What is demodulation?

The reverse of modulation — extracting original information from the carrier at the receiver. AM uses envelope detection; FM uses frequency discrimination or PLLs.

Why is modulation necessary?

Enables frequency multiplexing (multiple signals share the airwaves), efficient antenna radiation (small antennas work at high frequencies), and signal processing advantages.

What modulation does Wi-Fi use?

OFDM with up to 1024-QAM per subcarrier. Thousands of narrow-band signals each carry up to 10 bits per symbol, achieving multi-gigabit data rates.

Modulation Definition

Quick Answer

Modulation is the process of varying a carrier signal's properties (amplitude, frequency, or phase) according to an information signal. The carrier transports information across a communication channel. Demodulation is the reverse — extracting information from the modulated carrier. Modulation enables wireless communication, frequency multiplexing, and efficient spectrum use.

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Modulation: Carrier Meets Information

At its simplest, modulation means changing a steady wave to carry information. A perfectly steady tone at 100 MHz carries no information because it never changes. Vary its loudness according to your voice — amplitude modulation. Vary its pitch — frequency modulation. Vary its timing — phase modulation. The 100 MHz carrier wave acts as a vehicle, and your voice rides on it. The receiver's demodulation extracts the voice, discarding the carrier.

Key Formulas

Why Not Just Transmit the Original Signal?

Voice at 300-3400 Hz — if everyone transmitted at these frequencies, all conversations overlap. Modulation shifts each to a different carrier, enabling coexistence. Additionally, efficient antenna radiation requires size proportional to wavelength. At 3 kHz, wavelength is 100 km (impractical antenna). At 100 MHz, wavelength is 3 meters (practical). Modulation makes both multiplexing and efficient radiation possible.

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Types of Modulation at a Glance

Analog: AM (broadcast radio), FM (high-quality audio), PM (some communications). Digital: OOK (on-off keying, simplest), FSK (two frequencies for 0 and 1), PSK (phase changes encode bits), QAM (combination — highest data rates). Modern wireless uses OFDM — thousands of narrow subcarriers each independently modulated, used in Wi-Fi, 4G/5G, and digital TV.

Demodulation: Recovering the Original Signal

AM demodulation is simplest: an envelope detector (diode + capacitor) extracts amplitude variations. FM demodulation uses a frequency discriminator or phase-locked loop. Digital demodulation samples at symbol intervals and decides which symbol was sent using signal detection theory. Coherent demodulation (tracks carrier phase) achieves better performance than non-coherent, at the cost of receiver complexity.

Modulation in the Mathematical Framework

AM is multiplication in time: s(t) = x(t)·cos(ωt). By the convolution theorem, this shifts the message spectrum to the carrier frequency, creating sidebands. FM involves integration inside a cosine, creating nonlinear spectral properties described by Bessel functions. The Laplace transform analyzes modulation circuits — mixers, oscillators, filters, and PLLs all have transfer functions. The LAPLACE Calculator helps analyze these systems.

Frequently Asked Questions

Changing a carrier wave to carry information. Like Morse code varies a tone, modulation varies a radio wave's amplitude, frequency, or phase to transmit voice, music, or data.

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