Digital modes

See Wikipedia:Radio modulation modes for a list of all radio frequency modulation modes.

Signals sent by radio (or over long wires or when stored on magnetic media) must be modulated with some method that prevents their signal from degrading before the signals can be received. A transmitter and receiver must use the same mode of modulation to successfully communicate.

A digital mode converts a signal with a finite set of discrete symbols (i.e., 1 and 0, or the alphabet) into a modulated signal suitable for transmission.

Why use digital modes?

Typically, digital modes have the following advantages over analog modes:

• less bandwidth is used
• higher noise immunity
• lower error rate
• automatic operation is possible
• frequency sharing -- multiple conversations on a single frequency

Components of digital modes

A digital mode may have several components, some of which can be mixed and matched.

1. A modulation mode, which causes digital symbol data to be converted to analog signal.
2. An encoding method, which maps a set of digital symbols into symbols appropriate for the modulation mode.
3. A protocol, which formats data and gives it meaning beyond the symbols sent.

Examples

Morse code uses an encoding method that maps letters, numbers, and punctuation into two symbols (long dah, short dit) with spaces between letters and longer spaces between words. True CW modulates these symbols with either the presence or absence of a radio carrier wave. An alternate modulation method transmits the symbols as presence or absence of an audio tone, which can be then be FM modulated as if it were voice ("phone"). Morse code does does not require any protocol, but Q codes are frequently used as part of the protocol, partly to abbreviate messages, and partly to increase their international legibility.

APRS and packet radio use the same modulation and encoding methods, but use very different protocols which overlap only slightly.

Amateur TV (not a digital mode) modulates its video data into an analog signal which can then can be modulated for transmission with either AM or FM. (Commercial TV uses AM for video, FM for audio).

List of common digital modes

CW or morse code

Morse code is the only digital modulation mode designed for efficient transmission and reception by humans. Note that a skilled morse operator can achieve higher data bandwidth while using lower radio frequency bandwidth than a voice operator.

Slow-scan television

transmits pictures in 10 - 30 seconds using 3khz of bandwidth

PSK31

Modern narrow bandwidth digital mode which uses an encoding fine tuned for transmission of typed text.

Packet radio

Packet radio includes both a modulation mode and basic protocol (AX.25) that encapsulates a rich protocol intended for human interaction which includes mailboxes and sessions.

APRS

Automatic Position Reporting System; uses AX.25 from packet radio but adds its own protocol features suitable for instant messaging and position reporting. Commonly used for transmission of GPS position data, weather station data, other telemetry, and person to text messaging.

RTTY

or Radioteletype was one of the first digital modes. It can use any of several modulation modes including frequency-shift keying, and typically encodes text with 5 bit baudot code at a typical speed of 45 baud.

psk31

• PSK31 home page
• ARRL: psk31 specifications