# Blake: MCQ in Digital Communications

(Last Updated On: January 9, 2018)

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 α Law (alpha law)
• d. the µ Law (mu law)

ANS: D

17. In Europe, companding uses:

• a. the Logarithmic Law
• b. the A Law
• c. the α Law (alpha law)
• d. the µ 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:

µ

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 µ-law compressor if the input voltage is 0.388 volt?

ANS:

0.833 volt

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