# Blake: MCQs in Digital Communications

Multiple Choice Questions in Digital Communications from the book Electronic Communication Systems by Roy Blake. Make sure to familiarize each and every questions

This is the Multiple Choice Questions in Chapter 7: Digital Communications from the book Electronic Communication Systems by Roy Blake. If you are looking for a reviewer in Communications Engineering this will definitely help. I can assure you that this will be a great help in reviewing the book in preparation for your Board Exam. Make sure to familiarize each and every questions to increase the chance of passing the ECE Board Exam.

### Start Practice Exam Test Questions

Choose the letter of the best answer in each questions.

MULTIPLE CHOICE

1. The first digital code was the:

• a. ASCII code
• b. Baudot code
• c. Morse code
• d. none of the above

ANS: C

2. In digital transmission, signal degradation can be removed using:

• a. an amplifier
• b. a filter
• c. a regenerative repeater
• d. all of the above

ANS: C

3. TDM stands for:

• a. Time-Division Multiplexing
• b. Time-Domain Multiplexing
• c. Ten-Digital Manchester
• d. Ten Dual-Manchester

ANS: A

4. Hartley's Law is:

• a. I = ktB
• b. C = 2B log2M
• c. C = B log2(1 + S/N)
• d. SR = 2fmax

ANS: A

5. The Shannon-Hartley theorem is:

• a. I = ktB
• b. C = 2B log2M
• c. C = B log2(1 + S/N)
• d. SR = 2fmax

ANS: B

6. The Shannon Limit is given by:

• a. I = ktB
• b. C = 2B log2M
• c. C = B log2(1 + S/N)
• d. SR = 2fmax

ANS: C

7. The Nyquist Rate can be expressed as:

• a. I = ktB
• b. C = 2B log2M
• c. C = B log2(1 + S/N)
• d. SR = 2fmax

ANS: D

8. Natural Sampling does not use:

• a. a sample-and-hold circuit
• b. true binary numbers
• c. a fixed sample rate
• d. an analog-to-digital converter

ANS: A

9. Which is true about aliasing and foldover distortion?

• a. They are two types of sampling error.
• b. You can have one or the other, but not both.
• c. Aliasing is a technique to prevent foldover distortion.
• d. They are the same thing.

ANS: D

10. Foldover distortion is caused by:

• a. noise
• b. too many samples per second
• c. too few samples per second
• d. all of the above

ANS: C

11. The immediate result of sampling is:

• a. a sample alias
• b. PAM
• c. PCM
• d. PDM

ANS: B

12. Which of these is not a pulse-modulation technique:

• a. PDM
• b. PWM
• c. PPM
• d. PPS

ANS: D

13. Quantizing noise (quantization noise):

• a. decreases as the sample rate increases
• b. decreases as the sample rate decreases
• c. decreases as the bits per sample increases
• d. decreases as the bits per sample decreases

ANS: C

14. The dynamic range of a system is the ratio of:

• a. the strongest transmittable signal to the weakest discernible signal
• b. the maximum rate of conversion to the minimum rate of conversion
• c. the maximum bits per sample to the minimum bits per sample
• d. none of the above

ANS: A

15. Companding is used to:

• a. compress the range of base-band frequencies
• b. reduce dynamic range at higher bit-rates
• c. preserve dynamic range while keeping bit-rate low
• d. maximize the useable bandwidth in digital transmission

ANS: C

16. In North America, companding uses:

• a. the Logarithmic Law
• b. the A Law
• c. the Î± Law (alpha law)
• d. the Âµ Law (mu law)

ANS: D

17. In Europe, companding uses:

• a. the Logarithmic Law
• b. the A Law
• c. the Î± Law (alpha law)
• d. the Âµ Law (mu law)

ANS: B

18. Codec stands for:

• a. Coder-Decoder
• b. Coded-Carrier
• c. Code-Compression
• d. none of the above

ANS: A

19. A typical codec in a telephone system sends and receives:

• a. 4-bit numbers
• b. 8-bit numbers
• c. 12-bit numbers
• d. 16-bit numbers

ANS: B

20. Compared to PCM, delta modulation:

• a. transmits fewer bits per sample
• b. requires a much higher sampling rate
• c. can suffer slope overload
• d. all of the above

ANS: D

21. In delta modulation, "granular noise" is produced when:

• a. the signal changes too rapidly
• b. the signal does not change
• c. the bit rate is too high
• d. the sample is too large

ANS: B

22. Compared to PCM, adaptive delta modulation can transmit voice:

• a. with a lower bit rate but reduced quality
• b. with a lower bit rate but the same quality
• c. only over shorter distances
• d. only if the voice is band-limited

ANS: B

23. Which coding scheme requires DC continuity:

• a. AMI
• b. Manchester
• c. unipolar NRZ
• d. bipolar RZ

ANS: C

24. Manchester coding:

• a. is a biphase code
• b. has a level transition in the middle of every bit period
• c. provides strong timing information
• d. all of the above

ANS: D

25. The number of framing bits in DS-1 is:

• a. 1
• b. 2
• c. 4
• d. 8

ANS: A

26. Framing bits in DS-1 are used to:

• a. detect errors
• b. carry signaling
• c. synchronize the transmitter and receiver
• d. all of the above

ANS: C

27. So-called "stolen" bits in DS-1 are used to:

• a. detect errors
• b. carry signaling
• c. synchronize the transmitter and receiver
• d. all of the above

ANS: B

28. The number of bits per sample in DS-1 is:

• a. 1
• b. 2
• c. 4
• d. 8

ANS: D

29. The number of samples per second in DS-1 is:

• a. 8 k
• b. 56 k
• c. 64 k
• d. 1.544 x 106

ANS: A

30. The bit rate for each channel in DS-1 is:

• a. 1.544 Mb/s
• b. 64 kb/s
• c. 56 kb/s
• d. 8 kb/s

ANS: B

31. In DS-1, bits are transmitted over a T-1 cable at:

• a. 1.544 MB/s
• b. 64 kb/s
• c. 56 kb/s
• d. 8 kb/s

ANS: A

32. A T-1 cable uses:

• a. Manchester coding
• b. bipolar RZ AMI coding
• c. NRZ coding
• d. pulse-width coding

ANS: B

33. The number of frames in a superframe is:

• a. 6
• b. 12
• c. 24
• d. 48

ANS: B

34. A typical T-1 line uses:

• a. twisted-pair wire
• b. coaxial cable
• c. fiber-optic cable
• d. microwave

ANS: A

35. "Signaling" is used to indicate:

• a. on-hook/off-hook condition
• b. busy signal
• c. ringing
• d. all of the above

ANS: D

36. A vocoder implements compression by:

• a. constructing a model of the transmission medium
• b. constructing a model of the human vocal system
• c. finding redundancies in the digitized data
• d. using lossless techniques

ANS: B

37. Compared to standard PCM systems, the quality of the output of a vocoder is:

• a. much better
• b. somewhat better
• d. not as good

ANS: D

COMPLETION

1. Digitizing a signal often results in ____________________ transmission quality.

ANS:

improved

better

2. To send it over an analog channel, a digital signal must be ____________________ onto a carrier.

ANS: modulated

3. To send it over a digital channel, an analog signal must first be ____________________.

ANS: digitized

4. In analog channels, the signal-to-noise ratio of an analog signal gradually ____________________ as the length of the channel increases.

ANS:

decreases

gets worse

5. The ____________________ value of a pulse is the only information it carries on a digital channel.

ANS: binary

6. A ____________________ repeater is used to restore the shape of pulses on a digital cable.

ANS: regenerative

7. There are techniques to detect and ____________________ some errors in digital transmission.

ANS: correct

8. Converting an analog signal to digital form is another source of ____________________ in digital transmission systems.

ANS:

error

noise

9. ____________________-division multiplexing is easily done in digital transmission.

ANS: Time

10. All practical communications channels are band-____________________.

ANS: limited

11. ____________________ Law gives the relationship between time, information capacity, and bandwidth.

ANS: Hartley's

12. Ignoring noise, the _________________________ theorem gives the maximum rate of data transmission for a given bandwidth.

ANS: Shannon-Hartley

13. The ____________________ limit gives the maximum rate of data transmission for a given bandwidth and a given signal-to-noise ratio.

ANS: Shannon

14. ____________________ sampling is done without a sample-and-hold circuit.

ANS: Natural

15. The ____________________ Rate is the minimum sampling rate for converting analog signals to digital format.

ANS: Nyquist

16. ____________________ distortion occurs when an analog signal is sampled at too slow a rate.

ANS: Foldover

17. ____________________ means that higher frequency baseband signals from the transmitter "assume the identity" of low-frequency baseband signals at the receiver when sent digitally.

ANS: Aliasing

18. The output of a sample-and-hold circuit is a pulse-____________________ modulated signal.

ANS: amplitude

19. ____________________ modulation is the most commonly used digital modulation scheme.

ANS: Pulse-code

20. ____________________ noise results from the process of converting an analog signal into digital format.

ANS: Quantizing

21. ____________________ is used to preserve dynamic range using a reasonable bandwidth.

ANS: Companding

22. In North America, compression is done using the ____________________-law equation.

ANS:

Âµ

mu

23. In Europe, compression is done using the ____________________-law equation.

ANS: A

24. A ____________________ is an IC that converts a voice signal to PCM and vice versa.

ANS: codec

25. In a PCM system, the samples of the analog signal are first converted to ____________________ bits before being compressed to 8 bits.

ANS: 12

26. The number of bits per sample transmitted in delta modulation is ____________________.

ANS:

1

one

27. Delta modulation requires a ____________________ sampling rate than PCM for the same quality of reproduction.

ANS: higher

28. ____________________ noise is produced by a delta modulator if the analog signal doesn't change.

ANS: Granular

29. In delta modulation, ____________________ overload can occur if the analog signal changes too fast.

ANS: slope

30. The ____________________ size varies in adaptive delta modulation.

ANS: step

31. Adaptive delta modulation can transmit PCM-quality voice at about ____________________ the bit rate of PCM.

ANS: half

32. Unipolar NRZ is not practical because most channels do not have ____________________ continuity.

ANS: DC

33. In AMI, binary ones are represented by a voltage that alternates in ____________________.

ANS: polarity

34. Long strings of ____________________ should be avoided in AMI.

ANS: zeros

35. Manchester code has a level ____________________ in the center of each bit period.

ANS: transition

36. Manchester coding provides ____________________ information regardless of the pattern of ones and zeros.

ANS: timing

37. There are ____________________ channels in a DS-1 frame.

ANS: 24

38. DS-1 uses a ____________________ bit to synchronize the transmitter and receiver.

ANS: framing

39. In DS-1, each channel is sampled ____________________ times per second.

ANS: 8000

40. Data is carried over a T-1 line at a rate of ____________________ bits per second.

ANS: 1.544 x 106

41. A group of 12 DS-1 frames is called a ____________________.

ANS: superframe

42. From a group of twelve frames, signaling bits are "stolen" from every ____________________ frame.

ANS: sixth

43. ____________________ compression transmits all the data in the original signal but uses fewer bits to do it.

ANS: Lossless

1. Use Hartley's Law to find how much time it would take to send 100,000 bits over a channel with a bandwidth of 2,000 hertz and a channel constant of k = 10.

ANS:

5 seconds

2. Use the Shannon-Hartley theorem to find the bandwidth required to send 12,000 bits per second if the number of levels transmitted is 8.

ANS:

2000 hertz

3. What is the Shannon Limit of a channel that has a bandwidth of 4000 hertz and a signal-to-noise ratio of 15?

ANS:

16 kbps

4. What is the minimum required number of samples per second to digitize an analog signal with frequency components ranging from 300 hertz to 3300 hertz?

ANS:

6600 samples/second

5. What is the approximate dynamic range, in dB, of a linear PCM system that uses 12 bits per sample?

ANS:

74 dB

6. What is the approximate data rate for a system using 8 bits per sample and running at 8000 samples per second?

ANS:

64 kbps

7. If bits were "stolen" from every DS-1 frame, what would the useable data-rate be for each channel in the frame?

ANS:

56 kbps

8. Assuming maximum input and output voltages of 1 volt, what is the output voltage of a Âµ-law compressor if the input voltage is 0.388 volt?

ANS:

0.833 volt

### Complete List of MCQs in Electronic Communication Systems by Blake

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