1. Differentially Encoding
Method used to encode information in terms of signal transition
The original binary information is recovered simply by comparing the polarity of adjacent binary symbols to establish whether or not a transition has occured

2. 0 1 1 0 1 0 0 1 1 0 0 0 1 1 0 1 1 Original data Dif.enc. data
Dif.enc. data
A transition is used to designate symbol ‘0’ in the incoming data stream, no transition means ‘1’.
A reference bit is needed at the beginning

3. Modifications of PCM
Most favorite transmission scheme for transmission of analog signals
Advantages :
1- Same as digital ( easy to correct once the error is detected, secure communication use of encryption )
2- A uniform format for the transmission of different kinds of baseband signals ( can integrate different services in one network)

4. Disadvantages
1- System complexity ( use of DM)
2- Increase bandwidth ( availability of wide bandwidth common channels such as satellites and optical fibers(

5. Delta Modulator
The aim is to use simple quantizing strategy for constructing the encoded signal
The sample will be encoded using only one bit ( the difference will be encoded instead of the sample itself)
Use of fewer bits

6. In delta modulation (DM), the incoming signal is over sampled. i. e
In delta modulation (DM), the incoming signal is over sampled. i.e., the sampling rate is much higher than the Nyquist rate
DM provides a staircase approximation to the oversampled version of the message signal
The difference between the I/P m(t) and the approximation mq(t) is quantized into only two levels ±S

7. m(t) S Ts
mq(t)

8. Delta modulator includes a DM demodulator that generates a staircase approximation mq(t) of the analog message signal m(t) by an accumulator
if the staircase approximation mq(t) falls below the message signal m(t) at any sampling time instant then the DM modulator emits a bit 1 which means that the value of mq(t) has to be increased by S

9. If mq(t) lies above m(t) then the DM modulator emits a bit 0 which means that the value of staircase approximation mq(t) has to be reduced by S
DM wave is the binary sequence that shows if the actual value of message signal m(t) is above or below the previous reconstructed signal value mq(t)

10. Delta Modulator Block Diagram
So(t)
m(t)-mq(t)
So(t)
m(t)
mq(t) Ts
Delta Modulator Block Diagram

11. m(t) = The Base-band Signal
mq(t) = The quantized approximation of m(t)
If m(t) > mq(t) The comparator output = V(H)
If m(t) < mq(t) The comparator output = V(L)
If So(t) level is V(H), The Counter counts up
If So(t) level is V(L), The Counter counts down

12. Problems in delta modulation
Slope overload distortion
If the step size S is too small then the staircase approximation mq(t) cannot follow the fast variations in the input waveform m(t)
m(t) Ts S
mq(t)

13. To avoid slope overload, i. e
To avoid slope overload, i.e. in order for the sequence of samples mq[n] to increase as fast as the input sequence of samples m[n]
[ S / Ts ] ≥ max | m’ (t) |

14. 2. Start up response in DM:
m(t)
mq(t)

15. Granular noise
If the step size S is too large then the staircase approximation oscillates around the relatively flat segments of input waveform.
Granular noise is analogous to quantization noise in a PCM system
The choice of S compromises between the two problems
When the change in the I/P is too large choose large S and vice versa
We can use an adaptive DM to solve these problems

16. Figure 3–32 DM system waveforms.
Couch, Digital and Analog Communication Systems, Seventh Edition ©2007 Pearson Education, Inc. All rights reserved

17. Signal-to-noise ratio out of a DM system as a function of step size.
Couch, Digital and Analog Communication Systems, Seventh Edition ©2007 Pearson Education, Inc. All rights reserved

18. Comparison OF PCM and DM Systems

19. Differential PCM
We transmit the difference between the sample value m(k) at time k, and the sample value m(k-1) at time (k-1)
So, by adding up (accumulating) these changes, we shall generate at the receiver a waveform identical to m(t)
We transmit these differences by PCM
Since [m(k)-m(k-1)]<m(k), then we’ll need fewer levels, and fewer bits

20. Differential PCM Transmitter
d[k]
dq [k] to channel
m[k]
Quantizer
mq [k]
m1q [k]

21. Differential PCM Transmitter
So, mq[k] is the quantized version of m[k]
dq[k] is transmitted over the channel.

22. Differential PCM Receiver
From channel dq [k]
The output q[k]
Predictor
m1q [k]
Since the receiver is the same as the feed back portion of the transmitter, then:
The predictor output is the same (m1q[k])
The receiver output = The predictor input = mq[k]=m[k]+q[k]
So the added quantization noise is q[k], associated with the difference signal d[k] which is << m[k]

23. Adaptive Delta-Modulation
Adaptive Delta Modulation (ADM)
Adaptive Delta-Modulation

24. Adaptive Delta Modulation (ADM)
The step size S(k) is not kept fixed and always multiple of basic step So
When slope overload occurs, the step size becomes larger
mqa(t) will catch m(t) more rapidly
Step size:
The output e(k) = if m(t)> mq(t)
The output e(k) = if m(t)<

25. Adaptive Delta Modulator Block Diagram

26. Waveforms comparing Adaptive DM & linear DM
m(t)
mqa(t)
mq(t)
Waveforms comparing Adaptive DM & linear DM
mqa(t) will catch m(t) more rapidly
ADM has large quantization error
ADM has very low slop overload error