1. 4.2 Digital Transmission Pulse Modulation (Part 2.1)
Outlines
Pulse Modulation (Part 2.1)
Pulse Code Modulation (Part 2.2)
Delta Modulation (Part 2.3)
Line Codes (Part 2.4)

2. 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

3. Cont’d…

4. Operation of Delta Modulation

5. Cont’d... 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.

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

7. The receiver of a DM system

8. Delta Modulation - Example

9. DM circuit’s problem

10. Cont’d…
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).

11. Cont’d... 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

12. 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

13. Cont’d... 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.

14. 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)

15. 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

16. 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)

17. 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

18. 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.

19. 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

20. 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

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

22. 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.

24. 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.

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

27. END OF PART 2