1. EE 551/451, Fall, 2006 Communication Systems Zhu Han Department of Electrical and Computer Engineering Class 13 Oct. 3 rd, 2006

2. EE 541/451 Fall 2006 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, f s > 2f A (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.Alec Reeves1937 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.Bell Labs

3. EE 541/451 Fall 2006 Figure The basic elements of a PCM system. Pulse Code Modulation

4. EE 541/451 Fall 2006 Encoding

5. EE 541/451 Fall 2006 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 dBdigitalsignalingkbit/skHzpulse-code modulation Virtues, Limitations and Modifications of PCM

6. 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

7. EE 541/451 Fall 2006 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 w 1,w 2,…w p 3. Set of adders (  ) Linear Prediction Coding (LPC)

8. EE 541/451 Fall 2006 Block diagram illustrating the linear adaptive prediction process. Reduce the sampling rate

9. 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

10. EE 541/451 Fall 2006 Processing Gain

11. 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)

12. EE 541/451 Fall 2006 ADPCM Adaptive prediction with backward estimation (APB). 8-16 kbps with the same quality of PCM

13. EE 541/451 Fall 2006 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

14. EE 541/451 Fall 2006 Time-Division Multiplexing Figure Block diagram of TDM system.

15. EE 541/451 Fall 2006 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.T-carrierBell Labs telecommunicationsNorth AmericaJapanE1 A DS1 circuit is made up of twenty-four DS0circuit 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/sMbit/s History page 274

16. EE 541/451 Fall 2006 Synchronization Super Frame

17. EE 541/451 Fall 2006 Synchronization Extended Super Frame

18. EE 541/451 Fall 2006 T Carrier System Twisted Wire to Cable System

19. EE 541/451 Fall 2006 Fiber Communication

20. EE 541/451 Fall 2006 Delta Modulation (DM)

21. EE 541/451 Fall 2006 DM System: Transmitter and Receiver.

22. EE 541/451 Fall 2006 The modulator consists of a comparator, a quantizer, and an accumulator. The output of the accumulator is Slope overload distortion and granular noise

23. EE 541/451 Fall 2006 Slope Overload Distortion and Granular Noise ( differentiator )

24. 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)

25. EE 541/451 Fall 2006 delta-sigma modulation system.

26. EE 541/451 Fall 2006 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

27. EE 541/451 Fall 2006 Questions?