Data or DIGI modes
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.
RTTY - Radio Teletype
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.
Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time tactical digital communications of information of immediate value in the local area. In addition, all such data are ingested into the APRS Internet System (APRS-IS) and distributed globally for ubiquitous and immediate access.
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.
slow-scan television (SSTV)
Slow-scan television (SSTV) is a mode of video communications in which a sequence of fixed images is sent and received at intervals of several seconds. SSTV is practical for transmission of images over plain old telephone service ( POTS ) lines and in other applications where the available bandwidth is severely limited. The bandwidth of an SSTV signal is comparable to that of a voice signal in analog telephone or wireless communications (3 KHz or less).
SSTV was used to send the first video images back to earth from the Apollo 11 moon mission. SSTV is sometimes used in medical imaging, security systems, and in the remote monitoring of hazardous equipment or natural phenomena. Amateur radio operators occasionally use SSTV in their communications. Most SSTV communication is done in grayscale . However, color images can be sent and received in SSTV mode with some sacrifice in resolution , or a longer interval between images, or both.
An SSTV signal consists of a rapid sequence of audio tones having variable pitch. When heard directly, it has a characteristic warbling sound. An SSTV communications station consists of a telephone connection or radio transceiver, a personal computer, a program called a scan converter , and a video camera. The scan converter employs two data converters, one for receiving (by means of the microphone or line input port) and the other for transmitting (by means of the speaker or line output port). The scan converter changes incoming SSTV tones into images suitable for viewing on the computer. These images can be viewed in real time , stored individually, or stored as a sequence of video files. The scan converter also changes video data from the camera to audio tones that are fed to the telephone set or the microphone input of the radio transceiver.
DRM Digital Radio Mondiale (DIGISSTV)
THE DRM mode has become very popular. Why has the DRM mode become so popular? It takes no (or an insignificant) amount of time to decode/encode. This is not like RDFT. Everyone likes the decoding on the fly so that you can see instantly how the picture came through. The file data is sent faster, 3 times faster than RDFT and 2 times faster than DIGPAL. Now small standard Jpeg and Gif files can be sent in their original size. DRM allows larger files to be sent in less time which means better quality pictures in about the same amount of time. If you are not convinced that the picture quality of Digital SSTV is not better than analogue SSTV, then you should see "Analogue vs. Digital Test" on web page http://kb4yz.ham.org. When using DRM your ID (callsign) is sent continually. This would allow others to identify the transmitting station and turn an antenna for better reception. It allows viewing images with missing data (blocks) or progressively viewing as the data is received. This is somewhat like analogue SSTV. There are no critical periods. You could miss the start or end of a DRM transmission and still have enough data to be useful. Without the problem of "Bad block Zero", it is possible to expect even large files to make it through in spite of poor band conditions. A replay is exactly like the original. It would have the same file name, and the same file size.
There is also no picture slant adjustment required with digital TV modes which is often necessary with analogue SSTV.