EE 551/451, Fall, 2006 Communication Systems

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

EE 541/451 Fall 2006

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

EE 541/451 Fall 2006

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

EE 541/451 Fall 2006

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.

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EE 541/451 Fall 2006

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( differentiator )

EE 541/451 Fall 2006

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.

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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?