Pulse Digital Modulation MCQ Quiz - Objective Question with Answer for Pulse Digital Modulation - Download Free PDF

Explanation:

Digital Modulation Technique

Definition: Digital modulation refers to the process of modifying a carrier signal to encode digital information. This is achieved by altering the carrier's properties such as amplitude, frequency, or phase. Digital modulation techniques are fundamental in modern communication systems, as they enable efficient and reliable transmission of digital data over various communication channels.

Phase Shift Keying (PSK):

Definition: Phase Shift Keying (PSK) is a digital modulation technique where the phase of the carrier signal is varied to represent digital data. Each unique phase state corresponds to a specific binary or multi-bit symbol, allowing the transmission of information.

Working Principle: In PSK, the carrier signal's phase is shifted to encode digital information. For example:

• BPSK (Binary Phase Shift Keying): Two distinct phase states (e.g., 0° and 180°) are used to represent binary data (0 and 1).
• QPSK (Quadrature Phase Shift Keying): Four phase states (e.g., 0°, 90°, 180°, and 270°) are used, with each phase state representing two bits of data.
• Higher-Order PSK: More phase states (e.g., 8-PSK, 16-PSK) are used to represent more bits per symbol, increasing the data transmission rate.

Advantages of PSK:

• High spectral efficiency, allowing more data to be transmitted within a given bandwidth.
• Resilience to noise and interference, especially in low signal-to-noise ratio (SNR) environments.
• Widely used in modern digital communication systems, including wireless networks, satellite communications, and optical fiber systems.

Disadvantages of PSK:

• Complexity increases with higher-order PSK, requiring sophisticated demodulation techniques.
• Performance can degrade in the presence of severe phase noise or synchronization issues.

Applications: PSK is extensively used in various digital communication systems, including Wi-Fi, cellular networks, and satellite communications, due to its efficiency and robustness.

Correct Option Analysis:

The correct option is:

Option 3: Phase Shift Keying (PSK)

This option correctly identifies PSK as an example of a digital modulation technique. PSK is a widely used method for encoding digital information by varying the phase of the carrier signal, making it a fundamental concept in digital communication systems.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Single Sideband Modulation (SSB)

SSB is a type of analog modulation technique used in amplitude modulation (AM) systems. It involves transmitting only one sideband (either the upper or lower sideband) of the modulated signal while suppressing the carrier and the other sideband. SSB is primarily used in analog communication systems, such as amateur radio and long-distance voice communication, and is not an example of a digital modulation technique.

Option 2: Narrow Band FM

Narrow Band FM is another analog modulation technique where the frequency of the carrier signal is varied in proportion to the modulating signal. It is used in applications such as two-way radio communication and public safety communication. Similar to SSB, it is not a digital modulation technique.

Option 4: Double Sideband Amplitude Modulation

This is a classical analog modulation method where both the upper and lower sidebands are transmitted along with the carrier signal. It is commonly used in AM radio broadcasting but is not related to digital modulation techniques.

Conclusion:

Understanding the distinction between analog and digital modulation techniques is crucial for identifying the correct option. Among the options provided, only Phase Shift Keying (PSK) is a digital modulation technique, as it involves encoding digital information by varying the phase of the carrier signal. The other options, including Single Sideband Modulation (SSB), Narrow Band FM, and Double Sideband Amplitude Modulation, are examples of analog modulation techniques, which are fundamentally different from digital modulation.

In digital modulation schemes, raised cosine filter is used to reduce inter-symbol interference caused by timing error.

Additional Information

Inter-symbol Interference:

• This is the effect of dispersion on optical fibre.
• Intersymbol interference occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap.

Key Points

 The digitization of analog signals involves the rounding off of the values which are approximately equal to the analog values. The method of sampling chooses a few points on the analog signal and then these points are joined to round off the value to a near stabilized value. Such a process is called Quantization.

The quantizing of an analog signal is done by discretizing the signal with a number of quantization levels. 

Quantization is representing the sampled values of the amplitude by a finite set of levels, which means converting a continuous-amplitude sample into a discrete-time signal.

Additional InformationDelta Modulation is a technique, where the sampling rate is much higher and in which the stepsize after quantization is of a smaller value Δ, such modulation is termed delta modulation.

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony, and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.

Differential pulse code modulation is a technique of analog to digital signal conversion. This technique samples the analog signal and then quantizes the difference between the sampled value and its predicted value, then encodes the signal to form a digital value.

All given options are digital modulation schemes but the most appropriate option amongst them is ON-OFF keying which is also known as ASK.

Amplitude-shift keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave.

we can understand it by the given diagram

Important Points

Analog Modulation:

In this scheme, input needs to be in analog (continuous format)

Digital modulation:

In this scheme, input needs to be in digital format.

Bandwidth efficiency:

Spectral efficiency usually is expressed as “bits per second per Hertz,” or "bits/s/Hz".

In other words, it can be defined as the net data rate in bits per second (bps) divided by the bandwidth in hertz.

Bandwidth efficiency is given as:

\(\eta=\frac{R_b}{BW}\)

Note:

The bandwidth efficiency of M-ary PSK is

\(\eta= \frac{{{R_b}}}{BW}\)

\(BW = \frac{{2{R_b}}}{{{{\log }_2}M}}\)

To reduce the Quantization noise two things are done.

Oversampling and Noise shaping.

Explanation:

• An analog signal first undergoes the process of sampling before it is applied to ADC for conversion into a digital signal.
• Oversampling is a process in which an analog signal is sampled at a sampling frequency that is much greater than the Nyquist rate.

• Oversampling is:

  fs' = K × fs , ( K > 1)

• A Sigma-Delta ADC is an example of an ADC that employs oversampling.
• It is the same as “Delta modulation”. That is 1 sample is represented with 1 bit.
• In oversampling correlation increases and Quantization noise decreases.

Delta Modulation: 

A signal is Delta modulated to convert its analog information into a binary sequence, i.e., 1s and 0s. Here only one bit is required to represent the analog input. An over-sampled input is taken to make full use of the signal correlation. A stair-case approximated waveform will be the output of the delta modulator with the step-size as delta (Δ).

Delta Modulator has two main drawbacks:

1. Slope overload distortion: When the rate of rising of the input signal x(t) is so high that the staircase signal can not approximate it. We will see consecutive pulses of the same polarity in the output for such type of distortion. 

2. Granular noise: When the step size is too large compared to a small variation in the input signal (input is almost constant). Here, the output of the Delta modulator will be a sequence of alternate +ve and -ve pulses. 

∴ We can conclude that if the consecutive pulses are of opposite polarity during any time interval then the signal is almost constant during that interval.

Bandwidth efficiency:

Spectral efficiency usually is expressed as “bits per second per Hertz,” or "bits/s/Hz".

In other words, it can be defined as the net data rate in bits per second (bps) divided by the bandwidth in hertz.

Bandwidth efficiency is given as:

\(\eta=\frac{R_b}{BW}\)

Note:

The bandwidth efficiency of M-ary PSK is

\(\eta= \frac{{{R_b}}}{BW}\)

\(BW = \frac{{2{R_b}}}{{{{\log }_2}M}}\)

Concept: 1. In a PCM system with sinusoidal input, The SNR is given by 6n+1.8 dB. where n is a number of bits used for encoding the samples.

                2.  the number of quantization levels = \({\rm{L}} = {2^{\rm{n}}}\)

Application: The SNR is given by, \(6{\rm{n}} + 1.8 = 31.8{\rm{\;dB}}\)

\(\Rightarrow {\rm{n}} = 5{\rm{\;bits}}.\)

In digital modulation systems carrier parameters like phase, frequency or amplitude are varied with information signals.

According to this, there are three basic modulation schemes in digital modulation like:

• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)

The advantages provided by digital modulation techniques include:

• Better performance.
• Better Error detection and correction efficiency.
• Better Signal to noise ratio so less prone to interference or noise

In digital modulation schemes, raised cosine filter is used to reduce inter-symbol interference caused by timing error.

Additional Information

Inter-symbol Interference:

• This is the effect of dispersion on optical fibre.
• Intersymbol interference occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap.

All given options are digital modulation schemes but the most appropriate option amongst them is ON-OFF keying which is also known as ASK.

Amplitude-shift keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave.

we can understand it by the given diagram

Important Points

Analog Modulation:

In this scheme, input needs to be in analog (continuous format)

Digital modulation:

In this scheme, input needs to be in digital format.

Key Points

 The digitization of analog signals involves the rounding off of the values which are approximately equal to the analog values. The method of sampling chooses a few points on the analog signal and then these points are joined to round off the value to a near stabilized value. Such a process is called Quantization.

The quantizing of an analog signal is done by discretizing the signal with a number of quantization levels. 

Quantization is representing the sampled values of the amplitude by a finite set of levels, which means converting a continuous-amplitude sample into a discrete-time signal.

Additional InformationDelta Modulation is a technique, where the sampling rate is much higher and in which the stepsize after quantization is of a smaller value Δ, such modulation is termed delta modulation.

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony, and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.

Differential pulse code modulation is a technique of analog to digital signal conversion. This technique samples the analog signal and then quantizes the difference between the sampled value and its predicted value, then encodes the signal to form a digital value.

To reduce the Quantization noise two things are done.

Oversampling and Noise shaping.

Explanation:

• An analog signal first undergoes the process of sampling before it is applied to ADC for conversion into a digital signal.
• Oversampling is a process in which an analog signal is sampled at a sampling frequency that is much greater than the Nyquist rate.

• Oversampling is:

  fs' = K × fs , ( K > 1)

• A Sigma-Delta ADC is an example of an ADC that employs oversampling.
• It is the same as “Delta modulation”. That is 1 sample is represented with 1 bit.
• In oversampling correlation increases and Quantization noise decreases.

Delta Modulation: 

A signal is Delta modulated to convert its analog information into a binary sequence, i.e., 1s and 0s. Here only one bit is required to represent the analog input. An over-sampled input is taken to make full use of the signal correlation. A stair-case approximated waveform will be the output of the delta modulator with the step-size as delta (Δ).

Delta Modulator has two main drawbacks:

1. Slope overload distortion: When the rate of rising of the input signal x(t) is so high that the staircase signal can not approximate it. We will see consecutive pulses of the same polarity in the output for such type of distortion. 

2. Granular noise: When the step size is too large compared to a small variation in the input signal (input is almost constant). Here, the output of the Delta modulator will be a sequence of alternate +ve and -ve pulses. 

∴ We can conclude that if the consecutive pulses are of opposite polarity during any time interval then the signal is almost constant during that interval.

Bandwidth efficiency:

Spectral efficiency usually is expressed as “bits per second per Hertz,” or "bits/s/Hz".

In other words, it can be defined as the net data rate in bits per second (bps) divided by the bandwidth in hertz.

Bandwidth efficiency is given as:

\(\eta=\frac{R_b}{BW}\)

Note:

The bandwidth efficiency of M-ary PSK is

\(\eta= \frac{{{R_b}}}{BW}\)

\(BW = \frac{{2{R_b}}}{{{{\log }_2}M}}\)

Concept: 1. In a PCM system with sinusoidal input, The SNR is given by 6n+1.8 dB. where n is a number of bits used for encoding the samples.

                2.  the number of quantization levels = \({\rm{L}} = {2^{\rm{n}}}\)

Application: The SNR is given by, \(6{\rm{n}} + 1.8 = 31.8{\rm{\;dB}}\)

\(\Rightarrow {\rm{n}} = 5{\rm{\;bits}}.\)