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Subelement T8
SIGNALS AND EMISSIONS
Section T8D
Non-voice and digital communications: image signals and definition of NTSC, CW, packet radio, PSK, APRS, error detection and correction, amateur radio networking, Digital Mobile Radio, WSJT modes, Broadband-Hamnet
Which of the following is a digital communications mode?
  • Packet radio
  • IEEE 802.11
  • FT8
  • Correct Answer
    All these choices are correct

Digital communications methods are methods that send digital information (encoded in bits, 0 or 1) instead of sending an analog signal, such as voice or video.

The methods listed here are all digital modes:

  • Packet Radio is probably the best known digital mode which can be thought of as using a modem over a radio to allow computers to exchange data

  • JT65 is a digital protocol developed for amateur radio communication with extremely weak signals. It was designed by Joe Taylor, K1JT, to optimize Earth-Moon-Earth (EME) contacts on the VHF bands. The "65" refers to the 65 tones the protocol uses.

  • PSK31 is short for Phase Shift Keying, 31 Baud and is more of system for chat over radio; it allows realtime keyboard to keyboard informal chat between operators.

  • MFSK is short for Multiple frequency-shift keying and is a variation of FSK, a method used by some packet radio systems.

  • IEEE 802.11 is a set of specifications for implementing wireless local area network (WLAN) computer communication in the 900 MHz and 2.4, 3.6, 5, and 60 GHz frequency bands.

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Tags: digital modes arrl chapter 5 arrl module 12

What is a “talkgroup” on a DMR repeater?
  • A group of operators sharing common interests
  • Correct Answer
    A way for groups of users to share a channel at different times without hearing other users on the channel
  • A protocol that increases the signal-to-noise ratio when multiple repeaters are linked together
  • A net that meets at a specified time

A talk group on a DMR digital repeater is a way for groups of users to share a channel at different times without being heard by other users on the channel. This is accomplished by ID codes transmitted by the users. Users in a particular group will program their radios with the ID code for their group. Users' radios not programmed to their group's code will remain silent, even while on the same channel.


More in-depth information:

A number of different digital voice radio standards exist including DMR (Digital Mobile Radio) which implements talk groups defined for purposes of the exam as a way for groups of users to share a channel at different times without being heard by other users on the channel. None of the distractors make any sense.

That's all you need to know for this question, but Talk Groups can also be described as the DMR-MARC FAQ does here:

Q. What are Talk Groups?

A. Talk Groups are like different work groups that you communicate with. They are programmed as “channels”. Several can share a Time Slot on a repeater like multiple PL codes can share a community repeater. Each talk group can be isolated from the other, but may get a busy tone if the same time slot is in use by another. Greater efficiency can be achieved by bridge routing and trunking techniques. Each radio can have more than one talk group and may scan or roam based on talk group.

Digital repeaters can be operated stand-alone but often they are linked to other repeaters using the same standard in a network. See the bottom of this DMR-MARC FAQ for a list of networks, of which DMR-MARC is one.

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Tags: arrl chapter 6 arrl module 15

What kind of data can be transmitted by APRS?
  • GPS position data
  • Text messages
  • Weather data
  • Correct Answer
    All these choices are correct

APRS ( Automatic Position Reporting system) is a refinement of packet radio. Initially it only reported GPS location. Further refinements have added the ability to send short message and weather info.

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What type of transmission is indicated by the term "NTSC?"
  • A Normal Transmission mode in Static Circuit
  • A special mode for satellite uplink
  • Correct Answer
    An analog fast-scan color TV signal
  • A frame compression scheme for TV signals

NTSC is the name of the standard used to encode colors in an analog fast scan color TV signal.

If you ask a broadcast engineer, NTSC stands for Never The Same Color, because his job is to keep all the cameras looking the same. Now that TV is in the Digital Age the only people you see using the NTSC broadcast standard are Amateur Radio Operators.

Actually, NTSC stands for National Television Systems Committee. They created the rules that governed what the broadcast signal would be electronically so every TV would be able to display the correct picture.

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Which of the following is an application of APRS?
  • Correct Answer
    Providing real-time tactical digital communications in conjunction with a map showing the locations of stations
  • Showing automatically the number of packets transmitted via PACTOR during a specific time interval
  • Providing voice over internet connection between repeaters
  • Providing information on the number of stations signed into a repeater

It helps to know what APRS is and does. It transmits a station's GPS coordinates, so other stations can locate it. The components are a GPS receiver, a ham radio transmitter, and some logic to connect the two so the transmitter sends out the GPS coordinates. So:

It has nothing to do with counting packets, It doesn't require voice over Internet, and It doesn't count stations connected to a repeater.

It just provides real time communications that gives your location. In conjunction with a map, it shows your location to the stations that receive your GPS coordinates via the APRS system.

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Tags: aprs digital modes packet radio arrl chapter 5 arrl module 12

What does the abbreviation "PSK" mean?
  • Pulse Shift Keying
  • Correct Answer
    Phase Shift Keying
  • Packet Short Keying
  • Phased Slide Keying

Phase Shift Keying is a method for digitally transmitting data (with a computer of some sort) by varying (keying) the phase of the signal.

Phase refers to where you are in the cycle -- the peaks and valleys of the sine wave

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Tags: digital modes definitions arrl chapter 5 arrl module 12

Which of the following describes DMR?
  • Correct Answer
    A technique for time-multiplexing two digital voice signals on a single 12.5 kHz repeater channel
  • An automatic position tracking mode for FM mobiles communicating through repeaters
  • An automatic computer logging technique for hands-off logging when communicating while operating a vehicle
  • A digital technique for transmitting on two repeater inputs simultaneously for automatic error correction

DMR (Digital Mobile Radio) is one of several digital modes. It's the only one that uses time-multiplexing to allow two digital voice signals to be repeated through the same repeater, using the same 12.5 kHz repeater channel.

It's not a position tracking system.

It's not a logging technique.

The third distractor is tempting, because it talks about two simultaneous repeater inputs, but the part about time-multiplexing is unique to DMR, so the best answer.

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Tags: digital modes arrl chapter 6 arrl module 15

Which of the following is included in packet radio transmissions?
  • A check sum that permits error detection
  • A header that contains the call sign of the station to which the information is being sent
  • Automatic repeat request in case of error
  • Correct Answer
    All these choices are correct

A checksum is an error detection method used by many data transmission types including packet radio. Basically all bytes in the message are added (summed) up and sent as a "checksum". The receiving station repeates this process and "checks" the result against the checksum it received from the sending station.

If the checksum fails (the sums don't match) then an automatic repeat request is sent.

Since packet radio is a form of amateur radio communications the destination station is generally identified at least by call sign, so that information is often included in the header as well.

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Tags: digital modes packet radio arrl chapter 5 arrl module 12

What is CW?
  • A type of electromagnetic propagation
  • A digital mode used primarily on 2 meter FM
  • A technique for coil winding
  • Correct Answer
    Another name for a Morse code transmission

CW stands for "Continuous Wave" which originally distinguished it from other types of emissions. (see Wikipedia for more information) At this point the origin of the name is less important than just knowing that it's a name for Morse Code, which is really just something you'll need to remember.

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Which of the following operating activities is supported by digital mode software in the WSJT-X software suite?
  • Earth-Moon-Earth
  • Weak signal propagation beacons
  • Meteor scatter
  • Correct Answer
    All these choices are correct

WSJT [Weak Signal JT(after the program's initial author, Joseph Hooton Taylor Jr.)] is software that facilitates short, quick digital transmissions and is very useful for weak signals.

As such, it's great for moonbounce (Earth-Moon-Earth), weak signal beacons and meteor scatter, all of which result in very little of the transmitted signal reaching anyone's receiver.

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What is an ARQ transmission system?
  • A special transmission format limited to video signals
  • A system used to encrypt command signals to an amateur radio satellite
  • Correct Answer
    An error correction method in which the receiving station detects errors and sends a request for retransmission
  • A method of compressing data using autonomous reiterative Q codes prior to final encoding

ARQ stands for Automatic Repeat reQuest

When the receiving station detects an error, it automatically sends a repeat request to the sending station.

It has nothing to do with encryption (which would be illegal except for sending commands to a satellite), or video signals, or data compression. Indeed, it takes a long time to have the receiving station send a repeat request and to then re-transmit the message, or portion of the message.

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Which of the following best describes an amateur radio mesh network?
  • Correct Answer
    An amateur-radio based data network using commercial Wi-Fi equipment with modified firmware
  • A wide-bandwidth digital voice mode employing DMR protocols
  • A satellite communications network using modified commercial satellite TV hardware
  • An internet linking protocol used to network repeaters

WiFi is the only protocol listed in this question that takes advantage of meshing. "Mesh Networking" is defined by wikipedia as "a local network topology in which the infrastructure nodes connect directly, dynamically and non-hierarchically to as many other nodes as possible".

Repeaters may be networked but are often done via a hierarchically (e.g. a main node and child nodes). DMR is not mesh, and satellite communications are not as well.

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What is FT8?
  • A wideband FM voice mode
  • Correct Answer
    A digital mode capable of low signal-to-noise operation
  • An eight channel multiplex mode for FM repeaters
  • A digital slow-scan TV mode with forward error correction and automatic color compensation

FT8 is a relatively new digital mode which became popular in 2017. FT8 stands for Franke-Taylor 8-Frequency-Shift Keying modulation. FT8 was created by Joe Taylor, K1JT, and Steve Franke, K9AN.

It very quickly largely replaced JT65 because it's much faster. FT8 is usually used on HF bands and it can achieve very long distances because it is extremely tolerant of noise and interference. It requires that both transmitter and receivers synchronize their computer time with the same time source (which is easy with the internet).

When you transmit, you transmit for 15 seconds and send only a very short (75 bit) message with a 12 bit checksum. Hint: Remember FT as fifteen.

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