In modem-based data transmission, demodulation refers to the process of converting the incoming analog carrier back into the original digital data signal at the receiver.

Introduction / Context:
In data communication with modems, information is often sent over analog channels (such as telephone lines) by modulating a carrier. The receiver must reverse this process to recover the original digital bits. This reverse operation is called demodulation.

Given Data / Assumptions:

• We are discussing modem-based communication over analog media.
• Modulation maps digital bits onto an analog waveform for transmission.
• Demodulation recovers the digital data from the received waveform.

Concept / Approach:
Modulation = digital-to-analog mapping at the transmitter. Demodulation = analog-to-digital recovery at the receiver. The demodulator estimates symbol values from the waveform (e.g., amplitude, frequency, phase) and decides which bits they represent.

Step-by-Step Solution:
1) The transmitter modulates bits onto an analog carrier (e.g., PSK, FSK, QAM).2) The channel adds noise/impairments; the signal remains analog.3) The receiver's demodulator samples and filters the analog waveform.4) It estimates symbol parameters and maps them back to binary values.5) Output is a digital bitstream representing the original data.

Verification / Alternative check:
Loopback testing shows that applying modulation followed by demodulation reproduces the original digital data, within the error rate expected for the chosen scheme and channel quality.

Why Other Options Are Wrong:
Converting digital to analog signals: That is modulation, not demodulation.
Multiplexing various signals: That is a separate function (e.g., FDM, TDM), not demodulation.
Performing data description: Not a standard communication process term.
Filtering high-frequency noise: Filtering may be part of receivers, but it is not the definition of demodulation.

Common Pitfalls:
Assuming demodulation always yields analog output; in data modems it yields digital bits. Also confusing demodulation with decoding of higher-layer codes.

Final Answer:
converting analog to digital signals