PULSE ANALOG MODULATION

1. UNIT V-PULSE ANALOG
MODULATION
Types of Pulse modulation, PAM (Single polarity,
double polarity) PWM: Generation & demodulation of
PWM, PPM, Generation and demodulation of PPM,
Demodulation of Pulse Analog Modulated Signals,
Time Division Multiplexing, FDM,TDM Vs FDM

2. INTRODUCTION TO DIGITAL TRANSMISSION
• Digital transmission is the transmission digital data between two or more points in a
communication system.
• The signals can be binary or any other form of discrete-level digital pulses.
• The original information may be in digital form, or it could be analog signals that
have been converted to digital pulses prior to transmission and converted back to
analog signals at the receiver side.
• A pair of wires, coaxial cable, or an optical fiber cable, is used as transmission
medium or channel.
Advantages of Digital Transmission
Noise Immunity:
Multiplexing:
It is much simpler to store digital signals and process it further.(suitable for
digital signal processing).
Digital signals are easy to interface with different types of equipment.

3. PULSE MODULATION
• In analog modulation systems such as AM, FM and PM, the carrier signal is high
frequency sinusoidal signal.
• In pulse modulation system, the carrier is train of discrete pulses rather than being a
sine wave.
Definition
Pulse modulation consists of sampling the analog information signals and then
converting those samples into discrete pulses and transmitting the pulses from a source
to a destination over a physical transmission medium or channel.
• Any one characteristics of the pulse train i.e., amplitude, width or position can be
varied with respect to amplitude of the modulating signal.

4. (i) PWM - Pulse Width Modulation.
(ii) PPM - Pulse Position Modulation.
(iii) PAM - Pulse Amplitude Modulation
(iv) PCM - Pulse Code Modulation.
(v) DM - Delta Modulation
(vi) ADM - Adaptive Delta Modulation.

5. Pulse Amplitude Modulation (PAM) Definition:
"The amplitude of a constant width, constant-position pulse is varied with respect
to the amplitude of the sample of the modulating signal".
Types of Pulse Amplitude Modulation:
• Single Polarity PAM
• Double Polarity PAM
➢ Single polarity PAM is a situation where a suitable fixed DC bias is added
to the signal to ensure that all the pulses are positive.
➢ Double polarity PAM is a situation where the pulses are both positive and
negative.
Pulse Width Modulation (PWM) :
The width of a constant amplitude, carrier pulse is varied with respect to the
amplitude of the sample of the modulating signal"
Pulse Postion Modulation (PPM) :
The position of a constant –width pulse within a priscribed time slot is varied
according to the amplitude of the sample of the modulating signal"

6. The width and positions of the pulses are constant. Depending upon the shape of the
PAM pulse, it is mainly classified into
(i) Natural PAM
(ii) Flat top PAM.
Natural PAM
Generation of Natural PAM

7. Wave forms of Natural PAM generation

8. Detection of Natural PAM Signals:

9. Flat top PAM
• The natural sampling is rarely employed in practice. Instead, the other practical
sampling technique flat top sampling is employed in practice.
• Here, the message signal x(t) is sampled instantaneously at the rate 𝑓𝑠 = 1 /𝑇𝑠 , and
the duration of each sample is lengthened to a duration ‘τ’.
Generation of Flat top PAM

10. Advantages of PAM:
Simplicity of generation and detection.
Disadvantages of PAM
(i) The effect of additive noise is maximum in PAM. The added noise cannot be
removed easily.
(ii) The transmission bandwidth required for a PAM signal is too large.
(iii)Due to the changes in amplitudes of PAM pulses, the transmitted power is not
constant.

11. Pulse Width Modulation (PWM)
• PWM is also called Pulse Duration Modulation (PDM) or Pulse Length Modulation
(PLM).
Definition :
The width of a constant amplitude carrier pulse is varied with respect to the
amplitude of the modulating signal".
PWM Waveforms

12. Generation of PWM signal:
Block diagram of PWM and PPM generator

13. Waveforms of PWM and PPM

14. Detection of PWM signals:
•The output of the adder is clipped off at a threshold level to
generate a PAM signal at the output of the clipper.
• Finally, a low pass filter is used to recover the original
modulating signal from PAM signal .

15. Waveforms for PWM detection

16. Advantages of PWM
1. Effect of noise is very less.
2. Synchronization circuit is not necessary between transmitter and
receiver.
3. It is easy to recover the PWM signal from noise.
Disadvantages of PWM
1. Due to Variable pulse width, power of the pulse varies. Hence
transmission must be powerful to handle maximum width pulse.
2. Bandwidth requirement of PWM is larger than that of PAM to avoid
waveform distortion.

17. Pulse Position Modulation

20. TIME DIVISION MULTIPLEXING (TDM)
Time-division multiplexing is a method of putting multiple data streams in a single
signal by separating the signal into many segments, each having a very short
duration. Each individual data stream is reassembled at the receiving end based on
timing.
Working Operations of TDM
Block diagram of TDM system

21. Operation of TDM

22. The types of TDM are
(i) Synchronous TDM
(ii) Asynchronous TDM
Synchronous TDM (STDM)
In synchronous TDM, each device is given same time slot to transmit the data over the
link, irrespective of the fact that the device has any data to transmit or not.
• Synchronous TDM requires that the total speed of various input lines should not exceed
the capacity of path.
• Each device places its data onto the link when its time slot arrives i.e. each device is
given the possession of line turn by turn.

23. Disadvantages of STDM The disadvantages of STDM are
(i). The channel capacity cannot be fully utilized. Some of the slots go empty in certain
frames.
(ii). The capacity of single communication line that is used to carry the various transmission
should be greater than the total speed of input lines.
Asynchronous TDM (ATDM)
In asynchronous TDM, the time slots are not predefined and the slots are allocated to any
of the device that has the data to send. The multiplexer scans the various input lines,
accepts the data from the lines that have data to send, fills the frame and then sends the
frame across the link. It is also known as statistical time division multiplexing.
Asynchronous TDM

24. Transmission Hierarchy (T-Carrier Systems)
T carrier systems

25. T1 Carrier System
• In this system, 24 voice frequency signals, each sampled at 8000 samples per second,
encoded into an 8-bit word and transmitted along with one synchronizing bit per frame using
AMI code. T1 carrier system uses word interleaving.
Frame duration =18000sec=125𝜇sec Number of bits per frame =(24×8)+1=193
The PCM T1 frame using channel associated signaling
The 192 bits slots assigned to the encoded signal together with one extra timing
bit i.e., synchronizing bit which is necessary to provides synchronization
between channels, for a total of 193 bits. This is called a frame.
Hence, the transmission rate =193/125×10−6=1.544Mbps

26. T2 Carrier System
• In this system, 96 voice frequency signals are multiplexed. This is achieved by
multiplexing outputs of four T1 carriers and here 17 sync. bits are used.
Thus, the transmission rate =
(4×1.544)Mbps+(17×8)kbps =6.176Mbps+0.136Mbps =6.312Mbps
(3) T3 Carrier System
• In this system, 672 voice frequency signals are multiplexed. This is achieved by
multiplexing outputs of seven T2 carriers and here 69 sync. bits are used. Thus, the
transmission rate
=(7×6.312)Mbps+(69×8)kbps =44.184Mbps+0.552Mbps =44.736Mbps
𝑻𝟒 Carrier System
• In this system, 4032 voice frequency signals are multiplexed. This is achieved by
multiplexing outputs of six T3 carriers and here 720 sync. bits are used.
Thus, the transmission rate
=(6×44.736)Mbos+(720×8)kbps =268.416Mbps+5.760Mbps =274.176Mbps
T5 Carrier System
• In this system, 8064 voice frequency signals are multiplexed. This is achieved by
multiplexing outputs of two T4 carriers and here 1476 sync.bits are used.
Thus, the transmission rate
=(2×274.176)Mbps+(1476×8)kbps =548.352Mbps+11.808Mbps =560.16Mbps

27. Advantages
The advantages of TDM are
(i) Fully available channel bandwidth can be utilized for each channel.
(ii) Intermodulation distortion is absent.
(iii) TDM circuitry is not very complex.
(iv) The problem of crosstalk is not severe.
Disadvantages
The disadvantages of TDM are
(i) Synchronization is essential for proper operation.
(ii) Due to slow narrowband fading, all the TDM channels may get wiped out.

28. FREQUENCY DIVISION MULTIPLEXING (FDM)
𝐹𝐷𝑀 is a multiplexing technique that uses different frequencies to combine multiple
streams (signals) of data for transmission over a single communication medium. FDM
assigns a different frequency (sub channel) for every signal within the main channel
and then combines many modulated carrier frequencies for transmission.
FDM

29. Operation of FDM Systems
In this method of multiplexing, each message of maximum frequency 𝑓𝑚 is translated to a
different frequency spectrum by the use of the carriers. These messages are then combined
in an adder circuit.
• At the receiving end, a broad-band receiver receives this signal and passes it onto base
band receivers which receive signals corresponding to the respective base-band frequency.
At the output of these receivers, different signals are available. Here FDM operates in only
one direction.
Operations of FDM

30. Advantages
The advantages of FDM are
(i) A large number of signals (channels) can be transmitted simultaneously.
(ii) FDM does not need any synchronization between its transmitter and receiver for
proper operation.
(iii) Demodulation of FDM is also easy. (iv) Due to slow narrow band fading only a
single channel gets affected.
Disadvantages
The disadvantages of FDM are
(i) The communication channel must have a very large bandwidth.
(ii) Intermodulation distortion takes place.
(iii) Large number of modulators and filters are required.
(iv) FDM suffers from the problem of crosstalk.
(v) All the FDM channels get affected due to wideband fading.
Applications of FDM The applications of FDM are
(i) FDM is used for FM & AM radio broadcasting.
(ii) FDM is used in television broadcasting.
(iii) First generation cellular telephone also uses FDM.