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Amplitude modulation ('''AM''') is a technique used in electronic communication, most commonly for transmitting information via a Radio Carrier Wave . AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. (Contrast this with Frequency Modulation , also commonly used for audio transmissions, in which the Frequency is varied; and Phase Modulation , often used in Remote Controls , in which the Phase is varied.) In the mid-1870s, a form of amplitude modulation—initially called "undulatory currents"—was the first method to successfully produce quality audio over telephone lines. Beginning with Reginald Fessenden 's audio demonstrations in the early 1900s, it was also the original method used for audio radio transmissions, and remains in use by some forms of radio communication—"AM" is often used to refer to the Mediumwave broadcast Band (see AM Radio ). FORMS OF AMPLITUDE MODULATION As originally developed for the electric telephone, amplitude modulation was used to add audio information to the low-powered direct current flowing from a telephone transmitter to a receiver. As a simplified explanation, at the transmitting end, a telephone microphone was used to vary the strength of the transmitted current, according to the frequency and loudness of the sounds received. Then, at the receiving end of the telephone line, the transmitted electrical current affected an electromagnet, which strengthened and weakened in response to the strength of the current. In turn, the electromagnet produced vibrations in the receiver diaphragm, thus reproducing the frequency and loudness of the sounds originally heard at the transmitter. In contrast to the telephone, in radio communication what is modulated is a Continuous Wave radio signal ( Carrier Wave ) produced by a radio transmitter. In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent Sideband s. Each sideband is equal in Bandwidth to that of the modulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebands and a carrier is often called ''double sideband amplitude modulation'' (DSB-AM). Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information. To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal. This produces a Reduced-carrier Transmission or ''double-sideband suppressed-carrier'' (DSBSC) signal. A suppressed-carrier amplitude modulation scheme is three times more power-efficient than traditional DSB-AM. If the carrier is only partially suppressed, a ''double-sideband reduced-carrier'' (DSBRC) signal results. DSBSC and DSBRC signals need their carrier to be regenerated (by a Beat Frequency Oscillator , for instance) to be demodulated using Conventional Techniques . Even greater efficiency is achieved—at the expense of increased transmitter and receiver complexity—by completely suppressing both the carrier and one of the sidebands. This is Single-sideband Modulation , widely used in Amateur Radio due to its efficient use of both power and bandwidth. A simple form of AM often used for Digital communications is '' On-off Keying '', a type of '' Amplitude-shift Keying '' by which Binary data is represented as the presence or absence of a carrier wave. This is commonly used at radio frequencies to transmit Morse Code , referred to as Continuous Wave (CW) operation. In 1982, the International Telecommunications Union (ITU) designated the various types of amplitude modulation as follows: EXAMPLE Double Sideband AM A carrier wave is modelled as a simple sine wave, such as: : where the radio frequency (in Hz ) is given by: For generality, and are arbitrary constants that represent the carrier amplitude and initial phase. For simplicity, we set their respective values to 1 and 0. Let ''m''(''t'') represent an arbitrary waveform that is the message to be transmitted. And let the constant ''M'' represent its largest magnitude. For instance: : Thus, the message might be just a simple audio tone of frequency It is generally assumed that and that Then amplitude modulation is created by forming the product: represents another constant we may choose. The values ''A''=1, and ''M''=0.5, produce a ''y''(''t'') depicted by the graph labelled "50% Modulation" in Figure 4. For this simple example, ''y''(''t'') can be trigonometrically manipulated into the following equivalent form: : Therefore, the modulated signal has three components, a carrier wave and two sinusoidal waves (known as Sidebands ) whose frequencies are slightly above and below Also notice that the choice A=0 eliminates the carrier component, but leaves the sidebands. That is the DSBSC transmission mode. To generate double-sideband full carrier (A3E), we must choose: For more general forms of ''m''(''t''), trigonometry is not sufficient. But if the top trace of Figure 2 depicts the frequency spectrum, of ''m''(''t''), then the bottom trace depicts the modulated carrier. It has two groups of components: one at positive Frequencies (centered on ) and one at Negative Frequencies (centered on ). Each group contains the two sidebands and a narrow component in between that represents the energy at the carrier frequency. We need only be concerned with the positive frequencies. The negative ones are a mathematical artifact that contains no additional information. Therefore, we see that an AM signal's spectrum consists basically of its original (2-sided) spectrum shifted up to the carrier frequency. :::For those interested in the mathematics of Figure 2, it is a result of computing the Fourier Transform of: using the following transform pairs: :::: :::: ::::
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