Sunday, April 20, 2008

Digital Mode of the Week: RTTY (Part 2: Teleprinting over radio)

Having discussed the origins of the 5-bit teleprinting ("Baudot") code, now we can talk about one of the oldest digital modes. This is RTTY, for radioteletype. It has undergone several incarnations, from wire teleprinting (like news wires), through various types of hardware-based radio modems, and finally to just another digital option in multimode computer sound card packages.

RTTY is sent over the radio by keying a transmitter between two tone frequencies corresponding to binary states. These are called mark and space. The process is called frequency-shift keying (FSK, F1B emission).

Direct FSK is still used in some dedicated RTTY equipment, but audio frequency-shift keying (AFSK) is far more common today. It is done by generating the proper modulation of an audio tone, which is then sent to the audio input of a standard single-sideband voice radio. (Note that power is lowered due to RTTY having a continuous duty cycle.)

Either method produces the same signal in the receiver. To the ear, RTTY sounds like a continuous warbling tone with rather clicky bit transitions, and often kind of a busy, chattery pulsation to it.

The difference between mark and space frequencies is the shift (in Hz), and the number of bit transitions sent in a second is the speed (in baud). Sometimes you also see speed in words per minute.

Common speeds are 45 (actually 45.45), 50, and 75. Common shifts are 170, 450, and 850. Characters are sent asynchronously, as they are generated. The five data bits are preceded by a single start bit and then followed by 1, 1.5, or two stop bits. (In practice, a long stop bit just sounds like a brief pause on the tone, making RTTY sound even more chattery.)

RTTY's standard tone center (halfway between mark and space) varies a bit. For the most part, it's around 2210 Hz. RTTY used to require careful tuning to center the tones on their respective filters, but today's decoders are more forgiving. Usually, one simply clicks on or between the two peaks on a display, or the software jumps to the loudest signals. The tone center is sometimes relevant for frequency logging, however.

RTTY dial vs listed frequencies can get pretty ambiguous. One can easily be 2 or more kHz off, and not even be sure which way. If you hear nothing on the listed frequency, tune around.

The third important parameter is signal polarity, which can be "normal" or "reverse." This refers to whether mark or space is the lower of the two tones, preferably when tuned in LSB. Unknown signals require some trial and error to find the speed and polarity which print readable text. Shift is usually more evident.

RTTY has no error check, meaning that any problems with the signal will create gibberish, or no print at all. Missed characters on fades are just something we live with.

The good news is that RTTY may be primitive, but it's still heard all over the HF bands. The bad news is that most of this is encrypted, often in a secure military/ government mode called KG-84. There is absolutely no way to get meaningful copy from it. A few navies, particularly the French, still run RTTY test loops and an occasional real message in the clear. Remaining weather stations in Germany and Canada have regular RTTY schedules.

RTTY in the military can also be referred to as RATT.

Old time RTTY stations stored messages on long reels of perforated paper tape, a character at a time. A procedure named "tape relay" existed for the storage and forwarding of traffic. Tapes were punched by perforating devices attached to Teletype machines, for transmission later. Received traffic could be punched directly onto this tape by a reperforator, and relayed by sending the tape to a reader. Much of RTTY's operating nomenclature such as "brag tapes," "test slips," and "loops," is a holdover from this era.

1950s US Navy tape position

The American Radio Relay League (ARRL) headquarters station W1AW sends long RTTY bulletins daily, on time/frequency schedules listed all over the Internet. Shorter transmissions, by hams shooting the breeze or in operating contests, can be heard up or down maybe 10-15 kHz from 14080.