Virtual Labs

Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM), Pulse Position Modulation(PPM), Pulse Coded Modulation & their Demodulation

1. Which statement best explains why Pulse Amplitude Modulation (PAM) is more susceptible to noise compared to digital modulation schemes?

a: Because it encodes information in continuous levels susceptible to amplitude distortion. Explanation

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b: Because it uses wide bandwidth. Explanation

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c: Because it relies on frequency variation, which is inherently noisy. Explanation

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d: Because timing synchronization is not used. Explanation

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2. In PWM systems, why can higher signal amplitudes lead to intersymbol interference if the system is not properly designed?

a: Because increased pulse width can overlap adjacent symbols. Explanation

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b: Because amplitude fluctuations distort the pulse height. Explanation

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c: Because higher amplitudes cause frequency drift. Explanation

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d: Because signal reflection occurs at higher amplitudes. Explanation

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3. Which property of Pulse Position Modulation (PPM) makes it advantageous in low-power optical communication systems?

a: It transmits energy in short bursts, allowing efficient power usage. Explanation

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b: It has a constant pulse width for all signal levels. Explanation

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c: It requires no synchronization. Explanation

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d: It encodes amplitude variations efficiently. Explanation

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4. In the PCM system, why is oversampling generally avoided in practice?

a: It increases the data rate and storage without improving quality proportionally. Explanation

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b: It reduces the noise immunity of the quantizer. Explanation

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c: It causes aliasing in the signal. Explanation

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d: It leads to lower resolution of each sample. Explanation

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5. Why is PAM often used as an intermediate step in digital modulation schemes such as QAM or OFDM?

a: Because it allows simple mapping from digital bits to amplitude levels. Explanation

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b: Because it uses time division to reduce bandwidth. Explanation

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c: Because it requires no synchronization. Explanation

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d: Because it is robust against phase distortion. Explanation

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6. In PWM, which factor makes it more robust than PAM under varying channel gain conditions?

a: Information is encoded in time (width), not amplitude. Explanation

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b: It uses orthogonal frequency tones. Explanation

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c: Its pulses are less frequent than in PAM. Explanation

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d: It automatically adjusts pulse spacing. Explanation

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7. What limits the number of quantization levels in a PCM system?

a: The number of bits allocated per sample. Explanation

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b: The sampling frequency. Explanation

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c: The channel bandwidth. Explanation

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d: The analog signal frequency. Explanation

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8. Why does quantization in PCM inherently introduce noise?

a: Because it approximates real values with finite discrete levels. Explanation

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b: Because the signal is sampled at irregular intervals. Explanation

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c: Because pulses are affected by timing jitter. Explanation

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d: Because bandwidth limitation distorts the signal. Explanation

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9. A PCM system uses an 8-bit quantizer and a sampling rate of 8 kHz. What is its resulting bit rate?

a: 64 kbps Explanation

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b: 8 kbps Explanation

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c: 16 kbps Explanation

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d: 32 kbps Explanation

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10. In PPM, what is the key disadvantage when comparing synchronization requirements to PAM or PWM?

a: PPM requires precise timing alignment to decode correctly. Explanation

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b: PPM uses more bandwidth due to wider pulses. Explanation

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c: PPM encodes information in amplitude which is noise sensitive. Explanation

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d: PPM can only support binary encoding. Explanation

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