RTTY
| RTTY (radioteletype) | |
|---|---|
| Mode type | Digital text, two-tone frequency shift keying (FSK) |
| Code | 5-bit Baudot code (ITA2), with letters and figures shifts |
| Standard amateur speed | 45.45 baud, about 60 words per minute |
| Shift | 170 Hz between mark and space |
| On the air | |
| Where to find it | The data segment of each HF band, near but apart from FT8 |
| Busiest during | RTTY contests |
| Software | MMTTY, 2Tone, fldigi |
| Needs | An HF radio, a computer and a sound-card interface |
RTTY, short for radioteletype and usually pronounced "ritty", is the oldest digital mode in amateur radio. It sends text one character at a time by switching the transmitter between two frequencies, and it began as a way of driving mechanical teleprinters by radio. The machines are long gone from most shacks, replaced by a computer and a sound card, but the signal is the same one hams used in the 1950s: 45.45 baud, 170 Hz shift, Baudot code. Today RTTY survives mainly as a contest and DX mode, where its speed and simplicity still suit fast exchanges.
How it works
An RTTY signal uses frequency shift keying (FSK). The transmitter sends a steady carrier on one of two frequencies, called mark and space, which stand for the 1s and 0s of the code. The terms come from wire telegraphy, where current flowing in the line "marked" the paper tape and no current left a space. Amateur RTTY places the two 170 Hz apart, and by convention mark is the higher radio frequency. When nothing is being sent, the transmitter rests on mark.
Characters are sent in Baudot code, strictly the International Telegraph Alphabet No. 2 (ITA2). Each character is five bits, framed by a start bit and a stop bit one and a half bits long, so each takes about 165 milliseconds at 45.45 baud. Five bits allow only 32 combinations, too few for letters and numbers together, so two of them are shift characters: LTRS switches the receiver to letters and FIGS to figures and punctuation. If a shift character is lost to noise, the following text prints as the wrong set, turning a word into a row of numbers, which is why RTTY operators repeat call signs and exchanges. There are no lowercase letters and no error correction; a weak or fading signal simply prints garbled characters.
History
Teleprinters, electric typewriters that could send and receive over a wire, were used by news agencies, railways and the military for most of the 20th century. After World War II, surplus machines from makers such as the Teletype Corporation reached hams in large numbers, and amateur radioteletype grew through the 1950s and 1960s. A station of that era filled a corner of the shack: a heavy, clattering printer, a paper-tape punch and reader for prepared messages, and a terminal unit that converted the received audio tones into pulses to drive the printer.
Home computers began replacing the machines in the late 1970s and 1980s, first with dedicated interfaces and then with sound-card software. The computer could also show text on screen and store messages, and the clatter faded from ham shacks, but the over-the-air standard never changed, so a modern station can still work a restored teleprinter.
RTTY today
Everyday RTTY ragchewing has largely given way to other modes, first PSK31 and then FT8, but RTTY is far from dead. Major contests run on it, including the ARRL RTTY Roundup each January and the CQ World Wide and CQ WPX RTTY contests, and on those weekends the digital segments of the HF bands fill with RTTY signals. DXpeditions often run a RTTY station too. Compared with FT8, RTTY contacts are not limited to a fixed message format, and a busy station can work several contacts a minute.
On the air, RTTY occupies the data segment of each HF band, a little above the CW section; check the band plan for the exact range. RTTY is a data mode under Part 97, so in the US it is used where data emissions are permitted.
Software and FSK versus AFSK
Popular programs include MMTTY, a long-standing free Windows decoder by Makoto Mori JE3HHT; 2Tone, a decoder favored by contesters; and fldigi, which handles RTTY alongside many other digital modes. Contest loggers such as N1MM Logger+ can drive these decoders directly, often running two or more side by side on the same signal for better copy.
There are two ways to put the signal on the air:
- FSK (true frequency shift keying) uses the radio's own RTTY mode. The computer sends keying pulses, usually through a serial port line, and the radio shifts its carrier between mark and space. The radio's narrow RTTY filters can then be used on receive.
- AFSK (audio frequency shift keying) feeds the two audio tones into the microphone or data input with the radio in single sideband mode. Lower sideband turns the audio spectrum upside down, so the standard tones are 2125 Hz for mark and 2295 Hz for space with the radio on lower sideband, which keeps mark on the higher radio frequency.
Both produce the same signal when set up properly. With AFSK, the audio level must be kept below the point where the radio's automatic level control acts, or the signal becomes wider and distorted.