TRUE/FALSE
1. Analog signals cannot be sent using digital techniques.
ANS: F
2. Digitizing a signal can reduce distortion.
ANS: T
3. Digitizing a signal can improve the signal-to-noise ratio.
ANS: T
4. Morse code is an example of a binary digital transmission system.
ANS: F
5. Digital signals can be modulated onto an analog carrier.
ANS: T
6. Digitization removes noise and distortion from analog signals.
ANS: F
7. A digital signal can be changed from a 1 to a 0 by noise.
ANS: T
8. A regenerative repeater converts a degraded pulse into a new pulse.
ANS: T
9. TDM is easy to implement with digital signals.
ANS: T
10. Unlike analog, digital communications is not band-limited.
ANS: F
11. The amount of digital data that can be sent is limited by the channel capacity (C).
ANS: F
12. The digital data rate is limited by the bandwidth of the channel.
ANS: T
13. The digital data rate is limited by the SNR of the channel.
ANS: T
14. The digital data rate is limited by the number of levels transmitted.
ANS: T
15. The Shannon-Hartley theorem describes how to obtain the Shannon limit for transmission.
ANS: F
16. Digital data can be sent through a channel no matter how low the SNR is, as long as it is not zero.
ANS: T
17. Analog signals must be sampled before they can be sent in digital form.
ANS: T
18. Mathematically, it is not possible to completely reconstruct a band-limited signal from only samples of the signal.
ANS: F
19. The Nyquist Rate is equal to half the highest frequency component of the analog signal.
ANS: F
20. Natural sampling is also called "flat-top" sampling.
ANS: F
21. Flat-top sampling requires a sample-and-hold circuit.
ANS: T
22. Aliasing occurs when the sampling rate is too high.
ANS: F
23. Foldover distortion occurs when the sampling rate is too low.
ANS: T
24. Sampling is actually a form of modulation.
ANS: T
25. The output of a sampler is a PDM signal.
ANS: F
26. The most commonly used digital modulation scheme is PCM.
ANS: T
27. In PCM, the number of levels is the same as the number of bits.
ANS: F
28. Quantizing is converting a sample of an analog signal to a binary number.
ANS: T
29. Quantizing always introduces some error.
ANS: T
30. Quantizing always introduces some "noise".
ANS: T
31. The significance of quantizing noise increases as the number of bits per sample increases.
ANS: F
32. The dynamic range of a digital transmission system depends on the number of bits per sample.
ANS: T
33. The bandwidth required by a digital transmission system depends on the number of bits per sample.
ANS: T
34. Companding allows improved dynamic range for a given bandwidth.
ANS: T
35. The companding system used in America is known as "A-Law" companding.
ANS: F
36. Companding is basically a linear process.
ANS: F
37. Companding can be done with analog circuitry.
ANS: T
38. Digital companding is used by most modern telephone systems.
ANS: T
39. With delta modulation, only one bit is transmitted per cycle.
ANS: T
40. Delta modulation is particularly well suited to rapidly changing analog signals.
ANS: F
41. Delta modulation is prone to "granular noise".
ANS: T
42. Adaptive delta modulation reduces the occurrence of "slope-overload".
ANS: T
43. A disadvantage of adaptive delta modulation is that it requires a higher bit rate than PCM.
ANS: F
44. Digital data is put onto a cable using a line code.
ANS: T
45. Unipolar line coding requires DC continuity.
ANS: T
46. Bipolar RZ coding generates DC and low-frequency AC components.
ANS: F
47. The Manchester line code provides strong timing information.
ANS: T
48. The basic DS-1 signal consists of 12 voice channels.
ANS: F
49. In DS-1, each analog voice channel is sampled 8000 times per second.
ANS: T
50. In DS-1, the bit rate for each voice channel is 56 kbits per second.
ANS: F
51. A T-1 frame contains 193 bits.
ANS: T
52. T-1 uses AMI.
ANS: T
53. A T-1 cable can be twisted-pair copper wires.
ANS: T
54. A T-1 line runs at 1.544 Mbits per second.
ANS: T
55. In DS-1, bits are sometimes "stolen" from the voice channel to be used for signaling.
ANS: T
56. "Lossy" compression involves transmitting all the data in the original signal, but with fewer bits.
ANS: F
57. "Lossless" compression schemes look for redundancies in the data.
ANS: T
58. "Run-Length" encoding is a type of lossless compression.
ANS: T
MULTIPLE CHOICE
1. The first digital code was the:
a. | ASCII code | c. | Morse code |
b. | Baudot code | d. | none of the above |
ANS: C
2. In digital transmission, signal degradation can be removed using:
a. | an amplifier | c. | a regenerative repeater |
b. | a filter | d. | all of the above |
ANS: C
3. TDM stands for:
a. | Time-Division Multiplexing | c. | Ten-Digital Manchester |
b. | Time-Domain Multiplexing | d. | Ten Dual-Manchester |
ANS: A
4. Hartley's Law is:
a. | I = ktB | c. | C = B log2(1 + S/N) |
b. | C = 2B log2M | d. | SR = 2fmax |
ANS: A
5. The Shannon-Hartley theorem is:
a. | I = ktB | c. | C = B log2(1 + S/N) |
b. | C = 2B log2M | d. | SR = 2fmax |
ANS: B
6. The Shannon Limit is given by:
a. | I = ktB | c. | C = B log2(1 + S/N) |
b. | C = 2B log2M | d. | SR = 2fmax |
ANS: C
7. The Nyquist Rate can be expressed as:
a. | I = ktB | c. | C = B log2(1 + S/N) |
b. | C = 2B log2M | d. | SR = 2fmax |
ANS: D
8. Natural Sampling does not use:
a. | a sample-and-hold circuit | c. | a fixed sample rate |
b. | true binary numbers | 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 | c. | too few samples per second |
b. | too many samples per second | d. | all of the above |
ANS: C
11. The immediate result of sampling is:
a. | a sample alias | c. | PCM |
b. | PAM | d. | PDM |
ANS: B
12. Which of these is not a pulse-modulation technique:
a. | PDM | c. | PPM |
b. | PWM | 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 | c. | the a Law (alpha law) |
b. | the A Law | d. | the m Law (mu law) |
ANS: D
17. In Europe, companding uses:
a. | the Logarithmic Law | c. | the a Law (alpha law) |
b. | the A Law | d. | the m Law (mu law) |
ANS: B
18. Codec stands for:
a. | Coder-Decoder | c. | Code-Compression |
b. | Coded-Carrier | d. | none of the above |
ANS: A
19. A typical codec in a telephone system sends and receives:
a. | 4-bit numbers | c. | 12-bit numbers |
b. | 8-bit numbers | d. | 16-bit numbers |
ANS: B
20. Compared to PCM, delta modulation:
a. | transmits fewer bits per sample | c. | can suffer slope overload |
b. | requires a much higher sampling rate | d. | all of the above |
ANS: D
21. In delta modulation, "granular noise" is produced when:
a. | the signal changes too rapidly | c. | the bit rate is too high |
b. | the signal does not change | 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 | c. | only over shorter distances |
b. | with a lower bit rate but the same quality | d. | only if the voice is band-limited |
ANS: B
23. Which coding scheme requires DC continuity:
a. | AMI | c. | unipolar NRZ |
b. | Manchester | 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 | c. | 4 |
b. | 2 | d. | 8 |
ANS: A
26. Framing bits in DS-1 are used to:
a. | detect errors | c. | synchronize the transmitter and receiver |
b. | carry signaling | d. | all of the above |
ANS: C
27. So-called "stolen" bits in DS-1 are used to:
a. | detect errors | c. | synchronize the transmitter and receiver |
b. | carry signaling | d. | all of the above |
ANS: B
28. The number of bits per sample in DS-1 is:
a. | 1 | c. | 4 |
b. | 2 | d. | 8 |
ANS: D
29. The number of samples per second in DS-1 is:
a. | 8 k | c. | 64 k |
b. | 56 k | d. | 1.544 ´ 106 |
ANS: A
30. The bit rate for each channel in DS-1 is:
a. | 1.544 Mb/s | c. | 56 kb/s |
b. | 64 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 | c. | 56 kb/s |
b. | 64 kb/s | d. | 8 kb/s |
ANS: A
32. A T-1 cable uses:
a. | Manchester coding | c. | NRZ coding |
b. | bipolar RZ AMI coding | d. | pulse-width coding |
ANS: B
33. The number of frames in a superframe is:
a. | 6 | c. | 24 |
b. | 12 | d. | 48 |
ANS: B
34. A typical T-1 line uses:
a. | twisted-pair wire | c. | fiber-optic cable |
b. | coaxial cable | d. | microwave |
ANS: A
35. "Signaling" is used to indicate:
a. | on-hook/off-hook condition | c. | ringing |
b. | busy signal | 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 | c. | about the same |
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:
m
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 ´ 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
SHORT ANSWER
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 m-law compressor if the input voltage is 0.388 volt?
ANS:
0.833 volt
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