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26-2 The Morse code word eat as sent on CW.
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teleprinter machines than on-off switching. That is to shift the frequency of the carrier wave back and forth. It is called frequency-shift keying (FSK).
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The Morse code is not the only digital code of its kind. There are two common teleprinter codes used to send and receive radioteletype (RTTY) signals. These codes are known as the Baudot (pronounced Bo-doe ) and ASCII (pronounced ask-ee ) codes. (You needn t worry about where these names come from.) A carrier wave can be keyed on and off using either of these codes, at speeds ranging from 60 wpm to over 1000 wpm. In recent years, ASCII has been replacing Baudot as the standard teleprinter code. A special circuit, called a terminal unit, converts RTTY signals into electrical impulses to work a teleprinter or to display the characters on a monitor screen. The terminal unit also generates the signals necessary to send RTTY, as the operator types on the keyboard of a teleprinter terminal. A personal computer can be made to work as an RTTY terminal by means of terminal emulation software. This software is available in several different forms, and is popular among radio amateurs and electronics hobbyists.
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The trouble with using simple on-off keying for RTTY is that noise pulses, such as thunderstorm static crashes, can be interpreted by the terminal unit as signal pulses. This causes misprints. There is no problem if a crash takes place during the full-on, or mark, part of the signal; but if it happens during a pause or space interval, the terminal unit can be fooled into thinking it s a mark pulse instead. This problem can be helped greatly by sending a signal during the space part of the signal, but at a different frequency from the mark pulse. Then the terminal unit knows for sure that a mark is not being sent. Instead of sending Not mark, the transmitter sends Not mark, but space instead. The easiest way to do this is to send the mark part of the signal at one carrier frequency, and the space part at another frequency a few hundred hertz higher or lower. This is FSK. The difference between the mark and space frequencies is called the shift, and is usually between 100 Hz and 1 kHz. A frequency-versus-time graph of the Morse code word eat, sent using FSK, is shown in Fig. 26-3. Normally, Baudot or ASCII, rather than Morse, is used for teleprinter operation. A block diagram of an FSK transmitter is shown in Fig. 26-4. The FSK mode,
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Frequency-shift keying 477 like on-off code keying, is a digital form of communications. But unlike on-off Morse keying, FSK is frequency modulation (FM).
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26-3 The Morse code word eat as sent using FSK.
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26-4 A simple FSK transmitter.
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The telephone modem
Teleprinter data can be sent over the telephone lines using FSK entirely within the audio range. Two audio tones are generated, one for mark and the other for space. There are three sets of standard tone frequencies: 1200 Hz and 2200 Hz for general communications, 1070 Hz and 1270 Hz for message origination, and 2025 and 2225 Hz for answering. These represent shifts of 1000 Hz or 200 Hz. Because this FSK takes place at audio, it is sometimes called audio-frequency-shift keying (AFSK). A device that sends and receives AFSK teleprinter over the phone lines is known as a telephone modem. If you ve used a personal computer via the phone lines, you ve used a telephone modem. Perhaps you ve heard the bleep-bleep of the tones as data is sent or received.
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Amplitude modulation for voice
A voice signal is a complex waveform with frequencies mostly in the range 300 Hz to 3 kHz. Direct currents can be varied, or modulated, by these waveforms, thereby transmitting voice information over wires. This is how early telephones worked. Around 1920, when CW oscillators were developed to replace spark-gap transmitters, engineers wondered, Can a radio wave be modulated with a voice, like dc in a telephone If so, voices could be sent by wireless. Radio communications via Morse code was being done over thousands of miles, but CW was slow. The idea of sending voices was fascinating, and engineers set about to find a way to do it.
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