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UNIT- I1. What are the advantages of digital transmission?

The advantage of digital transmission over analog transmission is

noise immunity. Digital pulses are less susceptible than analog signals to variationscaused by noise.

Digital signals are better suited to processing and multiplexingthan analog signals.

Digital transmission systems are more noise resistant than theanalog transmissionsystems.

Digital systems are better suited to evaluate error performance.

2. What are the disadvantages of digital transmission?

i. The transmission of digitally encoded analog signals requiressignificantly more bandwidth than simply transmitting the originalanalog signal.ii. Analog signal must be converted to digital codes prior totransmission and converted back to analog form at the receiver, thusnecessitating additional encoding and decoding circuitry.

3. Define pulse code modulation.In pulse code modulation, analog signal is sampled and

converted to fixed length, serial binary number for transmission. The binary number varies according to the amplitude of the analog signal.This sample variable amplitude pulse is digitized by the analog todigital converter.

4. What is the purpose of the sample and hold circuit?The sample and hold circuit periodically samples the analog

input signal and converts those samples to multilevel PAM signal.

5. What is the Nyquist sampling rate? Nyquist sampling rate states that, the minimum sampling rate

is equal to twice the highest audio input frequency.

6. Define overload distortion.If the magnitude of sample exceeds the highest quantization

interval, overload distortion occurs.

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7. Define quantization.Quantization is process of approximation or rounding off.

Assigning PCM codes to absolute magnitudes is called quantizing.

8. Define dynamic range.Dynamic range is the ratio of the largest possible magnitude to

the smallest possible magnitude. Mathematically, dynamic range is

DR= Vmax/Vmin

9. Define Quantization error?The difference between the instantaneous values of the

quantized signal and the input signal is called as quantization error.

10. What are the three types of quantiser? Midtread quantiser Midriser quantiser Biased quantiser

11. Define companding.Companding is the process of compressing and expanding.

With companded systems, the higher amplitude analog signals arecompressed prior to transmission and then expanded at the receiver.

12. Define slope overload. How it is reduced.The slope of the analog signal is greater than the delta

modulator can maintain, and is called slope overload. Slope overloadis reduced by increasing the clock frequency and by increasing themagnitude of the minimum step size.

13. Define granular noise. How it is reduced.When the original input signal has relatively constant

amplitude, the reconstructed signal has variations that were not present in the original signal. This is called granular noise.Granularnoise can be reduced by decreasing the step size.

14. Define adaptive delta modulation.

Adaptive delta modulation is delta modulation system wherethe step size of the AC is automatically varied depending on theamplitude characteristics of the analog input signal.

15. Define delta modulation.It transmits only one bit per sample. It compared the value

with the previous sample value. It is used to reduce the signaling rateand transmission bandwidth.

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16. What are two main disadvantages of delta modulation? Slope overload distortion Granular noise

17. What are two types of companding? E-law companding A-law companding

18. What is DPCM?DPCM is nothing but differential pulse code modulation

where the differences in the amplitude of the two successive samplesare transmitted rather than the actual sample.

19. Define TDM.The signals to be multiplexed are transmitted sequentially one

after the other. Each signal occupies short time slot.

20. Define crosstalk and guard time.Crosstalk means interference between the adjacent TDM

channels. It is the unwanted coupling of information from onechannel to the other. Guard time is the spacing introduced betweenthe adjacent TDM channels.

21. Define Nyquist rate.The sampling rate of 2W samples per second, for signal of

bandwidth of Hertz, is called Nyquist rate.

22. What is Aliasing?Aliasing is the phenomenon of high frequency component in

the spectrum of the signal taking on the identity of lower frequency inthe spectrum of its sampled version. This effect is due to the samplingrate less than the Nyquist rate.

23. What are the measures to combat the effect of aliasing?i. Prior to sampling, low pass filter is used to attenuate those

high frequency Components of the signal that are not essential

to the information being Conveyed by the signal.ii. The filtered signal is sampled at rate slightly higher than the Nyquist rate.

24. What is Pulse Amplitude Modulation?Pulse Amplitude Modulation is process in which the

amplitudes of regularly spaced pulses are varied in proportion to thecorresponding sample values of continuous message signal.

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25. What are the operations involved in the generation of the PAMsignal? Instantaneous sampling of the message signal m(t) every Ts

seconds, where the sampling rate fs =1/ Ts is chosen inaccordance with the sampling theorem.

Lengthening the duration of each sample so obtained to someconstant value T.

26. What is the channel bandwidth required for PAM signal?The channel bandwidth required for PAM signal fc NfM

where fc/fM is the total number of signals which may be multiplexed.Multiplexing number of signals by PAM time division requires no

bandwidth that would be required to multiplex these signals by FDMusing SSB transmission.

27. What is natural sampling?In Natural sampling, the sampled signal consists of sequence

of pulses of varying amplitude whose tops are not flat but follow thewaveform of the message signal m(t).

28. What is flat top sampling?In flat top sampling, the duration of each sample is lengthened

to T, to avoid the use of an excessive transmission bandwidth, since bandwidth is inversely proportional to pulse duration.

29. Define Aperture effect.In flat top sampling, due to the lengthening of the sample,

amplitude distortion as well as delay of T/2 was introduced. Thisdistortion is referred to as Aperture effect.

30. How aperture effect can be corrected?Aperture effect can be corrected by connecting an equalizer in

cascade with the low pass reconstruction filter. This equalizer has theeffect of decreasing the in-band loss of reconstruction filter as thefrequency increases in such manner as to compensate for the apertureeffect.

31. How signal is recovered through holding?In signal recovery through holding, the sample pulses are

extended; that is, the sample value of each individual baseband signalis held until the occurrence of the next sample of that same basebandsignal. The output waveform consists of up and down staircasewaveform with no blank intervals. These voltage transitions arerounded as the capacitor charges and discharges exponentially.

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32. What is Pulse Width Modulation?In Pulse Width Modulation, the width of regularly spaced

pulses is varied in proportion to the corresponding sample values ofcontinuous message signal.

33. What is Pulse Position Modulation?Pulse Position Modulation, the positions of regularly spaced

pulses are varied in proportion to the corresponding sample values ofcontinuous message signal.

34. How channel synchronization is done in PAM systems?In PAM systems, channel synchronization is done by

transmitting marker pulse in addition to the message bearing pulses.This marker pulse can be identified by making its amplitude exceedthat of all possible message pulses.

UNIT – II

35. What is ISI?Inter Symbol Interference arises when the communication channel is

dispersive. Inter symbol interference is caused by overlapping tails of the pulse with adjacent pulses. The residual effect due to the occurrence of pulses before and after the sampling instant ti is called ISI. It is major sourceof bit errors in the reconstructed data stream at the receiver.

36. Define Eye pattern. Eye pattern is defined as the synchronized superposition of all possible

equalizations of the signal of interest viewed within particular signalinginterval. The interior region of the eye pattern is eye opening.

37. What is the information provided by the eye pattern?The width of the eye opening defines the time interval over which thereceived signal can be sampled without error from ISI. The sensitivity ofthe system to timing error is determined by the rate of change of eye asthe sampling time is varied.

38 . How many openings are there in an M-ary system? In the case of M-ary system, the eye pattern contains (M-1) eye

openings stacked up vertically one on the other, where is the number ofdiscrete amplitude levels used to construct the transmitted signal.

39. What is delta sigma modulation?Delta modulation scheme that incorporates integration at its input is calleddelta sigma modulation.

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40. What is the effect of thermal noise on PCM system?The effect of thermal noise on PCM system is to cause the matched filterdetector to make an occasional error in determining whether binary or binarywas transmitted. If the thermal noise is white and Gaussian, the probability oferror depends on the ratio E b/J.

41. What is the effect of thermal noise on DM system?

The effect of thermal noise in DM system is to cause the matched filterdetector to make an error in determining the polarity of the transmittedwaveform. Whenever such an error occurs, the received impulse streamexhibits an impulse of incorrect polarity.

42.State the Nyquist criterion for distortion less baseband binary criterion.The frequency function (f) eliminates ISI for samples taken at intervals Tb

provided that it satisfies the equation Eq(4.53) [262]

43. Define sinc pulse.

Sinc pulse is signal waveform that produces zero ISI and is defined by(t) sin (2MWt)/2MWt sinc (2Wt).

44. State an important property of the matched filter.The peak pulse signal to noise ratio of matched filter depends only on theSignal energy to the power spectral density of the white noise at the filteroutput.

45. Define raised cosine spectrum.The raised cosine spectrum consists of flat portion and roll off portion thathas sinusoidal form.

46. What are the practical difficulties in sinc pulse?The amplitude characteristics of the sinc pulse should be flat from –W to andzero elsewhere. This is physically unrealizable because of the abruptTransitions at the band edges ±W. The function decreases as 1/t for large t,resulting in slow rate of decay. There is practically no margin of error insampling times in the receiver.

47. Define roll off factor.The roll off factor f1/W, it indicates the excess bandwidth over the Idealsolution. The amount of ISI resulting from timing errors decreases as the rolloff factor is increased from zero to unity.

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Six Mark Questions

UNIT-11.With the help of neat diagrams, explain the transmitter and receiver of PCM.2.Draw the block diagram and explain briefly about TDM.3.Explain quantization error and derive an expression for maximum SNR in PCMsystem that uses linear quantization.4.What is slope overload distortion and granular noise in delta modulation and

how it is removed in ADM.

5.Explain DPCM. How is it different from PCM?6.Explain Delta modulation ?7. Explain Adaptive Delta modulation8.Explain Various errors in Delta modulation9.Explain Sampling and Reconstruction of a signal

10.Explain Different types of pulse Amplitude Modulation Techniques.

UNIT – II1. What is ISI? How is it avoided?

2. What is eye pattern. Explain measurement of different parameters usingeye pattern

3. Explain why adaptive equalizers are required for telephone channels.4. Obtain an expression for Nyquist criterian for distortionless basebandtransmission for zero ISI.5. Explain about the various equalization techniques.6. Explain Matched filter design.7.Explain the properties of Matched filter.

Ten Mark Question

UNIT I1. Explain Quantization process in PCM system and its types.2. Explain the generation of PAM signals?3. Explain PCM with suitable waveforms.4. Explain DM and ADM with suitable waveforms?5. Derive the expression for quantization noise and SNR in PCM?6. Explain in detail about TDM?7. Explain in detail about DPCM.

UNIT-II1. Derive the expression for the matched filter and prove any one property.2. Prove the effect ISI using necessary block diagram and design.3. Write brief notes on Nyquist’s criterion for distortion less base band binary

transmission.4. Explain in detail about the optimum demodulation of Digital signals in the

presence if ISI and AWGN.5. Explain about the various equalization techniques.