Baseband Modulation Techniques - PCM, DPCM, DM

Digital Communications
A4410
Dr. D. Praveen Kumar
Assistant Professor
Vardhaman College of Engineering
An autonomous institute, affiliated to JNTUH

2
Course Outcomes (Cos)
1. Develop the basic concepts of modulation, sampling, need for digital data
transmission with an insight into practical applications.
2. Compare and contrast ASK, FSK, PSK digital carrier modulation schemes in
terms of occupied bandwidth, complexity etc., and extend these into QPSK
for improved spectral efficiency
3. Apply the basics of information theory to calculate channel capacity and
other measurements.
4. Analyze the differences between the usage of systematic linear block codes
and convolutional codes for non-burst and burst channel applications
5. Examine various concepts of digital communications using trainer kits and
simulation tool

4
Elements of Digital Communication System

5
Electrical Representation of Binary Data

6

7

8

PULSE CODE MODULATION (PCM)
Low Pass
Filter
Sampler Quantizer Encoder
Cont.
msg sgl
PCM
wave
Regenerative
Ckt
Decoder Reconstruction
filter Destination
PCM
wave
Regenerative
Repeaters
Regenerative
Repeaters
PCM
wave
Regenerated
PCM wave
a. Transmitter
b. Transmission Path
c. Receiver
9

Contd.
• Sampler: Converts continuous signal to discrete time signal.
• Quantization is representing the sampled values of the amplitude by a
finite set of levels, which means converting a continuous-amplitude
sample into a discrete-time signal.
• Encoding: is the process of converting discrete set of samples suitable
for transmission.
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QUANTIZATION
• Two Types
– Uniform quantization
– Non-uniform quantization
• Uniform quantization
– Depending upon transfer characteristics three types
• Midtread Quantizer
• Midriser Quantizer
• Biased Quantizer
12

time
Ts
Midtread Quantizer
Sampled Signal Continuous Signal
13

δ/2
3δ/2
5δ/2
7δ/2
-7δ/2
-5δ/2
-3δ/2
-δ/2
Step size =δ
=0
=δ
=2δ
=3δ
=-δ
=-2δ
=-3δ
Contd.

Quantized Signal
xq(nTs) x(t)
15

16
δ/2 3δ/2 5δ/2 7δ/2
-δ/2
-3δ/2
-5δ/2
δ
2δ
3δ
-3δ
-2δ
-δ
x(nTs)
xq(nTs)
Transfer Characteristics of Midtread Quantizer

17
x(nTs)
δ/2
-δ/2
Quantization
Error
-δ/2 δ/2
Quantization Error
Quantization error lies between
Quantization error ε =x(nTs)- xq(nTs)

18
-δ/2 ≤ x(nTs) < δ/2; xq(nTs)=0
δ/2 ≤ x(nTs) < 3δ/2; xq(nTs)= δ
δ/2 3δ/2 5δ/2 7δ/2
-δ/2
-3δ/2
-5δ/2
δ
2δ
3δ
-3δ
-2δ
-δ
x(nTs)
xq(nTs)
Max. Quantization error

δ
2δ
3δ
4δ
-4δ
-3δ
-2δ
-δ
Step size =δ
= δ/2
=3δ/2
=5δ/2
=7δ/2
=-3δ/2
=-5δ/2
=-7δ/2
Midriser Quantizer
= -δ/2
19

21
Quantization error ε = xq(nTs) - x(nTs)

δ
2δ
3δ
4δ
-4δ
-3δ
-2δ
-δ
Step size =δ
= 0
=δ
=2δ
=3δ
=-2δ
=-3δ
=-4δ
Biased Quantizer
= -δ
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Baseband Modulation Techniques - PCM, DPCM, DM

Baseband Modulation Techniques - PCM, DPCM, DM

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