Editing Amplitude modulation (section)

== Foundation ==
In [[electronics]] and [[telecommunications]], [[modulation]] is the variation of a property of a [[continuous wave]] [[carrier signal]] according to an information-bearing signal, such as an [[audio signal]] which represents sound, or a [[video signal]] which represents images. In this sense, the carrier wave, which has a much higher frequency than the message signal, ''carries'' the information. At the receiving station, the message signal is extracted from the modulated carrier by [[demodulation]].

In general form, a modulation process of a [[sine wave|sinusoidal carrier wave]] may be described by the following equation:<ref>{{cite book |author=AT&T Bell Laboratories Staff |title=Telecommunication Transmission Engineering |volume=1—''Principles''|edition=2|publisher=AT&T Bell Center for Technical Education|year=1977}}</ref>

:<math>m(t) = A(t) \cdot \cos(\omega t + \phi(t))\,</math>.

''A(t)'' represents the time-varying amplitude of the sinusoidal carrier wave and the cosine-term is the carrier at its [[angular frequency]] <math>\omega</math>, and the instantaneous phase deviation <math>\phi(t)</math>. This description directly provides the two major groups of modulation, amplitude modulation and [[angle modulation]]. In angle modulation, the term ''A''(''t'') is constant and the second term of the equation has a functional relationship to the modulating message signal. Angle modulation provides two methods of modulation, [[frequency modulation]] and [[phase modulation]].

In amplitude modulation, the angle term is held constant and the first term, ''A''(''t''), of the equation has a functional relationship to the modulating message signal.

The modulating message signal may be analog in nature, or it may be a digital signal, in which case the technique is generally called [[amplitude-shift keying]].

For example, in AM radio communication, a continuous wave radio-frequency signal has its amplitude modulated by an audio waveform before transmission. The message signal determines the ''[[Envelope (waves)|envelope]]'' of the transmitted waveform. In the [[frequency domain]], amplitude modulation produces a signal with power concentrated at the [[carrier frequency]] and two adjacent [[sideband]]s. Each sideband is equal in [[bandwidth (signal processing)|bandwidth]] to that of the modulating signal, and is a mirror image of the other. Standard AM is thus sometimes called "double-sideband amplitude modulation" (DSBAM).

A disadvantage of all amplitude modulation techniques, not only standard AM, is that the receiver amplifies and detects [[Noise (radio)|noise]] and [[electromagnetic interference]] in equal proportion to the signal. Increasing the received [[signal-to-noise ratio]], say, by a factor of 10 (a 10 [[decibel]] improvement), thus would require increasing the transmitter power by a factor of 10. This is in contrast to [[frequency modulation]] (FM) and [[digital radio]] where the effect of such noise following demodulation is strongly reduced so long as the received signal is well above the threshold for reception. For this reason AM broadcast is not favored for music and [[high fidelity]] broadcasting, but rather for voice communications and broadcasts (sports, news, [[talk radio]] etc.).

AM is inefficient in power usage, as at least two-thirds of the transmitting power is concentrated in the carrier signal. The carrier signal contains none of the transmitted information (voice, video, data, etc.). However, its presence provides a simple means of demodulation using [[envelope detection]], providing a frequency and phase reference for extracting the message signal from the sidebands. In some modulation systems based on AM, a lower transmitter power is required through partial or total elimination of the carrier component, however receivers for these signals are more complex because they must provide a precise carrier frequency reference signal (usually as shifted to the [[intermediate frequency]]) from a greatly reduced "pilot" carrier (in [[reduced-carrier transmission]] or DSB-RC) to use in the demodulation process. Even with the carrier eliminated in [[double-sideband suppressed-carrier transmission]], carrier regeneration is possible using a [[Costas loop|Costas phase-locked loop]]. This does not work for [[single-sideband suppressed-carrier transmission]] (SSB-SC), leading to the characteristic "Donald Duck" sound from such receivers when slightly detuned. Single-sideband AM is nevertheless used widely in [[amateur radio]] and other voice communications because it has power and bandwidth efficiency (cutting the RF bandwidth in half compared to standard AM). On the other hand, in [[medium wave]] and [[short wave]] broadcasting, standard AM with the full carrier allows for reception using inexpensive receivers. The broadcaster absorbs the extra power cost to greatly increase potential audience.

=== Shift keying ===
A simple form of digital amplitude modulation which can be used for transmitting [[digital data|binary data]] is [[on–off keying]], the simplest form of amplitude-shift keying, in which [[binary numeral system|ones and zeros]] are represented by the presence or absence of a carrier. On–off keying is likewise used by radio amateurs to transmit [[Morse code]] where it is known as continuous wave (CW) operation, even though the transmission is not strictly "continuous". A more complex form of AM, [[quadrature amplitude modulation]] is now more commonly used with digital data, while making more efficient use of the available bandwidth.

=== Analog telephony ===
A simple form of amplitude modulation is the transmission of speech signals from a traditional analog telephone set using a common battery local loop.<ref>{{cite book |author=AT&T Bell Laboratories Staff |editor=R.J. Rey |title=Engineering and Operations in the Bell System|edition=2 |location=Murray Hill, NJ|year=1984|page=211|publisher=AT&T Bell Laboratories|isbn=0-932764-04-5}}</ref> The direct current provided by the central office battery is a carrier with a frequency of 0&nbsp;Hz. It is modulated by a microphone (''transmitter'') in the telephone set according to the acoustic signal from the speaker. The result is a varying amplitude direct current, whose AC-component is the speech signal extracted at the central office for transmission to another subscriber.

=== Amplitude reference ===
An additional function provided by the carrier in standard AM, but which is lost in either single or double-sideband suppressed-carrier transmission, is that it provides an amplitude reference. In the receiver, the [[automatic gain control]] (AGC) responds to the carrier so that the reproduced audio level stays in a fixed proportion to the original modulation. On the other hand, with suppressed-carrier transmissions there is ''no'' transmitted power during pauses in the modulation, so the AGC must respond to peaks of the transmitted power during peaks in the modulation. This typically involves a so-called ''fast attack, slow decay'' circuit which holds the AGC level for a second or more following such peaks, in between syllables or short pauses in the program. This is very acceptable for communications radios, where [[Dynamic range compression|compression]] of the audio aids intelligibility. However, it is absolutely undesired for music or normal broadcast programming, where a faithful reproduction of the original program, including its varying modulation levels, is expected.