Choose the right digital mode, read a waterfall, and explain codes, ARQ and forward error correction.
Syllabus: Digital emission modes
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 15 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
WSPR is the low-power beacon for judging HF propagation. FT8 is the narrow-band mode that copes with very low signal-to-noise: its report of +3 means +3 dB SNR in a 2.5 kHz bandwidth. Digital voice modes are DMR, D-STAR and System Fusion.
RememberWSPR = beacon. FT8 = weakest signals. DMR, D-STAR, Fusion = voice.
Each new line of signal is added as time passes. A clean signal is a neat stripe. Vertical lines on either side of a data or RTTY signal mean it is overmodulated.
RememberFrequency horizontal, time vertical, brightness = strength. Side lines = overmodulation.
Baudot (RTTY) is a 5-bit code with start and stop bits. PSK31 sends characters in Varicode: shorter codes for common characters, so upper case is longer and slows transmission. The two tones of an FSK signal are mark and space.
RememberBaudot = 5 bits. PSK31 = Varicode. FSK tones = mark and space.
A packet header carries the routing and handling information. In an ARQ mode a NAK means the packet had an error: retransmit it. If too many attempts fail, the connection is dropped. FEC instead sends redundant information with the data, so the receiver corrects errors without asking.
RememberNAK = send it again. Too many tries = disconnect. FEC = redundancy.
Every node links to several others. If one node fails, a packet may still reach its target by an alternate node. More nodes give more routes, not stronger signals.
RememberMesh = many routes, so one failure doesn't end the link.