Telegraphy in Electronic Communications:
Telegraphy in Electronic Communications that employs typewriter like machines operating at a maximum speed of about 60 words per minute (wpm) to send written messages from one point to another. In telegraphy, a user lodges a written message for transmission at a telegraph or post office. The message is subsequently transmitted to the office nearest to the addressee, and delivered in typewritten form, perhaps after having been first telephoned through, if that was part of the request. Telex combines the above system with subscriber dialing techniques. The originator of the message composes the address of the message and then its text and sends the message, which is automatically routed to the wanted subscriber, and printed out by machine. Thus the only difference between telex and telegraphy is in the signaling method and the actual sending procedure. All other aspects of the two systems are identical and will be considered together under the general title of telegraphy in this section. Note that Telegraphy in Electronic Communications and telex, together with facsimile and similar systems, are often collectively referred to as record services. This is because they provide a printed record, unlike telephony.
General System Description:
In either Telegraphy in Electronic Communications or telex, the transmitting teletypewriter produces a set of coded pulses when a given key is tapped. In the receiving mode, the same machine prints out the appropriate letter when a given code is received. A message is typed out at one end of a link and is printed out at the other end, either simultaneously or very soon afterward. The transmitting typewriter also prints a copy of what it sends, for checking and filing. Teletypewriters may be of the page-printer or tape-printer form. The former, perhaps the more common of the two, prints on a role of paper having much the same width as typing paper.
To avoid limitations due to the maximum typing speed and efficiency of the operator, there is an alternative system in which the message is typed out in advance on perforated tape or in a word processor, at any convenient speed. It can then be transmitted at maximum speed, so that the capacity of the channel and the machine are not wasted.
In the code used for teletype, each character has five binary elements, so that the code either resembles or (what is more likely) is identical to the CCITT-2-code. The only exception to this is that most HF radio links use the ARQ code. In practice, each element, i.e., each mark or space, occupies 22 ms. In addition each letter is preceded by a 22-ms space and followed by a 31-ms mark, so that each letter occupies approximately 7 1/2 elements, or 163 ms. Since the average telegraphic word is considered as consisting of six letters, it is seen that each word takes 1 s to transmit on the average; hence the 60-wpm limit mentioned earlier.
From the examples so far given, it would be excusable to think that simple AM is used for all radiotelegraphy. Nothing could be further from the truth. AM is a good system to use for illustrations and examples, but it is not a very good system for transmitting telegraphy over long distances.
Frequency Shift Keying (FSK):
It would be quite possible to transmit teletype by the ordinary ON-OFF keying of the transmitter. We could use amplitude modulation with pulses, ON corresponding to mark and OFF to space. Such a system has the inherent disadvantage that there is no real indication for the space. In addition, a system such as this would suffer from all the usual ailments of amplitude modulation, as a result of which it is never used for automatic telegraphy (it is, of course, widely used for manual Morse code CW operation). A system known as frequency-shift keying is generally used instead.
FSK is a system of frequency modulation. In it, the nominal unmodulated carrier frequency corresponds to the mark condition, and a space is represented by a downward frequency shift. The amount was 850 Hz in the original wideband FSK system designed for HF radio. For transmission by line or broadband systems, the current shift is 60 Hz, as laid down in CCITT Rec. R35. This is known as narrowband FSK, or frequency-modulated voice-frequency telegraph (FMVFT). FSK is still often used for HF radio transmissions, with a frequency shift that is commonly 170 Hz. As with other forms of FM, the main advantage of the wideband system is greater noise immunity, while the narrowband systems are used to conserve the allocated frequency spectrum. Note that FSK may be thought of as an FM system in which the carrier frequency is midway between the mark and space frequencies, and modulation is by a square wave. In practice, of course, only the fundamental frequency of the square wave is transmitted, and regeneration takes place in the receiver.
In the FSK generator, the frequency shift may be obtained by applying the varying dc output of the telegraph machine to a varactor diode in a crystal oscillator. At the receiving end, the signal is demultiplexed (if, as is common, FDM was used to send a number of telegraph or telex transmissions together) and applied to a standard phase discriminator. From the discriminator, signals of either polarity will be available. After some pulse shaping, they are applied to the receiving teletypewriter. If the telegraph transmission is by HF radio, the phase discriminator works at a (fairly low) intermediate frequency, although other methods are also possible for demodulation. An amplitude limiter is always used in the receiver, to take full advantage of the noise immunity of FSK.
Other Transmission Methods:
ON-OFF keying of the transmitter is sometimes used at MF but generally only for Morse code. In this system, carrier is present for a mark, and absent for a space. Some HF transmissions may still use two-tone modulation. This is an AM system, in which the carrier is modulated with one tone for a mark, and another audio for a space. In such HF telegraph transmissions, the two tones are generally 170 Hz apart. Neither system has the noise immunity of FSK.
Phase-shift keying (PSK) is a relatively new system, in which the carrier may be phase-shifted by +90° for a mark, and by – 90° for a space. In four-phase, or quadrature PSK, system, the possible phase shifts are + 135°, + 45°, – 45° and – 135°, so that 2 bits of information can be indicated instead of one as in the other systems. PSK has a number of similarities to FSK.
Multiplexing:
Where a telegraph transmission is sent over a single wire pair, or as the only modulation on a radio transmitter, no multiplexing is necessary. However, it is far more common to send telegraphy (and telex) over channels designed for telephony, where the available bandwidth is 300 to 3400 Hz. This is far more than an FMVFT channel requires, and so frequency-division multiplex, similar to the systems described is often used. For broadband systems, the subdivision of the telephone channel is governed by the previously mentioned CCITT Rec. R35. This allocates 120 Hz to each telegraph channel, so that 24 channels can be fitted into the telephone bandwidth. The nominal carrier frequency is 420 Hz for the lowest-frequency channel, while channel 24 is allocated a nominal carrier frequency of 3180 Hz. The 120-Hz bandwidth allocated to each channel is quite adequate for the 60-Hz frequency shift and leaves a further 60 Hz as a guard band.
Time-division multiplex systems for Telegraphy in Electronic Communications are now becoming widespread As explained earlier, pulses from various transmissions are interleaved in time, and here they happen to be pulses for telegraph transmitters. TDM systems naturally lend themselves to being used with other pulse-code transmissions, in wholly digital systems. It is expected that as many as 128 simultaneous telegraph transmissions, time-division multiplexed, could eventually be sent over one telephone channel.
Telegraph Speeds:
The maximum rate at which intelligence can be transmitted over a telegraph circuit is naturally proportional to the bandwidth of the circuit. The bandwidth, in turn, is governed by the duration of the shortest signal element and is inversely proportional to it. This comes about because bandwidth is dependent on the pulse repetition rate, and obviously the shorter each pulse, the greater the repetition rate per second. As a result of all these considerations, telegraph speed is expressed numerically as the reciprocal of the duration in seconds of the shortest signaling element. The baud is the name given to the unit of telegraphic speed. Thus the speed in bauds expresses the numbers of the shortest elements of signal that may occur per second. For the system so far described, in which each short element occupies 22 ms, the speed in bands is
where
L is the duration of the shortest signal element.