FCC Exam Question: 3-59H6

What is one way that voice is transmitted in a pulse-width modulation system?

A. A standard pulse is varied in amplitude by an amount depending on the voice waveform at that instant.
B. The position of a standard pulse is varied by an amount depending on the voice waveform at that instant.
C. A standard pulse is varied in duration by an amount depending on the voice waveform at that instant.
D. The number of standard pulses per second varies depending on the voice waveform at that instant.
Correct Answer: C

Explanation: In Pulse-Width Modulation (PWM), the instantaneous amplitude of the modulating signal, such as a voice waveform, directly controls the *duration* (or width) of a series of pulses. A louder (higher amplitude) portion of the voice signal will cause the generated pulse to be wider, while a quieter (lower amplitude) portion will result in a narrower pulse. This varying pulse duration then carries the voice information for transmission. Option A describes Pulse-Amplitude Modulation (PAM), where the pulse's amplitude is varied. Option B describes Pulse-Position Modulation (PPM), where the pulse's timing or position is varied. Option D describes Pulse-Frequency Modulation (PFM), where the number of pulses per second (frequency) varies.

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Includes Elements 1, 3, 6, 7R, 8, and 9.