Digital modes turn text, position reports, email or voice into radio signals that a computer or a radio’s built-in processor encodes and decodes. Some are built for weak signals on HF, some for messaging, and some for voice through VHF and UHF repeaters. FT8 and FT4, for example, need only an SSB radio, a computer and free software (WSJT-X User Guide).
Connecting a computer to a radio
Sound card modes such as FT8 work through audio. The software turns data into audio tones, the radio transmits them on SSB, and on receive the radio’s audio goes back into the computer for decoding. The WSJT-X system requirements describe the pieces (WSJT-X User Guide):
- Audio in and out. Many radios have a built-in USB audio codec, so one USB cable carries audio in both directions. Other stations use a separate audio interface between the radio and the computer.
- Transmit control. The computer keys the transmitter through a serial or USB control line, through the radio’s CAT (computer control) commands, or with VOX.
- Accurate time. The computer clock must be synchronized to UTC within 1 second.
The same guide asks for audio set to 48000 Hz, 16 bits, the radio in USB mode, and the receive filter at its widest setting, up to about 5 kHz. To set transmit audio, it suggests clicking Tune and lowering the Pwr slider from maximum until the radio’s RF output falls slightly. An overdriven transmitter makes a wide signal: the ON4UN and ON4WW guide warns that PSK31 is easy to overmodulate and advises keeping speech processing switched off (Ethics and Operating Procedures). RTTY keeps the transmitter at full output for the whole transmission (a 100 percent duty cycle, compared with about 50 percent for CW), so the same guide advises never running a 100 W radio above 50 W on RTTY for transmissions longer than a few seconds.
On VHF and UHF, packet and APRS also use audio, and a software modem such as Dire Wolf replaces the dedicated hardware terminal node controller (TNC) of earlier years by using the computer’s sound card (Dire Wolf).
FT8 and FT4
FT8 and FT4 are part of WSJT-X, open source software created and maintained by Joe Taylor, K1JT, and the WSJT Development Team, which decodes signals too weak to hear by ear (WSJT-X Official Home). It is free software licensed under the GNU General Public License (WSJT-X User Guide).
- FT8 sends in 15 second timed sequences. Its signal is 50 Hz wide, using 8-tone frequency shift keying at 6.25 baud (WSJT-X User Guide).
- FT4 uses 7.5 second sequences and is designed for contesting. It is 3.5 dB less sensitive than FT8 and uses 1.6 times the bandwidth, but offers about twice the contact rate (WSJT-X User Guide).
A contact is a short series of structured messages: call signs, a grid locator and a signal report in dB, measured against a 2500 Hz bandwidth. One station transmits in the even sequences and the other in the odd ones, and WSJT-X picks the right one when you double-click a station calling CQ. The user guide recommends answering on a clear transmit frequency rather than on top of the calling station, and notes that low power is the norm under most HF conditions (WSJT-X User Guide).
WSJT-X ships with a table of frequencies conventionally used for each mode, and the guide notes that these conventions can change (WSJT-X User Guide). Use that table (and what you see on the waterfall) rather than a printed list.
PSK31 and RTTY
PSK31 is a keyboard-to-keyboard mode for live conversation. It runs at 31.25 baud, occupies about 80 Hz in practice, and uses a variable-length code (Varicode) in which most lowercase letters use fewer bits than capitals, so typing in lowercase is faster (Ethics and Operating Procedures).
RTTY (radioteletype) dates from mechanical teleprinters. It uses the 5-bit Baudot code, one of the codes the rules name (47 CFR 97.309). On the amateur bands it is sent at 45 baud with a 170 Hz shift between the mark and space tones. It has no error correction, and if the receiver misses the shift between letters and figures, a report of “599” prints as “TOO” (Ethics and Operating Procedures).
fldigi includes modems for PSK, RTTY and many other modes, and uses the computer’s sound card (Fldigi Users Manual).
JS8
JS8 is a weak-signal mode for keyboard messaging, built on the WSJT-X FT8 engine. It adds directed messages, relays through other stations, store-and-forward messages and group messaging. The js8call.com site now hosts JS8Call-Improved, a community-driven evolution of the original JS8Call program that it describes as backward compatible with all JS8 stations.
Packet and APRS
Packet radio sends data in short bursts (packets) that carry addressing and error-detection information (ARRL glossary). The Dire Wolf software modem implements the AX.25 protocol used for amateur packet, and lists 1200 bps audio frequency shift keying as the most common speed on VHF and UHF, with 9600 bps as another option (Dire Wolf). The ARRL band plan notes that 145.01, 145.03, 145.05, 145.07 and 145.09 MHz are widely used for packet (ARRL band plan).
APRS uses packet to share GPS positions, weather data and short text messages, and can show stations on a map (Technician pool questions T8D03 and T8D05). Digipeaters retransmit packets addressed to them (ARRL glossary), and internet gateway (igate) stations forward packets to the APRS-IS network, where maps such as aprs.fi display them. Most of the positions on aprs.fi start out on amateur frequencies, typically around 145 MHz. Anyone can receive, store or republish what you transmit, so if you send your position, expect it to appear on the internet (aprs.fi FAQ).
Winlink
Winlink relays email by radio, using addresses based on amateur call signs (Technician pool question T2C08). Your station connects by radio to a gateway, which Winlink calls a Remote Message Server, and the gateway provides the internet connection (General pool questions G2E12 and G2E13; Technician pool question T8C11). The emergency communications guide covers how it is used in public service.
Winlink works on both VHF and HF and can run over packet radio (General pool question G2E12). HF connections use protocols such as PACTOR, one of the publicly documented techniques the rules mention (47 CFR 97.309), or VARA (General pool question G2E02), a modem whose developer sells licenses (EA5HVK).
Digital voice
On VHF and UHF, the General question pool names DMR, D-STAR and System Fusion as digital voice modes (General pool question G8C16). A D-STAR radio must have your call sign programmed into it before you transmit, and a digital mode hotspot connects nearby radios to a digital voice or data network (Technician pool questions T4B11 and T4A10).
DMR time-shares a single 12.5 kHz repeater channel between two voice signals. Traffic is organized into talkgroups, which you join by programming the group’s ID into your radio, and each repeater has a color code your radio must match to access it (Technician pool questions T8D07, T2B07 and T2B12).
On HF, FreeDV is an open source digital voice project, including its speech codec. It runs as a free application that lets an ordinary SSB radio carry digital voice.
The rules for data
The 2.8 kHz limit is recent. Until January 8, 2024, data on most HF segments was limited to 300 baud (1200 baud on 28.0 to 28.3 MHz). The FCC removed those symbol rate limits and set a 2.8 kHz bandwidth limit instead (88 FR 85126). At the same time it proposed removing the remaining symbol rate limits on 2200 m, 630 m, 6 m, 2 m, 1.25 m and 70 cm (88 FR 85171); as of October 2026 the Federal Register shows no final rule on that proposal.
Above 50 MHz the rules are looser. Stations may also use unspecified digital codes, as long as they are not used to obscure meaning (47 CFR 97.309(b)). The current limits are 19.6 kilobauds and 20 kHz bandwidth on 6 m and 2 m, and 56 kilobauds and 100 kHz on 1.25 m and 70 cm (47 CFR 97.305(c) and 97.307(f)).
Technicians can use all of these modes on VHF and UHF. On HF, Technicians have RTTY and data privileges from 28.0 to 28.3 MHz on 10 m (47 CFR 97.301; 97.305). Identification rules apply to digital contacts too. You must identify with an emission authorized on the frequency you are using: CW, voice in English, or RTTY using one of the named codes when the contact itself is RTTY or data (47 CFR 97.119).