In digital communications, what conversion does a modulator perform when preparing data for transmission over passband channels such as radio or telephone circuits? Choose the pair that correctly fills the blanks: A modulator converts a _____ signal to a(n) _____ signal.

Introduction:
A core task at the physical layer is adapting the form of information to suit the channel. Computers generate discrete-time, discrete-amplitude data (bits), but many practical channels—especially legacy copper loops and radio links—are optimized for band-limited, continuous-time waveforms. This question tests whether you know the direction of conversion performed by a modulator before transmission.

Given Data / Assumptions:

• Source data is digital (bits from a computer or digital interface).
• The transmission medium favors continuous, band-limited signals (for example, voice-band telephone line or RF channel).
• A modulator is used on the transmit side; a demodulator is used on the receive side.

Concept / Approach:
Modulation maps digital symbols onto the parameters of a carrier—amplitude, frequency, or phase—creating an analog passband waveform. Common schemes include ASK, FSK, PSK, and QAM. The receiver applies demodulation to recover the original symbols. This pair of operations (modulate/demodulate) explains the portmanteau “modem.” Therefore, the correct fill-in is: a modulator converts a digital signal to an analog signal suitable for the channel.

Step-by-Step Solution:
Identify the domain mismatch: digital source vs. analog-friendly channel.Recall the role of a modulator: map bits/symbols to carrier waveform parameters.State the transformation: digital → analog at the transmitter.Conclude that the correct pair is “digital; analog.”

Verification / Alternative check:
Consider a classic telephone-line modem: it accepts serial digital data and outputs audio-frequency tones. Similarly, cellular “modems” produce RF waveforms from digital packets.

Why Other Options Are Wrong:

• FSK; PSK and PSK; FSK (A, B): these are two types of analog modulation, not input/output categories.
• analog; digital (C): this describes demodulation, not modulation.
• None of the above (E): invalid because “digital; analog” is correct.

Common Pitfalls:
Confusing the direction of conversion (mixing up modulator vs. demodulator) or treating specific schemes (FSK, PSK) as if they described the input/output domains.

Final Answer:
digital; analog