Dc Quiz Bits 1 Mid

Quiz Bits – 1

Subject: Digital Communication Branch: 3rd B.Tech, ECE--------------------------------------------------------------------------------------------------------------------

1. The system which does not require Synchronization is a) Delta b) PCM c) PDM

2. One chief advantage of Continuous Variable Slope Delta Modulation over PCM isa) its high S/N ratio b) its high efficiency c) it can handle voice, at about half the bit rate

3. In delta modulation, in order to keep S/N ratio nearly constant, the step size ‘’ is kept small but increases as the input signal a) increases b) decreases c) remains unaffected

4. In continuous variable slope delta modulation, the squaring circuit makes a) the gain of the feedback amplifier to increase with signal amplitudeb) power to vary linearly with input signal variations c) both of these

5. One chief advantage of CVSD as compared to PCM is that it is not suitable fora) frequency sharing among multiple channelsb) time sharing among multiple channels c) none of these

6. As compared to message bandwidth, the PCM bandwidth isa) much smaller b) same c) much larger

7. As compared to ideal communication system, the PCM systema) has less complex circuitry b) compares most favorably with ideal systemc) requires 8 dB less power

8. Near threshold, the pulse system which gives most superior performance isa) Delta modulation b) PDM c) PCM

9. Regeneration capability are found in a) delta modulation b) PDM c) PCM

10. In PCM, increasing the transmitted power beyond threshold value of (S/N) leads toa) improvement in (S/N) b) no improvement in (S/N) c) both of these

11. In PCM, only decision at the receiver is required to be made is about the a) amplitude of the pulse b) width of the pulse c) presence of the pulse

12. In PCM, the high noise immunity is achieved with a) increased bandwidth b) decreased bandwidth c) none of these

13. A sample and hold circuit placed before A/D converter a) reduces the aperture time b) increases the aperture time c) none of these

14. In PCM, quantization noise appears as a) thermal noise b) shot noise c) white noise

15. Non uniform quantization is used for a) weak signals b) strong signals c) all types of signals

16. For weak signals, the step size is reduced for a) smaller sample amplitudes b) large sample amplitudes c) all sample amplitudes

17. One chief advantage of Continuous Variable Slope Delta Modulation over PCM is

A. S. Rao Page 1 of 11

Dept. of Electronics & Communication Engineering

Balaji Institute of Technology & Science

a) its high S/N ratio b) its high efficiency c) it can handle voice, at about half the bit rate

18. In PCM system, if the bandwidth of a channel is increased, the S/N ratio would a) decreases b) remains unaffected c) increases

19. To generate PCM, the signal is sampled and converted intoa) PWM b) PPM c) PAM

20. In PCM, for 128 standard quantizing levels, the maximum error will bea) 1/128 of the total amplitude range b) 1/256 of the total amplitude rangec) 1/64 of the total amplitude range

21. An audio channel which may require an analog bandwidth of 4 KHz when transmitted by a PCM system would require a bandwidth of a) 4 KHz b) 32 KHz c) 64 KHz

22. The PCM system employsa) FDM b) TDM c) either of these

23. Compander performs on a signala) compression before modulation b) expansion after demodulationc) both of these

24. The quantization error is a function ofa) intervals between levels b) signal amplitude c) both of these

25. For smaller signals, the signal to quantizing noise ratio is a) lower b) higher c) none of these

26. A Compander compresses the range of signal amplitudesa) before transmission b) at the receiving end c) both of these

27. Compander essentially makes amplitudes of a) weak signals greater b) weak and strong signals closer c) all signals greater

28. A Compander has a a) compressor at transmitting end b) expander at the receiving end c) both of these

29. In PCM, A is the pulse amplitude and N represents Gaussian noise. The error probability decreases rapidly if A/N isa) increased b) kept constant c) decreased

30. In PCM, if (S/N) exceeds a certain error threshold, the effect of random noise becomesa) pronounced b) significant c) negligible

31. As random noise is a limitation on conventional analog systems, the basic limitation on coded system is a) pulse rate b) transmission power c) quantization effect

32. The post detection S/N ratio for PCM increases when the number of quantum levelsa) decreases b) remains constant c) increases

33. The post detection S/N ratio is governed only bya) transmitted power b) no. of quantum levels c) both of these

34. In PCM, wide band noise reduction occurs because quantization noise decreases as the number of quantum levels a) increases b) decreases c) remains unaffected




35. For noise reduction in PCM, the exchange of bandwidth for S/N ratio is a) linear b) exponential c) uniform

36. The detection of PCM requires a knowledge of, a) code b) synchronization c) both of these

37. The bandwidth required for transmitting 4 KHz signal using PCM with 128 quantizing levels is a) 8 KHz b) 16 KHz c) 28 KHz

38. In a PCM system there are 256 quantizing levels, and pulse allocation width of 0.625μsec. many messages each of bandwidth 4 KHz are simultaneously transmitted on a single channel. The number of pulses per code group isa) 3 b) 4 c) 8

39. In the above question, the time period of each code group is a) 0.625μsec b) 2.5μsec c) 5μsec

40. In the same question, the time period between two samples isa) 5μsec b) 2.5μsec c) 125μsec

41. In the same question, the no. of messages that can be transmitted is a) 10 b) 15 c) 25

42. As compared to FDM, the terminal equipment in TDM is a) less b) same c) more

43. In Delta modulation, the binary output pulses has polarity which depends upon the difference between the modulating signal and a) reference signalb) feedback signal corresponding to the history of the signals previously sent c) none of these

44. In Delta Modulation, slope over load occurs when the modulating signal changesa) less rapidly than the encoder can flow b) more rapidly than the encoder can flowc) none of these

45. In Delta Modulation, slope overload can be delayed bya) increasing the sampling frequency b) increasing the step sizec) either of these

46. In an PCM system, to transmit voice frequencies below 3 KHz, seven bits are used for each sample coded at one of 128 permitted signal levels. Total no. of bits transmitted isa) 3000 bits/sec b) 6000 bits/sec c) 42,000 bits/sec

47. In the above question, minimum bandwidth required is a) 7 KHz b) 14 KHz c) 21 KHz

48. The quantization error is reduced as the number of quantization levels are a) decreased b) kept constant c) increased

49. The advantage of guard time between pulses is thata) it increases the efficiency of transmission b) it suppresses the cross talkc) message can be reconstructed with practical filter

50. To transmit a 4 KHz audio signal over a PCM channel, the sampling rate should be at leasta) 4000 samples/sec b) 8000 samples/sec c) 16000 samples/sec

51. If v is the no. of pulses in each code group in PCM, and fs is the sampling frequency, then the no. of coded pulses per second is




a) v/fs b) v fs c) (1-v) fs

52. In 24-channel PCM, the synchronization bit or s-bit is added to doa) channel synchronization b) frame synchronization c) none of these

53. The measurement of quantization noise, the signal used isa) sine wave b) white noise c) pulse wave

54. The main advantage of PCM isa) less bandwidth b) less power c) better performance in presence of noised) possibility of multiplexing

55. The main disadvantage of PCM is a) large bandwidth b) large power c) complex circuitry d) quantization noise

56. The main advantage of DM over PCM a) less bandwidth b) less power c) better S/N ratio d) simple circuitry

57. In base band transmission, the signals are transmitted ata) twice of their originating frequencies b) carrier frequencies c) their originating frequency

58. Base band pulses travel a) in space b) in waveguides c) along wires

59. The base band pulse signals can be sent over wire pairs of a few miles in length at speeds of up toa) 100 bps b) 300 bps c) 800 bps

60. Most digital transmission presently is via a) telephone voice circuits b) satellite links c) computer to computer links

61. Digital transmission via standard telephone voice circuits has top speeds of a) 800 bps b) 1000 to 5000 bps c) 109 bps

62. In contrast to analog transmission, digital system can employa) regeneration b) error control coding c) both of these

63. Synchronous digital signalsa) do not require any timing coordination at transmitter and receiverb) require timing coordination at transmitter and receiver c) none of these

64. Design wise, which of the following digital signals are more complicated? a) asynchronous signals b) unipolar signals c) synchronous signals

65. A quaternary signal is derived bya) grouping the binary digits in blocks of twob) using polar pulses for symbol transitions c) none of these

66. A baud is a unit of a) channel capacity b) signaling speed c) information

67. For 8-level system, there area) one bit per baud b) three bits per baud c) eight bits per baud

68. Data transfer rate represents a) transmission speed of the digital information b) conversion speed from analog to digitalc) average no. of bits, character or blanks transferred per unit time.

69. In digital transmission, impulse noise can cause-loss ofa) digital signal b) S/N ratio c) synchronization




70. Modems are used for a) modulating digital signals b) converting analog to digitals and vice versa c) either of these

71. The function which satisfies Nyquist first criterion is a) sine pulse b) sinc pulse c) triangular pulse

72. To achieve the same error probability, the transmitted power of an unipolar binary should bea) equal to that of polar binary b) half of polar binary c) twice that of polar binary

73. The output SNR in a DM system for a 1 KHz sinusoid sampled at 32 KHz without slope overload and followed by a 4 KHz post-reconstruction filter is given asa) 30.6 dB b) 8 dB c) 24.9 dB d) 20 dB

74. Binary data is transmitted using PSK signaling scheme with S1(t) = A cos ct, S2(t) = -A cos ct, 0 t Tb where bit duration Tb is equal to 0.2 nsec. The carrier frequency fc = 5fb. the carrier amplitude at the receiver input is 1mv and the PSD of the AWGN at the input is 10 -11 W/Hz. The probability of error for an optimum filter will bea) erfc 5.5 b) ½(erfc 5) c) erf 5.5 d)1/2(erf 5)

75. In an FSK system, the transmitted binary data rate = 2.5 Mbps, PSD of zero mean AWGN = 10-2 W/Hz and the amplitude of the received signal in the absence of noise is 1μV. The average probability of symbol error will bea) erf(6.6) b) ½ erf(6.6) c) erfc(2.45) d) ½ erfc(2.45)

76. In a system using k repeaters, if the S/N ratio is to be kept at original value; the transmitted power per station musta) increases linearly with k b) decreases linearly with k c) remains the same

77. In ASK, the transmission bandwidth is equal to a) base band bandwidth b) twice the base band bandwidthc) four times base band bandwidth

78. The advantage of coherent detection is a) simplicity of circuitry b) less vulnerability to noise c) none of these

79. The advantage of envelop detector isa) simplicity of circuitry b) less vulnerability to noise c) none of these

80. In coherent detection of ASK, ASK signal is a) frequency translated by carrier frequency and passed through a low pass filterb) multiplied with local oscillator and passed through a low pass filter c) none of these

81. Low speed modems for data rates below 1200 bps employa) ASK b) PSK c) FSK

82. In differential PCM, each word indicates a) difference between a sample amplitude and a reference signalb) difference in amplitude between a sample and previous sample c) none

83. In data transmission, the gross source rate dictatesa) error probability requirements b) signaling speed c) both of these

84. In contrast to analog transmission, the aim in digital transmission is to reproduce a) the message waveform b) the message information c) both of these

85. In data transmission, the transmission reliability dictatesa) signaling requirements b) volume of data transmitted c) error probability requirements




86. Analogous to analog systems, signaling speed in digital transmission is equivalent to a) SNR b) bandwidth c) none of these

87. Companding is used in PCM to a) reduce bandwidth b) reduce powerc) increase S/N ratio d) get almost uniform S/N ratio

88. The disadvantage of FSK is that a) it doesn’t provide sufficient S/N ratio b) it does not have low error probabilityc) it is not efficient in its use of spectrum space

89. In essence, FSK wave is two or more interleaved on-off a) ASK signals of different carrier frequencies b) PSK signals of different phasesc) FM signals of different carrier frequencies.

90. The disadvantage of coherent FSK detection is thata) it leads to high signal fading b) it requires two synchronized oscillators c) both of these

91. FSK is preferred to ASK in applications, wherea) fading of signal is prevalent b) synchronous detection not feasible c) both of these

92. In FSK, the threshold level is independent of a) carrier amplitude b) carrier frequency c) both of these

93. In ASK, the threshold level is a) independent of carrier amplitude b) function of carrier amplitude c) none of these

94. The binary PSK essentially a) ASK b) PCM c) phase reversal keying

95. In 8-PSK, each phase is represented bya) 2 bits b) 3 bits c) 8 bits

96. In detection, BPSK is similar to a) ASK b) PSK c) FSK

97. In differential PSK,a) Power requirement is twice that in coherent detectionb) Noise excursions cancel in the comparison process c) both of these

98. In differential PSK, the information is coded in terms of a) Absolute phase for each symbol b) phase changes between adjacent symbolsc) None of these

99. Main disadvantage of DPSK is thata) Power margin is very low b) it requires much large powerc) it is locked on a specific signaling speed

100. In M-ary PSK channel, the information rate(bps) can easily be increased by a) increasing M b) decreasing M c) none of these

101. In PCM, the quantization noise depends ona) sampling rate b) number of quantization levels c) signal power d) none

102. Which of the following modulation is digital in naturea) PAM b) PPM c) DM d) none

103. Which of the following modulation is analog in nature a) PCM b) DPCM c) DM d) none




104. In PCM, if the no. of quantization levels is increased from 4 to 64, then the bandwidth requirement will approximately be a) 3 times b) 4 times c) 8 times d) 16 times

105. Quantization noise occurs in a) PAM b) PWM c) DM d) none

106. In a DM system, the granular noise occurs when modulating signala) increases rapidly b) decreases rapidly c) change within the step sized) has high frequency component

107. The no. of bits per sample in a PCM system is increased from 8 to 16. the bandwidth of the system will increasea) 8 times b) 2 times c) ½ times d) 28 times

108. Companding is used a) to overcome quantizing noise in PCM b) in PCM transmitters, to allow amplitude limiting in the receiversc) to protect small signals in PCM from quantizing distortiond) in PCM receivers, to overcome impulse noise.

109. The biggest disadvantages of PCM is a) its inability to handle analog signalsb) the high error rate which its quantizing noise introduces

110. The standard data rate of a PCM voice channel is a) 8 kbps b) 8 bps c) 16 bps d) 64 kbps

111. An analog signal is sampled at the Nyquist interval of τ s seconds and quantized into L levels. The time duration of one bit of binary encoded signal is

a) b) c) d)

112. The signal to quantization noise ratio for a sinusoidal signal quantized using 10 bit PCM is a) 62 dB b) 10 dB c) 60 dB d) 32 dB

113. The rate at which information can be passed through a telecommunication channel depends on thea) a carrier frequency b) bandwidth c) transmission loss d) transmitted power

114. Signal to quantization noise ratio in n-bit PCM system a) depends on sampling frequency b) depends on no. of bitsc) increases with no. of bits d) decreases with no. of bits

115. The quantization noise in a PCM depends on a) dynamic range of the signal b) step size of the Quantizer c) both a & b d) none

116. Compression in PCM refers to relative compression of a) higher signal amplitudes b) lower signal amplitudes c) lower signal frequencyd) higher signal frequency

117. In PCM, if the transmission path is very long, thena) repeater stations are to be used b) pulse width is to be usedc) pulse amplitude is to be increased d) pulse shaping is to be reduced

118. The condition to avoid slope over error is




a) b) c) d)

[

119. Consider the following statements comparing DM with PCM system. DM requires1. a lower sampling rate 2. a higher sampling rate 3. a larger bandwidth 4. simpler hardware

a) 1, 2 and 4 b) 1, 2 and 3 c) 1, 3 and 4 d) 2, 3 and 4

120. The bit stream 01001 is differentially encoded using delay and EX-OR scheme for DPSK transmission. Assuming the reference bit as 1 and assigning phase of 0 and π for 1’s and 0’s respectively. In the encoded sequence, the transmitted phase sequence becomesa) π 0 π π 0 b) 0 π π 0 0 c) 0 π π π 0 d) π π 0 π π

121. The spectral density of White noise is a) exponential b) uniform c) linear increasing d) linearly decreasing

122. A Correlation receiver consists of a) a multiplier and an integrator b) an integrator only c) a multiplier onlyd) an adder and an integrator

123. The purpose of optimum receiver is to detect the a) weak signal embedded in background of white noise b) strong signals from the white noisec) both of these d) neither of the above

124. The output SNR of the matched filter can be a) η/2E b) 2E/η c) E/2η d) 2η/E

125. In 16-PSK the angular separation between adjacent output phases is a) 11.250 b) 22.50 c) 37.250 d) 21.250

126. QPSK is an example of M-ary data transmission with M equals to a) 2 b) 8 c) 4 d) 5

127. The no. of bits per sample in PCM system with sinusoidal input is increased from n to n+1. the improvement in signal to quantization noise ratio will bea) 3 dB b) 6 dB c) 2n dB d) n dB

128. Which of the following modulation schemes provides 4 bits per baud?a) QPSK b) 8-PSK c) 16_QAM d) 64-QAM

129. Which of the following gives maximum probability of error a) ASK b) FSK c) PSK d) DPSK

130. The output S/N ratio of a matched filter, fed at its input by a rectangular pulse of amplitude A and duration T, is given as (assume τ as noise power spectral density)a) 2A2T/τ b) AT c) A/T d) A/τ

131. Is it necessary to transmit a band limited time signal continuously over a channel in order to convey the information associated with the signal?a) yes b) no c) in most of the cases d) none

132. quantizing noise occurs in a) TDM b) FDM c) PWM d) PCM

State True or False




133. In DPSK, no synchronous carrier is needed at the receiver. [ True / False ]

134. In FSK, no synchronous carrier is needed at the receiver. [ True / False ]

135. Probability of error in DPSK is less than PSK . [ True / False ]

136. ASK is also known as on-off keying [ True / False ]

137. Correlator is a coherent system of signal reception [ True / False ]

Fill in the Blanks

138. In a PCM, the amplitude levels are transmitted in a unit code. The no. of levels is -------- (128)

139. In above question, sampling is done at the rate of 10 KHz. The BW required is ------- (35 KHz)

140. At the receiving end the time multiplexed signals ate separated by --------- (LPF)

141. The sampling rate that would be required for television signal of bandwidth 5 MHz is ---(10 MHz)

142. 6 telephone channels of 4 KHz BW and one hi-fi channel of 30 Hz- 1500 Hz are time division multiplexed. The pulse repetition rate required is ---------- (78 KHz)

143. The minimum bandwidth of BPSK scheme is equal to ---------

144. A channel has a SNR of 7 and BW 4 KHz. Keeping channel capacity (bps) same, the BW is reduced to accommodate more channels. Then SNR ---------- (increases)

145. The signal to quantization noise ratio is 25.8 dB for 16 quantization levels. The SQR for 128 quantization levels is ----------- (43.8 dB)

146. The bandwidth efficiency of 256-PSK is ----- bits/sec/Hz

147. A PCM system has a range of 4 Volts and is used to be encoded with 3 bits. The step size is ----(1 V)

148. An 8 level encoding scheme is used in a PCM system of 10 KHz channel BW. The channel capacity is ----------- (60 Kbps)

149. In a PCM system, the no. of quantization levels are 16 and the maximum signal frequency is 4KHz, the bit transmission rate is ------- (32 Kbps)

150. A PCM system has a range of 16V and is to be encoded with 3 bits. The step size is ----- (4V)

151. In a PCM system, the no. of quantization levels are 64 and the maximum signal frequency is 5 KHz, the bit transmission rate is ---------- (60 Kbps)

152. For the transmission of normal speech signal, the PCM channel needs a bandwidth of -- (64 KHz)

153. The ------ consists of difference between the input and output signals of the Quantizer. (quantizing error)

154. The minimum bandwidth of QPSK scheme is equal to ---------

155. The bandwidth efficiency of 64-FSK(non coherent) is ----- bits/sec/Hz

156. The minimum bandwidth of ASK scheme is equal to ---------

157. The minimum bandwidth of BFSK scheme is equal to ---------

158. The combination of a compressor and expander is called a --------. (Compander)




159. Any plan for representing each of the discrete set of values as particular arrangement of discrete events is called a -----------. (code)

160. The minimum bandwidth of 8- PSK scheme is equal to ---------

161. The signal to quantization of a 8 bit ADC is about -------- (48 dB)

162. The minimum bandwidth of 16-PSK scheme is equal to ---------

163. The minimum bandwidth of M-ary FSK (non coherent type) scheme is equal to ---------


165. The bandwidth efficiency of 16-FSK(non coherent)is ----- bits/sec/Hz

166. Bandwidth efficiency of M-ary PSK is ------------------

167. Bandwidth efficiency is defined as ---------------------

168. The essential operational in the transmitter of a PCM system are --------- (sampling, quantizing and encoding)

169. The quantizing and encoding are performed in the circuit which is called ------ (ADC)

170. The existence of a finite number of ------- is a basic condition of PCM. (discrete amplitude levels)

171. The conversion of an analog sample of the signal into a digital form is called the ------- process. (quantizing)


173. Error probability of PSK is -------. ( )

174. The difference between two adjacent discrete values is called------ (step size)

175. The use of a non uniform Quantizer is equivalent to passing the base band signal through a ------ and then applying the compressed signal to a ------- Quantizer. (compressor, uniform)

176. The probability of error (i.e., bit error rate) of DPSK is ----- than that of a BPSK. (higher)

177. One of the discrete events in a code is called a -------------. (code element or symbol)

178. A particular arrangement of symbols used in a code to represent a single value of the discrete set is called a ------------------. (code word or character)

179. Bandwidth efficiency of M-ary FSK is ------------------

180. The process in which the information in a binary PCM is encoded in terms of single transitions is referred to as--------. (differential encoding)

181. The capability of controlling the effect of distortion and noise produced by transmitting a PCM wave through a channel lies in reconstructing it by using ------. (regenerative repeaters)

182. The ------ process involves generating a pulse the amplitude of which is the linear sum of all the pulses in the code word, with each pulse weighted by its place value(20, 21, 22, ……) in the code.(decoding)

183. A minimum SQR of 40 dB is required. How many bits per code word are required in a linearly quantized PCM system? -------------- (7)

184. The audible range of 15Hz to 15 KHz is passed through a filter with cutoff at 7.5 KHz. The maximum sampling frequency should be ---------- (15KHz)




185. In the above question, the range is band limited to 1 KHz to 10 KHz. Then the minimum sampling frequency is ----------- (20 KHz)

186. The basic operations performed by ------ are equalization, timing and decision making. (regenerative repeater)

187. ------- noise may be introduced anywhere between the transmitter, output and the receiver input. (transmission noise)

188. The minimum error probability of matched filter is -------. ( )

189. ------- noise may be introduced in the transmitter and is carried along to the receiver output. (quantizing noise)

190. The important characteristic of a PCM system is its ----- to interference. (ruggedness)

191. The ------- is the one bit version of DPCM. (Delta modulation)

192. A Delta modulator using a fixed step size is often referred to as -----. (linear delta modulator)

193. The ----- noise occurs when the step size is too large relative to the local slope characteristics of the input waveform. ( granular noise)

194. The ----- and ------ are two noise effects in Delta modulation. (slope over load, granular noise)

195. The method in which the step size is adapted to the level of the input signal is called -----. (adaptive delta modulator)

196. The bandwidth requirement of DPSK is ----------- as compared to that of BPSK. (reduced)

197. The detection method must be performed with ----- error probability. (minimum)

198. Signal to noise ration of Integrate and Dump filter is ------. ( )

199. Error probability of ASK is -------. ( )

200. Error probability of FSK is -------. ( )




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Dc Quiz Bits 1 Mid