EE 551/451, Fall, 2006
Communication Systems
Zhu Han
Department of Electrical and Computer Engineering
Class 13
Oct. 3rd, 2006
EE 541/451 Fall 2006
Pulse Code Modulation (PCM)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 > 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.
EE 541/451 Fall 2006
Figure Figure The basic elements of a PCM system.The basic elements of a PCM system.
Pulse Code Modulation
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Encoding
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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, 8*6+1.8 dB
Virtues, Limitations and Modifications of PCM
EE 541/451 Fall 2006
Encode information in terms of signal transition; a transition is used to designate Symbol 0
Regeneration (reamplification, retiming, reshaping )
3dB performance loss, easier decoder
Differential Encoding
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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 ( )
Linear Prediction Coding (LPC)
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Block diagram illustrating the linear adaptive prediction process.
Reduce the sampling rate
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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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EE 541/451 Fall 2006
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).
Adaptive Differential Pulse-Code Modulation (ADPCM)
EE 541/451 Fall 2006
ADPCM
Adaptive prediction with backward estimation (APB).
8-16 kbps with the same quality of PCM
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Coded Excited Linear Prediction (CELP)Coded Excited Linear Prediction (CELP)
Currently the most widely used speech coding algorithm
Code books
Vector Quantization
<8kbps
Compared to CD
44.1 k sampling
16 bits quantization
705.6 kbps
100 times difference
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Time-Division MultiplexingTime-Division Multiplexing
Figure Block diagram of TDM system.
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DS1/T1/E1DS1/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
EE 541/451 Fall 2006
Synchronization Synchronization
Super Frame
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Synchronization Synchronization Extended Super Frame
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T Carrier SystemT Carrier System
Twisted Wire to Cable System
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Fiber CommunicationFiber Communication
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Delta Modulation (DM)
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DM System: Transmitter and Receiver.
EE 541/451 Fall 2006
The modulator consists of a comparator, a quantizer, and an accumulator. The output of the accumulator is
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EE 541/451 Fall 2006
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Slope Overload Distortion and Granular Noise
( differentiator )
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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 )
Delta-Sigma modulation (sigma-delta modulation)
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delta-sigma modulation system.
EE 541/451 Fall 2006
Adaptive Delta ModulationAdaptive 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
EE 541/451 Fall 2006
Questions?Questions?