Learning The Fundamental Digital Modulation Methods Part3

The  famous  Bell  103A  dataset  standard  was  also  introduced  by  Bell  Labs in 1962. It provided full-duplex service at 300 baud over normal phone  lines.  Frequency-shift  keying  was  used  with  the  call  originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at  2,025  or  2,225 Hz.  The  readily  available  103A2
gave  an  important boost to the use of remote low-speed terminals such as the  KSR33, the ASR33, and the  IBM 2741.  AT&T  reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.3.2    Significance of Digital Modulation The aim of digital modulation  is to transfer a  digital  bit stream over an analog  band  pass  channel;  for  example  over  the  public  switched telephone network  (where a  band pass filter  limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band.The aim of analog modulation is to transfer an analog baseband (or low pass) signal, for example an audio signal or  TV  signal, over an analog band  pass channel,  for  example  a  limited  radio  frequency  band  or  a cable TV network channel. Analog  and  digital  modulation  facilitate  Frequency  Division Multiplexing  (FDM),  where  several  low  pass  information  signals  are transferred simultaneously over the same shared physical medium, using  separate pass band channels. The aim of  digital base band modulation  methods, also known as  line coding,  is  to  transfer  a  digital  bit  stream  over  a  base band  channel, typically a non-filtered copper wire such as a  serial bus  or a wired  local area network. The  aim  of  pulse  modulation  methods  is  to  transfer  a  narrow band analog  signal,  for  example  a  phone  call  over  a  wide band  base band channel or, in some of the schemes, as a bit stream over another  digital transmission system. In music synthesisers, modulation may be used to synthesise waveforms with  a  desired  overtone  spectrum.  In  this  case  the  carrier  frequency  is typically  in  the  same  order  or  much  lower  than  the  modulating waveform.  See  for  example  frequency  modulation  synthesis  or  ring modulation. In  analog  modulation,  the  modulation  is  applied  continuously  in
response to the analog information signal.