the air with radio teletype transmissions. Nowadays, the majority of radio teletype transmissions consists of messages from radio amateurs and weather radio services. Dispatching of text messages in commercial short wave services became almost obsolete, email and SMS have taken it's place.