Fldigi

fldigi
Type	Multimode sound-card digital mode program
Developed by	Dave Freese, W1HKJ, and contributors
First released	2007
License	Free, open source (GNU GPL version 3)
In use
Platforms	Windows, macOS, Linux (including the Raspberry Pi) and other Unix-like systems
Common modes	PSK31, RTTY, Olivia, MFSK, MT63, THOR, Hellschreiber, CW
Companion programs	flmsg, flamp, flrig, fllog, flwrap and others
Main uses	Keyboard contacts, emergency message handling

fldigi (from Fast Light Digital, after the FLTK "Fast Light" toolkit it is built with) is a free program that turns a computer's sound card into a modem for dozens of amateur digital modes. Written by Dave Freese, W1HKJ, with help from other volunteers, it was first released in 2007 and has become the standard general-purpose tool for keyboard modes, the digital modes in which operators type to each other in real time, much like a text chat. It is also the center of a suite of companion programs, known as NBEMS, that emergency groups use to send exact written messages and files by radio.

What it does

Most digital programs handle one mode or one family. fldigi handles many. Its menus include PSK31 and its faster relatives, RTTY, Olivia and its cousin Contestia, MFSK, DominoEX, THOR, MT63, Hellschreiber, and CW, along with decoders for weather fax and some utility transmissions. All of them run through the same window: a waterfall display showing every signal in the receiver's passband, a receive pane where decoded text appears, and a transmit pane where the operator types.

Contacts are conversational rather than automatic. Operators click on a signal in the waterfall to tune it, read what the other station is sending, and type a reply. Macros, stored lines of text sent with a single click, take care of routine items such as a CQ call, a station description or the closing of a contact. A feature called RSID (Reed-Solomon identifier) sends a short burst at the start of a transmission that tells other copies of fldigi which mode is in use, so they can switch to it automatically, which is helpful with the less common modes.

fldigi can control most transceivers, either directly or through the companion program flrig, so frequency and mode changes follow the software. It can also log contacts and send reception reports to PSK Reporter.

Modes without their own article

Several of fldigi's modes have no article of their own on HamWiki. In brief, from the fldigi documentation:

MT63 spreads the text across 64 tones sent at once, with heavy error correction and interleaving, the spreading of each character's bits over several seconds so that a burst of noise damages only small parts of many characters. It comes in 500, 1000 and 2000 Hz widths; MT63-1000 is the usual calling mode. It shrugs off interference and tuning errors well but needs a clean, linear transmitter, since overdriving widens the signal.
MFSK steps a single carrier between 16 or 32 tones, with error correction running all the time. MFSK16, the most common version, is about 316 Hz wide and types at about 58 words per minute. It must be tuned accurately. The wider, faster MFSK32 and MFSK64 are intended for VHF and UHF, and fldigi's MFSK modes can also send small pictures.
DominoEX is a multi-tone mode using incremental frequency keying (IFK), in which the data is carried by the size of the step from one tone to the next rather than by the tones themselves, so the mode tolerates mistuning and drift well. It runs at several speeds; DominoEX16, for example, is about 355 Hz wide and types at about 115 words per minute.
THOR borrows from both MFSK and DominoEX, adding error correction to IFK. It is designed for HF and copes particularly well with static, and its speeds run from about 7 words per minute in its slowest form to more than 350 in THOR100.
Contestia, developed from Olivia by Nick Fedoseev, UT2UZ, in 2005, sounds almost the same and is about twice as fast, at a cost of roughly 1.5 to 3 dB of sensitivity. It uses a reduced character set with no lower case. The most common formats are 250 Hz with 8 tones, 500 Hz with 16 and 1000 Hz with 32.

NBEMS

NBEMS (Narrow Band Emergency Messaging Software) is the name for fldigi used together with its companion programs to move formatted messages and files, rather than casual conversation. The main pieces are:

flmsg, which creates and reads standard forms, including the ICS-213 general message and other FEMA Incident Command System forms, the ARRL radiogram used on the National Traffic System, and plain text. The form is compressed and checked so that it arrives exactly as sent.
flamp, which sends a file to many stations at once. It breaks the file into numbered, checksummed blocks; any station that misses a few blocks can ask for just those to be repeated.
flwrap, which wraps a file with a checksum for single transfers, and flrig and fllog for radio control and logging.

NBEMS needs no internet, no gateways and no special hardware: just radios, computers and sound-card connections. That independence is its main attraction compared with Winlink, which relies on gateway stations and servers, although many groups train with both.

Emergency use

ARES, RACES and other served-agency groups use NBEMS to pass lists, requests and situation reports that would be slow and error-prone to read aloud on a voice net. On VHF and UHF FM, a common choice is MT63, a wide, rugged mode that copes well with the audio distortion of FM radios and can even be decoded with a handheld held near a laptop's microphone, with no cable at all. Faster PSK modes are used where signals are strong. On HF, groups typically use narrower, more robust modes such as Olivia, MFSK or THOR, and agree on them in advance as part of their net plan.

Groups usually standardize on a set of modes, frequencies and form types, and hold regular practice nets where stations check in by sending a short form, so that settings are known to work before an emergency.

Agree on the mode before the net begins and turn on RSID for both transmit and receive. When everyone's software announces and recognizes the mode automatically, a station that starts in the wrong mode is spotted at once instead of sending pages of unreadable text.

Getting started

fldigi needs an HF transceiver or FM radio, a computer and a digital mode interface or radio with built-in USB audio. On first run a setup wizard asks for the call sign, audio devices and radio control method. For HF keyboard modes the radio is set to upper sideband, and the transmit audio should be kept below the point where the radio's ALC (automatic level control) acts, since over-driven PSK and MFSK signals become wide and distorted.

External links

Fldigi Users Manual, W1HKJ