1. Outlines Pulse Modulation Pulse Code Modulation Line Codes
4.2 Digital Transmission
Outlines
Pulse Modulation
Pulse Code Modulation
Line Codes

2. PULSE MODULATION (PM) Sampling analog information signal
Converting samples into discrete pulses
Transport the pulses from source to destination over physical transmission medium.

3. Cont’d... Four (4) Methods 1. PAM 2. PWM 3. PPM 4. PCM
Analog Pulse Modulation
Digital Pulse Modulation

4. Cont’d... Analog Pulse Modulation
Carrier signal is pulse waveform and the modulated signal is where one of the carrier signal’s characteristic (either amplitude, width or position) is changed according to information signal.

5. Pulse Amplitude Modulation (PAM)
The amplitude of pulses is varied in accordance with the
information signal.
Width & position constant.

6. Pulse Width Modulation (PWM)
Sometimes called Pulse Duration Modulation (PDM).
The width of pulses is varied in accordance to information signal.
Amplitude & position constant.

7. Cont’d...

8. Pulse Position Modulation (PPM)
Modulation in which the temporal positions of the pulses are varied in accordance with some characteristic of the information signal.
Amplitude & width constant.

9. Advantages & Drawbacks of Pulse Modulation
Requires greater BW to transmit & receive as compared to its analog counterpart.
Special encoding & decoding methods must be used to increased transmission rates & more difficult to be recovered.
Requires precise synchronization of clocks between Tx & Rx.
Noise immunity.
Relatively low cost digital circuitry.
Able to be time division multiplexed with other pulse modulated signal.
Storage of digital streams.
Error detection & correction

10. Sampling
A process of taking samples of information signal at a rate of Nyquist’s sampling frequency.
Nyquist’s Sampling Theorem :
The original information signal can be reconstructed at the receiver
with minimal distortion if the sampling rate in the pulse modulation
system equal to or greater than twice the maximum information
signal frequency.
fs >= fm (max)

11. Cont’d... Two basic techniques used to perform the sampling function:
Natural sampling
Flat-top sampling

12. Natural Sampling
Tops of the sample pulses retain their natural shape during the sample interval.
Frequency spectrum of the sampled output is different from an ideal sample.
Amplitude of frequency components produced from narrow, finite-width sample pulses decreases for the higher harmonics
Requiring the use of frequency equalizers

13. Natural Sampling

14. Flat-top Sampling Common used in PCM systems.
Accomplish in a sample-and-hold circuit
To periodically sample the continually changing analog input voltage & convert to a series of constant-amplitude PAM voltage levels.
The input voltage is sampled with a narrow pulse and then held relatively constant until the next sample is taken.

15. Cont’d…
Sampling process alters the frequency spectrum & introduces aperture error.
The amplitude of the sampled signal changes during the sample pulse time.
Advantages:
Introduces less aperture distortion
Can operate with a slower ADC

16. Flat-top Sampling

17. PULSE CODE MODULATION (PCM)
Basic scheme of PCM system
Quantization
Quantization Error
Companding
Block diagram & function of TDM-PCM communication system

18. Basic scheme of PCM system
The most common technique for using digital signals to encode analog data is PCM.
Example: To transfer analog voice signals off a local loop to digital end office within the phone system, one uses a codec.

19. Cont’d...
Because voice data limited to frequencies below 4000 Hz, a codec makes 8000 samples/sec. (i.e., 125 microsecond/sample).
If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal.

20. PCM Block Diagram Four step process
Most common form of analog to digital modulation
Four step process
Signal is sampled using PAM (Sample)
Integer values assigned to signal (PAM)
Values converted to binary (Quantized)
Signal is digitally encoded for transmission (Encoded)

21. 4 Steps Process

22. Cont’d… Analog signal is sampled.
Converted to discrete-time continuous-amplitude signal
(Pulse Amplitude Modulation)
Pulses are quantized and assigned a digital value.
A 7-bit sample allows 128 quantizing levels.
PCM uses non-linear encoding, i.e., amplitude spacing of levels is non-linear
There is a greater number of quantizing steps for low amplitude
This reduces overall signal distortion.
This introduces quantizing error (or noise).
PCM pulses are then encoded into a digital bit stream.
8000 samples/sec x 7 bits/sample = 56 Kbps for a single voice channel.

23. PCM Example

24. Quantization
A process of converting an infinite number of possibilities to a finite number of conditions (rounding off the amplitudes of flat-top samples to a manageable number of levels).

25. Cont’d...
Analog input signal
Sample pulse
PAM signal
PCM code

26. Cont’d… The quantization interval @ quantum
= the magnitude difference between adjacent steps.
The resolution = the magnitude of a quantum
= the voltage of the minimum step size.
The quantization error = the quantization noise
= ½ quantum
= (orig. sample voltage – quantize level)
PCM code = (sample voltage/resolution)

27. QUANTIZATION ERROR
A difference between the exact value of the analog signal & the nearest quantization level.

28. Types of Quantization
Midtread
Midrise

29. Types of Quantizer
1. Uniform type : The levels of the quantized amplitude are uniformly spaced. 2. Non-uniform type : The levels are not uniform.

30. Dynamic Range (DR)
Largest possible magnitude/smallest possible magnitude.
Where
DR = absolute value of dynamic range
Vmax = the maximum voltage magnitude
Vmin = the quantum value (resolution)
n = number of bits in the PCM code

31. Example 1
Calculate the dynamic range for a linear PCM system using 16-bit quantizing.
Calculate the number of bits in PCM code if the DR = dB

32. Coding Efficiency = Minimum number of bits x 100
A numerical indication of how efficiently a PCM code is utilized.
The ratio of the minimum number of bits required to achieve a certain dynamic range to the actual number of PCM bits used.
Coding Efficiency = Minimum number of bits x 100
Actual number of bits

33. Signal to Quantization Noise Ratio (SQR)
The worst-case voltage SQR
SQR for a maximum input signal
The signal power-to-quantizing noise power ratio
R =resistance (ohm)
v = rms signal voltage
q = quantization interval

34. Example 2
Calculate the SQR (dB) if the input signal = 2 Vrms and the quantization noise magnitudes = 0.02 V.
Determine the voltage of the input signals if the SQR = dB and q =0.2 V.

35. Effect of Non-Linear Coding

36. Nonlinear Encoding Quantization levels not evenly spaced
Reduces overall signal distortion
Can also be done by companding

37. Companding The process of compressing and then expanding.
The higher amplitude analog signals are compressed
prior to transmission and then expanded in receiver.
Improving the DR of a communication system.

38. Companding Functions

39. Method of Companding
For the compression, two laws are adopted: the -law in US and Japan and the A-law in Europe.
-law
A-law
The typical values used in practice are: =255 and A=87.6.
After quantization the different quantized levels have to be represented in a form suitable for transmission. This is done via an encoding process.
Vmax= Max uncompressed analog input voltage
Vin= amplitude of the input signal at a particular of instant time
Vout= compressed output amplitude
A, = parameter define the amount of compression

40. Example 3
A companding system with µ = 255 used to compand from 0V to 15 V sinusoid signal. Draw the characteristic of the typical system.
Draw an 8 level non-uniform quantizer characteristic that corresponds to the mentioned µ.

41. Cont’d...
A-law
μ-law

42. PCM Line Speed
The data rate at which serial PCM bits are clocked out of the PCM encoder onto the transmission line.
Where
Line speed = the transmission rate in bits per second
Sample/second = sample rate, fs
Bits/sample = no of bits in the compressed PCM code

43. Example 4
For a single PCM system with a sample rate fs = 6000 samples per second and a 7 bits compressed PCM code, calculate the line speed.

44. Virtues & Limitation of PCM
The most important advantages of PCM are:
Robustness to channel noise and interference.
Efficient regeneration of the coded signal along the channel path.
Efficient exchange between BT and SNR.
Uniform format for different kind of base-band signals.
Flexible TDM.

45. Cont’d…
Secure communication through the use of special modulation schemes of encryption.
These advantages are obtained at the cost of more complexity and increased BT.
With cost-effective implementations, the cost issue no longer a problem of concern.
With the availability of wide-band communication channels and the use of sophisticated data compression techniques, the large bandwidth is not a serious problem.

46. Time-Division Multiplexing
This technique combines time-domain samples from different message signals (sampled at the same rate) and transmits them together across the same channel.
The multiplexing is performed using a commutator (switch). At the receiver a decommutator (switch) is used in synchronism with the commutator to demultiplex the data.

47. Cont’d…
TDM system is very sensitive to symbol dispersion, that is, to variation of amplitude with frequency or lack of proportionality of phase with frequency. This problem may be solved through equalization of both magnitude and phase.
One of the methods used to synchronize the operations of multiplexing and demultiplexing is to organize the multiplexed stream of data as frames with a special pattern. The pattern is known to the receiver and can be detected very easily.

48. Block diagram of TDM-PCM communication system

49. LINE CODES
Converting standard logic level to a form more suitable to telephone line transmission.
The line codes properties:
Transmission BW should be small as possible
Efficiency should be as high as possible
Error detection & correction capability
Transparency (Encoded signal is received faithfully)

50. Cont’d...
Six factors must be considered when selecting a line encoding format;
transmission voltage & DC component
Duty cycle
Bandwidth consideration
Clock and framing bit recovery
Error detection
Ease of detection and decoding

51. Why Digital Signaling? Low cost digital circuits
The flexibility of the digital approach (because digital data from digital sources may be merged with digitized data derived from analog sources to provide general purpose communication system)

52. Digital Modulation Using Digital Signals to Transmit Digital Data
Bits must be changed to digital signal for transmission
Unipolar encoding
Positive or negative pulse used for zero or one
Polar encoding
Uses two voltage levels (+ and - ) for zero or one
Bipolar encoding
+, -, and zero voltage levels are used

53. Non-Return to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits.
Voltage constant during bit interval.
no transition, no return to zero voltage
More often, negative voltage for one value and positive for the other.

54. Non-Return to Zero Inverted (NRZ-I)
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding

55. Multilevel Binary(Bipolar-AMI)
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a problem)
No net dc component
Lower bandwidth
Easy error detection

56. Pseudoternary One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI

57. Manchester
There is always a mid-bit transition {which is used as a clocking mechanism}.
The direction of the mid-bit transition represents the digital data.
1  low-to-high transition
0  high-to-low transition
Consequently, there may be a second transition at the beginning of the bit interval.
Used in baseband coaxial cable and CSMA/CD twisted pair.

59. Differential Manchester
mid-bit transition is ONLY for clocking.
1  absence of transition at the beginning of the bit interval
0  presence of transition at the beginning of the bit interval
Differential Manchester is both differential and bi-phase.
[Note – the coding is the opposite convention from NRZI.]
Used in (token ring) with twisted pair.
* Modulation rate for Manchester and Differential Manchester is twice the data rate  inefficient encoding for long-distance applications.

61. Example 5 Sketch the data wave form for a bit stream 11010 using NRZL
Bipolar AMI
Pseudoternary

62. END OF PART 2