DIGITAL COMMUNICATION GNANAMANI COLLEGE OF ENGINEERING NH-7 A.K.SAMUTHIRAM, NAMAKKAL DEPARTMENT OF ELECTRONICS&COMMUNICATION ENGG, TWO MARK Q&A Sem/Branch : VI/ECE Subject : Digital communication Prepared by : R.Shankar, AP/ECE UNIT -1 SAMPLING AND WAVEFORM CODING 1. Define Nyquist rate. Let the signal be band limited to ‘w’ Hz. Then nyquist rate is given as, Nyquist=2w samples/sec. Aliasing will not take place if sampling rate is greater than nyquist rate. 2. What is meant by aliasing effect? Aliasing effect take place when sampling frequency is less than Nyquist rate under such condition, the spectrum of the sampled signal overlaps with itself. Hence higher frequency components are called aliasing effect. 3. Define PWM. PWM is basically width of the pulse changes according to amplitude of the modulating signal. It is also referred to as pulse duration modulation or PDM. 4. State sampling theorem. Sampling theorem states that a band limited signal of finite energy, which has no frequency components higher than W Hz, is completely described by specifying the values of the signal at the instants of time separated by 1/2w seconds. 5. Mention two merits of DPCM. i. Band width requirement of DPCM is less compared to PCM. ii. Quantization error is reduced because of prediction filter. iii. Numbers of bits used to represent one sample value are also reduced compared to PCM. 6. What is the main difference in DPCM and DM? Dm encodes the input sample by only one bit. It sends the information about +Ѕ or -Ѕ i.e. step rise or fall. DPCM can have more than one bit for encoding the sample. It sends the information about difference between actual sample value and predicated sample value. 7. How the message can be recovered from PAM? The message can be recovered from PAM by passing signal through reconstruction filter. The reconstruction filter integrates amplitudes of PAM pulses. Amplitude smoothing of reconstructed signal is done to remove amplitude discontinues due to pulses. GCE, NAMAKKAL Page 1 DIGITAL COMMUNICATION 8. Define Quantization. Quantization is a process of approximation or rounding off. Assigning PCM codes to absolute magnitudes is called quantization. 9.Define Companding. Companding is the process of compressing and expanding.With companded systems,the higher amplitude analog signals are compressed prior to transmission and then expanded at the receiver. 10.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. Granular noise can be reduced by decreasing the step size. 11.Define Aperture effect. In flat top sampling,due to the lengthening of the sample, amplitude distortion as well as a delay of T/2 was introduced.This distortion is referred to as Aperture effect. 12.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 the effect of decreasing the in-band loss of reconstruction filter as the frequency increases in such a manner as to compensate for the aperture effect. 13. Write an expression for band width of binary PCM with N messages with maximum frequency of FmHz ? If ‘v’ number bits are used to code each input sample then bandwidth of PCM is given as BT >=N.v.Fm .Here v.fm is the bandwidth required by one message. 14. How is PDM wave converted into PPM system? The PDM signal is given as clock signal to monostable multivibrator. The multivibrator triggers on the falling edge .hence the PPM pulse width is produced after falling edge of PDM pulse. PDM signal represent the input signal amplitude in the form of width of pulse. A PPM pulse after this width of PDM pulse. 15. Mention the use of adaptive quantizer in adaptive digital wave form coding scheme. Adaptive quantizer changes its step size according to variance of input signal. Hence quantization error is reduced. ADPCM uses adaptive quantization. The bit rate of such schemes reduced due to adaptive quantization. 16. What do you understand from adaptive coding? In adaptive coding quantization step size and prediction filter co-efficient are changed as per properties of input signals. This quantization error and number of bits used to represent the sample value. Adaptive coding is used at low bit rates. GCE, NAMAKKAL Page 2 DIGITAL COMMUNICATION 17. What is the advantage of delta modulation over PCM? Delta modulation uses one bit to encode one sample. Hence bit rate of delta modulation is low compared to PCM. 18. Define Dirac comb or ideal sampling function. What is its Fourier Transform? Dirac comb is nothing but a periodic impulse train in which the impulses are spaced by a time interval of Ts seconds. The equation for the function is given By The Fourier transform of is given by 19. Give the interpolation formula for the reconstruction of the original signal g(t) From the sequence of sample values {g(n/2W)}. Where, 2W is the bandwidth N is the number of samples. 20. Define Quadrature sampling. Quadrature sampling is used for uniform sampling of band pass signals Consider The in-phase component gI(t) and the Quadrature component gQ(t) may be respectively and then suppressing the sum-frequency components by means of appropriate low pass filter. Under the assumption that fc>W, we find that gI(t)&gQ(t) are both low-pass signals limited to -Wnumber of sub bands of equal bandwidths N->average number of bits fs/M->sampling rate for each sub band 20. Define Adaptive filter. It is a nonlinear estimator that provides an estimate of some desired response without requiring knowledge of correlation functions, where the filter coefficients are data dependent. A popular filtering algorithm is the LMS algorithm. 21. Define data signaling Rate. Data signalling rate is defined as the rate measured in terms bits per second which data are transmitted. Data signaling rate Rb=I/Tb Where Tb=bit duration. (b/s) at 22. Define modulation rate. It is defined as the rate at which signal level is changed depending on the nature of the format used to represent the digital data. It is measured in bauds or symbols per second. 23. State NRZ unipolar format. In this format binary 0 is represent by no pulse and binary 1 is represented by the positive pulse. GCE, NAMAKKAL Page 10 DIGITAL COMMUNICATION 24. State NRZ polar format. Binary 1 is represented by a positive pulse and binary 0 is represented by a negative pulse. 25. State NRZ bipolar format. Binary 0 is represented by no pulse and binary one is represented by the alternative positive and negative pulse. 26. State Manchester format. Binary the first half bit duration negative pulse and the second half bit duration positive pulse. Binary first half bit duration positive pulse and the second half bit duration negative pulse. 27. What is the width of the eye? It defines the time interval over which the received waveform can be sampled without error from intersymbol interference. 28. What is sensitivity of an eye? The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied. 29. What is margin over noise? The height of the eye opening at a specified sampling time defines the margin over noise. 30. How eye pattern is obtained? The eye pattern is obtained by applying the received wave to the vertical deflection plates of an oscilloscope and to apply a saw tooth wave at the transmitted symbol rate to the horizontal deflection plate. 31. Mention the need of optimum transmitting and receiving filter base band data transmission. When binary data is transmitted over base band channel noise interference with it. Because of this noise interference error introduced in signal detection. Optimum filter perform to functions while receiving the noisy signal. i. Optimum filter integrate a signal during the bit interval and check the output at the time instant where signal to noise ratio is maximum. ii. Transfer function of the optimum filter is selected so as to maximize signal to noise ratio. iii. Optimum filter minimizes the probability of error. iv. 32. How is eye pattern obtained on the CRO? Eye pattern can be obtained on the CRO by applying the signal tonone of the input channels giving an external trigger of (1/Tb)hz.this makes one sweep of beam equal to ‘Tb seconds. 33. What is the condition for zero inter symbol interference? Zero ISI can be obtained if the transmitted pulse satisfies the following condition: Time domain: Þ[(i-k)]= {1 for i=k 0 for i≠k Frequency domain: ∞ = ∑ p (f - nfb) =Tb k=-∞ GCE, NAMAKKAL Page 11 DIGITAL COMMUNICATION 34. From the eye pattern, how is the best time for sampling determined? It is preferable to sample the instant at which eye is open widest. At this instant chances of error are minimum. 35. What is the purpose of using an eye pattern? Eye pattern can be used for: 1. To determined an interval over which the received wave can be sampled without error due to ISI. 2. To determine the sensitivity of the system to timing error. 3. The margin over the noise is determined from the eye. 36. Why do you need adaptive equalization in a switched telephone network? In a switched telephone network the distortion depends upon 1. Transmission characteristics of individual links. 2. Number of links in connection. Hence fixed pair of transmit and receive filters will not serve the equalization problem. The transmission characteristics keep on changing. The adaptive equalization is used. 37. What is an ideal nyquist channel? P(t)=sin(2πBot)/2πBot Such pulses have the spectrum of, P(f)={1/2B0 for –b0