Pulse Code Modulation (PCM) System MCQ || Pulse code Modulation Questions And Answers

Answer.1. PCM

Explanation:-

• Quantization is the process through which a range of continuous analog values are quantized or rounded off to a single value, thereby forming samples of a discrete digital signal.
• Quantization Error occurs when there is a difference between an input value and its quantized value. 
• Quantization occurs when an analog signal is converted into its digital form, thus it occurs in Pulse Code Modulation (PCM).

Answer.2. 64 kHz

Explanation:-

Number of samples produced each sec : 2 × 4000 = 8000 (Nyquist criteria)

Each sample is encoded into 8 bits

Bit rate = 8 × 8000 = 64 kilo bits/sec

For PCM bit rate = Transmission bandwidth

Hence transmission bandwidth is 64 kHz

Answer.2. Better performance in presence of a noise 

Explanation:-

• PCM (Pulse Code Modulation) is a digital scheme for transmitting analog data.
• The amplitude of an analog signal can take any value over a continuous range, i.e. it can take on infinite values.
• But, digital signal amplitude can take on finite values.
• Analog signals can be converted into digital by sampling and quantizing.

Advantages of PCM:

• Encoding is possible in PCM.
• Very high noise immunity, i.e. better performance in the presence of noise.
• Convenient for long-distance communication.
• Good signal-to-noise ratio.

Answer.4. R/5 KHz

Explanation:-

For M-ary signaling B.W ≥ $\frac{{{R_b}}}{{{{\log }_2}M}}$

Bit Rate R_b = R k bits/sec

32-ary signaling is used:

$\frac{R}{{{{\log }_2}32}} \cdot kHz$

Minimum Bandwidth = R/5 KHz

Answer.1. 12

Explanation:-

The number of levels for an n-bit PCM system is given by:

L = 2ⁿ

We can also state that the number of bits for a given quantization level will be:

n = log₂ L

Calculation:

The number of levels given = 4096, i.e. L = 4096

The number of bits used will be:

n = log₂ (4096)

= log2 (2¹²)

= 12 log₂ (2)

n = 12

Answer.3. Larger bandwidth is required

Explanation:-

Advantages of FM over AM are:

• Improved signal to noise ratio.
• Smaller geographical interference between neighbouring stations.
• Less radiated power.
• Well-defined service areas for given transmitter power.

Disadvantages of FM:

• Much more Bandwidth
• More complicated receiver and transmitter.

Answer.4. $A \le \frac{{{\rm{\Delta }}{f_s}}}{{2\pi {f_m}}}$

Explanation:-

Two types of distortions (Errors) occurs in Delta modulation system i.e.

1) Slope overload error

2) Granular Error

To avoid the slope overload error, the optimum or desired condition is:

$\frac{{\rm{\Delta }}}{{{T_s}}} = \frac{{d\;m\left( t \right)}}{{dt}}$

i.e. the step rise of the quantized output must follow the input

Slope overload occurs when:

$\frac{{\rm{\Delta }}}{{{T_s}}} < \frac{{d\;m\left( t \right)}}{{dt}}$

To prevent/avoid slope overload error, the condition that shall be satisfied is:

$\frac{{d\;m\left( t \right)}}{{dt}} \le \frac{{\rm{\Delta }}}{{{T_s}}}$

m(t) is a sinusoidal waveform given as:

m(t) = A_m sin ω_m t

The condition to avoid slope overload, therefore, becomes:

$\frac{d}{{dt}}({A_m}\sin {\omega _m}t) \le \frac{{\rm{\Delta }}}{{{T_s}}}$

${A_m}\cos {\omega _m}t\;\left( {{\omega _m}} \right) \le \frac{{\rm{\Delta }}}{{{T_s}}}$

A_m cos (2π (mt)).2πf_m ≤ Δ⋅f_s

${A_m} \le \frac{{{\rm{\Delta }}{f_s}}}{{2\pi {f_m} \cdot \cos \left( {2\pi {f_m}f} \right)}}$

For maximum Amplitude, the above condition becomes:

${A_m} \le \frac{{{\rm{\Delta }} \cdot {f_s}}}{{2\pi {f_m}}}$

Answer.3. 22%

Explanation:-

The transmission efficiency of an AM signal is given by the expression:

$\eta = \frac{{{\mu ^2}}}{{2 + {\mu ^2}}} \times 100$

With μ = 0.75, the transmission efficiency will be

$\eta = \frac{{{{\left( {0.75} \right)}^2} \times 100}}{{2 + {{\left( {0.75} \right)}^2}}}$

μ = 21.9% ≅ 22%

Answer.4. only the video signals

Explanation:-

In TV transmission:

• A video is Vestigial sideband modulated, which is a type of amplitude-modulated waveform
• The Video signals are thus encoded in amplitude variations of the carriers
• The Audio signal is encoded in FM waveform
• Thus, the audio signals are encoded as frequency variations of the carrier

Noise is the signal that affects amplitude majorly. Thus, the video signal is distorted from amplitude variations.

Answer.3. Modulation used in FM but pre-emphasis is not used

Explanation:-

In FM broadcasting, pre-emphasis improvement is the improvement in the signal-to-noise ratio of the high-frequency portion of the baseband. The higher frequency signal is not to be detected properly because modulation is used in FM but pre-emphasis is not used.

In the process of detecting a frequency modulated signal, the receiver produces a noise spectrum that rises in frequency (a so-called triangular spectrum). This is the reason why a preemphasis is required.

Without pre-emphasis, the received audio would sound unacceptably noisy at high frequencies, especially under conditions of low carrier-to-noise ratio, i.e., during fringe reception conditions.

Preemphasis increases the magnitude of the higher signal frequencies, thereby improving the signal-to-noise ratio.