DIGITAL COMMUNICATION VIVA QUESTIONS WITH SOLUTIONS

By Ishan Sharma

A quick revision of concepts

• 1.Define Modulation.?• Ans . Modulation is the addition of information (or the signal) to

an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals.

• • 2.Need for modulation?• Ans 1) TO REDUCE THE ANTENNA HEIGHT

2) TO MULTIPLEX THE MORE NUMBER OF SIGNALS3) TO REDUCE THE NOISE & DISTORTIONS4) TO NARROW BANDING THE SIGNAL5) TO REDUCE EQUIPMENT COMPLEXITY

• • 3.Define AM?• Ans Modulation in which the amplitude of a carrier wave is varied

in accordance with some characteristic of the modulating signal.

4.Merits and demerits of AM.? Ans advantages>1. It is simple to implement2. It can be demodulated using a circuit consisting of very few components3. AM receivers are very cheap as no specialised components are needed. Disadvantages> 1. An amplitude modulation signal is not efficient in terms of its power usage 2. It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency 3. An amplitude modulation signal is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.

5.Applications of AM?• Ans Broadcast transmissions, Air band radio ,Single sideband, Quadrature amplitude modulation.

6.Define FM?

Ans frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave

7.Merits and demerits of FM?

Ans

Advantages

• Amplitude of FM wave is independent of depth of modulation (In AM, it is dependent on depth of modulation).

• In FM, Transmitted power remains same.

• FM is immune to noise than AM (SNR is high).

• Noise is lesser for increased frequency deviation.

• Adjacent channel interference is less in FM than AM (as there are guard bands between FM stations).

• FM operates at upper VHF and UHF.

Disadvantages

• Wider channel -> 7 to 15 times larger than AM.

• FM transmitter and receiver equipment are complex and expensive.

• Line of sight -> area of reception is smaller than AM.

8..Applications of FM?

Ans 1. Magnetic tape storage

2. FM synthesis

3. An FM signal can also be used to carry a stereo signal.

9.Define pulse modulation?

Ans. A type of modulation in which pulses are varied in some respect, such as width or amplitude

10. PAM.,PWM,PPM?

Ans pulse amplitude modulation. Analoge modulation technique.

pulse width modulation. Analoge modulation technique.

pulse position modulation. Analoge modulation technique.

11.Differentiate between phase and frequency modulation?

Ans. For FM the frequency deviation(maximum instantaneous difference between an FM modulated frequency and the nominal carrier frequency. ) is independent of frequency and the phase deviation (In phase modulation, the maximum difference between the instantaneous phase angle of the modulated wave and the phase angle of the unmodulated carrier) decreases with increasing frequency.

For PM the phase deviation is independent of frequency and the frequency deviation increases with increasing frequency.

> Pm Used in WiFi, GSM and satellite television

12. Define pulse code modulation (PCM).?

Ans. digital s/g modulation technique.

13..Define delta modulation.?

Ans Scheme in which difference of s/g d(t) is encoded into single bit.

14. quantization?

in mathematics and digital signal processing, is the process of mapping a large set of input values to a (countable) smaller set – such asrounding values to some unit of precision.

15. What is slope overload error?

Ans excessive disparity b/w signal and step is described as slope overload error.

16. What is granular error?

Granular noise occurs when the original analog signal has a relatively constant amplitude and the reconstructed signal has variations that were not present in the original information signal.

17. 17.Define adaptive delta modulation.?Ans. modification of D.M. is ADM in which step size changes according to increasing probability of slope overload error. 18.Distinguish between DM and ADM.?Ans DM : slope overload error but in ADM: Quantization error

19.Define ASK, PSK and FSK.?Ans All are digital modulation schemes.ASK refers to a type of amplitude modulation that assigns bit values to discrete amplitude levelsFSK refers to a type of frequency modulation that assigns bit values to discrete frequency levels. PSK in a digital transmission refers to a type of angle modulation in which the phase of the carrier is discretely varied—

either in relation to a reference phase or to the phase of the immediately preceding signal element—to represent data being transmitted

20.Differentiate ASK and FSK.?ANS. 1. Frequency modulation scheme other is amplitude …2. Fsk less susceptible to errors while ask is more..3. Fsk spectrum is 2 x Ask spectrum.

21.What is meant by PLL?

Ans A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly adjusted to match in phase (and thus lock on) the frequency of an input signal

22.Applications of PLL.?

Ans 1. Frequency Divider and Multiplier.

2. PLL Amplifier for Angle Modulated Signals.

3. Frequency Synthesis and Angle Modulation by PLL.

23.Define pre-emphasis & de-emphasis?

Ans

Pre-emphasis:

Pre-emphasis refers to boosting the relative amplitudes of the modulating voltage for higher audio frequencies from 2 to approximately 15 KHz.

De-emphasis:

De-emphasis means attenuating those frequencies by the amount by which they are boosted.

However pre-emphasis is done at the transmitter and the de-emphasis is done in the receiver.

24.Why is ASK called as ON-OFF keying?

Ans In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero.

25.What are the advantages of digital transmission?

Ans Produces fewer errors

Permits higher maximum transmission rates

More efficient

More secure

Simpler to integrate voice, video and data

26.What is meant by frequency synthesizer?

Ans A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator.

27.Applications of frequency synthesizer.?

Ans . They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc.

28.What is meant by line coding?

Ans Output of the multiplexer (TDM) is coded into electrical pulses or waveforms

for the purpose of transmission over the channel

29.Define polar encoding.?

Ans . The signal does not return to zero, it is either a +ve voltage or a -ve voltage. Polar line encoding is also called None Return To Zero (NRZ).

30.Define bipolar encoding.?

Ans bipolar encoding is a type of line code, where two nonzero values are used, so that the three values are +,-, and zero.example Alternate Mark Inversion (AMI)

31.Define unipolar encoding.?Ans Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a

binary 0.32.Define Manchester encoding.?Ans. Each bit is transmitted in a fixed time (the "period").A 0 is expressed by a low-to-high transition, a 1 by high-to-low transition

33. Differentiate between RZ,NRZ,NRZI,AMI codes?ANS

36.Explain ARQ,FEC.HARQ?

Automatic Repeat reQuest (ARQ), also known as Automatic Repeat Query, is an error-control method for data transmission that uses acknowledgements and timeouts to achieve reliable data transmission over an unreliable service.

forward error correction (FEC) or channel coding[1] is a technique used for controlling errors in data transmission over unreliable or noisy communication channels. The central idea is the sender encodes his message in a redundant way by using an error-correcting code (ECC).

Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high-rate forward error-correcting coding and ARQ error-control.

37.What is sampling?

Ans sampling is the reduction of a continuous signal to a discrete signal.

38.State sampling theorem.?

Ans . The Sampling Theorem shows that a continuous-time band-limited signal may be represented perfectly by its samples at uniform intervals of T seconds if T /is small enough.

39.What is aliasing?

Ans .Aliasing is an effect that causes different signals to become indistinguishable when sampled.

• 40.How to avoid aliasing effect.?• 1. The easiest way to prevent aliasing is the application of a steep sloped low-pass filter with half the sampling frequency

before the conversion. • 2. Aliasing can be avoided by keeping Fs>2Fmax• 41.Define Nyquist Criteria.• Ans states that a signal must be sampled at a rate greater than twice the highest• frequency component of interest in the signal to capture the highest frequency component of interest;• otherwise the high otherwise, the high-frequency content will alias at a frequency inside the spectrum of interest• 42.Purpose of sampling?• Ans. Sampling is done first so that each sample can be represented by n digits corressponding to 2n levels of quantization.• 43. what is DTV?• Ans. is the transmission of audio and video by digitally processed and multiplexed signal• 44. How many digital television terrestrial broadcasting standards (DTTB) are used?• Ans There are four different widely used digital television terrestrial broadcasting standards (DTTB):

• Terrestrial Integrated Services Digital Broadcasting (ISDB-T)• Digital Video Broadcasting-Terrestrial (DVB-T) • Advanced Television System Committee (ATSC)• Digital Terrestrial Multimedia Broadcasting (DTMB)

45. Difference in HDTV and SDTV?

Ans. HDTV: 720p or 1080i(interlaced video) + 16:9 aspect ratio.( ratio of width to height)

SDTV: 4:3 aspect-ratio broadcasts, the 640 × 480 format is used in NTSC countries, while 720 × 576 is used in PAL countries

46. What is aperture effect?

Ans In flat top sampling,due to the lengthening of the sample, amplitude distortion as well as adelay of T/2 was introduced.This distortion is referred to as Aperture effect.

47.How aperture effect can be corrected?

Ans . Aperture effect can be corrected by connecting an equalizer in cascade with the low pass reconstruction filter.

48. what is pulse stuffing?

Ans. Process of adjusting bit rates to make them equal involves adding bits to the slower bit stream in an operation.

49.Explain nysquist criterion and interval?

Ans. the Nyquist criterion is often stated as fs > 2B. And 2B is called the Nyquist rate for function with bandwidth B.

Nyquist interval: The maximum time interval between equally spaced samples of a signal that will enable the signal waveform to be completely determined.

Note 1: The Nyquist interval is equal to the reciprocal of twice the highest frequency component of the sampled signal.

50 A law and µ law?

Ans. An A-law algorithm is a standard companding algorithm, used in European 8-bit PCM digital

communications systems to optimize, i.e., modify, the dynamic range of an analog signal for

digitizing. It is similar to the μ-law algorithm used in North America and Japan.

51.Make block diagram of ASK modulator and demodulator?

52.Make block diagram of PSK modulator and demodulator?

54. non-return-to-zero mark (NRZ-M)

A binary encoding scheme in which a signal parameter, such as electric current or voltage, undergoes a change in a significant condition or level every time that a "one" occurs, but when a "zero" occurs, it remains the same, i.e., no transition occurs.

> Can be used for ask generation.

53.Make block diagram of FSK modulator and demodulator?

55. why pm is better than fm ?Ans . The reason PM is better is because in PM and FM there can be non-zero energy

produced at 0 Hz, which in FM will produce a shift in pitch if the FM wave is used again as a modulator, however in PM the DC component will only produce a phase shift

56. Difference in digital and discrete signals?Ans. digital : Representation of a sequence of discrete values Discrete : after sampling of continuous signal the individual or distinct packets of

signals we obtain is called discrete signal.57. why we need quantization?ANS. After sampling the discrete signal has to be rounded to its nearer digital value.

hence we go for quantization.58.Sample and Hold circuit?Ans.

END OF PRESENTATION