T8D

Digital modes, packet and APRS

What CW, PSK, packet, APRS, FT8 and DMR are, and how checksums and ARQ keep data correct.

12 pool questions1 on the exam

Syllabus: Non-voice and digital communications: image signals and definition of NTSC, CW, packet radio, PSK, APRS, error detection and correction, amateur radio networking, DMR, WSJT modes, Broadband-Hamnet

CW switches the carrier. PSK flips its phase.

CW is Morse code: the transmitter is keyed on and off. PSK is Phase Shift Keying: the carrier stays on and its phase jumps to send data.

CW: Morse by switching the carrier. PSK: data by flipping the wave's phase.

RememberCW = Morse code. PSK = Phase Shift Keying.

Digital modes: pick one for the job

Mode	What it is	Good for
FT8	Digital mode that works at low signal-to-noise	Weak-signal contacts
WSJT-X	Software suite for FT8 and other weak-signal modes	EME (moonbounce), meteor scatter, propagation beacons
Packet	Data in addressed packets	Messages, data
IEEE 802.11	Wi-Fi	Wireless networks

All of these are digital modes.

RememberFT8 and WSJT-X: digital modes for very weak signals.

Packets carry an address and a checksum. ARQ re-asks.

A packet has a header (call signs), the data, and a checksum that lets the receiver detect errors. With ARQ, the receiver spots an error and requests a retransmission.

Checksum detects the error. ARQ asks for the resend.

RememberChecksum detects. ARQ asks for a resend.

APRS puts packet data on a map

APRS (Automatic Packet Reporting System) sends short packets with GPS position, weather data and text messages. Software plots them on a map for real-time tactical communications.

APRS = packets (position, weather, messages) plotted on a map in real time.

RememberAPRS = position, weather and messages, shown on a map.

DMR splits a channel in two. Mesh is Wi-Fi. NTSC is TV.

Slots alternate fast enough that each conversation sounds continuous.

Term	What it is
DMR	Time-multiplexes two digital voice signals on one 12.5 kHz channel
Mesh network	Data network using commercial Wi-Fi gear with modified firmware
NTSC	Analog fast-scan color TV signal

RememberDMR: two talks, one channel. Mesh: Wi-Fi hardware. NTSC: analog TV.

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