Digital communications refers to the transmission of digital data, in contrast to analog communications.
Analog communications use a continuously varying signal, a digital transmission can be broken down into discrete messages. Transmitting data in discrete messages allows for greater signal processing capability. The ability to process a communications signal means that errors caused by random processes can be detected and corrected. Digital signals can also be sampled instead of continuously monitored and multiple signals can be multiplexed together to form one signal.
Because of all these advantages, and recent advances in wideband communication channels and solid-state electronics, scientists have fully realised these transmission systems. Digital communication has grown quickly in the commercial world, edging out analog communication, because of the vast demand to transmit computer data and the ability of digital communications to do so. Digital communication has always had a place in Amateur Radio.
It is often overlooked as being a digital mode but CW (Morse Code) is really the first digital mode utilised in radio communications. From the birth of radio until just after the second world war CW was the only digital mode in common use.
The end of the second world war saw a large amount of surplus military teletype equipment available so radio teletype (RTTY) became very popular.
This changed little until the 1970's when the very first personal computers arrived so noisy teletype machines were replaced with keyboards and screens. This enabled amateurs such as Peter Martinez G3PLX to devise a new system for transmitting data over radio, called AMTOR(AMateur Teletype Over Radio).
AMTOR was loosely based on a commercial protocol called SITOR and for the first time, allowed very simple error detection techniques to be incorporated into the data stream.
Throughout the 70's development progressed across the world with different digital modes until 1982 when the X.25 packet radio standards were finalised. Development has continued with further enhancements to AMTOR and the new PACTOR and CLOVER modes were introduced.
Summary of digital modes:
- AMTOR.
Hear what it sounds like.- Seven pulses (four marks and three spaces representing letters, numbers and symbols).
- Modulated by frequency shift keying.
- Mode A:
- ARQ - Automatic Repeat reQuest (repeat characters only sent at the request of the receiving station).
- Mode B:
-
FEC - Forward Error Correction (each character sent twice at all times)
- CLOVER. Hear what it sounds like.
- Conceived by Ray Petit, W7GHM in 1987.
- Based on Coherent CW (CCW) where narrow band CW of 12Hz to 15Hz bandwidth is transmitted at 12 wpm Data sent as 8ms pulses every 32ms.
- Highly complex modulation method using phase and amplitude modulation that is varied depending on band conditions.
- Length of data blocks also varied depending on band conditions.
- Error detection and error correction provided by Reed-Solomon coding.
- CCW (Coherent Continuous Wave).
- Both stations synchronised by a precise clock with all symbols beginning at a known point in time.
- CW (Continuous / Carrier Wave).
- On-Off keying. One example being Morse Code.
- D-STAR.
- Is an open protocol, available to be implemented by anyone.
- Supports two types of digital streams:
- Digital Voice (DV) stream used on 144 and 440 MHz contains both digitised voice (3600 bps including error correction) and digital data (1200 bps).
- Digital Data (DD) stream, used only on 1.2 GHz, is entirely data with a bit rate of 128k bps.
See also: Glossary appendix entry.
- G-TOR. Hear what it sounds like.
- HELLSCHREIBER. Hear what it sounds like.
- Uses facsimile technology.
- Developed by Germany prior to World War II!
- Recent use of PC sound cards as DSP units has increased the interest in this mode.
- The single-tone version (Feld-Hell) is the method of choice for HF operation.
- It is an on-off keyed system with 122.5 dots/second, or about a 35 wpm text rate, with a narrow bandwidth (about 75 Hz).
- Text characters were originally printed in 3 identical diagonal lines on a narrow paper tape so synchronisation was not a problem.
- Today they are 'painted' on the screen, as apposed to being decoded and printed.
- High Speed CW.
- Utilised during the Second World War to reduce the time for a transmission to take place and hence defeat direction finding techniques.
- Message is recorded manually onto magnetic tape and played back at 300 wpm for transmission.
- Packet Radio (X.25). Hear what it sounds like.
- First true amateur digital protocol.
- Data sent in blocks called frames.
- Checksum error detection methods.
- Uses digipeaters to cover longer distances.
- Possible to utilise Bulletin Board Systems (BBS) and mailboxes.
- Requires a terminal unit / modem.
- PACTOR. Hear what it sounds like.
- PACTOR II. Hear what it sounds like.
- A robust and powerful PSK mode which operates well under varying conditions.
- It uses strong logic and automatic frequency tracking.
- DSP based and as much as 8 times faster than PACTOR.
- Compatible with PACTOR.
- Like GTOR, it is a proprietary mode owned by SCS and only available with their line of multi-mode TNC controllers.
- Project 25 (P25).
- Standards produced through the joint efforts of the Association of Public Safety Communications
Officials International (APCO), the National Association of State Telecommunications Directors (NASTD), selected Federal Agencies and the National Communications System (NCS), and standardized under the Telecommunications Industry Association (TIA).
- Open architecture, user driven suite of system standards.
- P25 compliant radios can communicate in analog mode with legacy radios and in either digital or analog mode with other P25 radios.
- The P25 standard exists
in the public domain, allowing any manufacturer to produce a P25 compatible radio product.
- Developed primarily for North American public safety services, but not limited to public safety alone.
- May be deployed in other private system applications worldwide.
See also: Glossary appendix entry.
- PSK31. Hear what it sounds like.
- Digital mode optimised for human generated text, allows two operators to communicate by typing their messages real-time using computer keyboards.
- Simple variable length code.
- Modulated by phase shift keying.
- Baud rate of 31 baud.
- RTTY. Hear what it sounds like.
- Five pulses representing letters, numbers and symbols.
- Modulated by frequency shift keying.
- Code known as Murray Code in the UK and the Baudot Code in the USA.
- Speeds of 45 to 75 baud.
- Prone to fading and noise.
- Requires a terminal unit.
- Voice Over Internet Protocol (VoIP).
- Digitised voice (telephony) transmission using the Internet Protocol.
- Combined with radio technology to provide worldwide access.
- Utilised by Echolink, amongst others, to allow radio stations to communicate at great distances, with the internet providing the backbone channel and local radio communications providing the peripheral connections at each end.
- Argued to not be 'real radio' by some, hailed as the future by others.
