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Digital Communication - Sampling & Quantization - Important Short Questions and Answers: Sampling & Quantization

__SAMPLING &QUANTIZATION__

**1. Draw the typical digital communication system**

**2. ****How can BER of an system be improved.**

·
Increasing the transmitted signal power

·
Employing modulation and demodulation technique

·
Employing suitable coding and decoding methods

·
Reducing noise interference with help of improved
filtering

**3. ****Give an example for time limited and time unlimited
signals **

Time
limited- rectangular pulse, triangular pulse

Time
unlimited signals - sinusoidal signal,exponential signal and step signal.

**4. ****Give the advantages and disadvantages of digital
communication.**

**Advantage**

Speech,
video and other data can be transmitted simultaneously Wide dynamic range is
possible since data is digital

**DisAdvantage**

digital
communication required synchronization

data rate
are very high

**5. Which parameter is called figure of merit of a
digital communication system and why?**

The
ration Eb/No or bit energy to noise power spectral density is called figure of
merit of a digital communication system.

**6. What is meant by distortion less transmission?**

For
distortion less transmission, the transfer function of the system if given as,
H(w)=Ke^{-jwto}

K-
Constant magnitude response

The
transfer function impose two requirements on the system

1. The
system response must have constant magnitude response

2. The
system phase shift response must be linear with frequency

**7. Why prefilterring done before sampling**

The
signal must be limited to some highest frequency W Hz before sampling.

Then the
signal is sampled at the frequency of fs=2W of higher. Hence the single should
be prefiltered at higher that W Hz.If the signal is not prefiltered, then
frequency component higher that W Hz will generate aliasing in the sampled
signal spectrum.

**8. Define quantization noise power **

Quantisation
noise power is the noise power due to quantisation noise. Let the quantization noise
has the pdf of f€(Є).Then Quantisation noise power is given as, E[Є]^{2}
= Є^{2} f_{€}(Є) d Є.

**9. State sampling theorem.**

A band limited
signal of finite energy, which has no frequency components higher than W Hz,
may be completely recovered from the knowledge of its samples taken at the rate
of 2W samples per second.

**10. What is quantization error **

Because
of quantization inherent error are introduces in the signal. The error is
called Quantization error Є=xq(nTs)-x(nTs)

xq(nTs)-
quantis ed value of the signal

x(nTs)-
value of te sample before quantization

**11. Compare uniform and non uniform quantization**

**Uniform Quantization**

1. The
quantization step size remains same throughout the dynamic range of the signal

2. SNR
ratio varies with the input signal amplitude

**Non-uniform Quantization**

1. The
quantization step size varies with the amplitude of the input signal.

2. SNR
ratio can be maintained constant

**12. What is meant by quantization.**

The
conversion of analog sample of the signal into digital form is called
quantizing process

13. **Differentiate the principle of temporal
waveform coding and model based coding**

**TEMPORAL WAVEFORM CODING**

The
signal which varying with time can be digitized by periodic time sampling and
amplitude quantization. This process is called temporal waveform coding
.DM,ADM,DPCM are example of temporal waveform coding

**MODEL BASED CODING**

The
signal is characterised in various parameter. This parameter represent the
model of the signal.LPC is an example model based coding.

14. **What is meant by aliasing effect?**

Aliasing
effect takes place when sampling frequency is less than Nyquist rate.Under such
condition, the spectrum of the sampled signal overlaps with itself.Hence higher
frequencies take the form of lower frequencies. This interference of the
frequency components is called aliasing effects.

**15.
****What is
the effects of aliasing? How it is reduced?**

(i)
Since high and low frequencies interfere with each
other,distortion is generated.

(ii) The data
is lost and it can not be recovered.

*Aliasing can be avoided by two methods:*

(i)
Sampling rate fs ≥ 2W samples/sec. Where W---> Max.frequency present in the
signal

(ii)Strictly
bandlimit the signal to „W‟Hz.

**16. What is the function of Low pass filter on
sampling?**

A low
pass filter basically a reconstruction of filter.This filter should pass all
the frequencies between(-W,W), Since original signal was having maximum
frequency of „W‟Hz.Therefore cut-off frequency of this low pass reconstruction
filter will be „W‟Hz.

**17. Define Non-uniform quantization.**

In
non-uniform quantization, the step size is not fixed. It varies according to
certain level of input signal amplitude. Step size is small at low input signal
levels and the step size is higher at high input levels. Hence signal to noise
power ratio remains almost same throughout the input signal.

**18. ****What is
meant by compander?**

The
non-uniform quantization(variable stepsize „∂‟)becomes very difficult to
implement.

Therefore
the signal is amplified at low signal levels and attenuated high signal
levels.After this process,uniform quantization is used.This is equivalent to
more stepsize at low signal levels and small step size at high signal levels.At
the receiver a reverse process is done.That is the signal is attenuated at low
signal levels and amplified at high signal levels to get original signal.Thus
the compressionbof signal at transmitter and expansion at receiver is called
combinely as Companding.

**IMPORTANT TERMS**

**Quantization:**

The
conversion of analog sample of the signal into digital form is called
quantizing process.

**Aliasing effect:**

Aliasing
effect takes place when sampling frequency is less than Nyquist rate.Under such
condition, the spectrum of the sampled signal overlaps with itself.Hence higher
frequencies take the form of lower frequencies. This interference of the
frequency components is called aliasing effects.

**Sampling theorem:**

A band
limited signal of finite energy, which has no frequency components higher than
W Hz, may be completely recovered from the knowledge of its samples taken at
the rate of 2W samples per second.

**Quantization error:**

Because
of quantization inherent error are introduces in the signal. The error is
called Quantization error Є=xq(nTs)-x(nTs)

xq(nTs)-
quantis ed value of the signal x(nTs)- value of te sample before quantization

**Non-uniform quantization:**

In
non-uniform quantization, the step size is not fixed. It varies according to
certain level of input signal amplitude. Step size is small at low input signal
levels and the step size is higher at high input levels. Hence signal to noise
power ratio remains almost same throughout the input signal.

**Compander:**

The
non-uniform quantization(variable stepsize „∂‟)becomes very difficult to
implement.

Therefore
the signal is amplified at low signal levels and attenuated high signal
levels.After this process,uniform quantization is used.This is equivalent to
more stepsize at low signal levels and small step size at high signal levels.At
the receiver a reverse process is done.That is the signal is attenuated at low
signal levels and amplified at high signal levels to get original signal.Thus
the compressionbof signal at transmitter and expansion at receiver is called
combinely as Companding.

**Quantization noise power:**

Quantisation
noise power is the noise power due to quantisation noise. Let the quantization noise
has the pdf of f€(Є).Then Quantisation noise power is given as, E[Є]^{2}
= Є^{2} f_{€}(Є) d Є.

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