Digital modes overview

What a data mode is

A digital mode turns text or files into a pattern of audio tones, sends them through an ordinary radio, and has a computer turn them back into text at the far end. The radio is just a transmitter and receiver; the mode is the agreement between the two computers about how bits become sound. Different modes exist because different jobs want different trade-offs.

One trade-off underneath everything

A slower signal gives the receiver more time to decide what it heard, so it can pull data from below the noise (see Noise and signal-to-noise ratio and Bandwidth, symbol rate and data rate). Add error correction and it gets tougher still, at the cost of more symbols. So the weakest-signal modes are the slowest, and the fast modes need a good path.

The families

Job	Modes	Idea
Keyboard chat, no connection	PSK31, RTTY, MFSK, Olivia and Contestia	Type, and the other end sees it. No acknowledgement.
Weak-signal contacts	FT8 and FT4, JT65, JT9 and the WSJT family	Short, timed, heavily error-corrected messages.
Keyboard messaging, weak-signal	JS8Call	FT8-style modulation with free text.
Propagation beacons	WSPR	Transmit a tiny report; listeners map where it arrived.
Position and short messages	Packet radio and AX.25, APRS	Addressed frames, often relayed by digipeaters.
Email and files	Winlink and radio email, VARA, ARDOP and PACTOR	A two-way link that asks for retransmission of errors.

Digital voice (DMR, D-STAR and others) is a different thing: it sends speech, digitised, and is covered separately.

Three ideas that recur

• Keying: how bits change the carrier. Frequency shifts (FSK) and phase shifts (PSK) are the common ones; see Digital modulation: FSK, PSK and QAM.
• Error handling: either FEC (extra bits let the receiver fix errors alone) or ARQ (the receiver detects an error and asks for a resend). ARQ needs a two-way path; FEC works one-way.
• Timing: a few modes need clocks agreed to about a second, because transmissions happen in fixed slots.

Related

• PSK31Keyboard chat in 31 Hz.
• FT8 and FT4Weak-signal 15-second and 7.5-second modes.
• Packet radio and AX.25Digital messaging with nodes and BBSs.
• Winlink and radio emailEmail over HF and VHF.
• Digital mode softwareWSJT-X, fldigi and friends.
• Digital modulation: FSK, PSK and QAMHow bits become tones and phase shifts.
• Error detection and correctionParity, CRC and forward error correction.