EE 551451, Fall, 2006 Communication Systems

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EE 551/451, Fall, 2006Communication Systems

• Zhu Han
• Department of Electrical and Computer Engineering
• Class 13
• Oct. 3rd, 2006

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Pulse Code Modulation (PCM)

• Pulse code modulation (PCM) is produced by
  analog-to-digital conversion process. Quantized
  PAM
• As in the case of other pulse modulation
  techniques, the rate at which samples are taken
  and encoded must conform to the Nyquist sampling
  rate.
• The sampling rate must be greater than, twice the
  highest frequency in the analog signal,
• fs gt 2fA(max)
• Telegraph time-division multiplex (TDM) was
  conveyed as early as 1853, by the American
  inventor M.B. Farmer. The electrical engineer
  W.M. Miner, in 1903.
• PCM was invented by the British engineer Alec
  Reeves in 1937 in France.
• It was not until about the middle of 1943 that
  the Bell Labs people became aware of the use of
  PCM binary coding as already proposed by Alec
  Reeves.

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Pulse Code Modulation
Figure The basic elements of a PCM system.
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Encoding
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Virtues, Limitations and Modifications of PCM
Advantages of PCM 1. Robustness to noise
and interference 2. Efficient regeneration
3. Efficient SNR and bandwidth trade-off 4.
Uniform format 5. Ease add and drop 6.
Secure DS0 a basic digital signaling rate of
64 kbit/s. To carry a typical phone call, the
audio sound is digitized at an 8 kHz sample rate
using 8-bit pulse-code modulation. 4K baseband,
861.8 dB
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Differential Encoding

• Encode information in terms of signal transition
  a transition is used to designate Symbol 0
• 
  
• Regeneration (reamplification, retiming,
  reshaping )
• 
  
• 
  
• 
  

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Linear Prediction Coding (LPC)

• Consider a finite-duration impulse response (FIR)
  
• discrete-time filter which consists of three
  blocks
• 1. Set of p ( p prediction order) unit-delay
  elements (z-1)
• 2. Set of multipliers with coefficients w1,w2,wp
• 3. Set of adders ( ? )

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Reduce the sampling rate
Block diagram illustrating the linear adaptive
prediction process.
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Differential Pulse-Code Modulation (DPCM)
Usually PCM has the sampling rate higher than the
Nyquist rate. The encode signal contains
redundant information. DPCM can efficiently
remove this redundancy. 32 Kbps for PCM Quality
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Processing Gain
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Adaptive Differential Pulse-Code Modulation
(ADPCM)

• Need for coding speech at low bit rates , we have
  two aims in mind
• 1. Remove redundancies from the speech
  signal as far as possible.
• 2. Assign the available bits in a
  perceptually efficient manner.
• 
  

Adaptive quantization with backward estimation
(AQB).
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• ADPCM
• 
  

8-16 kbps with the same quality of PCM
Adaptive prediction with backward estimation
(APB).
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Coded Excited Linear Prediction (CELP)

• Currently the most widely used speech coding
  algorithm
• Code books
• Vector Quantization
• lt8kbps
• Compared to CD
• 44.1 k sampling
• 16 bits quantization
• 705.6 kbps
• 100 times difference

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Time-Division Multiplexing
Figure Block diagram of TDM system.
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DS1/T1/E1

• Digital signal 1 (DS1, also known as T1) is a
  T-carrier signaling scheme devised by Bell Labs.
  DS1 is a widely used standard in
  telecommunications in North America and Japan to
  transmit voice and data between devices. E1 is
  used in place of T1 outside of North America and
  Japan. Technically, DS1 is the transmission
  protocol used over a physical T1 line however,
  the terms "DS1" and "T1" are often used
  interchangeably.
• A DS1 circuit is made up of twenty-four DS0
• DS1 (8 bits/channel 24 channels/frame 1
  framing bit) 8000 frames/s 1.544 Mbit/s
• A E1 is made up of 32 DS0
• The line data rate is 2.048 Mbit/s which is split
  into 32 time slots, each being allocated 8 bits
  in turn. Thus each time slot sends and receives
  an 8-bit sample 8000 times per second (8 x 8000 x
  32 2,048,000). 2.048Mbit/s
• History page 274

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Synchronization

• Super Frame

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Synchronization

• Extended Super Frame

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T Carrier System
Twisted Wire to Cable System
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Fiber Communication
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Delta Modulation (DM)




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DM System Transmitter and Receiver.
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Slope overload distortion and granular noise
The modulator consists of a comparator, a
quantizer, and an accumulator. The output of the
accumulator is
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Slope Overload Distortion and Granular Noise
( differentiator )
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Delta-Sigma modulation (sigma-delta modulation)

• The modulation which has an integrator
  can
• relieve the draw back of delta modulation
  (differentiator)
• Beneficial effects of using integrator
• 1. Pre-emphasize the low-frequency content
• 2. Increase correlation between adjacent
  samples
• (reduce the variance of the error signal
  at the quantizer input )
• 3. Simplify receiver design
• Because the transmitter has an integrator ,
  the receiver
• consists simply of a low-pass filter.
• (The differentiator in the conventional DM
  receiver is cancelled by the integrator )

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delta-sigma modulation system.
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Adaptive Delta Modulation

• Adaptive adjust the step size according to
  frequency, figure 6.21
• Out SNR
• Page 286-287
• For single integration case, (BT/B)3
• For double integration case, (BT/B)5
• Comparison with PCM, figure 6.22
• Low quality has the advantages.
• Used in walky-talky

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Questions?