Signal modulation
Modulation varies a carrier wave to transmit information.
Signal modulation alters one or more traits of a repeating waveform in electronics and telecommunications to carry information. This process takes a message signal—like audio from a microphone, video from a camera, or a digital bitstream from a computer—and impresses it onto a carrier signal for transmission. Carrier waves become necessary when the message signal’s frequency is too low for practical transmission; a radio antenna must be about one-quarter of the transmitted wave’s wavelength, and for low-frequency waves, that wavelength can stretch kilometers, making such an antenna unfeasible.
Another reason to modulate is to send multiple information channels over a single medium using frequency-division multiplexing (FDM). In cable television, for example, many carriers, each modulated with a different TV channel, travel through one cable. Because each carrier occupies its own frequency range, the channels don’t interfere. At the receiving end, a demodulator extracts the original modulation signal from the carrier.
A modulator is a device or circuit that performs modulation, while a demodulator (sometimes called a detector) does the reverse. A modem—short for modulator–demodulator—handles both tasks in bidirectional communication. The lower-frequency band occupied by the modulation signal is called the baseband; the higher-frequency band of the modulated carrier is the passband.
Modulation techniques are fundamental to wireless communication, encoding data by varying a carrier’s amplitude, frequency, or phase.
- Antenna length ratio
- one-fourth of the wavelength
- Symbol rate example
- 1000 symbols/second (1000 baud)
- Bit rate example
- 2000 bits per second
- M equals 2 to n
- M = 2^N alternative symbols
Lore & Background
Signal modulation techniques are fundamental methods used in wireless communication to encode information onto a carrier wave by varying its amplitude, frequency, or phase. Analog modulation methods include amplitude modulation (AM), where the amplitude of the carrier signal is varied by the modulation signal, and frequency modulation (FM), where the frequency is varied. These were the earliest types of modulation, used to transmit audio signals in AM and FM radio broadcasting. Other analog methods include double-sideband modulation, single-sideband modulation, vestigial-sideband modulation, quadrature amplitude modulation (QAM), phase modulation (PM), and transpositional modulation (TM).
Digital modulation methods impress a digital signal consisting of a sequence of binary digits onto the carrier by mapping bits to elements from a discrete alphabet. Fundamental digital techniques include amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK). In quadrature amplitude modulation (QAM), an in-phase signal and a quadrature phase signal are amplitude modulated with a finite number of amplitudes and summed, equivalent to a combination of PSK and ASK. More advanced digital methods include orthogonal frequency-division multiplexing (OFDM), which splits data across multiple closely spaced sub-carriers, each modulated separately, and is used in WiFi networks, digital radio stations, and digital cable television transmission.
Reader's Guide
Signal modulation is significant because it enables practical wireless communication by allowing low-frequency message signals to be transmitted via higher-frequency carrier waves, overcoming antenna size constraints. It also permits multiple channels of information to be transmitted through a single communication medium using frequency-division multiplexing (FDM), as in cable television where many carrier signals, each modulated with a different television channel, are transported through a single cable without interference. The legacy of modulation includes both analog systems like AM and FM broadcasting, which were the earliest types, and modern digital systems such as Wi-Fi, Bluetooth, cellular networks, and satellite communications that use techniques like PSK, QAM, OFDM, and spread spectrum. Digital modulation methods, considered a form of digital-to-analog conversion, allow efficient data transmission by encoding multiple bits per symbol, as illustrated by a modem using four tones to represent two bits each, achieving a bit rate twice the symbol rate. The article notes that most textbooks consider digital modulation schemes as a form of digital transmission, synonymous to data transmission, while very few consider it as analog transmission.
Did You Know?
- Receiving a radio wave requires an antenna length of one-fourth the wavelength of the transmitted wave.
- A modem playing 1000 tones per second at 2 bits per tone achieves a bit rate of 2000 bits per second.
More in Radio Modulation Modes 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced