EC6501 Digital Communication, Third year, Department of Electronics & Communication Engineering, First and second Units Questions

PART A – (10 x 2 = 20 Marks)
1.      What is aliasing?
2.      States sampling theorem for band limited signal and the filter to avoid aliasing.
3.      Write the two fold effects of quantization process.
4.      Define APF and APB.
5.      Write the limitations of delta modulation
6.      What is companding? Sketch the input, output characteristics of a compressor and expander.
7.      What are the advantages of delta modulation?
8.      What is a linear predictor on what basis is the predictor coefficient determined.
9.      What is the signal to noise ratio of PCM system if number of quantization level is 28?
10.  For the uniform quantizer discuss the way in which the number of quantization level influence the bandwidth and quantization noise?
PART B-(5*16=80 Marks)
11.  Describe the process of sampling and how the message signal is reconstructed from its samples also illustrate the effect of aliasing with neat sketch.             (16)
12.  Describe PCM waveform coder and decoder with neat sketch and list the merits compare with analog codes.                                                                                  (16)
13.   i) Describe and illustrate delta modulation and its quantization error.          (8)
ii) Explain how adaptive delta modulation performs better and gains more signal to noise ratio then delta modulation.                                                                    (8)
14.  Illustrate how the adaptive time domain coder the speech at low bit rate and compare it with the frequency domain coder ADPCM.                                    (16)
15.   i)The signal x(t) =4cos 400Πt+12cos 360Πt is ideally sampled at a frequency of 300 samples per second. The sampled signal is passed through a unit gain low pass filter with a cutoff frequency of 200 Hz list the frequency components present at the output of the low pass filter.                                                       (8)
 ii) A delta modulator with the fixed step size of 0.75 V is given a sinusoidal message signal. If the sampling frequency is 30 times of the nyquist rate, determine the maximum permissible amplitude of the message signal if slope overload is to be avoided.                                                                                      (8)