T8D: Digital and Non-Voice Communications – Ham Radio Technician License Study Guide

T8D covers the wide variety of digital and non-voice modes available to amateur radio operators. These modes range from Morse code — the oldest form of radio communication — to modern weak-signal digital modes that can make contacts under conditions where voice communication would be impossible.

This group covers CW, packet radio, APRS, PSK, DMR, NTSC television, FT8, WSJT-X, ARQ error correction, and amateur mesh networks.

CW: Morse Code

CW stands for continuous wave and is simply another name for a Morse code transmission. In Morse code, information is conveyed by switching the carrier on and off in patterns of short and long pulses (dots and dashes). CW is not a digital mode in the modern sense, but it is a non-voice mode that has been used since the earliest days of radio. As covered in T8A, CW has the narrowest bandwidth of any common amateur mode — about 150 Hz.

Packet Radio

Packet radio is a digital mode that transmits data in structured packets. Each packet transmission includes all three of the following:

• A checksum — a value appended to the packet that permits error detection at the receiving end
• A header — contains the call sign of the station to which the information is being sent (the destination address)
• Automatic Repeat reQuest (ARQ) — if the receiving station detects an error, it requests the transmitting station to resend the packet

APRS: Automatic Packet Reporting System

APRS is a digital communications system that can transmit GPS position data, text messages, and weather data — all three types. Its primary application is providing real-time tactical digital communications in conjunction with a map showing the locations of stations. APRS is widely used in public service events, emergency communications, and mobile tracking. Stations equipped with APRS transmit their position automatically, and anyone with an APRS receiver or internet connection can see where they are on a map in real time.

PSK: Phase Shift Keying

PSK stands for Phase Shift Keying. It is a modulation technique that encodes digital data by shifting the phase of the carrier signal. PSK31 is the most common PSK mode in amateur radio — a narrow, efficient digital mode popular on HF bands for keyboard-to-keyboard communication.

DMR: Digital Mobile Radio

DMR (Digital Mobile Radio) is a digital voice mode that time-multiplexes two digital voice signals on a single 12.5 kHz repeater channel. It does this by dividing time into alternating slots — one for each voice path. This doubles the capacity of a single repeater channel compared to analog FM.

A DMR talkgroup is a way for groups of users to share a channel at different times without hearing other users on the channel. Talkgroups let different communities of interest use the same repeater infrastructure without interfering with each other — you only hear traffic from your own talkgroup.

NTSC Television

NTSC stands for National Television System Committee. In amateur radio, NTSC indicates an analog fast-scan color TV signal. Amateur television (ATV) operators use NTSC to transmit live video on UHF bands. As noted in T8A, NTSC TV signals require about 6 MHz of bandwidth — by far the widest of any common amateur mode.

WSJT-X and FT8

The WSJT-X software suite is a collection of weak-signal digital modes developed for challenging propagation paths. It supports all three of the following operating activities: Earth-Moon-Earth (bouncing signals off the moon), weak signal propagation beacons, and meteor scatter communications.

FT8 is one of the modes in the WSJT-X suite. It is a digital mode capable of low signal-to-noise ratio operation — it can decode signals that are far too weak to hear by ear. FT8 has become the dominant HF digital mode for making long-distance contacts under difficult band conditions.

ARQ Error Correction

An ARQ (Automatic Repeat reQuest) transmission system is an error correction method in which the receiving station detects errors and sends a request for retransmission. When the receiving end identifies a corrupted packet or block of data, it signals the transmitting station to send that data again. ARQ is used in packet radio and various other digital modes to ensure reliable data delivery. It differs from forward error correction (FEC), where redundant data is included in the original transmission so errors can be corrected without retransmission.

Amateur Mesh Networks

An amateur radio mesh network is an amateur-radio based data network using commercial Wi-Fi equipment with modified firmware. The modified firmware changes the operating frequency and behavior to comply with amateur radio rules and to extend the range and flexibility of the network. Mesh networks can provide high-bandwidth data connectivity over distances far beyond normal Wi-Fi range, and are increasingly used for emergency communications infrastructure.

T8D Practice Questions