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One area of amateur radio that has grown by a significant amount since the introduction of personal computers is the use of digital modes of transmission.
These digital modes provide a very interesting way in which to communicate over ham radio, offering different challenges to those presented by the more traditional modes of transmission.
What can be thought of as the first of the family of digital modes was RTTY - radio teletype. Originally this used large and heavy mechanical teleprinters or teletypes. With the introduction of computers in the home, these teleprinters became redundant and it was possible to incorporate much greater levels of flexibility. This also saved a significant amount of space, a commodity often short in ham radio stations.
With the developments in computer techniques as well as transmission technology and general hardware and software it is possible to utilise many advanced techniques and as a result a variety of digital modes of transmission modes have arisen. A large number of different digital modes has grown up, and it is possible to select the right mode for a given application.
General properties of ham radio digital modes
Apart from the obvious fact that amateur radio digital modes employ digital techniques there are several other similarities between them, although there are obviously many differences as well.
One of the main similarities is that in the main they are narrow band modes, and although they are wider than CW transmissions, many of them occupy bandwidths of around 200 Hz or so. This makes these digital modes very efficient in terms of their bandwidth usage. It also has advantages in terms of their resilience to interference. Occupying a smaller bandwidth, the receiver bandwidth can be made narrower, and this means that less interference will be received, and this can be a distinct advantage when signals are low in strength or when interference levels are high.
Handshaking & error correction
One of the advantages of using digital modes is that they generally employ computer processing technology, often that provided by linking the rig or transceiver to a computer.
The first technique that is used is handshaking. This is where the transmitter sends data and the receiver acknowledges when it has arrived. To achieve this, the transmitter sends a small group of characters and when they have been successfully received, the receiver sends an acknowledgement. To check the data has been received, performs a parity check to give an indication of whether it has been received correctly.
Some systems employ more complicated forms of error detection and correction. Schemes like forward error correction, FEC may be used.
The basic concept behind FEC is that the sender encodes the message in a redundant way by using an error-correcting code.
The redundancy allows the receiver to detect a limited number of errors that may occur anywhere in the message, and often it enables the receiver to correct these errors without retransmission. In this way FEC enables the receiver to correct errors without needing a reverse channel to request retransmission of data, but at the cost of a fixed, higher forward channel bandwidth.
Commonly used digital modes
When talking about digital modes, references will be heard to a great variety of different types of digital mode radio transmission. Many of these digital modes have arisen steadily over the years and some are used more widely than others. Some have been specifically developed for particular applications and therefore they tend to be used in small areas of amateur radio.
A list of some of the more commonly used digital modes is given below and then they are covered in greater detail later.
- RTTY - Radio Teletype: Radio teletype is the earliest of the digital modes. As already mentioned it was originally typified by large mechanical teleprinters or teletypes.
Data was sent at a data rate of either 45.5 or 50 baud using a two tone scheme. On HF the carrier signal was frequency shift keyed, whereas on VHF and above an FM signal had an audio tone that was frequency shift keyed. Data was sent using the Baudot code rather than ASCII which is used for many transmissions today.
Although there are many other amateur radio digital modes that can be used, RTTY is still quite widely used, and is likely to remain so for some time.
- Packet Radio: As the name implies, packet radio sends data out in packets. Once received, the receiving station checks that the data has been received correctly before allowing the next packet to be sent. If errors are detected the packet can be resent.
The packet radio system allows a number of other facilities such as digipeaters to relay messages and the use of mailboxes, etc. In view of the length of the packets this form of digital mode is more suited to VHF / UHF usage and is not normally used at HF.
- AMTOR: The letters stand for AMateur Telex Over Radio. This digital mode was one of the first computer style modes to be used at HF. Data is sent out in small groups and when acknowledgements are received the next small group is sent.
As data is sent out in small bursts, this mode is far more suited to HF operation where it finds most of its use. This form of digital mode is typically used at HF, and although once one of the main forms of digimode used on the HF bands, its use is giving way to more sophisticated forms of digital mode such as PSK31.
Read more about . AMTOR.
- PSK31: PSK31 derives its name from the modulation format and rate used. The modulation used is phase shift keying (PSK) and it transmits data at a rate of 31.25 bits per second - the rate being chosen to enable the rate to be easily derived from the 8 kHz sampling used in many digital signal processors.
This amateur radio digital mode is widely used on HF and is resilient to interference. It allows real-time "chat" style contacts to be made and in view of all its advantages it has gained widespread acceptance.
Read more about . PSK31.
- PACTOR: PACTOR is a digital mode scheme that uses Frequency Shift Keying (FSK) modulation and is used primarily on the HF portion of the radio spectrum.
PACTOR is a digital mode that combines elements of AMTOR and packet radio and this gives rise to the name PACTOR. It was developed in order to improve the reception of digital data when the received signal was weak or noisy.
PACTOR combines the bandwidth efficiency of packet radio with the error-correction and automatic repeat request of AMTOR. Since it was first released there have been developments and PACTOR II and PACTOR III are now available.
Read more about . PACTOR.
- Clover: CLOVER is a digital communications mode that is particularly suited for narrow band HF communications. One of its key features is that it monitors the link between the transmitter and receiver and adapts the modulation format accordingly. CLOVER sends 8-bit digital data and is able to carry ASCII text as well as executable computer files without using the additional control characters required in other digital modes, which decrease throughput.
CLOVER uses Reed-Solomon data encoding to provide forward error correction (FEC) within each data block to repair many errors without the need for retransmission.
- MFSK-16: MFSK is an amateur radio digital mode that has been designed to provide an easy-to-use chat mode for real-time amateur contacts, nets and bulletin transmissions although it is not really suitable for contesting or Bulletin Board System use.
The letters MFSK stand for multi frequency shift keying, and this gives an insight into its operation. It takes the two tone concept of RTTY and expands this to several tones and this makes for a system that is more robust than even PSK31 in the face of interference.
MFSK16 is a half-duplex using a non-Automatic Repeat ReQuest, ARQ forward-error-correcting, FEC mode. In view of all the error protection, it performs well in conditions where there is fading or interference.
- WSJT: The full name for WSJT is Weak Signal communication by K1JT. As the name implies it is a form of digital communication mode optimised for weak signal communication. Originally developed by K1JT, the software is now open source and its development has continued and it is now managed by a small team of radio amateurs.
WSJT incorporates a number of modes within the overall specification for specific applications. These variants include: FSK441, JT6M, JT65, JT4, FT8 and others. Interestingly the take up of FT8 has been very rapid as its advantages for HF long distance communications have become very apparent.
- WSPR: WSPR is a software set that is used for weak signal communication between stations. In fact the letters WSPR stand for Weak Signal Propagation Reporter.
WSPR is a software set initially developed by Joe Taylor, K1JT, although now it is now open source and managed by a small team of radio amateurs.
Essentially the programme is designed for sending and receiving low-power transmissions to test propagation paths on the MF and HF bands. To this end it implements a protocol designed for probing potential propagation paths with low-power transmissions. Transmissions carry the station callsign, Maidenhead grid locator, and transmitter power in dBm. The program can decode signals with S/N as low as -28 dB in a 2.5 kHz bandwidth. Stations with Internet access can upload their reception reports to a central database called WSPRnet which includes a mapping facility. In this way propagation paths that may be open can be seen and used.
Read more about . WSPR.
Although these amateur radio digital are mentioned in the list others are being developed and introduced, however these are some of the most popular types used in amateur radio. Although it may seem that there is a very large number of digital modes available, it is possible to use a single PC to accommodate all them and this means that it is relatively simple to switch between them, and large amounts of equipment are not needed.
While it may seem that there is an enormous variety of different digital modes that can be used, this results from the fact that people are experimenting and trying to improve the effectiveness of digital modes of transmission as technology moves forward. While there are many digital modes to choose from, the situation may not be as difficult as many may imagine because it is often possible to receive and send a large number of types of transmission using a single interface. Simply by using different software, it is possible to receive and send the required digital mode format. While this may not always be possible for every format, it at least reduces the amount of equipment required.