Discussion
What is the fundamental reason modulation is used at all?
Answer: C
Modulation exists to translate the message spectrum to a band where radiation, propagation and channel allocation all work: antennas of practical size, a channel the regulator has assigned, and a propagation mechanism that reaches the receiver. A three-kilohertz speech signal cannot be radiated efficiently by any antenna anyone would build, which is the whole problem. Bandwidth is generally preserved or increased rather than reduced, and in wideband schemes it is deliberately increased in exchange for noise performance. Nothing about the translation improves the source's own signal-to-noise ratio. And conversion between analogue and digital is a separate operation that may or may not precede it.
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