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Pulse Code Modulation (PCM) Viva Questions

Pulse Code Modulation Viva Questions and Answers

1. Mention channel noise?


The combined effect of thermal noise, interference from other users, and interference from circuit switching transients are termed channel noise. It can enter the transmission path at any moment. It can be represented as Additive White Gaussian Noise (AWGN) with a zero mean and power spectral density of No/2.


2. Discuss Inter Symbol Interference (ISI)?


A pulse waveform passing via a band-limited channel is dispersed or spread. Spreading will exceed symbol duration when the channel bandwidth is near to the signal bandwidth, causing signal pulses to overlap. Inter Symbol Interference (ISI) is the term used for this overlapping.


3. What do you know about PCM?


PCM is a type of baseband signal generated by encoding each quantized sample into a digital word from quantized PAM signals. Sampling, quantizing, and encoding are the three most important operations in the PCM transmitter.


4. What is companding?


The process of signal compression and expansion is known as companding. The compressor enhances weak signals and attenuates strong signals at the transmitter. The expander performs the opposite of compression at the receiver. The companding process is used to solve the problem of quantization noise.


5. Discuss Line Coding?


Binary digits are coded into electrical pulses or waveforms to deliver multiplexed message signals in the form of a sequence of bits across a baseband channel.  This process is termed Line coding or transmission coding.


6. What do you know about the PCM waveform? Discuss its types?


A Pulse Code Modulation (PCM) waveform is generated when pulse modulation is applied to binary symbols. These PCM waveforms are known as Line Codes in telephone applications. The four types of PCM waveforms are as follows.


1. Non-Return to Zero (NRZ)

2. Return to Zero (RZ)

3. Phase encoded

4. Multilevel Binary


7. Illustrate about Non-Return to Zero (NRZ) waveform?


Non-Return to Zero (NRZ) waveforms occur when the PCM waveform remains at any non-zero level for the whole bit interval T. The following subgroups are:


(i) NRZ – L (L for level)

(ii) NRZ – M (M for mark)

(iii) NRZ – S (S for space)


8. Discuss about Return to Zero (RZ) waveform?


Return to Zero (RZ) waveform refers to a PCM waveform that returns to zero after a portion of the bit interval T. It can be classified into the subcategories listed below:


(i) Unipolar RZ

(ii) Bipolar RZ

(iii) RZ-AMI (AMI for Alternate Mark Inversion)


9. What is phase encoded waveform?


The time position of the occurrence or transition of a pulse waveform is used to discriminate between various logic levels in a phase encoded system. It can be classified as:


(i) Bi-φ-L (Bi-phase-Level) or Manchester coding

(ii) Bi-φ-M (Bi-phase-Mark)

(iii) Bi-φ-S (Bi-phase-Space)

(iv) Delay Modulation (DM) or Miller Coding


10. What you know about Multilevel binary waveform?


Multilevel binary waveforms are binary waveforms that encode binary data using three levels instead of two. The formats used in multilevel binary are namely:


(i) Dicode NRZ

(ii) Dicode RZ

(iii) Duobinary signalling

(iv) Bipolar RZ

(v) RZ – AMI


11. Mention the factors that were utilised to choose a particular PCM waveform type.


The most common criteria for assessing PCM waveforms and determining a waveform type are as follows:


1. Spectral attributes or characteristics

2. Bit synchronization capabilities

3. Error detection performance

4. Interference and noise immunity

5. Cost and complexity or implementation


12. Interpret PCM wordsize.


The PCM wordsize is the number of bits necessary to indicate the quantization levels of an analogue sample. Each speech sample has 256 quantization levels for digital telephone channels. As a result, PCM word size is 8 bits (28 = 256).


13. List the different types of Pulse Modulation.


Analog Pulse Modulation

Digital Pulse Modulation

1. Pulse Amplitude Modulation (PAM)

1. Pulse Code Modulation (PCM)

2. Pulse Width Modulation (PWM)

2. Differential Pulse Code Modulation (DPCM)

3. Pulse Position Modulation (PPM)

3. Delta Modulation (DM)

 

4. Adaptive Delta Modulation (ADM)

 

14. Discuss in detail about M-ary Pulse Modulation Waveforms.


The generated waveform is called M-ary Pulse Modulation Waveform in pulse modulation schemes, when the output of the pulse generator consists of pulses with M possible amplitude levels. M is usually an integer power of two.


15. Mention the  advantages and disadvantages of M-ary Pulse Modulation schemes?


Advantages are:


1. The M-ary system, also known as multilayer signalling, decreases the amount of bandwidth needed for transmission.

2. A high signal-to-noise ratio is important.

3. The M-ary technique is log2M times quicker than the binary representation.


Disadvantages


1. In comparison to the binary scheme, the M-ary strategy uses more energy to achieve equal detection performance.

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