Digital Communication MCQ with answers

Digital Communication MCQ with answers 

1. Aliasing error occurs in below condition.

 A. fs= 2W                B. fs > 2W                C. fs < 2W                D. None of these

 

2. In ideal sampling process, the sampling signal is-

A. Train of rectangular pulses                     B. Train of impulses

C. Sine wave                                       D. None of these

 

3. Spectrum of signal extends from 200Hz to 3200 Hz. The signal is-

A. Low pass signal    B. Band pass signal   C. High pass signal    D. Band reject signal

 

4. The minimum sampling frequency for signal extends from 200 Hz to 3200 Hz is

A. 6.4 KHz     B. 3.2 KHz    C. 6 KHz    D. 200 Hz

 

5. Aliasing error can be eliminated by-

A. Using anti aliasing filter                                     B. Reducing sampling frequency  

C. Increasing sampling frequency                         D. Both A & C

 

6. Circuit required to generate flat topped signal is-

A. Integrator          B. Differentiator           C. Sample and hold         D. Chopper

 

7. Bandwidth requirement for the following is Highest

A. PCM                         B. DM               C. ADM      D. DPCM

 

8. In PCM, information is transmitted in-

A. Amplitude    B. Frequency   C. Phase      D. Codeword of N Bit length

 

9. If the number of quantization levels in PCM system is 64, then the number of bits per word will be-

A. 8       B. 16        C. 6        D. 32

 

10. Companding is used to improve SNR of-

A . High frequency signal                                 B. Low frequency signal

C. Weak Signal                                                   D. Strong signal

 

11. Signaling rate of DM system having sampling frequency 16 KHz is-

A. 8 KHz    B.16 KHz     C. 64 KHz    D. 4 KHz

 

12. Following modulation technique have lowest bandwidth requirement-

A. ASK            B. FSK           C. PSK                D. QAM

 

13. The bit rate is …………than baud rate in QPSK.

A. Half    B. Double   C. Same    D. Quarter

 

14. Number of bits per symbol in QPSK is-

A. 2       B.  4      C. 3             D. 8

 

15)   In uniform quantization process

a. The step size remains same
b. Step size varies according to the values of the input signal
c. The quantizer has linear characteristics
d. Both a and c are correct

ANSWER: (d) Both a and c are correct

 

16)   The process of converting the analog signal into discrete form is called

a. Modulation
b. Multiplexing
c. Quantization
d. Sampling

ANSWER:(d) Sampling

 

 17)   The characteristics of compressor in μ-law companding are

a. Continuous in nature
b. Logarithmic in nature
c. Linear in nature
d. Discrete in nature

ANSWER: (a) Continuous in nature

 

 18)   The modulation techniques used to convert analog signal into digital signal are

a. Pulse code modulation
b. Delta modulation
c. Adaptive delta modulation
d. All of the above

ANSWER: (d) All of the above

 

 19)   The sequence of operations in which PCM is done is

a. Sampling, quantizing, encoding
b. Quantizing, encoding, sampling
c. Quantizing, sampling, encoding
d. None of the above

ANSWER:(a) Sampling, quantizing, encoding

 

 20)   In PCM, the parameter varied in accordance with the amplitude of the modulating signal is

a. Amplitude
b. Frequency
c. Phase
d. None of the above

ANSWER: (d) None of the above

 

 21)   One of the disadvantages of PCM is

a. It requires large bandwidth
b. Very high noise immunity
c. Cannot be decoded easily
d. All of the above

ANSWER: (a) It requires large bandwidth

 

 22)   The expression for bandwidth BW of a PCM system, where v is the number of bits per sample and f_m is the modulating frequency, is given by

a. BW ≥ vf_m
b. BW ≤ vf_m
c. BW ≥ 2 vf_m
d. BW ≥ 1/2 vf_m

ANSWER: (a) BW ≥ vf_m

  

23)   The error probability of a PCM is

a. Calculated using noise and inter symbol interference
b. Gaussian noise + error component due to inter symbol interference
c. Calculated using power spectral density
d. All of the above

ANSWER: (d) All of the above

 

 24)   In Delta modulation,

a. One bit per sample is transmitted
b. All the coded bits used for sampling are transmitted
c. The step size is fixed
d. Both a and c are correct

ANSWER: (d) Both a and c are correct

 

 25)   In digital transmission, the modulation technique that requires minimum bandwidth is

a. Delta modulation
b. PCM
c. DPCM
d. PAM

ANSWER: (a) Delta modulation

 

 26)   In Delta Modulation, the bit rate is

a. N times the sampling frequency
b. N times the modulating frequency
c. N times the nyquist criteria
d. None of the above

ANSWER: (a) N times the sampling frequency

 

 27)   In Differential Pulse Code Modulation techniques, the decoding is performed by

a. Accumulator
b. Sampler
c. PLL
d. Quantizer

ANSWER: (a) Accumulator

  

28)   DPCM is a technique

a. To convert analog signal into digital signal
b. Where difference between successive samples of the analog signals are encoded into n-bit data streams
c. Where digital codes are the quantized values of the predicted value
d. All of the above

ANSWER: (d) All of the above

  

29)   DPCM suffers from

a. Slope over load distortion
b. Quantization noise
c. Both a & b
d. None of the above

ANSWER:(c)  Both a & b

 

 30)   The noise that affects PCM

a. Transmission noise
b. Quantizing noise
c. Transit noise
d. Both a and b are correct

ANSWER: (d) Both a and b are correct

 

 31)   The factors that cause quantizing error in delta modulation are

a. Slope overload distortion
b. Granular noise
c. White noise
d. Both a and b are correct

ANSWER:(d) Both a and b are correct

 

 32)   Granular noise occurs when

a. Step size is too small
b. Step size is too large
c. There is interference from the adjacent channel
d. Bandwidth is too large

ANSWER: (b) Step size is too large

 

 33)   The digital modulation technique in which the step size is varied according to the variation in the slope of the input is called

a. Delta modulation
b. PCM
c. Adaptive delta modulation
d. PAM

ANSWER: (c) Adaptive delta modulation

 

 34)   The digital modulation scheme in which the step size is not fixed is

a. Delta modulation
b. Adaptive delta modulation
c. DPCM
d. PCM

ANSWER:(b) Adaptive delta modulation

 

 35)   In Adaptive Delta Modulation, the slope error reduces and

a. Quantization error decreases
b. Quantization error increases
c. Quantization error remains same
d. None of the above

ANSWER: (b) Quantization error increases

 

 36)   The number of voice channels that can be accommodated for transmission in T1 carrier system is

a. 24
b. 32
c. 56
d. 64

ANSWER: (a) 24

 

 37)   The maximum data transmission rate in T1 carrier system is

a. 2.6 megabits per second
b. 1000 megabits per second
c. 1.544 megabits per second
d. 5.6 megabits per second

ANSWER: (c) 1.544 megabits per second

  

38)   T1 carrier system is used

a. For PCM voice transmission
b. For delta modulation
c. For frequency modulated signals
d. None of the above

ANSWER: (a) For PCM voice transmission

 

 39)   The process of coding multiplexer output into electrical pulses or waveforms for transmission is called

a. Line coding
b. Amplitude modulation
c. FSK
d. Filtering

ANSWER:(a)  Line coding

 

 40)   For a line code, the transmission bandwidth must be

a. Maximum possible
b. As small as possible
c. Depends on the signal
d. None of the above

ANSWER: (b) As small as possible

 

 41)   Regenerative repeaters are used for

a. Eliminating noise
b. Reconstruction of signals
c. Transmission over long distances
d. All of the above

ANSWER:(d) All of the above

 

 42)   Scrambling of data is

a. Removing long strings of 1’s and 0’s
b. Exchanging of data
c. Transmission of digital data
d. All of the above

ANSWER: (a) Removing long strings of 1’s and 0’s

 

 43)   In polar RZ format for coding,

a. Symbol ‘0’ is represented by Zero voltage
b. Negative voltage
c. Pulse is transmitted for half the duration
d. Both b and c are correct

ANSWER: (d) Both b and c are correct

 

 44)   In a uni-polar RZ format,

a. The waveform has zero value for symbol ‘0’
b. The waveform has A volts for symbol ‘1’
c. The waveform has positive and negative values for ‘1’ and ‘0’ symbol respectively
d. Both a and b are correct

ANSWER: (d) Both a and b are correct

 

 45)   Polar coding is a technique in which

a. 1 is transmitted by a positive pulse and 0 is transmitted by negative pulse
b. 1 is transmitted by a positive pulse and 0 is transmitted by zero volts
c. Both a & b
d. None of the above

ANSWER: (a) 1 is transmitted by a positive pulse and 0 is transmitted by negative pulse

  

46)   The polarities in NRZ format use

a. Complete pulse duration
b. Half duration
c. Both positive as well as negative value
d. Each pulse is used for twice the duration

ANSWER: (a) Complete pulse duration

 

 47)   The format in which the positive half interval pulse is followed by a negative half interval pulse for transmission of ‘1’ is

a. Polar NRZ format
b. Bipolar NRZ format
c. Manchester format
d. None of the above

ANSWER: (c) Manchester format

 

 48)   The maximum synchronizing capability in coding techniques is present in

a. Manchester format
b. Polar NRZ
c. Polar RZ
d. Polar quaternary NRZ

ANSWER: (a) Manchester format

 

 49)   The advantage of using Manchester format of coding is

a. Power saving
b. Reduces Bandwidth
c. DC Component becomes zero
d. Both b and c are correct

ANSWER: (d) Both b and c are correct

 

 50)   Alternate Mark Inversion (AMI) is also known as

a. Pseudo ternary coding
b. Manchester coding
c. Polar NRZ format
d. None of the above

ANSWER:(a) Pseudo ternary coding

  

51)   In DPSK technique, the technique used to encode bits is

a. AMI
b. Differential code
c. Uni polar RZ format
d. Manchester format

ANSWER: (b)Differential code

  

52)   The channel capacity according to Shannon’s equation is

a. Maximum error free communication
b. Defined for optimum system
c. Information transmitted
d. All of the above

 ANSWER: (d) All of the above

 

 53)   The technique that may be used to increase average information per bit is

a. Shannon-Fano algorithm
b. ASK
c. FSK
d. Digital modulation techniques

ANSWER: (a) Shannon-Fano algorithm

 

 54)   Code rate r, k information bits and n as total bits, is defined as

a. r = k/n
b. k = n/r
c. r = k * n
d. n = r * k

ANSWER: (a) r = k/n

  

55)   The information rate R for given average information H= 2.0 for analog signal band limited to B Hz is

a. 8 B bits/sec
b. 4 B bits/sec
c. 2 B bits/sec
d. 16 B bits/sec

ANSWER:(b) 4 B bits/sec

  

56)   Information rate is defined as

a. Information per unit time
b. Average number of bits of information per second
c. rH
d. All of the above

ANSWER: (d) All of the above

 

 57)   Entropy is

a. Average information per message
b. Information in a signal
c. Amplitude of signal
d. All of the above

ANSWER: (a) Average information per message

 

  

58)   The memory less source refers to

a. No previous information
b. No message storage
c. Emitted message is independent of previous message
d. None of the above

ANSWER: (c) Emitted message is independent of previous message

  

59)   The information I contained in a message with probability of occurrence is given by (k is constant)

a. I = k log₂1/P
b. I = k log₂P
c. I = k log₂1/2P
d. I = k log₂1/P²

ANSWER: (a) I = k log₂1/P

  

60)   The average information contained in a message is called

a. Entropy
b. Efficiency
c. Coded signal
d. None of the above

ANSWER: (a) Entropy

  

61)   Overhead bits are

a. Framing and synchronizing bits
b. Data due to noise
c. Encoded bits
d. None of the above

ANSWER: (a) Framing and synchronizing bits

 

 62)   ISI may be removed by using

a. Differential coding
b. Manchester coding
c. Polar NRZ
d. None of the above

ANSWER: (a) Differential coding

 

 63)   Probability density function defines

a. Amplitudes of random noise
b. Density of signal
c. Probability of error
d. All of the above

ANSWER: (a) Amplitudes of random noise

 

 64)   Impulse noise is caused due to

a. Switching transients
b. Lightening strikes
c. Power line load switching
d. All of the above

ANSWER: (d) All of the above

 

 65)   In coherent detection of signals,

a. Local carrier is generated
b. Carrier of frequency and phase as same as transmitted carrier is generated
c. The carrier is in synchronization with modulated carrier
d. All of the above

ANSWER: (d) All of the above

  

66)   Synchronization of signals is done using

a. Pilot clock
b. Extracting timing information from the received signal
c. Transmitter and receiver connected to master timing source
d. All of the above

ANSWER:(d) All of the above

  

67)   A linear code

a. Sum of code words is also a code word
b. All-zero code word is a code word
c. Minimum hamming distance between two code words is equal to weight of any non zero code word
d. All of the above

ANSWER: (d) All of the above

  

68)   For a (7, 4) block code, 7 is the total number of bits and 4 is the number of

 a. Information bits

b. Redundant bits

c. Total bits- information bits

d. None of the above

 

ANSWER: (a) Information bits

  

69)   Parity bit coding may not be used for

 a. Error in more than single bit

b. Even bit is in error

c. Both a & b

d. None of the above

 ANSWER: (c) Both a & b

 

70)   Parity check bit coding is used for

 a. Error correction

b. Error detection

c. Error correction and detection

d. None of the above

 ANSWER: (b) Error detection

 

71)   For hamming distance dmin and t errors in the received word, the condition to be able to correct the errors is

 a. 2t + 1 ≤ dmin

b. 2t + 2 ≤ dmin

c. 2t + 1 ≤ 2dmin

d. Both a and b

 ANSWER: (d) Both a and b

 

72)   For hamming distance dmin and number of errors D, the condition for receiving invalid codeword is

 a. D ≤ dmin + 1

b. D ≤ dmin – 1

c. D ≤ 1 – dmin

d. D ≤ dmin

 ANSWER:(b)  D ≤ dmin – 1

  

73)   The capacity of Gaussian channel is

 a. C = 2B(1+S/N) bits/s

b. C = B2(1+S/N) bits/s

c. C = B(1+S/N) bits/s

d. C = B(1+S/N)2 bits/s

 

ANSWER: (c) C = B(1+S/N) bits/s

 

74)   For M equally likely messages, the average amount of information H is

 

a. H = log10M

b. H = log2M

c. H = log10M2

d. H = 2log10M

 

ANSWER:(b)  H = log2M

 

 75)   The channel capacity is

 a. The maximum information transmitted by one symbol over the channel

b. Information contained in a signal

c. The amplitude of the modulated signal

d. All of the above

 

ANSWER: (a) The maximum information transmitted by one symbol over the channel

 

76)   According to Shannon Hartley theorem,

 a. The channel capacity becomes infinite with infinite bandwidth

b. The channel capacity does not become infinite with infinite bandwidth

c. Has a tradeoff between bandwidth and Signal to noise ratio

d. Both b and c are correct

 

ANSWER: (d) Both b and c are correct

 

77)   The negative statement for Shannon’s theorem states that

 

a. If R > C, the error probability increases towards Unity

b. If R < C, the error probability is very small

c. Both a & b

d. None of the above

 

ANSWER: (a) If R > C, the error probability increases towards Unity

 

78)   For M equally likely messages, M>>1, if the rate of information R ≤ C, the probability of error is

 

a. Arbitrarily small

b. Close to unity

c. Not predictable

d. Unknown

 

ANSWER: (a) Arbitrarily small

 

79)   For M equally likely messages, M>>1, if the rate of information R > C, the probability of error is

 

a. Arbitrarily small

b. Close to unity

c. Not predictable

d. Unknown

 

ANSWER: (b) Close to unity

 

80)   In Alternate Mark Inversion (AMI) is

 

a. 0 is encoded as positive pulse and 1 is encoded as negative pulse

b. 0 is encoded as no pulse and 1 is encoded as negative pulse

c. 0 is encoded as negative pulse and 1 is encoded as positive pulse

d. 0 is encoded as no pulse and 1 is encoded as positive or negative pulse

 

ANSWER: (b) 0 is encoded as no pulse and 1 is encoded as positive or negative pulse

 

81)   Advantages of using AMI

 

a. Needs least power as due to opposite polarity

b. Prevents build-up of DC

c. May be used for longer distance

d. All of the above

 

ANSWER: (d) All of the above

 

82)   The interference caused by the adjacent pulses in digital transmission is called

 

a. Inter symbol interference

b. White noise

c. Image frequency interference

d. Transit time noise

 

ANSWER: (a) Inter symbol interference

 

 

83)   The number of bits of data transmitted per second is called

 

a. Data signaling rate

b. Modulation rate

c. Coding

d. None of the above

 

ANSWER: (a) Data signaling rate

 

 

84)   Nyquist criterion helps in

 

a. Transmitting the signal without ISI

b. Reduction in transmission bandwidth

c. Increase in transmission bandwidth

d. Both a and b

 

ANSWER: (d) Both a and b

 

85)   The Nyquist theorem is

 

a. Relates the conditions in time domain and frequency domain

b. Helps in quantization

c. Limits the bandwidth requirement

d. Both a and c

 

ANSWER: (d) Both a and c

 

 

86)   In On-Off keying, the carrier signal is transmitted with signal value ‘1’ and ‘0’ indicates

 

a. No carrier

b. Half the carrier amplitude

c. Amplitude of modulating signal

d. None of the above

 

ANSWER: (a)  No carrier

 

87)   ASK modulated signal has the bandwidth

 

a. Same as the bandwidth of baseband signal

b. Half the bandwidth of baseband signal

c. Double the bandwidth of baseband signal

d. None of the above

 

ANSWER:  (a) Same as the bandwidth of baseband signal

 

88)   Coherent detection of binary ASK signal requires

 

a. Phase synchronization

b. Timing synchronization

c. Amplitude synchronization

d. Both a and b

 

ANSWER:  (d) Both a and b

 

89)   The probability of error of DPSK is ______________ than that of BPSK.

 

a. Higher

b. Lower

c. Same

d. Not predictable

 

ANSWER:  (a) Higher

 

90)   In Binary Phase Shift Keying system, the binary symbols 1 and 0 are represented by carrier with phase shift of

 

a. Π/2

b. Π

c. 2Π

d. 0

 

ANSWER: (b)  Π

 

91)   BPSK system modulates at the rate of

 

a. 1 bit/ symbol

b. 2 bit/ symbol

c. 4 bit/ symbol

d. None of the above

 

ANSWER: (a) 1 bit/ symbol

 

92)   The BPSK signal has +V volts and -V volts respectively to represent

 

a. 1 and 0 logic levels

b. 11 and 00 logic levels

c. 10 and 01 logic levels

d. 00 and 11 logic levels

 

ANSWER:  (a) 1 and 0 logic levels

 

93)   The binary waveform used to generate BPSK signal is encoded in

 

a. Bipolar NRZ format

b. Manchester coding

c. Differential coding

d. None of the above

 

ANSWER:  (a) Bipolar NRZ format

 

94)   The bandwidth of BFSK is ______________ than BPSK.

 

a. Lower

b. Same

c. Higher

d. Not predictable

 

ANSWER:  (c) Higher

 

95)   In Binary FSK, mark and space respectively represent

 

a. 1 and 0

b. 0 and 1

c. 11 and 00

d. 00 and 11

 

ANSWER:  (a) 1 and 0

 

 

96)   The maximum bandwidth is occupied by

 

a. ASK

b. BPSK

c. FSK

d. None of the above

 

ANSWER: (c) FSK

 

97)   QPSK is a modulation scheme where each symbol consists of

 

a. 4 bits

b. 2 bits

c. 1 bits

d. M number of bits, depending upon the requireme

 

ANSWER: (b)  2 bits

 

98)   The data rate of QPSK is ___________ of BPSK.

 

a. Thrice

b. Four times

c. Twice

d. Same

 

ANSWER:  (c) Twice

 

99)   QPSK system uses a phase shift of

 

a. Π

b. Π/2

c. Π/4

d. 2Π

 

ANSWER:  (b) Π/2

 

100)   Minimum shift keying is similar to

 

a. Continuous phase frequency shift keying

b. Binary phase shift keying

c. Binary frequency shift keying

d. QPSK

 

ANSWER: (a) Continuous phase frequency shift keying