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The number of different characters a system can convey or display is essentially down to the number of bits used to send or store 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 lower end, 3 bits allows us to handle 8 values, 0 to 7, called "Octal".
For 10 digits (0-9), we need 4 bits, called Binary Coded Decimal, BCD. 4 bits also allowing 16 values, 0 to 9, then A to F, called Hexadecimal. (A byte (8 bits) can be expressed using two "hex" characters, such as E3.)
Off topic: If you had an earlier Palm Pilot handheld computer you had 2 bit grey-scale, providing the light green background, light and dark grey, and black; although most apps just used 1 bit, meaning green or black. Later ones had 4 bits (16 levels), then colour. The PNGs of my sketches and photos on this site are 8 bits times 3 colours RGB (red, green, and blue), meaning 24 bits per pixel. Lossless data compression is used, not JPEGs which can leave artefacts around black lines. An Alpha channel is available to provide transparency, this mode being RGBA. GREY and GREYA are also options. 16 bits per colour is also available in all these modes. I only use transparency for T-shirts, as this allows the fabric around text or irregularly shaped features, while also allowing white elements over coloured fabric. Examples: VK-73 and Be Great, Don't Over-modulate!.
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 is 11111. 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. Messages could be stored on paper tape with space for 5 holes across its width.
It is formalised as the International telegraphy alphabet No. 2 (Baudot–Murray code).
Heavy mechanical teletype machines are featured in "Good Morning Vietnam", printing news re the war, etc. More modern units used dot matrix printers, and eventually a small screen as well as the printer.
If you have ever wondered why older air tickets and boarding passes only used UPPER CASE, this is due to a 6 bit system, which allows 64 values, covering letters, numerals, and a basic punctuation. That said, remote typesetting used 6 bits with four shift levels, to allow upper and lower case, plus upper and lower case in italics or bold. 6 bits is a distractor below, incorrectly attributed to 5-bit Baudot.
This is a 7 or 8 bit system, with 128 or 256 characters (not all printing). The original version was a formalisation of a 7 bit system used by Teletype Corporation, allowing 33 control characters (many no longer needed) and 95 printable characters: Upper and lower case characters used in English, numerals, and a good selection of punctuation and symbols. It stands for "American Standard Code for Information Interchange", and is also listed in MIL-STD-188-100, and ISO and other standards.
Control characters include Line Feed, Carriage Return, and Form Feed (which advances to the next page is printing). BEL / BELL is also available on some systems, sometimes invoked using Ctrl-G. ACK and NAK may be used to indicate an acknowledgement or negative-acknowledgement, but IGNORE the reference to "special binary codes" in the ARQ question.
These were also used on glass screen terminals, such as the Digital Equipment Corporation VT (Video Terminal) series. Home computers and PCs had terminal emulators with various features.
Home computers using BASIC, such as the MicroBee and VZ-200/300 (Laser 210/310), along with BASIC on DOS used ASCII, although some (including the VZ) only displayed upper case. ASC() and CHR$ could be used thus: M = ASC(MID$(A1$,P,1)) is used to return the ASCII value of a single character in a string (in decimal), then conditionally subtract (or add) 32 from it, and add it to a new string in only upper (or lower) case using N1$ = N1$ + CHR$(M). If a teenager can work this out then if someone actually wanted to censor a "cooker" dribbling about "vAcCiNaTiOn" on farcebook, I doubt that alternating case would hide the term. The very good, and free BASIC-256 includes UPPER and LOWER commands, which also work on additional European letters, such as the Spanish ñ, pronounced "ny", as in canyon.
Using 8 bits allows the use of an extended character set with values 128 - 255. This provided "Latin" or European characters such as vowels with acutes (é in café), diaeresis / umlauts (used to indicate the second vowel in Zoë is a new sound), ligatures (eg: Æ, æ), special consonants (Ç, ñ), currency symbols, fractions, and more maths symbols such as μ and Ω. 227 or E3 (above) is π. Lines to make boxes and solid or shaded blocks are also included.
Another trick is "Code Pages", with different characters in part of the upper block, for use in countries with additional extended Latin or totally different character sets. Code page 861 is Icelandic, with the thorn character, Þ and þ once used in English, although replaced by "y" when using imported lead type sets, hence ye for þe, meaning and pronounced "the". This is not the second person plural pronoun, now replaced by you. Others code pages include Hebrew, Greek, Cyrillic, Nordic, French Canadian, and Portuguese.
One standard terms these "8-bit single-byte coded graphic character sets". An example is ISO/IEC 8859-4:1998 - Latin alphabet No. 4, which includes Greenlandic characters.
Another work-around, used to display text in Nepali is using the Preeti font along with standard ASCII values.
Parity bits are added to data streams containing ASCII characters. Those who worked with serial data (RS-232), terminals, and/or configured modems, etc, will know 7-E-1 means 7 bits of data, an even parity bit, and one stop bit. Even parity means that the value of the final bit is set to ensure that the number of 1s is even. 1010001 has 3 ones, so a 1 is added to the end; 1101001 has 4 ones, so a 0 is added, giving 11010010. Odd parity does the opposite.
8-N-1 was also common, 8 bits, no parity bit, and 1 stop bit.
EBCDIC is an alternative, used in various IBM systems, it is natively 8 bit. It stands for Extended Binary Coded Decimal Interchange Code, which is a spoiler below. The BCD bit is because the second nibble (4 bits) for letters is encoded using values from 1 to 9, and numbers using values from 0 to 9. Some sorting in OS/2 follows its order, such as numbers after letters, not before. It has multiple code pages, which in modern cases now includes the euro symbol, €.
This is a more complex system, allowing over 1.1 million characters, meaning pretty much every character from every language can be displayed, plus a great many maths and other symbols. In UTF-8 encoding 1 byte is used for basic ASCII, 2 for European and West Asian (Near and Middle Eastern) languages; 3 when using East Asian languages, and a 4th when using things like Emojis. 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. I suppose it is useful when passing messages in languages which use other characters, in preference to ASCII code-pages, Preeti font, etc.
As an example, the "Almost equal to" character, ≈ can be added to a web page by several means, such as using decimal: ≈, hexadecimal: ≈, as an entity: ≈ or ≈, or directly in the text, like other characters: ≈
Some display in colour, depending on the browser: 🌮 gives: 🌮 the TACO emoji, and it looks like he already has re Greenland.
During the initial response to a major disaster having a PC and printer (using toner rather than ink which runs when damp) able to produce signs in various languages is very useful. They might warn of contaminated water, etc.
Just as there are various ways to modulate voice onto a signal there a range of methods for sending data. These have various bandwidths.
Morse Code, in the form where a continuous wave is interrupted, also termed CW, has a narrow occupied bandwidth. A commercially available radio in CW mode limits the steepness of the turn on and turn off of the carrier emission, to limit bandwidth, and to avoid "key-clicks" across the band. 13 WPM in the example equates to 10.8333 elements per second. The examiner has determined that the bandwidth of this signal is 52 Hz. This is also the width of a very narrow DSP filter for CW.
Frequency Shift Keying has been used for radio teletype for many decades, for things such as news services; and diplomatic communications, with appropriate encryption. One tone is a Mark or 1, the other Space or 0. AX-25 packet also uses it. More complex versions are used in things like the WSJT modes.
Each operator's FT8 transmission is very narrow, with a bandwidth of 50 Hz. The DXpedition's transmission may be the exception, in Fox and Hound operation.
The bandwidth is approximately the frequency shift plus the symbol rate. A 170 Hz shift and a 300 baud rate adds up to 470 Hz or around half a kilohertz (the answer to the removed question was 0.5 kHz). Note that this applies when using an SSB radio, not necessarily to tones sent via an FM one.
FSK is also termed AFSK as in the vast majority of cases it is generated by applying an audio signal to the microphone or line input of the radio. True FSK also exists, where the transmit frequency is directly switched. This was used in the Soviet Union on Long Wave, to send teletype to the Far East.
Higher speed packet operates at 9600-baud, and uses a 4800-Hz frequency shift. Add these and get 14,400 Hz or 14.4 kHz. The only similar answer is 15.36 kHz. This uses a special "9600" or High Speed Data / Packet port on radios, often with a mini-DIN connector.
Phase-Shift Keying is used for a range of modes. The most famous is PSK31. "Important" items are: The phase of a PSK signal should be changed at the zero crossing point of the RF signal to minimise (minimize) bandwidth. The use of sinusoidal data pulses in PSK31 also helps minimise bandwidth.
PSK31 is most often used on MF and HF, but can be used on VHF and UHF. See: SigID Wiki: PSK
Quadrature Amplitude Modulation (QAM) is a method which provides a good data throughput. It works by modulating the amplitude of two carriers of the same frequency which are 90 degrees out of phase.
The FSK, PSK, and QAM modes are typically sent using SSB (LSB) on MF, HF, VHF, and UHF. As an AM related mode having too high an audio level passing from the modem or soundcard (or motherboard sound connections or USB sound module), or having the microphone / input gains set too high results in overmodulation, as stated in the question. When using 1200 bps AFSK FM the risk is instead over-deviation, a DISTRACTOR below.
A constellation diagram of a QAM or QPSK signal shows the possible phase and amplitude states for each symbol.
You an see them on Wikipedia: QAM and Wikipedia: PSK; as well as Wikipedia: Constellation diagram.
A vector signal analyser can display the Constellation diagram of a received signal. Noise means that they are not perfect spots, but balls or clouds of points around the ideal point.
Previously the preserve of military systems, various spread spectrum techniques are now used more widely, especially in CDMA, and newer W-CDMA telephony systems, likewise Bluetooth. Some amateurs use various spread spectrum techniques. Note that these need radios able to emit a signal across a much wider bandwidth than an off-the-shelf VHF-UHF all mode radio does. Use is permitted on 70 cm and shorter.
The military version is often frequency hopping, rapidly varying the frequency of a transmitted signal according to a pseudorandom sequence, known only to allied stations (if any remain).
Many other systems use direct sequence spread spectrum (DSSS), which uses a high-speed binary bit stream to shift the phase of an RF carrier.Its wide bandwidth means interference on discrete frequencies causes little degradation of the received signal. Thus some random suburban priest / vicar / father using a wireless microphone across 50 kHz at a few milliwatts is hardly going to take out a TV station spreading across around 7 MHz with many kilowatts into a high gain antenna array.
You could try: Spread Spectrum Scene
Hedy Kiesler Markey (Hedy Lamarr) was a co-inverter of Frequency Hopping, as presented in U.S. patent 2,292,387. Andrew Viterbi and his research are very interesting. A pre-WW2 refugee to the US escaping Italian fascism, he helped develop the phase-locked loop, and later CDMA and direct sequence spread spectrum. His name below is however a distractor.
While her patent is cited as "prior art" in many other patents, I think "An actress invented Wi-Fi!" or similar claims, are overstating the situation.
Baud rate is often conflated with bit rate, and in cases where there are only two possible states this is true. Simple FSK and Binary PSK (BPSK) are examples of this. However, QPSK (Quadrature PSK or 4-PSK) has 4 positions, and these allow 2 bits to be send for each baud. 8-PSK, 16-PSK are further examples.
QAM, has between 4 and 64 states, meaning growing numbers of bits per baud. Microwave systems running 1024-QAM are getting a gigabit per second in 56 MHz of bandwidth.
Each signal element in termed a symbol, so symbol rate is the same as baud. With PSK31 that means 31 symbols per second, or 31 baud.
It we do not wish to increase bandwidth (or are not allowed to) but want a greater throughput we need to make each symbol work harder, meaning having more bits carried in each symbol. Going from BPSK to QPSK doubles throughput from 31 bits to 62 bits per second, but this assumes a reasonable quality signal path, as attempting to go too fast means error rates increase*. PACTOR IV is a great example, able to go from 46.9 bps to 5512.5 bps over 10 steps, from 2 tone chirp to 32-QAM.
* Due to the debacle around ending 3G in Australia I needed a new 'phone to make voice calls on Telstra. I chose a 5G capable ZTE and it has worse throughput than the old 3G/4G handset I am now using as a modem, even with it on an inferior network. Basically, the modulation system is fragile. In fact, on the ZTE my PC's browser often reports an error before loading the page a second or so later. I wonder if it actually stepping down from 5G to 4G.
The above are the physical aspect of data transmission, but many systems divide a data stream into packets. AX-25 packet and Pactor are Ham radio examples. The transmitting station calculates a parity or check character and includes it in the packet. If there the receiving station finds its calculation on the content does not match the check character it asks for the packet to be resent using the ARQ system.
AX.25 is a simple system which DOES NOT use forward error correction. FEC sends extra "redundant" data which helps the receiving station reconstruct the transmitted data (and right now in clear weather the FEC system in my DVB(T) set-top box is doing a great job ensuring no visual or audible glitches on the show I have on, but it reaches its limit during heavy rain). The examiner makes no reference to FX.25, nor Q15X25, both of which do use this (each has a Wikipedia article, as does Improved Layer 2 Protocol (IL2P)).
Some Hams use modified Wi-Fi routers, which operate in the 2.4 GHz ISM band, part of which is also the 13 cm Ham band, including using gain antennas. They use IP addresses, which stands for Internet Protocol.
Just as your 'phone might search for and establish a connection to stored networks (yours, your friend's. the Library's, etc), they use discovery and link establishment protocols.
One way to detect the degree of rotation of a disc is to cut a series slots in it, and pass beams of light through these, to light sensors; or to reflect light of light or dark strips. A bunch of microswitches or direct electrical contacts could also be used; so could magnets and reed switches.
If we want 16 positions we could use 4 bit binary, and position 0 to position 15, before returning to 0. However, going from position 7 to 8 we go from 0111 to 1000. A slight overlap of transmission and we have 1111, position 15, around 180 degrees of error. Between 15 and 0 an uneven transition mean a jump to the value for pretty much any position.
Named for Frank Gray of Bell Labs, and also termed the reflected binary code (RBC), Gray Code removes these errors.
The PP column in the position or sector number; HEXD is the situation above using Binary Coded Hexadecimal, with up to 4 transitions at once; and GRAY is the (or a) Gray Code.
PP HEXD GRAY
00 0000 0000
01 0001 0001
02 0010 0011
03 0011 0010
04 0100 0110
05 0101 0111
06 0110 0101
07 0111 0100
08 1000 1100
09 1001 1101
10 1010 1111
11 1011 1110
12 1100 1010
13 1101 1011
14 1110 1001
15 1111 1000
As you can see (or perhaps have heard via a text reader, and noted; sorry if this is difficult), moving from 1 to 2 involves a single change instead of two. Thus at the boundary of these two ranges the maximum error can only be at the few degrees between the two sectors of the disc, and the displayed values can only be for the values relating to those sectors, it cannot glitch to a value several sectors or more away.
Balanced Gray code is a more complex version, with 4 transitions for each position. Single-track Gray code is also cool. Read more on the various types at: Wikipedia: Gray code
Gray Code is also used in the placement of values on a constellation diagram.
These are actual questions from the Extra exam pool.
E8C01
What is Quadrature Amplitude Modulation or QAM?
A. A technique for digital data compression used in digital television which removes redundancy in the data by comparing bit amplitudes
B. Transmission of data by modulating the amplitude of two carriers of the same frequency but 90 degrees out of phase
C. A method of performing single sideband modulation by shifting the phase of the carrier and modulation components of the signal
D. A technique for analog modulation of television video signals using phase modulation and compression
This is AM of two carriers which are at 90 degrees to each other, answer B.
E8C02
What is the definition of symbol rate in a digital transmission?
A. The number of control characters in a message packet
B. The maximum rate at which the forward error correction code can make corrections
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.
E8C03
Why should the phase of a PSK signal be changed 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.
E8C04
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.
E8C05
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.
E8C06
What is the bandwidth of an FT8 signal?
A. 10 Hz
B. 50 Hz
C. 600 Hz
D. 2.4 kHz
FT8 is very narrow, with multiple channels able to fit within one SSB voice channel. It is 50 Hz, answer B.
E8C07
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.
E8C08
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.
E8C09
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. Extended ASCII
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 tuning 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.
Off topic, as with old ball mouse encoders, free turning dials often use two bits which generate a pulse train which is in quadrature, and the relative changes determine the direction.
E8C10
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.
E8C11
What is the relationship between symbol rate and baud?
A. They are the same
B. Baud is twice the symbol rate
C. Baud rate is half the symbol rate
D. The relationship depends on the specific code used
These are the same, answer A.
E8C12
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.
E8C13
What is described by the constellation diagram of a QAM or QPSK signal?
A. How many carriers may be present at the same time
B. The possible phase and amplitude states for each symbol
C. Frequency response of the signal stream
D. The number of bits used for error correction in the protocol
If you read articles (including on Wikipedia) on these modes you will see diagrams showing the possible phase and amplitude states for each symbol, answer B.
E8C14
What type of addresses do nodes have in a mesh network?
A. Email
B. Trust server
C. Internet Protocol (IP)
D. Talk group
These are networks formed using modified Wi-Fi routers, often otherwise used to provide internet access within homes, colleges, and businesses, etc. Thus they use Internet Protocol (IP) addresses, answer C.
E8C15
What technique do individual nodes use to form a mesh network?
A. Forward error correction and Viterbi codes
B. Acting as store-and-forward digipeaters
C. Discovery and link establishment protocols
D. Custom code plugs for the local trunking systems
Just as your 'phone might search for and establishes a connection to stored networks, they use discovery and link establishment protocols, answer C.
E8D01
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. Built-in error correction codes minimize interference
D. If the receiver detects interference, it will signal the transmitter to change frequencies
Signals not recognised by the algorithm are ignored, answer A.
E8D02
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.
E8D03
Which describes spread spectrum frequency hopping?
A. If interference is detected by the receiver, it will signal the transmitter to change frequencies
B. RF signals are clipped to generate a wide band of harmonics which provides redundancy to correct errors
C. A binary bit stream is used to shift the phase of an RF carrier very rapidly in a pseudorandom sequence
D. Rapidly varying the frequency of a transmitted signal according to a pseudorandom sequence
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.
E8D04
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. More difficult to tune
This causes key clicks which can extend across the band in use, and interfere with other users, answer C.
E8D05
What is the most common method of reducing key clicks?
A. Increase keying waveform rise and fall times
B. Insert low-pass filters at the transmitter output
C. Reduce keying waveform rise and fall times
D. Insert high-pass filters at the transmitter output
We need to increase the rise and fall times, answer A.
E8D06
What is the advantage of including parity bits in ASCII characters?
A. Faster transmission rate
B. Signal-to-noise ratio is improved
C. A larger character set is available
D. Some types of errors can be detected
It allows some errors to be detected, answer D.
If a glitch causes a 7 bit + odd parity byte 10110011 to be read as 10110111 the number of ones changes from odd to even. This would trigger the error related process. I however think "appending parity bits to ASCII characters" would be more accurate. However, if 11 was lost, and made 00 then three ones remain, clearly still odd. Y would become A.
E8D07
What is a common cause of overmodulation of AFSK signals?
A. Excessive numbers of retries
B. Excessive frequency deviation
C. Bit errors in the modem
D. Excessive transmit audio levels
Note this asks the cause, and this is an incorrectly set up soundcard, or modem putting an excessive audio level into the transmitter, or having its input gain too high, answer D.
Excessive frequency deviation would the symptom, in the case of FM, although this appears to be referring to SSB, as used with digital modes on MF & HF.
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.
E8D09
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.
E8D10
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 large 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 "á" in "Olá!", and Greek derived mathematical symbols such as Ω and ω.
E8D11
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.
Interestingly, the use of ASCII in US government systems was mandated by Democratic President Lyndon B. Johnson in 1968.
ASCII is a standard registered with what is now ANSI, the American National Standards Institute.
What was referred to as "ANSI text" by some PC users, etc, used escape sequences (the ESC character followed by codes) to instruct screen based terminals to move the cursor and change text and background colours, and to flash text. The ANSI.SYS driver on DOS PC allowed these to be displayed.
| J-DOS v7.77 SELF TEST: |
| DISK 1 PARITY TEST: PASSED |
| DISK 2 PARITY TEST: FAILED |
| DISK 3 PARITY TEST: .... |
Copy and Paste this section into OpenOffice and watch for a moment. :-)
A skilled Telex or RTTY operator can decode a message with a missing shift:
"MA'AM, OUR CURRENT PRICE IS HUE Z VAT." can be decoded to "MA'AM, OUR CURRENT PRICE IS £73 + VAT."
"SIR, WE CAN ONLY OFFER £67 + =-5." can be decoded to "SIR, WE CAN ONLY OFFER £67 + VAT.". This might leave the recipient making the weighing-up motion while repeating "6 7, 6 7".
By the way, VAT, like GST, is a consumption tax applied on both local and imported goods, and some services, but is not a tariff. In some countries baked beans (local, single market, or imported) are untaxed as a basic food, but a bean and meat product (chilli / chili) is considered a luxury, and is taxed, whether local or a US import. However like a US tariff, they are included in the price offered to consumers, but often not in business-to-business sales, where it is added, as above. Taxed items on a supermarket bill are indicated with * or #, and the tax amount shown at the bottom.
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, April 2026.
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