Frequency-shift keying (FSK) is most commonly applied in which practical communication service?

Introduction / Context:
Frequency-shift keying (FSK) is a digital modulation method that conveys binary symbols by shifting the frequency of a carrier between discrete values. Understanding which services predominantly use FSK clarifies the mapping between modulation families and application domains in communication systems.

Given Data / Assumptions:

• FSK = digital keying by frequency shifting between symbol states.
• We compare with telephony, satellite analog TV, broadcast radio, etc.

Concept / Approach:

FSK was historically and is currently used in telegraphy and low-to-moderate data-rate links (e.g., RTTY, paging, low-speed modems, wireless sensor links). These applications require robust symbol recognition under noise and fading at modest bandwidths. Analog telephony and analog broadcast systems primarily use amplitude or frequency modulation of continuous waveforms—not symbol-by-symbol frequency hopping between tones as in FSK.

Step-by-Step Solution:

Verification / Alternative check:

Standard textbooks list FSK, MSK, GFSK for telemetry, paging, and control channels; telegraphy is the archetypal example of FSK usage.

Why Other Options Are Wrong:

• Telephony (analog PSTN) uses analog voice; not digital FSK over the entire call path.
• Analog satellite TV used FM video carriers, not symbol-by-symbol FSK.
• AM/FM radio broadcasting uses analog modulation, not FSK for program audio.
• FM stereo pilot is a single fixed tone (19 kHz), not FSK.

Common Pitfalls:

Assuming “frequency modulation” used in broadcast FM is the same as “frequency-shift keying.” FM is continuous deviation; FSK uses discrete frequency states for symbols.

Final Answer:

Telegraphy / data keying (digital keying systems)