One area of amateur radio or ham radio that has grown by a significant amount since the introduction of personal computers is the use of digital modes or transmission. These digimodes as they are often called 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.
The first of the family of digimodes was RTTY. 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 digimodes have arisen. Each one of these digimodes has its own advantages and as a result different types tend to be used for different applications.
Commonly used digimodes
When talking about digimodes, references will be heard to a great variety of different types of digimode radio transmission. Many of these digimodes have arisen steadily over the years and some digimodes 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 digimodes is given below and then they are covered in greater detail later.
- RTTY - Radio Teletype
- Packet Radio
Although these digimodes 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 digimodes 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.
General properties of digimodes
Apart from the obvious fact that digimodes 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 digimodes 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.
Many of the more advanced forms of digimode transmission employ various forms of error correction. There are various forms of error correction that can be employed. Some enable errors to be detected so that a request is made to resend a particular packet or data burst. This data can then be resent until it is received correctly. Alternatively additional data can be sent to enable errors to be detected and then corrected. By using one of more of these methods, the levels of accuracy of many of these digimodes is very high. Some digimodes enable data to be sent over very marginal channels when conditions are very poor.
RTTY - Radio Teletype
radio teletype is the earliest of the digimodes. 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 digimodes that can be used, RTTY is still quite widely used, and is likely to remain so for some time.
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 digimode is more suited to VHF / UHF usage and is not normally used at HF.
The letters stand for AMateur Telex Over Radio. This digimode 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 digimode 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..
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. The scheme 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.
PACTOR is a digimode scheme that uses Frequency Shift Keying (FSK) modulation and is used primarily on the HF portion of the radio spectrum,. It 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.
While it may seem that there is an enormous variety of different digimodes 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 digimodes 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 digimode format. While this may not always be possible for every format, it at least reduces the amount of equipment required.