E8C: Digital signals: digital communications modes; CW; information rate vs. bandwidth; spread-spectrum communications; modulation methods
Which one of the following digital codes consists of elements having unequal length?
What are some of the differences between the Baudot digital code and ASCII?
Baudot uses five data bits per character, ASCII uses seven; Baudot uses two characters as shift codes, ASCII has no shift code
Baudot uses four data bits per character, ASCII uses seven; Baudot uses one character as a shift code, ASCII has no shift code
Baudot uses six data bits per character, ASCII uses seven; Baudot has no shift code, ASCII uses two characters as shift codes
Baudot uses seven data bits per character, ASCII uses eight; Baudot has no shift code, ASCII uses two characters as shift codes
What is one advantage of using the ASCII code for data communications?
It is possible to transmit both upper and lower case text
It includes built-in error-correction features
It contains fewer information bits per character than any other code
It uses one character as a shift code to send numeric and special characters
What technique is used to minimize the bandwidth requirements of a PSK-31 signal?
Use of sinusoidal data pulses
Zero-sum character encoding
Reed-Solomon character encoding
Use of trapezoidal data pulses
What is the necessary bandwidth of a 13-WPM international Morse code transmission?
Approximately 52 Hz
Approximately 13 Hz
Approximately 26 Hz
Approximately 104 Hz
What is the necessary bandwidth of a 170-hertz shift, 300-baud ASCII transmission?
What is the necessary bandwidth of a 4800-Hz frequency shift, 9600-baud ASCII FM transmission?
What term describes a wide-bandwidth communications system in which the transmitted carrier frequency varies according to some predetermined sequence?
Amplitude compandored single sideband
Time-domain frequency modulation
Which of these techniques causes a digital signal to appear as wide-band noise to a conventional receiver?
What spread-spectrum communications technique alters the center frequency of a conventional carrier many times per second in accordance with a pseudo-random list of channels?
Time-domain frequency modulation
Frequency compandored spread-spectrum
What spread-spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier?
Binary phase-shift keying
Phase compandored spread-spectrum
What makes spread-spectrum communications resistant to interference?
Only signals using the correct spreading sequence are received
Interfering signals are removed by a frequency-agile crystal filter
Spread-spectrum transmitters use much higher power than conventional carrier-frequency transmitters
Spread-spectrum transmitters can hunt for the best carrier frequency to use within a given RF spectrum
What is the advantage of including a parity bit with an ASCII character stream?
Some types of errors can be detected
Faster transmission rate
The signal can overpower interfering signals
Foreign language characters can be sent
What is one advantage of using JT-65 coding?
Virtually perfect decoding of signals well below the noise
Uses only a 65 Hz bandwidth
Easily copied by ear if necessary
Permits fast-scan TV transmissions over narrow bandwidth
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