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Dar es Salaam institute of Technology (DIT)

ET 7308

Principles of Modulation and Demodulation

Ally, J

jumannea@gmail.com

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Pulse Modulation

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The Chapter includes:

• Pulse Amplitude Modulation

• Pulse Width Modulation

• Pulse Position Modulation

• Pulse Code Modulation

PULSE MODULATIONThe process of transmitting signals in the form of pulses (discontinuous signals) by using special techniques.

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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.

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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.

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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;

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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

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* 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)

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* 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)

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PAM, PWM and PPM at a glance:

Analog Signal

Amplitude Modulated Pulses

Width Modulated Pulses

Position Modulated Pulses

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* 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)

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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.

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Components of PCM Encoder

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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

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Three different sampling methods for PCM

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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.

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Example 1A complex low-pass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal?

SolutionThe 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 samplesper second.

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Time

Voltage

76543210

111 110 101 100 011 010 001 000

Levels

Binary

Codes

Time

Time

Voltage

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

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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

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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.

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Example 2We want to digitize the human voice. What is the bit rate,assuming 8 bits per sample?

SolutionThe human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows:

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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.

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Components of a PCM decoder

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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

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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.

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Delta Modulation (DM)In DM, an incoming message signal is oversampled to purposely increasethe 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 () 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

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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 staircasefunction.

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Delta modulation and demodulation components

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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

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