Digital Communications Lab Viva Questions with Answers part - II

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26.  Define Pulse code modulation?
Each and every quantized samples will be encoded with sequence of zeros and ones with 'n' bits within sampling interval (Ts), So the bit duration will be Ts/n. as no. of bits (n) increases error decreases but bandwidth increases.

27.  How bits are needed to encode N different levels?
      Log₂ N

28.  Define step size?
It is the difference between two  successive no.s (levels), i.e. voltage difference between one digital level and the next digital level. (Ex: 000 & 001, 1011 & 1100)
29. How to calculate Step size in PCM?
step size(Δ) =   (V_FS-V_min)/L
where L =2ⁿ, n = no. of bits, VFS = full scale voltage
30.  Define Quantization error?
It is the difference between sampled signal and Quantized signal.

31.  What is the max value of Quantization error?
Half of the Step size.

32.  What are the applications of PCM?
computer Disk, Digital Telephony, Digital Audio Applications, etc.

33.  What are the disadvantages of Pulse code modulation?
To get less Quantization Noise no. of bits should be increased, then bandwidth will be increased. So we have to compromise with either Quantization error or bandwidth.

34.  Define Differential pulse code modulation?
In DPCM, The difference between the consequent samples will be encoded with sequence of zeros and ones.

35.  Why DPCM is better than PCM?
Instead of encoding each sample, Its better to encode the difference between samples then Quantization error will be minimized with less no. of bits, then bandwidth also get decreased.

36.  Define Delta modulation? Why it is better?
It is same as DPCM with no. of bits to encode is one bit only (either 0 or 1). by this bandwidth will be decreased.

37.  What is granular noise? Define slope overload?
The Delta modulation is efficient when and only when signal is varying continuously with less variations. if signal varies suddenly then we get two different Noises. those are slope overload and granular noise
38.  When granular noise and slope overload occur in Delta modulation?
Granular Noise:  Δ / T_s > slope of signal

Slope Overload Noise: Δ / T_s < slope of signal

39.  What is Adaptive Delta Modulation and what are the advantages?

If the step size varies according to the slope of the signal then that is called as Adaptive Delta modulation. granular and slope over load noise will be desuced.

40.  Compare all Digital pulse modulation techniques (PCM, DPCM, DM, ADM)?

41.  What is multiplexing? How many types of multiplexing possible in communication?

Combining two or more signals to pass through a channel is called as multiplexing.
The different Multiplexing techniques are:1) Frequency division Multiplexing, 2) Time division Multiplexing, 3) Wavelength division multiplexing, 4) Orthogonal frequency division multiplexing.
42. What is Line coding and what are the different line coding techniques?
Line coding is the representation of digital data (0,1) as puses. the types in line coding are: Non Return to Zero (NRZ), Return to Zero (RZ), Biphase, Differential Manchester, Bipolar
43. What is the difference between Source coding and Line coding?
Source Coding is used to represent analog signals  in form of Digital data (like PCM,DPCM,DM), where Line Coding is used to represent digital data in the form of pulses (like NRZ, RZ)
44. Define ISI (Inter symbol Interference)?
It is a Distortion in digital signal that one symbol interferes with other symbol. 
45. What is Matched filter?
It is an optimal linear filter for maximizing the SNR (Signal to Noise Ratio) in the presence of additive random noise.
46. What is the cause of inter symbol Interference (ISI) and it can be reduced?
ISI caused by multipath propagation and inherent non linear frequency response of channel. It can be reduced by pulse shaping.
47.What is multipath Interference?
When a signal reaches to transmitter in various paths then delays exist in each path so at receiver all delayed signals will be received, then original signal will be interfered by its delayed versions. That is called multipath interference. 

51. What is frequency synthesizer?

it is an oscillator which can generate any range of frequencies.

Applications of frequency synthesizer?

52. Define polar encoding?

It is a line code of  RZ (Return to Zero) in which binary 1 represented by +ve voltage and 0 represented by zero volts.

53. Define bipolar encoding?

It is a line code of  NRZ (Non Return to Zero) in which two non zero values are for encoding binary data.

54. Define Manchester encoding?

It is a line code which will generated based on clock and binary data. The code is XOR logic of clock and digital data.

55. What are the features of Code Division Multiple Access?

The CDMA technique is more secure communication over all, this is used in military, If we know the code which is used to encode the data, then only we can receive the data. But No. of users are limited (depends on code length)

56. Explain about Frequency Division Multiple Access?

In FDMA Spectrum is divided into segments and each segment is permanently assigned to each transmitter. so to get more efficiency in using spectrum, each signal bandwidth should be less and perfect modulation technique should be used.

57. Explain about Time division Multiple Access?

In this a fraction of time slot will be assigned to each transmitter, so as no. of transmitters increases complexity increases. for this perfect synchronization is required.