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 k^th 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 (A_out/A_in) 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
      
    •