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ication a continuous wave radio frequency signal has its amplitude modulated by an audio waveform before transmission the message signal determines the envelope of the transmitted waveform in the frequency domain amplitude modulation produces a signal with power concentrated at the carrier frequency and two adjacent sidebands each sideband is equal in 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 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 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 phase locked loop 6 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 edit a simple form of digital amplitude modulation which can be used for transmitting binary data is on off keying the simplest form of amplitude shift keying in which 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 7 analog telephony edit a simple form of amplitude modulation is the transmission of speech signals from a traditional analog telephone set using a common battery local loop 8 the direct current provided by the central office battery is a carrier with a frequency of 0 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 edit 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 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 itu type designations edit in 1982 the international telecommunication union itu designated the types of amplitude modulation designation description a3e double sideband a full carrier the basic amplitude modulation scheme r3e single sideband reduced carrier h3e single sideband full carrier j3e single sideband suppressed carrier b8e independent sideband emission c3f vestigial sideband lincompex linked compressor and expander a submode of any of the above itu emission modes history edit one of the crude pre vacuum tube am transmitters a telefunken arc transmitter from 1906 the carrier wave is generated by 6 electric arcs in the vertical tubes connected to a tuned circuit modulation is done by the large carbon microphone cone shape in the antenna lead one of the first vacuum tube am radio transmitters built by meissner in 1913 with an early triode tube by robert von lieben he used it in a historic 36 km 22 mi voice transmission from berlin to nauen germany compare its small size with the arc transmitter above amplitude modulation was used in experiments of multiplex telegraph and telephone transmission in the late 1800s 9 however the practical development of this technology is identified with the period between 1900 and 1920 of radiotelephone transmission that is the effort to send audio signals by radio waves the first radio transmitters called spark gap transmitters transmitted information by wireless telegraphy using pulses of the carrier wave to spell out text messages in morse code they could not transmit audio because the carrier consisted of strings of damped waves pulses of radio waves that declined to zero and sounded like a buzz in receivers in effect they were already amplitude modulated 10 11 continuous waves edit the first am transmission was made by canadian born american researcher reginald fessenden 12 on 23 december 1900 13 using a spark gap transmitter with a specially designed high frequency 10 khz interrupter 14 over a distance of one mile 1 6 km at cobb island maryland us his first transmitted words were hello one two three four is it snowing where you are mr thiessen 13 though his words were perfectly intelligible the spark created a loud and unpleasant noise 14 fessenden was a significant figure in the development of am radio he was one of the first researchers to realize from experiments like the above that the existing technology for producing radio waves the spark transmitter was not usable for amplitude modulation and that a new kind of transmitter one that produced sinusoidal continuous waves was needed this was a radical idea at the time because experts believed the impulsive spark was necessary to produce radio frequency waves and fessenden was ridiculed he invented and helped develop one of the first continuous wave transmitters the alexanderson alternator with which he made what is considered the first am public entertainment broadcast on christmas eve 1906 he also discovered the principle on which am is based heterodyning and invented one of the first detectors able to rectify and receive am the electrolytic detector or liquid baretter in 1902 other radio detectors invented for wireless telegraphy such as the fleming valve 1904 and the crystal detector 1906 also proved able to rectify am signals so the technological hurdle was generating am waves receiving them was not a problem 11 36 55 75 195 10 76 77 116 117 125 133 134 162 early technologies edit early experiments in am radio transmission conducted by fessenden valdemar poulsen ernst ruhmer quirino majorana charles herrold and lee de forest were hampered by the lack of a technology for amplification the first practical continuous wave am transmitters were based on either the huge expensive alexanderson alternator developed 1906 1910 or versions of the poulsen arc transmitter arc converter invented in 1903 the modifications necessary to transmit am were clumsy and resulted in very low quality audio modulation was usually accomplished by a carbon microphone inserted directly in the antenna or ground wire its varying resistance varied the current to the antenna the limited power handling ability of the microphone severely limited the power of the first radiotelephones many of the microphones were water cooled 15 1102 vacuum tubes edit the 1912 discovery of the amplifying ability of the audion tube invented in 1906 by lee de forest solved these problems the vacuum tube feedback oscillator invented in 1912 by edwin armstrong and alexander meissner was a cheap source of continuous waves and could be easily modulated to make an am transmitter modulation did not have to be done at the output but could be applied to the signal before the final amplifier tube so the microphone or other audio source didn t have to modulate a high power radio signal wartime research greatly advanced the art of am modulation and after the war the availability of cheap tubes sparked a great increase in the number of radio stations experimenting with am transmission of news or music the vacuum tube was responsible for the rise of am broadcasting around 1920 the first electronic mass communication medium amplitude modulation was virtually the only type used for radio broadcasting until fm broadcasting began after world war ii 11 203 205 229 230 237 242 10 174 177 235 355 357 at the same time as am radio began telephone companies such as at t were developing the other large application for am sending multiple telephone calls through a single wire by modulating them on separate carrier frequencies called frequency division multiplexing 9 single sideband edit in 1915 john renshaw carson formulated the first mathematical description of amplitude modulation showing that a signal and carrier frequency combined in a nonlinear device creates a sideband on both sides of the carrier frequency passing the modulated signal through another nonlinear device can extract the original baseband signal 9 his analysis also showed that only one sideband was necessary to transmit the audio signal and carson patented single sideband modulation ssb on 1 december 1915 9 this advanced variant of amplitude modulation was adopted by at t for longwave transatlantic telephone service beginning 7 january 1927 after ww ii it was developed for military aircraft communication analysis edit illustration of amplitude modulation the carrier wave sine wave of frequency f c and amplitude a is expressed by c t a sin 2 π f c t displaystyle c t a sin 2 pi f_ c t the message signal such as an audio signal that is used for modulating the carrier is m t and has a frequency f m much lower than f c m t m cos 2 π f m t ϕ a m cos 2 π f m t ϕ displaystyle m t m cos left 2 pi f_ m t phi right am cos left 2 pi f_ m t phi right where m is the amplitude sensitivity m is the amplitude of modulation if m 1 1 m t a is always positive for undermodulation if m 1 then overmodulation occurs and reconstruction of message signal from the transmitted signal would lead in loss of original signal amplitude modulation results when the carrier c t is multiplied by the positive quantity 1 m t a y t 1 m t a c t 1 m cos 2 π f m t ϕ a sin 2 π f c t displaystyle begin aligned y t left 1 frac m t a right c t left 1 m cos left 2 pi f_ m t phi right right a sin left 2 pi f_ c t right end aligned in this simple case m is identical to the modulation index discussed below with m 0 5 the amplitude modulated signal y t thus corresponds to the top graph labelled 50 modulation in figure 4 using prosthaphaeresis identities y t can be shown to be the sum of three sine waves y t a sin 2 π f c t 1 2 a m sin 2 π f c f m t ϕ sin 2 π f c f m t ϕ displaystyle y t a sin 2 pi f_ c t frac 1 2 am left sin left 2 pi left f_ c f_ m right t phi right sin left 2 pi left f_ c f_ m right t phi right right therefore the modulated signal has three components the carrier wave c t which is unchanged in frequency and two sidebands with frequencies slightly above and below the carrier frequency f c 4 spectrum edit figure 2 double sided spectra of baseband and am signals a useful modulation signal m t is usually more complex than a single sine wave as treated above however by the principle of fourier decomposition m t can be expressed as the sum of a set of sine waves of various frequencies amplitudes and phases carrying out the multiplication of 1 m t with c t as above the result consists of a sum of sine waves again the carrier c t is present unchanged but each frequency component of m at f i has two sidebands at frequencies f c f i and f c f i the collection of the former frequencies above the carrier frequency is known as the upper sideband and those below constitute the lower sideband the modulation m t may be considered to consist of an equal mix of positive and negative frequency components as shown in the top of figure 2 one can view the sidebands as that modulation m t having simply been shifted in frequency by f c as depicted at the bottom right of figure 2 16 75 76 figure 3 the spectrogram of an am voice broadcast shows the two sidebands green on either side of the carrier red with time proceeding in the vertical direction the short term spectrum of modulation changing as it would for a human voice for instance the frequency content horizontal axis may be plotted as a function of time vertical axis as in figure 3 it can again be seen that as the modulation frequency content varies an upper sideband is generated according to those frequencies shifted above the carrier frequency and the same content mirror imaged in the lower sideband below the carrier frequency at all times the carrier itself remains constant and of greater power than the total sideband power power and spectrum efficiency edit the rf bandwidth of an am transmission refer to figure 2 but only considering positive frequencies is twice the bandwidth of the modulating or baseband signal since the upper and lower sidebands around the carrier frequency each have a bandwidth as wide as the highest modulating...