Signal modulation

For the musical use of "modulation", see modulation (music).

Modulation is the process of varying a carrier signal, typically a sinusoidal signal, in order to use that signal to convey information. One of the three key characteristics of a signal are usually modulated: its phase, frequency or amplitude. A device that performs modulation is known as a modulator and a device that performs demodulation is known as a demodulator. A device that can do both operations is a modem, a contraction of the two.

In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represnted on a constellation diagram.

In analogue modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.

Modulation is generally performed to overcome the signal transmission issues. Some of the reasons are:

• For easy (low loss, low dispersion) propagation as electromagnetic waves
• Simultaneously transmitted without interference from other signals
• Smaller and audience oriented antennas
• Combine multiple data signals on one carrier(multiplex)

Normally carrier signals are high frequency electromagnetic waves.

Analogue Modulation Techniques

• Phase modulation (PM)
• Frequency modulation (FM)
• Amplitude modulation (AM)
  • Single-sideband modulation (SSB, or SSB-AM), very similar to single-sideband suppressed carrier modulation (SSB-SC)
  • Vestigial-sideband modulation (VSB, or VSB-AM)
• Sigma-delta modulation (∑Δ)

Digital Modulation Techniques

• Phase-shift keying (PSK)
• Frequency-shift keying (FSK) and audio frequency-shift keying (AFSK)
  • Minimum-shift keying (MSK)
  • Gaussian minimum-shift keying (GMSK)
  • Very minimum-shift keying (VMSK)
• Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)
• Quadrature amplitude modulation (QAM), a combination of PSK and ASK.
• Trellis coded modulation (TCM) also known as trellis modulation

Often incorrectly referred to as a modulation scheme, orthogonal frequency division multiplexing (OFDM) usually takes advantage of one of the digital techniques. It is also known as discrete multitone (DMT). When OFDM is used in conjunction with channel coding techniques, it is described as Coded orthogonal frequency division multiplexing (COFDM). OFDM is strictly a channel access method and not a modulation scheme.

Pulse modulation

These are hybrid digital and analogue techniques.

• Pulse-code modulation (PCM)
• Pulse-width modulation (PWM)
• Pulse-amplitude modulation (PAM)
• Pulse-position modulation (PPM)
• Pulse-density modulation (PDM)

Miscellaneous Techniques

• The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.
• Adaptive modulation
• Wavelet modulation

See also

• Types of radio emissions
• Communications channel
• Channel access methods
• Channel coding
• Line code
• Telecommunication
• Modem
• RF modulator
• Codec

External links

• "Data Encoding Techniques" and "Specifications for Data Encoding" discuss the various encoding techniques that have been used with various types of Ethernet.