Spring 2009 Issue

Digital Radio

Digital Modes
They’re Not Just About Data Anymore

By Mark Thompson, WB9QZB

 

 

Until recently, when ham radio operators considered operating the digital modes, that usually meant some form of data transmission.

Hams became involved in digital in the late 1040s by using surplus Baudot-format mechanical teletype equipment. The teletype machines were large, heavy, and noisy. They printed on yellow roll-type paper. Some machines had the optional paper tape punch. With it you could record material in advance on paper tape. The paper tape could then be transmitted at full speed later. This was especially useful for bulletin transmissions.

Ham radio teletype was called RTTY. RTTY equipment was used on HF to make keyboard QSOs and receive bulletins, such as those from the ARRL. RTTY had no error correction, so signal fading and noise on HF could cause many errors.

In the 1970s some hams interfaced their RTTY equipment to 2-meter FM transceivers to monitor messages from local hams in a mode called auto-start. In some areas FM repeaters were used to extend the range of RTTY auto-start activity and to disseminate bulletins.

For those more than 30 years, Baudot RTTY is what digital meant in ham radio. Up until the early the 1980s five-bit Baudot was the only mode the FCC allowed on ham radio. Baudot only contained upper-case letters. You might have seen a copy of a Western Union telegram or an AT&T Telex printed using Baudot.

However, with the advent of personal computers, in the 1970s hams wanted to use ASCII. ASCII code has more characters, including lower case and additional punctuation. Hams appealed to the FCC, which approved the use of ASCII in the early 1980s. They used more modern teletype and computer terminals to transmit and receive ASCII RTTY. Some hams at the time also developed programs for the early PCs to transmit and receive both Baudot and ASCII RTTY.

In the 1980s a group of hams invented packet radio, which used the ASCII code. Packet radio required that a packet be received correctly or it was retransmitted. On the VHF/UHF bands this led to very accurate transmission of data. However, noise and signal fading on HF caused packets to be retransmitted over and over again, resulting in the 300-baud transmission speed dropping dramatically.
Hams developed BBS (bulletin board systems) accessible on VHF and UHF in most areas of the country. Terrestrial forwarding networks were developed to forward e-mails across the country and the world. HF gateways were developed to allow long-distance forwarding of messages between BBSes.
In the late 1980s and early 1990s many of the packet radio TNCs (terminal node controllers) were extended to include other ASCII-based text modes such as AMTOR, Pactor, and G-TOR. Modes such as AMTOR had error correction, which RTTY did not.

In the mid-1990s the internet became available. Many hams abandoned packet radio to use the internet. As a result, the BBSes and forwarding networks fell into disuse and many were taken off the air.

In the late 1990s major developments occurred in ham radio data modes. PSK31 was developed. It performed much better in weak-signal conditions on HF than the other data modes. In addition, it was the first mode to use a PC soundcard with a computer program and did not require dedicated hardware customized for a specific mode.

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