Digital Communications

Question File Number 30



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Digital Communications

Contributed by Murray Greenman ZL1BPU

The original digital means of electrical communication was the Morse code. It is still in use today as a very successful method for transferring information by means other than voice. Today Morse has been joined by some other methods each with its own advantages and disadvantages.

RTTY, AMTOR, PACTOR, PSK31, Packet Radio and other modes have all been given a great boost with the arrival of the computer as a generally available appliance. In fact some of the new modes would be impossible without the computer and the PC sound card. The advent of satellites with store and forward facilities has also enhanced digital operations.

Digital

It is now possible to pass information to many parts of the world with a hand-held transceiver, modem, and computer, and also to have real-time conversations around the world using an HF radio and a computer. Each of these means of digital communication has its own protocol.

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How Digital Modes are Generated

Two common digital coding schemes are used; the ITU-R ITA2 alphabet, (often misnamed the Baudot code), and the ITU-R ITA5 alphabet (or ASCII - American Standard Code for Information Interchange). ITA2 codes each character as a number between 0 and 31 to represent the various letters, digits and punctuation marks. To fit more than 32 different characters into the code, most numbers are used twice, and a special character (a shift character) is used to switch between the two meanings. The number can be represented by a 5-digit binary number (e.g. 14 = 01110 in binary). RTTY is one of the few systems that use the ITA2 alphabet today.

The ITA5 alphabet has 128 combinations, so a comprehensive alphabet, including lower and upper case letters, can be represented in seven binary bits. ITA-5 is used by PACTOR, packet radio and many other modes. Some digital modes (such as Morse!) use a scheme called a Varicode where the different characters are represented as numbers of different lengths. If the more frequently used characters are shorter, the transmission of plain text is therefore more efficient.

The numbers to be transmitted must then be modulated onto a radio signal in some way. There are three main properties of a radio signal; frequency, phase and strength (amplitude), so there are three common modulation methods, and some modes use a combination of two or more of these. Many modes are transmitted using Frequency Shift Keying (FSK). This in principle consists of switching between two adjacent frequencies which are used to designate the 0 or 1 data bits. The two tones must maintain a fixed frequency separation or shift and of course the radio frequencies must also be stable. The most common shifts used by amateurs on HF for FSK are 170 Hz and 200 Hz. Wider shifts are used on VHF where data rates and signal bandwidths can be higher. Other modes use more tones (Multiple Frequency Shift Keying, MFSK), or one of the other techniques, such as Phase Shift Keying (PSK), where the phase of the tone or carrier is varied, or Amplitude Shift Keying (ASK), where the signal strength is varied or even keyed on and off.

To send a character over the radio, one bit (binary digit), 0 or 1, is assigned to one of two states, or if there are more than two possible states (say if there are four tones or four PSK phases), then two or more bits at a time may define the state to transmit. The data changes the properties of the signal to be transmitted (i.e. modulates the signal), as each state is fed successively to the transmitter modulator, to define and transmit each symbol.

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For the receiving end to be able to accurately decode the characters sent, the bits must be sent at a constant speed. The signalling speed of serial data transmissions on wires is measured in bits per second (bps), since the bits are always sent one at a time. However, the signalling speed on a radio link is not measured in bits, but in symbols per second (the unit of symbols/sec is the baud). The symbol is the basic modulated signalling entity on a radio link, and represents the state of each signalling interval. Each symbol may carry one or more (or even less) data bits, depending on the modulation technique. For RTTY, each symbol (a short duration of one tone or another) carries one data bit, so the speed in bps is the same as the baud rate.

The device that produces a modulated tone symbol for each data state, or creates a data state for each received tone symbol, is called a modem (a modulator /demodulator). The modem may be a special separate unit rather like a telephone modem, or sometimes the modulation is performed directly on a transmitter oscillator or a modulator, and a separate modem device may not be necessary except perhaps for receiving. Equally, the function of a modem now often takes place in a computer sound card, with the signals fed from it and to it by an SSB transceiver.

RTTY (Radio TeleTYpe) is one of the oldest of the machine-generated digital modes. It does not necessarily require a computer, as it is simple enough to be handled by a mechanical device similar to a typewriter - a teleprinter. RTTY, like most other digital modes, works by encoding characters into a digital alphabet.

Common speeds used by amateurs for RTTY are: 45.5, 50 and 75 baud, equivalent to 60 wpm, 66 wpm, and 100 wpm. (There are five letters and a space in the average word).

AMTOR is a form of RTTY, now little used, that uses error checking to ensure that the data sent is received correctly. The message being sent is broken up into groups of three characters each. A special alphabet is used which has seven bits per character; every valid character always has a 4:3 ratio of 0s and 1s. This small packet is then transmitted through the modem to the radio. AMTOR always operates at 100 baud and uses 170 Hz shift FSK.

The system can operate in two modes, mode A and mode B. Mode A uses Automatic Repeat Request (ARQ) to ask the sending station to resend any packets that are not received properly (correct 4:3 ratio) once contact is established. Mode B sends the data twice, and checks the data but will not ask for a repeat. It is used for establishing contact (i.e. calling CQ) and for net and bulletin transmissions.

Packet Radio is an ARQ system like AMTOR, but with more powerful error checking and message handling abilities. Larger packets are used, and encoded in each packet are the sender and destination addresses, and a very efficient error detection scheme called a Cyclic Redundancy Check (CRC).

The Packet protocol allows a limited number of stations to carry on independent conversations on the same frequency without interference. The effective communication rate will be reduced if many stations are using the same frequency and excessive packet collisions occur.

Packets are assembled and prepared for transmission by a Terminal Node Controller (TNC), which manages the packet radio protocol and also contains a modem. The individual characters are usually in the ASCII alphabet, and a packet protocol called AX25 is usually used. The assembled packet is then passed to the modem and a radio in the same way as AMTOR or PACTOR.

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Packet radio allows automated message forwarding throughout the world. Most activity is on VHF and higher bands where more stable propagation prevails and FM transmitters and receivers are used.

Large cities are centres of activity and cities are connected to each other by a series of relay stations. For longer distances the cities are connected by HF links (using PACTOR) or via internet or satellite gateways. Store-and-forward relaying is used. Most cities have a Bulletin Board System (BBS) for packet radio users. These can be used for the circulation of amateur radio information. They can be accessed by stations comprising a home computer, a simple modem and a VHF FM transceiver.

Another popular application of Packet Radio and AX25 is a telemetry technique sometimes called the Automatic Position Reporting System (APRS), although it is used for much more than reporting position. Stations with information to pass on send regular standard format messages in the manner of a beacon, which can be retransmitted by other stations. Applications of this type do not use bi-directional error correction, but do use automatic forwarding much the same as conventional packet systems.

PACTOR is derived from AMTOR. Like AMTOR it is a two-way error correcting system, but PACTOR dynamically adapts to conditions, switching from 100 baud to 200 baud. PACTOR can accept a series of imperfect data packets and reassemble them into the correct text. A recent version of PACTOR, called PACTOR II, uses the same protocol, but uses PSK modulation for even higher performance.

PSK31 is the most popular of the new digital modes. It is used like RTTY, for live keyboard-to-keyboard contacts. It uses differential binary PSK modulation at 31.25 baud. It is easy to tune in and to operate. The signal is very narrow (only 50Hz) and the performance very good, due to the high sensitivity and noise rejection of the PSK technique. PSK31 uses advanced digital signal processing (DSP), and can be run on many computer platforms, including Windows with a SoundBlaster type soundcard. The software is available free.

All you need to get going is a stable HF SSB Transceiver of conventional design, and a computer with a soundcard. You run two shielded audio cables between the rig and the sound card. The computer with its soundcard does the job of the modem. You can download FREE software from a web page. When all is set up, you have a live-keyboard system for chatting with other HF stations around the world. This is a really exciting mode. You can get further details about PSK31 from: http://aintel.bi.ehu.es/psk31.html

Other modes: There are numerous other digital modes in use, and more being introduced all the time. Many of these are designed for specific applications. For example, MFSK16 was designed for very long distance low power real-time conversations, and also is most effective on lower bands with strong multi-path reception and burst noise. CLOVER is an ARQ mode designed for reliable long distance file transfer under poor conditions, while MT63 was designed for net operation under severe interference. Some of these modes use interesting modulation methods such as single or multi-carrier Binary Phase Shift Keying BPSK, Quadrature Phase Shift Keying QPSK, or Orthogonal Frequency Division Multiplex OFDM. There are even special modes for moon-bounce, auroral signals, very weak LF comunications and satellite operation. Many of these new modes also use a simple sound card modem and free software.

Don't overlook Hellschreiber. This is a mode with an interesting history. Hellschreiber is a method for sending text by radio or telephone line that involves dividing each text character into little pieces and sending them as dots. Hellschreiber was invented by the German inventor, Rudolf Hell who patented Hellschreiber in 1929.

The same SSB transceiver and computer set-up used for PSK31 can be used for Hellschreiber. Most Hellschreiber operation uses ASK modulation at 122.5 baud. You can check out the world of Hell on the web site at: http://www.qsl.net/zl1bpu and download the latest Hell software from there.

Hellschreiber is becoming popular with HF digital operators, as it provides very good performance with simple equipment and is easy to use. Its application is as a point-to-point mode for live contacts in a similar way to RTTY and PSK31. Modern variations such as PSK-Hell and FM-Hell provide even better performance with features to overcome specific ionospheric limitations of other digital modes.

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Digital Modes and Propagation

While sensitivity and therefore rejection of Broadband Noise is an important property of digital modes, there are other specific ionospheric problems that affect digital modes more than is apparent on either Morse or voice modes. Burst Noise (electrical machinery, lightning) causes errors, interferes with synchronisation of data modes and impedes error correction systems, while Carrier Interference, (TV and mains harmonics, other radio transmissions) will obviously impair reception of most modes.

There are two other effects which are not so obvious. Multi-path Reception, where the signal arrives from different paths through the ionosphere with different time delays, can have a devastating effect on digital modes such as RTTY, that no increase in transmitter power will correct. The best solution to this problem is to use a mode with a very low baud rate, such as MFSK16 or MT63, to limit the timing errors. Doppler Modulation, caused mostly by fast moving air streams in the ionosphere or the movement of the apparent reflective height through changes in ion density, also has a serious effect, changing especially the phase and even the frequency of signals. This is best countered by using higher baud rates, or avoiding PSK modes. Doppler can be a big problem with long distance PSK31 operation.

Because the requirements for best performance conflict to some extent, and there is no one mode which will defeat all the problems, however in all cases the use of an effective error correction system (designed for the conditions) will provide significant improvements. The best solution is to choose an appropriate mode for the conditions prevailing at the time.

ZL1BPU writes regular feature articles for the NZART Journal Break-In



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Further details about NZART can be obtained from the web at  NZART. There are several categories of NZART membership which include Transmitting and Non-Transmitting. Anyone interested in radio can join. E-mail enquiries to  nzart@nzart.org.nz will bring details about NZART Membership. On-line details about  Joining NZART - Membership has advantages.
Contact Webmaster at e-mail  webmaster@nzart.org.nz
Compiled Sun Nov 28 2010 at 8:41:44pm


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