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In days of old, digital modes required strong signals to provide a coherent signal into a modem based on physical filters. Modern digital signal processing (DSP) either in dedicated ICs, or using a PC sound-card and software processing allows suitable designed digital signals to be resolved at very low received levels.
One family of modes uses WSJT software, standing for Weak Signal by Joe Taylor. Examples are JT9 and JT65.
The inventor, Joseph Hooton Taylor Jr. K1JT is an astrophysicist who received the Nobel Prize in Physics for discovering a "new type of pulsar". He previously held VK2BJX.
Another popular mode is PSK31, a mode which uses phase shift keying, and 31 symbols per second. To speed communications, instead of a fixed number of bits per character, this mode uses "varicode", meaning common characters are send using a shorter sequence than longer ones. Lower case also have shorter varicodes than upper case ones, so SHOUTY upper-case messages take longer than ones using normal cases.
Stations operating using these modes usually use only modest power, maybe 20 watts, but even lower power (QRP) also works well.
WSPR, said "whisper", stands for "Weak Signal Propagation Reporter". Is a low powered beaconing system. It assists in understanding propagation. See: https://physics.princeton.edu//pulsar/K1JT/wspr.html
It has gained attention as reflections from the hull or contrails of MH370 in an otherwise empty region of sky provide a possible location for its end point.
These are packet based data modes, used on HF especially.
In Packet and PACTOR, a an ACK is an acknowledgement reply, meaning the last packet was decoded OK, and the next packet should be sent. A NAK is a negative acknowledgement which means the packet most be resent.
If a station connects to another, but the error rates are excessive, the link will be "dropped", or disconnected.
This is a 5 bit code, although it does use stop and start bits. It only supports English language upper-case characters, and via shift characters, numerals, and selected punctuation. It is used in RTTY.
These FSK (Frequency Shift Keying) modes, including RTTY, name one frequency the mark, the other the space. These equate to 1s and 0s.
These are actual questions from the General exam pool.
On what band do amateurs share channels with the unlicensed Wi-Fi service?
A. 432 MHz
B. 902 MHz
C. 2.4 GHz
D. 10.7 GHz
This is 2.4 GHz, which is also an Industrial, Scientific, and Medical band, answer C.
Which digital mode is used as a low-power beacon for assessing HF propagation?
This is WSPR, answer A.
What part of a data packet contains the routing and handling information?
This is the header, answer C.
Which of the following describes Baudot code?
A. A 7-bit code with start, stop and parity bits
B. A code using error detection and correction
C. A 5-bit code with additional start and stop bits
D. A code using SELCAL and LISTEN
Baudot, used in TELEX and RTTY uses 5 bit codes, answer C.
In the PACTOR protocol, what is meant by an NAK response to a transmitted packet?
A. The receiver is requesting the packet be retransmitted
B. The receiver is reporting the packet was received without error
C. The receiver is busy decoding the packet
D. The entire file has been received correctly
This stands for Negative Acknowledgement, and means the racket was not received correctly, and thus needs to be retransmitted, answer A.
What action results from a failure to exchange information due to excessive transmission attempts when using PACTOR or WINMOR?
A. The checksum overflows
B. The connection is dropped
C. Packets will be routed incorrectly
D. Encoding reverts to the default character set
The connection is dropped (disconnected), answer B.
How does the receiving station respond to an ARQ data mode packet containing errors?
A. It terminates the contact
B. It requests the packet be retransmitted
C. It sends the packet back to the transmitting station
D. It requests a change in transmitting protocol
If the packet cannot be decoded, it must he retransmitted, answer B.
Which of the following statements is true about PSK31?
A. Upper case letters are sent with more power
B. Upper case letters use longer Varicode signals and thus slow down transmission
C. Error correction is used to ensure accurate message reception
D. Higher power is needed as compared to RTTY for similar error rates
Only a semi-literate RWNJ on Facebook would believe the first option, and given normal messages are mostly lower case, it makes sense that these use shorter varicodes, and upper case uses longer, answer B.
In most cases only modest power is used on PSK31.
What does the number 31 represent in "PSK31"?
A. The approximate transmitted symbol rate
B. The version of the PSK protocol
C. The year in which PSK31 was invented
D. The number of characters that can be represented by PSK31
There are about 31 symbols per second, answer A.
How does forward error correction (FEC) allow the receiver to correct errors in received data packets?
A. By controlling transmitter output power for optimum signal strength
B. By using the varicode character set
C. By transmitting redundant information with the data
D. By using a parity bit with each character
This is done by sending "redundant information", meaning that content is repeated to some extent, answer C.
How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified?
A. Dot and dash
B. On and off
C. High and low
D. Mark and space
This is Mark and Space, answer D.
Which type of code is used for sending characters in a PSK31 signal?
Like in Morse Code, the most common characters have the shortest length, and this variable-length code is called Varicode, answer A.
What is indicated on a waterfall display by one or more vertical lines on either side of a digital signal?
A. Long path propagation
B. Backscatter propagation
C. Insufficient modulation
Overmodulation can cause spurious signals either side of the transmitted signal, answer D.
Which of the following describes a waterfall display?
A. Frequency is horizontal, signal strength is vertical, time is intensity
B. Frequency is vertical, signal strength is intensity, time is horizontal
C. Frequency is horizontal, signal strength is intensity, time is vertical
D. Frequency is vertical, signal strength is horizontal, time is intensity
Frequency increases across the screen, while brightness indicates the signal strength. Over time the displayed information scrolls down the screen, as water flows over a waterfall, answer C.
Intensity may be replace by a false colour representation. Some digital modes appear as two parallel lines, while slow Morse may be visible as short and long elements. Ionospheric sounding transmissions and some interference from poor quality power supplies may be seen as diagonal lines wandering across the waterfall. To see and example click the green image on this page.
There is even software which can generate a signal which will place text and symbols directly into the waterfall display.
If you have been watching the question numbers, you may notice that we only have 2 sections to go.
On to: Feedlines and SWR
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, April 2022.
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