Codes: how characters become bits
Three character encodings the pool distinguishes.
Which of the following describes Baudot code?
A 5-bit code with additional start and stop bits.
Baudot is what RTTY uses, and it predates computing — five bits gives only 32 symbols, so letters and figures share codes and a shift character switches between them. It is why RTTY sometimes degenerates into gibberish after a missed shift.
Which type of code is used for sending characters in a PSK31 signal?
Varicode.
Which of the following statements is true about PSK31?
Upper case letters use longer Varicode bit sequences and thus slow down transmission
Varicode is variable-length: common characters get short codes, rare ones get long ones. Since ordinary text is mostly lowercase, PSK31 optimises for it — and TYPING IN CAPITALS genuinely slows the mode down. A design decision with a visible behavioural consequence.
Error control: two strategies
What is the role of ARQ in a transmission system? (Technician material) Detect an error, ask again.
In an ARQ mode, what is meant by a NAK response to a transmitted packet?
Request retransmission of the packet.
NAK is negative acknowledgement — “that arrived corrupt, send it again.” Its counterpart ACK confirms good receipt.
What action results from a failure to exchange information due to excessive transmission attempts when using an ARQ mode?
The connection is dropped
ARQ does not retry forever. After a set number of failures the link gives up, which is the correct behaviour: a path that cannot carry the data is not improved by trying harder.
How does forward error correction (FEC) allow the receiver to correct data errors?
By transmitting redundant information with the data.
The other strategy: send extra bits so that errors can be corrected at the receiver without asking again. It costs bandwidth on every transmission and needs no return path.
Choosing between them: ARQ where there is a return path and the data must be exact (Winlink, packet). FEC where there is no return path or latency matters (WSPR, FT8, QPSK31).
The narrow-band weak-signal modes
Which digital mode is used as a low-power beacon for assessing HF propagation?
WSPR.
Weak Signal Propagation Reporter: very low power, extremely narrow, and its whole purpose is to be received and reported. Stations worldwide log what they hear to a database, so a fraction of a watt tells you exactly which paths are open.
Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?
FT8.
And its report format:
What does an FT8 signal report of +3 mean?
The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth
Read that carefully. FT8’s own bandwidth is about 50 Hz, but reports are normalised to 2.5 kHz so they are comparable with SSB. A report of −20 dB — routine in FT8 — means a signal 20 dB below the noise in an SSB-width channel, which is why you cannot hear FT8 contacts you are making.
Packet structure
What part of a packet radio frame contains the routing and handling information?
Header.
Header, then payload, then a frame check sequence. The header is what makes routing possible: it names the sender, the destination, and any digipeaters along the way.
Mesh networks
Which is true of mesh network microwave nodes?
If one node fails, a packet may still reach its target station via an alternate node
Self-healing by design. A mesh has no single path and no single point of failure, which is exactly the property that makes it attractive for emergency infrastructure — and the reason it is worth building before an emergency rather than during one.
Digital voice
Which of the following provide digital voice modes?
DMR, D-STAR, and SystemFusion.
Three incompatible systems, all in current amateur use, none able to talk to the others on the air. Which one is worth having is a question about what your local repeaters run, not about technical merit.
Reading a waterfall
Which of the following describes a waterfall display?
Frequency is horizontal, signal strength is intensity, time is vertical.
Three dimensions on a flat screen, which is why it beats a plain spectrum plot for digital work: you see not just what is present but what it has been doing.
What is indicated on a waterfall display by one or more vertical lines on either side of a data mode or RTTY signal?
Overmodulation
Those extra lines are intermodulation products from an overdriven transmitter, sitting either side of the intended signal. They are visible to everyone using that band.
This is the single most useful diagnostic in digital operating, because your own radio’s meters will not show it and other operators will not always tell you. If you can see your own signal on a WebSDR or a second receiver, look for those lines — and if they are there, reduce your drive until they vanish.
Check yourself
- Why does typing in capitals slow PSK31 down?
- An FT8 report says −18. What does that mean, and could you hear the station?
- You see faint vertical lines either side of your own digital signal on a WebSDR. What is wrong and what is the fix?
Answers
- Varicode is variable-length and optimised for lowercase, so uppercase letters use longer bit sequences.
- The signal-to-noise ratio is 18 dB below the noise in a 2.5 kHz bandwidth — well beyond what an ear can decode. No.
- Overmodulation — intermodulation products from too much drive. Reduce transmit audio level and drive until the lines disappear.