Pulse modulation

1. DIT
Dar es Salaam institute of Technology (DIT)
ET 7308
Principles of Modulation and Demodulation
Ally, J
jumannea@gmail.com

2. DIT
Pulse Modulation

3. DIT
The Chapter includes:
• Pulse Amplitude
Modulation
• Pulse Width Modulation
• Pulse Position
Modulation
• Pulse Code Modulation
PULSE MODULATION
The process of transmitting signals in the form of
pulses (discontinuous signals) by using special
techniques.

4. DIT
Pulse Modulation
 There are two types of Pulse Modulation which are analog pulse
modulation and digital pulse modulation
 In analog pulse modulation
 A periodic pulse train is used as the carrier wave, and some characteristic
feature of each pulse (e.g., amplitude, duration, or position) is varied in a
continuous manner in accordance with the corresponding sample value of
the message signal.
 Thus in analog pulse modulation, information is transmitted basically in
analog form, but the transmission takes place at discrete times.
 In digital pulse modulation
The message signal is represented in a form that is discrete in both time and
amplitude, thereby permitting its transmission in digital form as a sequence of
coded pulses; this form of signal transmission has no CW counterpart.

5. DIT
Analog Pulse Modulation Digital Pulse Modulation
Pulse Amplitude (PAM)
Pulse Width (PWM)
Pulse Position (PPM)
Pulse Code (PCM)
Delta (DM)
Pulse Modulation
Pulse Amplitude Modulation (PAM):
* The signal is sampled at regular intervals such that each sample
is proportional to the amplitude of the signal at that sampling
instant. This technique is called “sampling”.
* For minimum distortion, the sampling rate should be more than
twice the signal frequency.

6. DIT
Pulse Amplitude Modulation (PAM)
 In pulse-amplitude modulation (PAM), the amplitudes
of regularly spaced pulses are varied in proportion to
the corresponding sample values of a continuous
message signal; the pulses can be of a rectangular
form or some other appropriate shape.
 Pulse-amplitude modulation as defined here is
somewhat similar to natural sampling, where the
message signal is multiplied by a periodic train of
rectangular pulses. However, in natural sampling the
top of each modulated rectangular pulse varies with
the message signal, whereas in PAM it is maintained
flat;

7. DIT
AND
Gate
Pulse Shaping
Network
FM
Modulator
Analog
Signal
PAM - FM
Pulses at sampling frequency HF Carrier Oscillator
PAM
Pulse Amplitude Modulator
Analog Signal
Amplitude Modulated
Pulses

8. DIT
* In this type, the amplitude is maintained constant but the duration
or length or width of each pulse is varied in accordance with
instantaneous value of the analog signal.
* The negative side of the signal is brought to the positive side by
adding a fixed d.c. voltage.
Analog Signal
Width Modulated Pulses
Pulse Width Modulation (PWM or PLM or PDM)

9. DIT
* In this type, the sampled waveform has fixed amplitude and
width whereas the position of each pulse is varied as per
instantaneous value of the analog signal.
* PPM signal is further modification of a PWM signal. It has
positive thin pulses (zero time or width) corresponding to the
starting edge of a PWM pulse and negative thin pulses
corresponding to the ending edge of a pulse.
* This wave can be
further amended
by eliminating the
whole positive
narrow pulses.
The remaining
pulse is called
clipped PPM.
PWM
PPM
Pulse Position Modulation (PPM)

10. DIT
PAM, PWM and PPM at a glance:
Analog Signal
Amplitude Modulated Pulses
Width Modulated Pulses
Position Modulated Pulses

11. DIT
* Analog signal is converted into digital signal by using a digital
code.
* Analog to digital converter employs two techniques:
1. Sampling: The process of generating pulses of zero width
and of amplitude equal to the instantaneous amplitude of the
analog signal. The no. of pulses per second is called
“sampling rate”.
2. Quantization: The process of dividing the maximum value
of the analog signal into a fixed no. of levels in order to
convert the PAM into a Binary Code.
The levels obtained are called “quanization levels”.
* A digital signal is described by its ‘bit rate’ whereas analog
signal is described by its ‘frequency range’.
* Bit rate = sampling rate x no. of bits / sample
Pulse Code Modulation (PCM)

12. DIT
Explanation of PCM
 Pulse Code Modulation is the one of the basic form of digital pulse
modulation
 The basic operations performed in the transmitter of a PCM system
are sampling, quantizing, and encoding
 The low-pass filter prior to sampling is included to prevent aliasing
of the message signal.
 The quantizing and encoding operations are usually performed in
the same circuit, which is called an analog-to-digital converter.
 The basic operations in the receiver are regeneration of impaired
signals, decoding, and reconstruction of the train of quantized
samples.
 Regeneration also occurs at intermediate points along the
transmission path as necessary.
 When time-division multiplexing is used, it becomes necessary to
synchronize the receiver to the transmitter for the overall system to
operate satisfactorily.

13. DIT
Components of PCM Encoder

14. DIT
Sampling
 Analog signal is sampled every TS secs.
 Ts is referred to as the sampling interval.
 fs = 1/Ts is called the sampling rate or sampling
frequency.
 There are 3 sampling methods:
 Ideal - an impulse at each sampling instant
 Natural - a pulse of short width with varying amplitude
 Flattop - sample and hold, like natural but with single
amplitude value
 The process is referred to as pulse amplitude
modulation PAM and the outcome is a signal with
analog (non integer) values

15. DIT
Three different sampling methods for PCM

16. DIT
Nyquist sampling rate for low-pass and
bandpass signals
 According to the Nyquist theorem, the sampling rate must be at
least 2 times the highest frequency contained in the signal.

17. DIT
Example 1
A complex low-pass signal has a bandwidth of 200 kHz.
What is the minimum sampling rate for this signal?
Solution
The bandwidth of a low-pass signal is between 0 and f,
where f is the maximum frequency in the signal. Therefore,
we can sample this signal at 2 times the highest frequency
(200 kHz). The sampling rate is therefore 400,000 samples
per second.

18. DIT
Time
V
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7
6
5
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3
2
1
0
111
110
101
100
011
010
001
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L
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v
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s
B
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C
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Time
Time
V
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0 1 0 1 0 1 1 1 0 1 1 1 1 1 0 1 0 1 0 1 0
Sampling,
Quantization
and Coding

19. DIT 5/21
Pulse Code Modulation (PCM) ExamplePulse Code Modulation (PCM) Example
 The signal is assumed to be band-limited with bandwidth B
 The PAM samples are taken at a rate of 2B, or once every Ts=1/(2B)
seconds
 Each PAM sample
is quantized into
one of 16 levels
 Each sample is
then represented
by 4 bits.
 8 bits→256 level
→better quality
 4000Hz voice→
(8000sample/s)*
8bits/sample=
64Kbps

20. DIT
Bit rate and bandwidth requirements of
PCM
 The bit rate of a PCM signal can be calculated form the
number of bits per sample x the sampling rate
Bit rate = nb x fs
 The bandwidth required to transmit this signal depends on
the type of line encoding used. Refer to previous section for
discussion and formulas.
 A digitized signal will always need more bandwidth than the
original analog signal. Price we pay for robustness and other
features of digital transmission.

21. DIT
Example 2
We want to digitize the human voice. What is the bit rate,
assuming 8 bits per sample?
Solution
The human voice normally contains frequencies from 0 to
4000 Hz. So the sampling rate and bit rate are calculated
as follows:

22. DIT
PCM Decoder
 To recover an analog signal from a
digitized signal we follow the following
steps:
 We use a hold circuit that holds the amplitude
value of a pulse till the next pulse arrives.
 We pass this signal through a low pass filter
with a cutoff frequency that is equal to the
highest frequency in the pre-sampled signal.
 The higher the value of L, the less
distorted a signal is recovered.

23. DIT
Components of a PCM decoder

24. DIT
Important advantages of PCM
 Robustness to channel noise and interference.
 Efficient regeneration of the coded signal along the
transmission path.
 Efficient exchange of increased channel bandwidth for
improved signal-to-noise ratio, obeying an exponential
law.
 A uniform format for the transmission of different kinds of
baseband signals, hence their integration with other
forms of digital data in a common network.
 Comparative ease with which message sources may be
dropped or reinserted in a time-division multiplex system.
 Secure communication through the use of special
modulation schemes or encryption

25. DIT
Limitations and modifications of PCM
 PCM advantages, however, are attained at the cost of
increased system complexity and increased channel
bandwidth.
 Although the use of PCM involves many complex
operations, today they can all be implemented in a cost-
effective fashion using commercially available and/or
custom-made very-large-scale integrated (VLSI) chips.
 The requisite device technology for the implementation
of a PCM system is already in place. So improvements
in VLSI technology, we are likely to see an ever-
expanding use of PCM for the digital transmission of
analog signals.
 If, however, the simplicity of implementation is a
necessary requirement, then may use Delta Modulation
(DM) as an alternative to pulse-code modulation.

26. DIT
Delta Modulation (DM)
In DM, an incoming message signal is oversampled to purposely increase
the correlation between adjacent samples of the signal.
 An analog input is approximated by a staircase function that moves up
or down by one quantization level (δ) at each sampling interval (Ts).
 A 1 is generated if the staircase function is to go up during the next
interval; a 0 is generated otherwise.
 The staircase function tracks the original waveforms

27. DIT
Delta Modulation Operation
 For transmission:
 the analog input is compared to the most recent value of
the approximating staircase function.
 If the value of the analog input exceeds that of the
staircase function, a 1 is generated; otherwise, a 0 is
generated.
 Thus, the staircase is always changed in the direction of
the input signal.
 For reception:
 The output of the DM process is therefore a binary
sequence that can be used at the receiver to reconstruct
the staircase
function.

28. DIT
Delta modulation and demodulation components

29. DIT
Pulse Code Modulation (PCM) versus Delta
Modulation (DM)
 DM has simplicity compared to PCM
 DM has worse SNR compared to PCM
 PCM requires more bandwidth
 e.g. for good voice reproduction with PCM
 want 128 levels (7 bit) & voice bandwidth 4khz
 need 8000 sample/s x 7bits/sample = 56kbps
 PCM is more preferred than DM for analog signals

30. Merits of Digital Communication:
1. Digital signals are very easy to receive. The receiver has to just detect
whether the pulse is low or high.
2. AM FM signals become corrupted over much short distances as compared
to digital signals. In digital signals, the original signal can be reproduced
accurately.
3. The signals lose power as they travel, which is called attenuation. When
AM and FM signals are amplified, the noise also get amplified. But the
digital signals can be cleaned up to restore the quality and amplified by
the regenerators.
4. The noise may change the shape of the pulses but not the pattern of the
pulses.
5. AM and FM signals can be received by any one by suitable receiver. But
digital signals can be coded so that only the person, who is intended for,
can receive them.
6. AM and FM transmitters are ‘real time systems’. I.e. they can be received
only at the time of transmission. But digital signals can be stored at the
receiving end.
7. The digital signals can be stored, or used to produce a display on a
computer monitor or converted back into analog signal to drive a loud
speaker. END