Basic Communication theory

1. PULSE MODULATION

2. Pulse Modulation
 Advantages of pulse modulation:
 Noise immunity, because pulses are evaluated
based on precise time interval and amplitude
compared with reference level
 Multiplexing is possible
 Signal regeneration is used instead of
amplification
 Simpler to measure and evaluate

3. Pulse Modulation
 Disadvantages of pulse modulation:
 Use more bandwidth – generation of pulses
require more bandwidth
 Need additional encoding and decoding circuitry
 Require precise time synchronization in receiver
and transmitter
 Incompatible with older analog transmission
systems

4.  Basic types of pulse modulation:
 Phase amplitude modulation (PAM)
 Pulse width modulation (PWM)
 Pulse position modulation (PPM)
 Pulse code modulation (PCM)

5. Pulse Amplitude Modulation (PAM)
 PAM waveform characteristics:
 Pulse width is constant
 Position of pulse is constant
 Amplitude of pulse is varied according to the
amplitude of the sample of the analog signal
 Application: PAM is used as an intermediate
form of modulation with PSK (phase-shift
keying), QAM (quadrature amplitude
modulation) and PCM

6. Pulse Width Modulation (PWM)
 Width of pulse is varied proportional to the
amplitude of the analog signal at the time the
signal is sampled
 The resulting PWM waveform has constant
amplitude
 Application: Special-purpose communication
systems, mainly for military, rarely for
commercial digital transmission systems

7. Pulse Position Modulation (PPM)
 Position of the pulse is varied according to the
amplitude of the analog signal
 The resulting PPM waveform has constant
amplitude and constant width
 Application: Special-purpose communication
systems, mainly for military, rarely for
commercial digital transmission systems

8. Comparing PAM, PWM and PPM
Resulting
Pulse
Pulse
Amplitude
Modulatio
n (PAM)
Pulse
Width
Modulatio
n (PWM)
Pulse
Position
Modulatio
n (PPM)
Pulse width
(duration)
Constant Variable Constant
Pulse
position
Constant Constant Variable
Pulse height
(amplitude)
Variable Constant Constant

9. Pulse Code Modulation (PCM)
 A form of digitally encoding analog signal
 Pulses are of fixed length and amplitude
 Is a binary system i.e. represented by logic 1
or 0
 PCM is the most widely used pulse modulation
technique

10. Pulse Code Modulation (PCM)
Bandpass
filter
Sample
and hold
Analog to
digital
converter
Parallel to
serial
converter
Regenerative
repeater
Regenerative
repeater
Serial to
parallel
converter
Digital to
analog
converter
Hold Low pass
filter
Analog
input
signal
Output
signal
PCM Transmitter
PCM Receiver

11. PCM
 At transmitter:
 Bandpass filter limits frequency of analog signal
 Sample-and-hold periodically samples analog signal to
obtain PAM samples
 ADC change PAM samples to parallel PCM codes
 Parallel-to-serial converter change parallel PCM codes to
serial digital codes
 Repeaters regenerate digital pulses in the transmission
line
 Integrated circuit which performs encoding and
decoding is called codec (coder/decoder)
 At receiver, the operation is reverse to that of the
transmitter. The “hold” circuit converts PAM signals
to original analog form

12.  In summary, 3 steps in PCM:
 Sampling
 Quantization
 Encoding

13. PCM sampling
 PCM sampling will periodically sample
continually changing analog input voltage
 Sampling rate must fulfill Nyquist sampling
theorem i.e. sampling rate must be at least
twice the highest frequency of input signal
 2 types of PCM sampling:
 Natural sampling
 Flat-top sampling

14. Natural sampling
 Top of the pulse takes shape of input
waveform for the sample interval
 Difficult for ADC to convert to PCM code
because amplitude is not constant

15. Flat-top sampling
 Top remains at the sampled value for the
duration of the sample
 Works with sample-and-hold circuit
 Most commonly used

16. Quantization
 Quantization  convert sampled amplitudes
to discrete amplitudes taken from a set of
possible amplitudes
 Quantization level = the individual voltage
levels
 Quantization interval (or quantum) =
difference between adjacent voltage levels
 No. of quantization levels, L
L = 2n
Where n = no. of bits used in the code

17.  Maximum quantization level is given by
2(n-1) – 1, where n is the no. of bits

18. Encoding
 Encoding  assigning each quantization
level to a set of codes
 The codes are sign-magnitude codes,
where the most significant bit (MSB) is the
sign bit, and the others represent magnitude

19. Dynamic Range (DR)
 Dynamic Range (DR):
DR = V
max
V
min
Where Vmax = maximum voltage magnitude
Vmin = quantum value (interval size)

20. Example 1
 Given a PCM system with these parameters:
maximum analog input frequency = 4 kHz
no. of bits used in PCM code = 8
 Find
 Minimum sampling rate
 Number of quantization levels, and thus the
number of codes
 Maximum quantization level

21. Delta Modulation
 Only a single bit is transmitted, which indicates
whether that sample is larger or smaller than
the previous sample
 If current sample is smaller than previous
sample, a ‘0’ is transmitted
 If current sample is larger than previous
sample, a ‘1’ is transmitted

22. Delta Modulation Transmitter
Sample
and
hold
Analog
input
Digital to
analog
converter
Up/down
counter
Delta
PCM
+
-
Sampling
pulse
clock
1 = up
0 = down

23. Delta Modulation Transmitter
 Analog input is sampled and converted to PAM signal
 PAM signal is compared with output of Digital-to-Analog
Converter (DAC)
 Output of DAC is a voltage equal to the regenerated
magnitude of the previous sample which was stored in
the up-down counter as a binary number
 Up-down counter is incremented/decremented
depending on whether the previous sample is
larger/smaller than current sample
 Up-down counter is clocked at a rate equal to the sample
rate

24. Delta Modulation Receiver
Low
pass
filter
Digital to
analog
converter
Delta
Up/down
PCM counter
clock
Recovered
analog
signal

25. Delta Modulation Receiver
 As logic ‘1’ or ‘0’ are received, the up-down
counter is incremented/decremented
accordingly
 Output of DAC in the decoder is identical to
the output of DAC in the transmitter

26. Problems associated with Delta
Modulation
 1) Slope overload
 Slope of analog signal will be greater than the
delta modulator can maintain
 Happens when analog input signal changes at a
faster rate than the DAC can maintain
 How to avoid:
 Increase clock frequency
 Increase magnitude of minimum step size

27. Problems associated with Delta
Modulation
 2) Granular noise
 The reconstructed signal has variations that were
not present in the original signal
 Happens when the original analog input signal
has a relatively constant amplitude
 How to avoid:
 Decrease step size

28. Problems associated with Delta
Modulation
 Small resolution is needed to reduce granular
noise, but large resolution is needed to reduce
slope overload. Hence, a compromise is
needed
 Granular noise – more prevalent in analog
signals that have gradual slopes
 Slope overload – more prevalent in analog
signals that have steep slopes or rapid
variations in amplitude

29. Differential PCM (DPCM)
 Often in PCM, there are successive samples
that are almost of equal amplitudes. This
cause several identical PCM codes to be
transmitted, which is redundant
 DPCM can solve this problem by
transmitting the amplitude difference of
the two successive samples instead of the
actual samples
 Hence, fewer bits are required for DPCM

30. DPCM Transmitter
 Low pass filter limits the input signal to half the sample
rate
 Differentiator subtractor compares the bandlimited
input signal with the preceding accumulated signal level in
the differentiator
 The difference between the two signals is PCM encoded
and transmitted
Low
pass
filter
Sample
and
hold
Analog to
digital
converter
Parallel to
serial
Differentiator
subtractor
Analog converter
input
Digital to
analog
converter
Integrator Binary
adder
Serial
DPCM

31. DPCM Receiver
 Each received sample is converted to analog,
stored and summed with the next sample
received
Serial to
parallel
converter
Digital to
analog
converter
Low pass Hold
filter
Sum
signal
out
Adder +
Integrator
Serial
DPCM
in
Analog
out