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Amateur Radio Info & Exams - Signals & Emissions 2 - Digital

The number of different characters a system can convey or display is essentially down to the number of bits in each character. The number is 2n, where n is the number of bits. (The same concept can also be applied to the number of colours or shades a digital image system can handle.)

At the low end, 3 bit allow use to handle 8 values, 0 to 7, called "Octal". For 10 digits (0-9), we need 4 bits, called Binary Coded Decimal, BCD; with this also allowing 16 values, 0 to 9, then A to F, called Hexadecimal.


Used for RTTY and Telex, Baudot uses only 5 bits, but with a clever cheat, called SHIFT characters. Thus the first bank of 32 allowed 26 uppercase letters, the all-important Shift to Figures (11011), and common space, carriage return, line feed, and NULL (00000). The second bank has numerals, brackets, a slash, a couple of maths symbols, and punctuation; Shift to Letters (11111), and the BELL, which in older machines rang a metal bell, but on more modern ones sounded a buzzer, in both cases to attract the attention of the operator.


If you have ever wondered why older air tickets and boarding passes only use upper case, this is due to a 6 bit system, which allows 64 values, covering letters, numerals, and a basic punctuation.


This is a 7 or 8 bit system, with 128 or 256 characters, allowing upper and lower case characters, numerals, lots of punctuation and symbols, and optionally, "Latin" or European characters such as vowels with umlauts, more maths symbols, etc.

Again, there are "cheats", such as "Code Pages", allowing additional characters to be displayed, or for use in countries with extended Latin or totally different character sets.

EBCDIC is an alternative, used by some large IBM systems, and a spoiler below.


This is a 16 bit system, used to allow a theoretical 65,536 characters, meaning pretty much every character from every language can be displayed, plus a great many symbols. This does not have specific uses in Amateur Radio that I am aware of, although some of the symbols on these pages are part of the Unicode system.

Spread Spectrum

Previously the preserve of military systems, various spread spectrum techniques are now used more widely, especially in CDMA, and newer W-CDMA telephony systems. Some amateurs use various spread spectrum techniques.

Its wide bandwidth means interference on discrete frequencies causes little degradation of the received signal.

Relevant Questions

These are actual questions from the Extra exam pool.

How is Forward Error Correction implemented?
A. By the receiving station repeating each block of three data characters
B. By transmitting a special algorithm to the receiving station along with the data characters
C. By transmitting extra data that may be used to detect and correct transmission errors
D. By varying the frequency shift of the transmitted signal according to a predefined algorithm

FEC involves transmitting extra data, to allow errors to be detected, and potentially corrected, answer C.

What is the definition of symbol rate in a digital transmission?
A. The number of control characters in a message packet
B. The duration of each bit in a message sent over the air
C. The rate at which the waveform of a transmitted signal changes to convey information
D. The number of characters carried per second by the station-to-station link

This is the rate at which the waveform of a transmitted signal changes, noting that each change may convey more than one bit of information, answer C.

Why should phase-shifting of a PSK signal be done at the zero crossing of the RF signal?
A. To minimize bandwidth
B. To simplify modulation
C. To improve carrier suppression
D. All these choices are correct

In true phase shift keying, having the phase change at the zero crossing point reduces bandwidth, answer A.

What technique minimizes the bandwidth of a PSK31 signal?
A. Zero-sum character encoding
B. Reed-Solomon character encoding
C. Use of sinusoidal data pulses
D. Use of trapezoidal data pulses

It uses sinusoidal data pulses, presumably because sinewaves have low harmonic content, answer C.

What is the approximate bandwidth of a 13-WPM International Morse Code transmission?
A. 13 Hz
B. 26 Hz
C. 52 Hz
D. 104 Hz

This is around 52 Hertz, answer C.

What is the necessary bandwidth of a 170-hertz shift, 300-baud ASCII transmission?
A. 0.1 Hz
B. 0.3 kHz
C. 0.5 kHz
D. 1.0 kHz

An addition of the shift and the bit-rate allows an estimation of the bandwidth, and in this case it is 500 Hz, or 0.5 kHz, answer C.

What is the necessary bandwidth of a 4800-Hz frequency shift, 9600-baud ASCII FM transmission?
A. 15.36 kHz
B. 9.6 kHz
C. 4.8 kHz
D. 5.76 kHz

The bandwidth exceeds the symbol and the baud rate, so it must be answer A, 15.36 kHz.

This is a little more than the sum of the shift and data rate.

How does ARQ accomplish error correction?
A. Special binary codes provide automatic correction
B. Special polynomial codes provide automatic correction
C. If errors are detected, redundant data is substituted
D. If errors are detected, a retransmission is requested

This is used in modes such as AX-25 packet, where if an error is detected, a retransmission request is sent, answer D.

Which digital code allows only one bit to change between sequential code values?
A. Binary Coded Decimal Code
B. Extended Binary Coded Decimal Interchange Code
C. Excess 3 code
D. Gray code

This clever system is called Gray code, answer D.

It is especially useful for when we want to read the absolute position of a rotating object (rather than relative movement, as on the dial of a digitally controlled transceiver). Apparently it is also used in digital TV data streams, as a change in more than one bit indicates an error.

How may symbol rate be increased without increasing bandwidth?
A. It is impossible
B. Increasing analog-to-digital conversion resolution
C. Using a more efficient digital code
D. Using forward error correction

A more efficient digital code allows more data to be transmitted in a given bandwidth, answer C.

What is the relationship between symbol rate and baud?
A. They are the same
B. Baud is twice the symbol rate
C. Symbol rate is only used for packet-based modes
D. Baud is only used for RTTY

These are the same, answer A.

What factors affect the bandwidth of a transmitted CW signal?
A. IF bandwidth and Q
B. Modulation index and output power
C. Keying speed and shape factor (rise and fall time)
D. All these choices are correct

This is the keying speed, and the shape of the Morse elements (how fast the signal rises and falls), answer C.

Why are received spread spectrum signals resistant to interference?
A. Signals not using the spread spectrum algorithm are suppressed in the receiver
B. The high power used by a spread spectrum transmitter keeps its signal from being easily overpowered
C. The receiver is always equipped with a digital blanker
D. If interference is detected by the receiver it will signal the transmitter to change frequencies

Signals not recognised by the algorithm are ignored, answer A.

What spread spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier?
A. Frequency hopping
B. Direct sequence
C. Binary phase-shift keying
D. Phase compandored spread spectrum

This is direct sequence, answer B.

How does the spread spectrum technique of frequency hopping work?
A. If interference is detected by the receiver it will signal the transmitter to change frequencies
B. If interference is detected by the receiver it will signal the transmitter to wait until the frequency is clear
C. A binary bit stream is used to shift the phase of an RF carrier very rapidly in a pseudorandom sequence
D. The frequency of the transmitted signal is changed very rapidly according to a pseudorandom sequence also used by the receiving station

As the name implies, the signal rapidly jumps from one frequency to the other, followed by a sequence known by equipment at both stations (but not by the enemy's stations), answer D.

What is the primary effect of extremely short rise or fall time on a CW signal?
A. More difficult to copy
B. The generation of RF harmonics
C. The generation of key clicks
D. Limits data speed

This causes key clicks which can extend across the band in use, and interfere with other users, answer C.

What is the most common method of reducing key clicks?
A. Increase keying waveform rise and fall times
B. Low-pass filters at the transmitter output
C. Reduce keying waveform rise and fall times
D. High-pass filters at the transmitter output

We need to increase the rise and fall times, answer A.

What is the advantage of including parity bits in ASCII characters?
A. Faster transmission rate
B. The signal can overpower interfering signals
C. Foreign language characters can be sent
D. Some types of errors can be detected

It allows some errors to be detected, answer D.

If a glitch causes 10110011 to be read as 10110111 the number of ones changes from odd to even. This would no longer correlate with the value of the parity bit accompanying the byte.

What is a common cause of overmodulation of AFSK signals?
A. Excessive numbers of retries
B. Ground loops
C. Bit errors in the modem
D. Excessive transmit audio levels

Note this asks the cause, and this is an incorrectly set up soundcard, modem, or transceiver putting an excessive audio level into the transmitter, or having its input gain too high, answer D.

E8D08 (D)
What parameter evaluates distortion of an AFSK signal caused by excessive input audio levels?
A. Signal to noise ratio
B. Baud rate
C. Repeat Request Rate (RRR)
D. Intermodulation Distortion (IMD)

Excessive levels can cause a high level of "intermod", answer D.

What is considered an acceptable maximum IMD level for an idling PSK signal?
A. +10 dB
B. +15 dB
C. -20 dB
D. -30 dB

This is the lowest level, -30 dB, answer D.

What are some of the differences between the Baudot digital code and ASCII?
A. Baudot uses 4 data bits per character, ASCII uses 7 or 8; Baudot uses 1 character as a letters/figures shift code, ASCII has no letters/figures code
B. Baudot uses 5 data bits per character, ASCII uses 7 or 8; Baudot uses 2 characters as letters/figures shift codes, ASCII has no letters/figures shift code
C. Baudot uses 6 data bits per character, ASCII uses 7 or 8; Baudot has no letters/figures shift code, ASCII uses 2 letters/figures shift codes
D. Baudot uses 7 data bits per character, ASCII uses 8; Baudot has no letters/figures shift code, ASCII uses 2 letters/figures shift codes

Cutting a long question short, Baudot has 5 bits, seen in the 5 holes on Telex paper tape, answer B.

To ensure an entire message is not garbled if a single shift is lost, separate characters are used as figures shift, and letters shift (remembering that there are only 32 possible characters using 5 bits). ASCII uses 7 or 8 bits, and there is no need for shift codes, as there are 128 or 256 characters which can be displayed, the latter allowing display of European accented characters such as "é" in Café, and Greek derived mathematical symbols such as Ω and ω.

What is one advantage of using ASCII code for data communications?
A. It includes built in error correction features
B. It contains fewer information bits per character than any other code
C. It is possible to transmit both upper and lower case text
D. It uses one character as a shift code to send numeric and special characters

ASCII, and related codes, such as EBCDIC, have enough codes to allow (26 letters × 2) + 10 numerals + lots of punctuation in the space allowed by 7 bits, 27 = 128 options; so upper and lower case text is possible, answer C.

On to: Antennas 1 - Parameters and Patterns

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, June 2022.

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