Digital Transmission Study Notes

  • Digital-to-digital conversion involves three techniques: line coding, block coding,
    and scrambling.
  • Line coding is the process of converting digital data to a digital signal.
  • We can roughly divide line coding schemes into five broad categories: unipolar,
    polar, bipolar, multilevel, and multitransition.
  • Block coding provides redundancy to ensure synchronization and inherent error
    detection. Block coding is normally referred to as mB/nB coding; it replaces each
    m-bit group with an n-bit group.
  • Scrambling provides synchronization without increasing the number of bits. Two
    common scrambling techniques are B8ZS and HDB3.
  • The most common technique to change an analog signal to digital data (digitization) is called pulse code modulation (PCM).
  • The first step in PCM is sampling. The analog signal is sampled every Ts s, where Ts
    is the sample interval or period. The inverse of the sampling interval is called the
    sampling rate or sampling frequency and denoted by fs, where fs = lITs. There are
    three sampling methods-ideal, natural, and flat-top.
  • According to the Nyquist theorem, to reproduce the original analog signal, one
    necessary condition is that the sampling rate be at least twice the highest frequency
    in the original signal.
  • Other sampling techniques have been developed to reduce the complexity of PCM.
    The simplest is delta modulation. PCM finds the value of the signal amplitude for
    each sample; DM finds the change from the previous sample.
  • While there is only one way to send parallel data, there are three subclasses of
    serial transmission: asynchronous, synchronous, and isochronous.
  • In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or
    more stop bits (1 s) at the end of each byte.
  • In synchronous transmission, we send bits one after another without start or stop
    bits or gaps. It is the responsibility of the receiver to group the bits.
  • The isochronous mode provides synchronized for the entire stream of bits must. In
    other words, it guarantees that the data arrive at a fixed rate.


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