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
This group covers common amateur digital modes, the abbreviations used for them, and a few related systems: APRS, television, DMR and mesh networking.
Digital modes
Many modes carry data rather than voice. Packet radio, IEEE 802.11 (Wi-Fi) and FT8 are all digital communication modes. PSK stands for Phase Shift Keying, in which information is carried by changes in the signal’s phase. CW, short for continuous wave, is simply another name for Morse code transmission.
Packet radio and error control
Packet radio sends data in short bursts. Each packet includes:
A header containing the call sign of the station to which the data is addressed.
A checksum that permits error detection.
Support for ARQ (automatic repeat request), an error-correction method in which the receiving station detects errors and requests retransmission.
APRS
The Automatic Packet Reporting System sends short packets that can carry GPS position data, text messages and weather data. Its typical application is real-time tactical digital communication combined with a map showing the locations of stations. It does not tally PACTOR packets or link repeaters.
WSJT-X and FT8
The WSJT-X software suite supports weak-signal modes. FT8 is a digital mode capable of operating at low signal-to-noise ratios, decoding signals far below what you could hear. WSJT-X activities include Earth-Moon-Earth (moonbounce), weak-signal propagation beacons and meteor scatter.
DMR
DMR (Digital Mobile Radio) uses time-division multiplexing to carry two digital voice signals on a single 12.5 kHz repeater channel, by alternating time slots.
Television
NTSC is the analog fast-scan color television standard used in North America. Amateurs can send it as fast-scan ATV, which needs about 6 MHz of bandwidth. It is not a digital encryption or compression standard.
Mesh networks
An amateur radio mesh network is a data network built from commercial Wi-Fi equipment with modified firmware. Nodes automatically link to each other, forming a high-speed network useful for emergency communications. It is not a DMR voice mode, a satellite network or a repeater linking protocol.
Key facts to memorize
Packet, IEEE 802.11 and FT8 are all digital modes.
FT8 works at low signal-to-noise ratios.
APRS carries GPS positions, text messages and weather data.
PSK means Phase Shift Keying.
DMR time-multiplexes two voice signals on a 12.5 kHz channel.
Packet includes a header with the destination call sign, a checksum and ARQ.
CW is another name for Morse code.
WSJT-X supports EME, weak-signal beacons and meteor scatter.
ARQ means the receiver requests retransmission when it detects errors.
NTSC is analog fast-scan color TV.
Mesh networks use modified commercial Wi-Fi equipment.
Memory hooks
ARQ: the receiver asks for the packet again (Automatic Repeat reQuest).
DMR gives you two time slots, two conversations, one channel.
Questions in this group (12): answers shown
T8D01
Which of the following is a digital communications mode?
A. Packet radio
B. IEEE 802.11
C. FT8
D. All these choices are correct ✓ Correct answer
Packet radio is a digital mode, IEEE 802.11 (Wi-Fi) is a digital data standard also used by amateurs, and FT8 is a weak-signal digital mode. Each of them qualifies, so all the choices are correct.
T8D02
What is FT8?
A. A wideband FM voice mode
B. A digital mode capable of low signal-to-noise operation ✓ Correct answer
C. An eight-channel multiplex mode for FM repeaters
D. A digital slow-scan TV mode with forward error correction and automatic color compensation
FT8 is a digital mode designed for weak-signal work, decoding signals well below the noise level in short 15-second transmissions. It is not an FM voice mode, a repeater multiplex mode or slow-scan TV.
T8D03
What kind of data can be transmitted by APRS?
A. GPS position data
B. Text messages
C. Weather data
D. All these choices are correct ✓ Correct answer
APRS sends short packets that can carry GPS position data, text messages and weather station data, among other things. Since all three are valid APRS content, all of the choices are correct.
T8D04
What is meant by the term "NTSC?"
A. A digital transmission standard for encrypting data
B. A special mode for satellite uplink
C. An analog fast-scan color TV signal ✓ Correct answer
D. A frame compression scheme for TV signals
NTSC is the analog fast-scan color television standard used in North America. It is not an encryption standard, a satellite uplink mode or a frame compression scheme, though amateurs use it for fast-scan ATV.
T8D05
Which of the following is an application of APRS?
A. Providing real-time tactical digital communications in conjunction with a map showing the locations of stations ✓ Correct answer
B. Automatically showing the number of packets transmitted via PACTOR during a specific time interval
C. Providing voice over internet connection between repeaters
D. Providing information on the number of stations signed into a repeater
APRS provides real-time tactical digital communication combined with a map showing the locations of stations. It does not count PACTOR packets, link repeaters over the internet or report repeater check-ins.
T8D06
What does the abbreviation "PSK" mean?
A. Pulse Shift Keying
B. Phase Shift Keying ✓ Correct answer
C. Packet Sampled Keying
D. Power Sampled Keying
PSK stands for Phase Shift Keying, a digital mode in which information is carried by changes in the phase of the signal. PSK31 is a popular narrowband HF example. The other expansions are not real modulation terms.
T8D07
Which of the following describes DMR?
A. A technique for time-multiplexing two digital voice signals on a single 12.5 kHz repeater channel ✓ Correct answer
B. An automatic position tracking mode for FM mobiles communicating through repeaters
C. An automatic computer logging technique for hands-off logging when communicating while operating a vehicle
D. A digital technique for transmitting on two repeater inputs simultaneously for automatic error correction
DMR (Digital Mobile Radio) uses time-division multiplexing to carry two digital voice conversations on one 12.5 kHz repeater channel by alternating time slots. It is not a position-tracking mode, logging technique or dual-input scheme.
T8D08
Which of the following is included in packet radio transmissions?
A. A checksum that permits error detection
B. A header that contains the call sign of the station to which the information is being sent
C. Automatic repeat request in case of error
D. All these choices are correct ✓ Correct answer
Packet transmissions include a header with the destination call sign, a checksum for error detection, and automatic repeat request (ARQ) so errors are retransmitted. Each of these is part of packet radio, so all are correct.
T8D09
What is CW?
A. A type of electromagnetic propagation
B. A digital mode used primarily on 2-meter FM
C. Error correction for digital transmission using code words
D. Another name for a Morse code transmission ✓ Correct answer
CW, for continuous wave, is another name for Morse code transmission, made by switching a carrier on and off. It is not a propagation type, a 2-meter FM digital mode or an error correction scheme.
T8D10
Which of the following operating activities is supported by digital mode software in the WSJT-X software suite?
A. Earth-Moon-Earth
B. Weak signal propagation beacons
C. Meteor scatter
D. All these choices are correct ✓ Correct answer
WSJT-X software supports Earth-Moon-Earth (moonbounce) contacts, weak-signal propagation beacons and meteor scatter, using modes such as FT8, JT65 and MSK144. All of the choices are supported activities.
T8D11
What is the role of ARQ in a transmission system?
A. A special transmission format limited to video signals
B. A system used to encrypt command signals to an amateur radio satellite
C. An error correction method in which the receiving station detects errors and sends a request for retransmission ✓ Correct answer
D. A method of compressing data using autonomous reiterative Q codes prior to final encoding
ARQ, automatic repeat request, is an error-correction method where the receiving station detects errors and asks the sender to retransmit. It is not a video format, an encryption method or a compression scheme.
T8D12
Which of the following best describes an amateur radio mesh network?
A. An amateur-radio data network using commercial Wi-Fi equipment with modified firmware ✓ Correct answer
B. A wide-bandwidth digital voice mode employing DMR protocols
C. An amateur-radio satellite communications network using modified commercial satellite TV hardware
D. An internet linking protocol allowing communication through repeaters around the world
An amateur radio mesh network uses commercial Wi-Fi equipment with modified firmware to build a high-speed data network. It is not a DMR voice mode, a satellite network or a worldwide repeater linking protocol.