Non-Voice and Digital Modes: CW, Packet, PSK, APRS, DMR, and Mesh Networking

Concept

T8A settled how to choose between FM and SSB for voice, a question of bandwidth against path reliability. But voice is only one of the things a Technician license lets you send over the air. This lesson opens up nearly everything else: Morse code, still very much alive generations after it stopped being required for a license; text and position data riding on packet radio; a live-picture format from the broadcast television era that some hams still use; software-decoded modes that can pull a signal out of noise no human ear could ever separate from static; a purpose-built digital voice mode that fits two conversations into the space one analog voice channel used to need; and a way of stringing radios together into a data network that never touches the commercial internet. Nine-plus names crowd into this single group, and on first glance they can look like an arbitrary vocabulary list to memorize. They aren't. Each one is a different answer to the same question T8A already asked: what needs to be sent, and what does the available bandwidth and signal path actually allow? This lesson works outward from the oldest and simplest of those answers toward the newest and most elaborate.

Concept

Start with the oldest mode of all: CW, continuous-wave telegraphy, better known today by the name of the code it carries, Morse code. A CW transmitter does nothing more sophisticated than turn a single steady carrier on and off in patterns of short and long pulses, timed by a hand key or an electronic keyer at the sending end and decoded, by ear or by machine, at the receiving end. That may sound primitive next to a mode that pushes a whole photograph or a live conversation through the air, but on-off keying is actually the simplest digital signal there is: exactly two states, present or absent, nothing in between. Everything else in this lesson is a more elaborate version of the same basic idea CW already embodies, encoding information as a sequence of discrete states rather than as a continuously varying voice waveform. T8A already placed CW at the narrow end of the bandwidth scale, the tightest of any mode discussed there, because it carries the least information per second of anything in common amateur use; that same narrowness is exactly what lets a weak CW signal be pulled out by a trained ear when a voice signal at the same transmitter power would already be lost in noise, which is a large part of why CW is still very much a going concern on crowded HF bands today.

Concept

At the opposite end of the information scale sits NTSC, the analog color television standard the United States broadcast industry relied on for decades before switching over to digital broadcasting, and which some amateurs still use for fast-scan amateur television, sending a live, continuously redrawn picture over the air much the way an old broadcast station once did. T8A already placed fast-scan TV at the widest end of the bandwidth spectrum, wider even than FM voice, and NTSC is the reason why: a picture redrawn many times a second simply carries far more information than a voice waveform does, and a signal built to carry that much information necessarily takes up far more spectrum to do it. NTSC is worth knowing by name specifically because it names one particular analog television format, closing the loop T8A opened when it ranked fast-scan TV as the widest-bandwidth mode without yet saying what a fast-scan television signal actually is.

Concept

CW's on-off keying is digital in the most stripped-down sense possible, but most of what follows in this lesson encodes information in more elaborate ways than simply switching a carrier on and off. Phase-shift keying, universally abbreviated PSK, is one of the more common encoding techniques in amateur use: rather than turning the carrier on and off, or shifting its frequency the way FM does, a PSK transmitter shifts the carrier's phase, meaning the timing of its wave cycle relative to a reference point, in patterns that encode data. The most familiar amateur application, a narrow keyboard-to-keyboard chat mode, packs an entire typed conversation into less bandwidth than one SSB voice signal needs, letting two operators type back and forth in something close to real time, the same basic exchange CW operators have carried on for over a century, just with a keyboard standing in for a hand key. The thread running through every digital mode covered in this lesson, PSK included, is that a computer or a dedicated modem is doing the actual encoding and decoding, translating between the radio's analog waveform on one side and data, whether that data is typed text, a position report, or digitized voice, on the other.

Concept

Some digital modes exist specifically to push weak-signal performance further than any human ear can manage, and that is the territory the WSJT-X software suite covers. FT8 is its best-known mode: a digital protocol built to stay decodable even when a signal sits well below the level a voice or CW operator would need just to make out a call sign, because a computer can integrate a weak, precisely structured signal over many seconds and extract meaning from it in a way human hearing simply cannot. The same suite supports several operating activities that lean on exactly that sensitivity: Earth-Moon-Earth work, bouncing a signal off the lunar surface and catching the faint return; meteor scatter, catching a brief reflection off a meteor's ionized trail as it burns up; and monitoring dedicated beacon stations, low-power transmitters left running continuously so operators elsewhere can judge whether a distant band has opened. T8A already established that SSB, not FM, is the mode of choice once a contact turns marginal, because SSB degrades gradually instead of collapsing at a hard threshold; WSJT-family modes take that same underlying idea a step further by handing the entire weak-signal problem to a computer built specifically to solve it, rather than asking a human ear to keep working harder as a signal fades.

Concept

Packet radio bundles data into short, self-contained chunks, and every one of those chunks carries more than just its payload. It also carries a header naming the destination station's call sign, and a checksum, a short mathematical summary of the chunk's contents that lets the receiving end tell whether anything was corrupted along the way. That checksum is amateur radio's most common example of error detection in practice: it doesn't prevent an error, it just reliably notices one happened. Automatic repeat request, universally shortened to ARQ, is usually what happens next: if the checksum on the receiving end comes back wrong, that station sends a request back asking the transmitting station to resend that chunk, rather than silently accepting corrupted data or leaving a gap in the message. Together, the addressed header, the checksum, and ARQ are what let a packet link move data reliably across a connection that, like any radio path, is going to have moments of noise and interference; each piece does its own job, but the combination is what lets a packet connection be trusted to deliver exactly what was sent, not a scrambled approximation of it.

Concept

APRS, the Automatic Packet Reporting System, takes packet radio and puts it to a specific and enormously popular use: broadcasting a station's position, plus whatever else that station is set up to send, out over a shared frequency where any station or app running APRS can pick it up. Position data drawn from GPS is the anchor of the system, letting a network of stations and digital relay stations called digipeaters plot every reporting station on something close to a live map, but APRS traffic isn't limited to position alone; the same packets can carry short typed messages and telemetry such as temperature or weather readings from an unattended station. Put those pieces together and APRS becomes a genuinely tactical tool: a continuously updating map of who is where, layered with messages and sensor data moving over the air. That combination is exactly why APRS shows up constantly in served-agency and emergency-communications work, ARES and CERT deployments especially, where a net control station can watch reporting stations move across a map instead of reconstructing everyone's position from voice check-ins alone.

Concept

DMR, Digital Mobile Radio, takes a different piece of this puzzle: rather than moving text or position data, it moves voice, but digitally instead of as the analog FM signal T8A already covered. Its defining trick is splitting time on a single repeater channel: a DMR channel can occupy the same slice of spectrum an analog FM channel would use, but divides that channel's airtime into alternating slots, carrying two entirely independent digital voice conversations on what looks, from the outside, like one channel, with each conversation getting every other slot. That is a direct answer to the same bandwidth-versus-capability tradeoff T8A raised when comparing FM and SSB, just approached from a different angle: instead of narrowing a signal's bandwidth the way SSB does, DMR keeps the channel width the same and instead divides the time available on it, doubling how many separate conversations a single repeater slot can support.

Concept

The last major idea in this group scales up from a single voice or data channel to something closer to a whole computer network: amateur radio mesh networking, most commonly built today using Broadband-Hamnet. The hardware involved is ordinary commercial Wi-Fi gear, the same kind found in a home wireless router, but running modified firmware that lets it operate under amateur licensing terms instead of the unlicensed rules that same hardware normally follows, and that lets each node automatically discover and relay through its neighbors to extend a network well past what any single radio link could reach alone. Because it is built from commodity hardware and does not depend on the commercial internet at all, a mesh network like this becomes especially valuable when ordinary infrastructure is down, letting a served agency or an emergency operations center move data, images, or even voice across a disaster area using nothing but amateur-owned equipment and licensed spectrum.

Concept

Lined up together, this whole group answers one question from several directions: once voice stops being the only thing worth sending, what else is there, and what does each option trade away to get it? CW and PSK carry small amounts of information, code and typed text, in very little bandwidth. FT8 and the rest of the WSJT-X suite chase the weakest signals imaginable by handing the decoding job to a computer instead of a human ear. NTSC sits at the opposite bandwidth extreme, carrying a whole moving picture. Packet radio, and APRS built on top of it, add addressing, reliability, and position-awareness to data moving through the air. DMR takes the same repeater-channel discipline T8A introduced for analog FM and applies it to digital voice instead. And mesh networking scales the whole idea up from a single link to a self-organizing network. None of this competes with the FM and SSB voice modes T8A covered; it extends the same menu into everything voice was never built to carry.