Digital Communications Lab Viva Questions with Answers part - I

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1.      What are the disadvantages of Analog communication?
Its not reliable, Noise effect is more on the signals, Power required for signal transmission also more, Circuit complexity is more and costly.

2.      What are the Advantages of Digital Communication?
Reliable, Noise effect is very less, power consumption is very less, various Digital ICs are available so circuits not complex , cheap, Error detection and correction is also possible. 

3.      What are different types of digital modulation?
ASK, FSK, PSK, PCM, DPCM,Delta modulation, Adaptive Delta modulation etc..

4.      How to convert an analog signal into digital signal?
Blocks:  Anti aliasing filter, Sampler, Quantizer, encoder.

5.      Define the functionality of Sampler, Quantizer?
Sampler: converts a continuous time signal into discrete time signal.
Quantizer: converts continuous in amplitude signal into discrete in amplitude signal. 

6.      What are some Coding techniques?
Pulse code modulation, Differential pulse code modulation, Delta modulation, Adaptive delta modulation.

7.      What is Sampling?
Converting a continuous time signal into discrete in time signal is called as Sampling (similar to cutting a bread into slices)

8.      Define Sampling theorem?
To reconstruct the Continuous time signal from discrete time signal ,the sampling frequency should be more than equal to twice of Continuous time signal frequency(max).

9.      What is Nyquist Rate?
If the sampling frequency is twice of Continuous time signal frequency(max), then that is called as Nyquist rate.

10.  How many types of samplings are their? Explain briefly?
Impulse Sampling, Natural sampling, Flat top sampling.

11.  What is aliasing effect?  How to overcome it?
Due to imperfect sampling the signals will be interfered in frequency domain i.e called aliasing effect in sampling. if sampling theorem satisfied in sampling or first by passing signal from anti aliasing filter before sampling then aliasing effect will be reduced

12.  What are the Analog pulse modulation methods?
Pulse amplitude modulation, pulse width modulation and pulse position modulation..

13.  Define Pulse amplitude modulation?
The carrier pulse height (amplitude) proportional to amplitude of messege signal.

14.  Define Pulse width modulation?
The carrier pulse width proportional to amplitude of message signal.

15.  Define Pulse position modulation?
The carrier pulse position proportional to amplitude of message signal.

16.  Compare PAM, PWM, PPM?

17.  What is Amplitude shift Keying (ASK) ?
It represent the digital data as variations in amplitudes in carrier wave. i.e '1' represented by transmitting a fixed amplitude carrier wave for the bit duration with constant frequency.

18.  What is Phase shift keying ( PSK)?
It represent the digital data as variations in phase shift in carrier wave. i.e '1' represented by 0 phase shift carrier wave , where '0' represented as 180 phase shift incarrier wave for the bit duration with constant frequency

19.  What is Frequency shift keying (FSK) ?
It represent the digital data as variation in frequency in carrier wave, i.e for '1' more than carrier frequency , for '0' less than carrier frequency.

20.  What is Binary Phase shift Keying (BPSK) ?
for each one bit of binary data (0 & 1) carrier phase will be changed (two different shifts: 0, 180)

21.  What is Quadrature Phase shift Keying (QPSK) ?
for each two bits of binary data (00,01,10 & 11) carrier phase will be changed (four different shifts : 45, 135, -45, -135)
22.  What is the difference between Bit Rate and Baud Rate?
Bit rate represents Bits per sec,Baud rate represents no. of symbols per second i.e. in communications the no. of bits transmitted per sec is called as Bit Rate (units bps) and The no. of times a signal (here carrier) changes its state (change in freq, phase, amplitude) per sec is called as Baud rate.
23.  What is bandwidth of BPSK signal?
2Fc, if Fc represents carrier frequency
24.  Compare ASK, PSK and FSK.?
Bandwidth: ASK< PSK < FSK
Power:       ASK <PSK = FSK
Probability of error: ASK > PSK > FSK
Signal to Noise Ratio: ASK < PSK < FSK
25.  Why is ASK called as ON-OFF keying?

When input data is 1 then output is carrier, if input is 0 out put is zero. so its looks like a switch which will switch on when input is 1 and off when input is zero

                                                                                                         

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Digital signal processing lab viva questions on Z transforms and DFT, FFT

Home/ lab viva questions/ DSP Part-II& III

Part-II : Z Transforms
1. Define one-sided and two-sided Z-transform.

2. What is region of convergence (ROC)?
The region of Z magnitude (|Z|) for which  x(n).|Z|^-n is absolutely summable

3. State the  Initial value and final value theorem with regard to Z-transform.

4. Define Z-transform of unit step signal.
                  Z/(Z-1)        ROC |z|>1

5. What are the different methods available for inverse Zs-transform?
Residue method, Partial fractions method, Long division method, Convolution integral method

6. When the z-domain transfer function of the system can be directly obtained from s-domain transfer function?
When input is an Impulse sampled signal.

7. Define the transfer function of an LTI system.
For Discrete time systems: Z transform of impulse response of system
For Continuous time systems: Laplace transform of impulse response

8. Write the transfer function of Nth order LTI system.

9. Give the importance of convolution and deconvolution operations using Z-transform.

10. Give the conditions for stability of an LTI discrete time system in z-plane.

The ROC should contain unit circle with in it.
11. Explain when an LTI dicrete time system will be causal.

The ROC should contain infinite. i.e system impulse response is a right sided signal.

12. State the convolution property of Z-transform.
  -------------> 
      -------------> 
13. state the properties of Z transform?
            click here
14. Z-transform  of various signals?
            click here

Part-III: DFT & FFT

1. Define DFT of a discrete time sequence?

2. Define inverse DFT.

3. What is the relation between DTFT and DFT?

4. What is the drawback in Fourier transform and how is it overcome?

Fourier transform is that it is not truly realizable in practice but we can get closer, it is not applicable to all signals, so we go for Laplace in continuous , Z in discrete. 
5. List any four properties of DFT .
linearity Property :  ax1(n)+bx2(n)→aX1(ω)+bX2(ω)







Duality Property:  X(N)⟷Nx[((−k))N]

Complex conjugate property:  x∗(n)⟷X∗((K))N=X∗(N−K)

Circular shift property: x(n)exp(j2ΠKn/N)⟷X((K−L)

6. What is FFT, What it's importance?

FFT stands for Fast Fourier Transform, this is same as DFT but algorithm is different by FFT with in lees time we can compute Fourier transform compared to DFT.
7. compare FFT and DFT?

DFT and FFT both are used to represent a discrete time signal in frequency domain, But DFT procedure is formula based where FFT is algorithm based, FFT is more efficient and faster than DFT, i.e if a sequence contains N samples then to calculate 
DFT  no. of multiplications and additions  required are:   N² ,  N(N-1) 
FFT no. of multiplications and additions required are : (N/2) log₂(N), N log₂(N)

8. What are the various algorithms to calculate FFT?

Decimation In Time (DIT), Decimation In frequency (DIF)
9. Draw the DIT FFT structure with the length of 8?

10. Draw the DIF FFT structure with the length of 8?

11. What is phase factor or twiddle factor?
It is defined as W_N = e^-j2π/N

12. What are the phase factors involved in all stages of computation in the 8-point DIT radix-2 FFT?

First stage: W₈⁰

Second stage: W₈⁰_, W₈²

Third stage: W₈⁰_, W₈¹_, W₈², W₈³

13. Draw the basic butterfly diagram or flow graph of DIT radix-2 FFT?

14. Draw the basic butterfly diagram or flow graph of DIF radix-2 FFT?

15. What are the phase factors involved in all stages of computation in 8-point DIF radix-2 FFT?

First stage: W₈⁰_, W₈¹_, W₈², W₈³

Second stage: W₈⁰_, W₈²

Third stage: W₈⁰

16. What is magnitude and phase spectrum? 
Magnitude spectrum is the graph between Fourier transform magnitudes and frequency. Phase spectrum is the graph between Fourier transform phases and frequency.

                                                                                                             

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