Q.Why is AM transmission noisy?
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AM carries information as amplitude, and noise is mostly amplitude variation, so noise corrupts the message and cannot be removed.
In amplitude modulation the message is contained in the changing amplitude (envelope) of the carrier. Almost all interference — lightning, electrical machinery, ignition systems, other sparks — produces sharp variations in amplitude and adds directly to the envelope of the AM wave.
When this noise-contaminated wave reaches the receiver, the detector responds to the whole envelope and cannot tell which amplitude changes are the wanted message and which are due to noise. Moreover, an amplitude limiter (which would clip amplitude variations) cannot be used, because that would also destroy the message. As a result, the noise passes straight through to the output. …
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