Amplitude Modulation (AM) plus frequency division multiplexing (FDM) is one
way of solving above problem. Each conversation is shifted to a different part of
the  frequency  spectrum  by using a high-frequency waveform to "carry" each
individual speech signal. These high frequencies are called carrier frequencies .

     Amplitude modulation is the process of varying the amplitude of the sinusoidal
carrier wave by the amplitude of the modulating signal, and is illustrated in Fig. 5
 

     The unmodulated carrier wave has a constant peak  value  and  a  higher
frequency  than  the modulating signal , but, when  the modulating signal is applied,
the peak  value  of  the  carrier varies in accordance with the instantaneous value
of the modulating signal, and the outline wave shape or  "envelope" of the
modulated wave's peak values is the same as the original modulating signal wave
shape. The modulating signal  waveform  has  been  superimposed  on  the  carrier
wave.

     When a sinusoidal carrier wave of frequency  fc Hz  is amplitude - modulated
by a sinusoidal modulating  signal  of  frequency  fm Hz , then  the  modulated
carrier  wave  contains  three frequencies .

             1) fc Hz  : Original carrier frequency
             2) ( fc + fm ) Hz  :  The sum of carrier and modulating signal frequencies
             3) ( fc - fm )  Hz  :  The difference between carrier and modulating signal

     This is illustrated in Fig. 6
 

     It should be noted that two of these frequencies are new, being produced by
the amplitude-modulation process, and are called side-frequencies.

     The sum of carrier and modulating signal frequencies is called the upper
side-frequency. The difference between carrier and modulating signal frequency is
called the lower side-frequency. This is illustrated in the frequency spectrum
diagram of  Fig. 7
 
   

     The bandwidth of the modulated carrier wave is

                ( fc + fm ) - ( fc - fm ) = 2 fm
 
            i.e. double the modulating signal frequency
 

     The complete amplitude-modulated wave band of lower sideband plus carrier
plus upper sideband shown in Fig. 8  takes up more frequency bandwidth than is
really necessary to transmit the information signal since all the information is
carried by either one of the sidebands alone . The carrier component is of constant
amplitude and frequency so does not carry any of the information signal at all . It is
possible by using special equipment to suppress both the carrier and one sideband
and to transmit just the other sideband with no loss of information. This method of
working is called single sideband working ( SSB ) . This method is not used for
domestic radio broadcasting , but it is used for some long-distance radio telephony
systems and for multi-channel carrier systems used in national telephone networks.
 
 
   

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