Interview Questions and Answers on Waveform Coding Techniques

Q.1. How is the ‘information’ transmitted in a PCM system ?

Ans. In PCM, the ‘information’ is transmitted in the form of ‘code words’ . The amplitude, width and position of the  pulses at the PCM transmitter output are always constant .

Q.2. What is quantization ?

Ans. It is the process of approximation. The samples are approximated or rounded off to the nearest quantization      level .

Q.3. What is the relation between number of quantization levels and number of bits per word ?

Ans. Q = 2^N

Q.4. What is quantization error ? What is the maximum value ?

Ans. It is the difference between quantized signal and original signal . Its maximum value is ±Δ/2 .

Q.5. How can we reduce the quantization error ?

Ans. By reducing the step size Δ . This is possible by increasing the number of quantization level Q.

Q.6. What are the advantages of PCM ?

Ans.

1. High noise immunity
2. Repeaters can be used
3. Storage and processing of the signal is possible

Q.7. What is the disadvantage of PCM ?

Ans. Very large bandwidth requirement .

Q.8. What is the signaling rate in terms of N and  f_s ?

Ans. Signaling rate  = N f_s .

Q.9. Why is companding used ?

Ans. In order to improve the signal to quantization noise ratio of weak signals .

Q.10. How bandwidth required for the DM signal is less than that of PCM ?

Ans. In PCM, we have to transmit N bits per quantized sample but in DM, we have to transmit only 1 bit per sample .This reduces the bandwidth .

Q.11. What information do you transmit in DM system ?

Ans. In delta modulation (DM) , the information about the result of comparison between present and previous            samples is transmitted .

Q.12. What is the cause of slope overload error in DM ?

Ans. When the slope of analog input signal is much higher than the slope of approximated staircase signal, the       slope overload error is observed .

Q.13. How can we reduce the slope overload error in DM ?

Ans. By increasing the sampling frequency f_s  or by increasing the step size Δ .

Q.14. What is the signaling rate of a DM system ?

Ans. As only one bit is transmitted per sample, the signaling rate is equal to sampling rate f_s .

Q.15. What is granular noise ?

Ans. Granular noise is similar to the quantization noise in PCM .

Q.16. How can we reduce granular noise ?

Ans. By reducing the step size Δ .

Q.17. Can we use DM practically as an alternative to PCM ?

Ans. No, we can not use because in order to eliminate the slope overload, practically, the sampling rate  f_s  is              required to be so high that the bandwidth requirement of DM exceeds that of PCM .

Q.18. How is slope overload reduced in ADM ?

Ans. In ADM, the step size is increased or reduced progressively to track the analog signal faithfully. This reduces    the possibility of slope overload .

Q.19. Is it possible to use the baseband transmission in PCM ?

Ans. Yes, it is possible .

Q.20. What are the advantages of digital representation of a signal ?

Ans. The advantages of the digital representation are :

1. immunity to noise
2. regeneration of the coded signal is possible
3. communication can be kept secret or private due to the use of codes

Q.21. What are the disadvantages of digital representation of a signal ?

Ans. Some of the disadvantages are :

1. Increased transmission bandwidth
2. Increased system complexity

Q.22. Why is PCM not used for radio broadcasting ?

Ans. Due to higher BW requirement and complex circuitry .

Q.23. The granular noise in ADM will be ______ than that in DM .

Ans. Higher

Q.24. In DM, we actually transmit _______ .

Ans. The encoded error signal.

Q.25. The predictive coding theory tells that _________ .

Ans. It is possible to reconstruct the original message signal by accumulating a series of error signals .

Q.26. For increasing the signal to quantization noise by 6 dB, we have to increase the number of bits per PCM word by _____ .

Ans. 1

Q.27. Name different source coding techniques used in digital communication systems.

Ans. Various source coding techniques are :

1. Pulse code modulation (PCM)
2. Delta modulation (DM)
3. Differential pulse code modulation (DPCM)
4. Adaptive delta modulation (ADM)

Q.28. Are the signal and noise separable in PCM ?

Ans. Yes, the signal and noise are separable in PCM .

Q.29. Is it possible to use the repeaters (regenerators) in a PCM system ?

Ans. Yes, it is possible to use repeaters which are actually regenerators . They can separaten noise from the PCM signal and regenerate the original PCM signal .

Q.30. What is the advantage of using repeaters ?

Ans. The advantage of using repeaters is reduced effect of noise.

Q.31. What is the relation between number of quantization levels and the number of digits per word ?

Ans. The relation is Q = 2^N , where , Q is the number of quantization levels and N is the number of digits per word.

Q. 32. What is the relation between the quality of PCM signal and the number of digits per word ?

Ans. The quality of signal will improve with increase in number of digits per word N .

Q.33. Why is quantization necessary ?

Ans. If we do not use quantization, then, we will have to convert each and every sampled value into a separate          digital word. This will need a large number of bits per word (N) . This will increase the signaling rate (bits/sec),        and hence the bandwidth requirement of the system .

Q.34. What is the maximum value of signal to quantization noise of a PCM system ?

Ans. The maximum value of signal to quantization noise with a sinusoidal input signal is SNR_Q(max) = (1.8 + 6N) dB .

Q.35. How is uniform quantization is different from non-uniform quantization ?

Ans. The quantization is called as unform quantization if the step size remains constant throughout the input range . However, if the step size varies depending on the size of input, then the quantization is known as non-                          uniform quantization .

Q.36. Where is non-uniform quantization used ?

Ans. Non-uniform quantization is used to quantize the speech and music signal, as they have low values of crest      factor .

Q.37. What is the bandwidth of a PCM system ?

Ans. The minimum bandwidth of a PCM system is half the signaling rate i.e. it is 1/2(N f_s) .

Q.38.  What is the relation between power contents and crest factor of a signal ?

Ans. The relation between normalized signal power and its crest factor is given by : P =1 / CF²

Q.39. State the types of non-uniform quantizers ?

Ans. There are two types namely the μ-law and A-law companding .

Q.40. What is the difference between the μ-law and A-law compressor characteristics ?

Ans. The μ-law compressor characteristics is linear characteristics for smaller amplitudes and it  is logarithmic for  the large values of the input signal.

The A-law compressor characteristics is made up of piecewise linear segments for the low level inputs and                    piecewise logarithmic segments for the high level inputs .

Q.41. What is the main disadvantage of a PCM system ?

Ans. The main disadvantage of a PCM system is its high signaling rate and therefore large bandwidth requirement.     This is because each quantized sample is converted into an N bit digital word .

Q.42. How is this disadvantage overcome using the linear delta modulation ?

Ans. This disadvantage is overcome using the linear delta modulation because in delta modulation system, only one   bit is transmitted per clock cycle. Instead of transmitting the actual sampled value (as in case of PCM) , the                   information about the comparison of present and previous value of the signal is transmitted . The signaling                   rate of delta modulation is thus only ‘f_s‘ instead of N f_s  in case of PCM .

Q.43. Practically is the signaling rate of DM system less than that of the PCM system ?

Ans. No, practically, the signaling rate of DM system is higher than that of a PCM system .

Q.44. What is the condition for avoiding the slope overload error ?

Ans. The condition for avoiding the slope overload error is that the slope of the DM approximated signal , should be greater than or equal to the continuous time signal .

Q.45. What is the difference between DM and ADM ?

Ans. The step size of the ADM approximated signal is variable. This is the difference between DM and ADM                systems.

Q.46. State the advantages of ADM over DM.

Ans. Some of the important advantages of ADM are :

1. Reduction in slope overload error
2. Improvement in signal to quantization noise ratio
3. Wide dynamic range
4. Better utilization of bandwidth as compared to DM

Q.47. What is the operating principle of differential PCM system ?

Ans. In the DPCM system, the error between a signal and its approximated version is encoded and transmitted,        instead of transmitting the original signal .

Q.48. Why is the signaling rate of DPCM is less than that of a PCM ?

Ans. As the number of bits required for encoding an error signal is always less than the number of bits needed to        encode the actual input signal, the bit rate  (signaling rate ) of DPCM is always less than that of a PCM signal .

Q.49. What is the role of a predictor in DPSM system ?

Ans. A predictor will predict the value of the  next sample by taking into account all the previous sample values.          This  will reduce the error to a very small value which needs a smaller number of bits for its encoding. This will            reduce the signaling rate of DPCM to a great extent.

Q.50. What is the spectrum occupied by the speech signal ?

Ans. Typically, the speech signal occupies the range from 200 Hz to 3200 Hz .