Learning The Fundamental Digital Modulation Methods Part2

A low-frequency message signal (top) may be carried by an  AM  or  FM radio wave. Common analog modulation techniques are as listed below. Amplitude Modulation  (AM)-  the amplitude of the carrier signal
is  varied  in  accordance  to  the  instantaneous  amplitude  of  the  modulating signal Double-Side band Modulation (DSB) Double-side band  modulation with carrier (DSB-WC) is used on  the AM radio broadcasting band Double-side band suppressed-carrier transmission
(DSB-SC)  Double-side band reduced carrier transmission (DSB-RC) Single-side band modulation (SSB, or SSB-AM), SSB with carrier (SSB-WC) SSB suppressed carrier modulation (SSB-SC) Vestigial side band modulation (VSB, or VSB-AM) Quadrature Amplitude Modulation (QAM) Angle modulation Frequency  Modulation  (FM)-  here  the  frequency  of  the  carrier signal is  varied in accordance to the instantaneous amplitude of  the modulating signal. Phase Modulation  (PM)-  here the phase shift of the carrier signal is  varied  in accordance  to  the  instantaneous  amplitude  of  the  modulating signal The accompanying figure shows the results of (amplitude-) modulating a signal onto a carrier (both of which are sine waves). At any point along
the  y-axis, the amplitude of the modulated signal is equal to the  sum of  the carrier signal. In  digital  modulation, an analog carrier signal is modulated by a digital  bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding  demodulation  or detection as
analog-to-digital  conversion.  The  changes  in  the  carrier  signal  are chosen from a  finite  number of  M  alternative symbols (the  modulation  alphabet) A simple example-  a telephone line is designed for transferring audible sounds-  for  example  tones,  and  not  digital  bits  (zeros  and  ones). Computers may however communicate over a telephone line by means of  modems,  which  are  representing  the  digital  bits  by  tones,  called symbols.  If  there  are  four  alternative  symbols  (corresponding  to  a musical instrument that can generate four different tones, one at a time), the first symbol  may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since  each  tone  (i.e.,  symbol)  represents  a  message  consisting  of  two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dial up modems
as opposed to DSL modems.