1. 4.2 Digital Transmission Pulse Modulation Pulse Code Modulation
Outlines
Pulse Modulation
Pulse Code Modulation
Delta 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. continue… 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. continue...

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. PULSE CODE MODULATION (PCM)
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.
The pulses are of fixed length and fixed amplitude
PCM is a binary system where a pulse or lack of a pulse within a prescribed time slot represents either a logic 1 or a logic 0 condition.

10. 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)

11. 4 Steps Process

12. PCM Sampling
The function of a sampling circuit in PCM transmitter is to periodically sample the continually changing analog input voltage and convert those samples to a series of constant-amplitude pulses that can more easily be converted to binary PCM code
There are two basic techniques used to perform the sampling function:
Natural sampling
Flat-top sampling

13. 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

14. Natural Sampling

15. Flat-top Sampling
The most common method used for sampling voice signals 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.

16. continue…
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

17. Flat-top Sampling

18. Sampling rate
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 >=2 fm (max)

19. 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).
Analog signals contain an infinite number of amplitude possibilities. Thus, converting an analog signal to a PCM code with a limited number of combinations requires quantization.
With quantization, the total voltage range is subdivided into a smaller number of subranges as shown in Table.
The PCM code shown in the Table is a 3-bit sign-magnitude code with 8 possible combinations (4 positive and 4 negative).
The leftmost bit is the sign bit (1 = + and 0 = -) and the two rightmost bits represent magnitude.

20. continue…
Each voltage level has one code assigned to it except zero volts, which has two codes, 100 (+0) and 000 (-0).
The magnitude difference between adjacent steps is called the quantization interval or quantum.

21. continue...
Analog input signal
Sample pulse
PAM signal
PCM code

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

23. Dynamic Range (DR)
The ratio of the largest possible magnitude to the smallest possible magnitude that can be decoded by the digital-to-analog converter in the receiver.
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

24. 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

25. 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

26. 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

27. 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 (max) = dB and q =0.2 V.

28. 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 dynamic range of a communication system.

29. Companding Functions

30. Methods 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

31. continue...
A-law
μ-law

32. 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

33. 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.

34. 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.

35. continue…
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.

36. DELTA MODULATION (DM)
A single-bit PCM code to achieve digital transmission of analog.
Logic ‘0’ is transmitted if current sample is smaller than the previous sample
Logic ‘1’ is transmitted if current sample is larger than the previous sample

37. Cont’d…

38. Operation of Delta Modulation

39. continue... Analog input is approximated by a staircase function
Move up or down one level () at each sample interval (by one quantization level at each sampling time)  output of DM is a single bit.
Binary behavior
Function moves up or down at each sample interval
In DM the quantization levels are represented by two symbols: 0 for - and 1 for +. In fact the coding process is performed on eq.
The main advantage of DM is its simplicity.

40. The transmitter of a DM System
Cont’d...
The transmitter of a DM System

41. The receiver of a DM system

42. Delta Modulation - Example

43. DM circuit’s problem

44. continue…
Slope overload distortion is due to the fact that the staircase approximation mq(t) can't follow closely the actual curve of the message signal m(t ). In contrast to slope-overload distortion, granular noise occurs when  is too large relative to the local slope characteristics of m(t). granular noise is similar to quantization noise in PCM.
It seems that a large  is needed for rapid variations of m(t) to reduce the slope-overload distortion and a small  is needed for slowly varying m(t) to reduce the granular noise. The optimum  can only be a compromise between the two cases.
To satisfy both cases, an adaptive DM is needed, where the step size  can be adjusted in accordance with the input signal m(t).

45. continue... In summary Slope overload
Due to the input analog signal amplitude changes faster than the speed of the modulator
to minimize : the product of the sampling step size and the sampling rate must be equal to or larger than the rate of change of the amplitude of the input analog signal.
Granular noise
Due to the difference between step size and sampled voltage.
To minimize : increase the sampling rate, decrease the step size of modulator

46. DM Performance Good voice reproduction PCM - 128 levels (7 bit)
Voice bandwidth 4khz
Should be 8000 x 7 = 56kbps for PCM
Data compression can improve on this
e.g. Interframe coding techniques for video

47. continue... Adaptive Delta Modulation (ADM)
A Delta Modulation system where the step size of the DAC is automatically varied depending on the amplitude characteristics of the analog signal.
A well designed ADM scheme can transmit voice at about half the bit rate of a PCM system with equivalent quality.

48. 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)

49. continue...
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

50. 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)

51. 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

52. 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.

53. Non-Return to Zero Inverted (NRZ-I)
Non-return to zero inverted on ones
Constant voltage pulse for duration of bit
The polarity of the bit is reversed when a 1 bit is encountered
All subsequent 0s following the 1 are recorded at the same polarity

54. 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

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

56. 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.

58. 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.

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

61. END OF PART 2