Thursday, October 2, 2008

Lecture 7

Lecture 7


  • Pulse Code Modulation
    • A way of digitizing voice signals
    • Voice signal is band limited to 4kHz ( sampling rate = 8 ksamples/s)
    • 8-bit nonuniform quantizer is used to quantize each sample (data rate = 64kbits/s)
    • It can be shown the SNR for PCM = (6m-10)dB

  • How fast and reliable can a digital transmission occur through a channel?
  • Depends on a number of factors:
    • Amount of energy present in the signal
    • Noise properties of the channel
    • Distance for the signal to propagate
    • Bandwidth(BW) of the transmission medium
  • Bandwidth:
    • Determines the range of frequencies that can be transmitted through a channel
    • Consider a sinusoidal wave:
    • Frequency present in the wave = f0Hz or 2πf0 radians / s
  • Effective bandwidth
    • Most energy
    • Wave of
  • Square waves have infinite bandwidth, the kth component kf = 1/k
    • Therefore most energy and amplitude is in the first few components
    • S(t) = A 4/π[
      • Square wave with infinite frequency
    • A bandwidth of 4Mhz has a data rate of 2Mbps
  • Cos(Ѳ) = sin( Ѳ + π/2)
  • Amplitude response A(f): is the ratio of the output amplitude to input amplitude (Aout/Ain) as a function of frequency
  • Phase shift: is a variation in φ(f) as a function of frequency
  • Signal power


  • P power distributed across resistance R
  • V voltage across resistance R
  • Instantaneous power is proportional to s(t)2
  • Average power from (t1, t2)
  • Take P =
  • Transmission impairment
    • Attenuation
      • When he signal falls off as a measure of distance

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