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Chapter: Digital Communication - Waveform Coding

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Important Short Questions and Answers: Waveform Coding

Digital Communication - Waveform Coding - Important Short Questions and Answers: Waveform Coding

WAVEFORM CODING

 

1.What is meant by temporal waveform coding

 

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

 

2. 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 exampleof 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.

 

3. What is meant by DPSK?

 

In DPSK, the input sequence is modified. Let input sequence be d(t) and output sequence be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0.

 

When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence the technique is called Differential PSK.

 

4. Mention the merits of DPCM.

 

1 .Bandwidth requirement of DPCM is less compared to PCM.

2. .Quantization error is reduced because of prediction filter

3. .Numbers .of bits used to represent .one sample .value are also reduced

compared to PCM.

 

5. What is the main difference in DPCM and DM?

 

DM encodes the input sample by one bit. It sends the information about + δ or -δ, ie step rise or fall. DPCM can have more than one bit of encoding the sample. It

sends the information about difference .between .actual sample value and the predicted sample value.

 

6. What is the advantage of delta modulation over pulse modulation schemes?

Delta modulation encodes one bit per samples. Hence signalling rate is reduced in DM.

 

7. What is meant by adaptive delta modulation?

 

In adaptive delta modulation, the step size is adjusted as per the slope of the input signal. Step size is made high if slope of the input signal is high. This avoids slope overload distortion.

 

8.What are the two limitations of delta modulation?

1 Slope of overload distortion.

2. Granular noise.

 

9. How does Granular noise occurs?

 

It occurs due to large step size and very small amplitude variation in the input signal.

 

10.      What are the advantages of the Delta modulation?

 

1.           Delta modulation transmits only one bit for one sample. Thus the signalling rate and transmission channel bandwidth is quite small for delta modulation.

 

2.           The transmitter .and .receiver .implementation is very much simple for delta modulation. There is no analog to digital converter involved in delta modulation.

 

11.      What do you understand from adaptive coding?

 

In adaptive coding, the quantization step size and prediction filter coefficients are changed as per properties of input signal. This reduces the quantization error and number of bits to represent the sample value. Adaptive coding is used for speech coding at low bits rates.

 

 

12.Mention the use of adaptive quantizer in adaptive digital waveform cod ing schemes.

 

Adaptive quantizer changes its .step .size .according variance of .the .input signal. Hence quantization error is significantly reduced due to the adaptive quantization. ADPCM uses adaptive quantization. The bit rate of such schemes is reduced due to adaptive quantization.

 

 

WAVEFORM CODING - IMPORTANT TERMS

 

Temporal waveform coding

 

The singal which varying with time can be digitzed 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.

 

DPSK

 

In DPSK, the input sequence is modified. Let input sequence be d(t) and output sequence be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0.

 

When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence the technique is called Differential PSK.

 

DPCM

 

1 .Bandwidth requirement of DPCM is less compared to PCM.

2. .Quantization error is reduced because of prediction filter

3. .Numbers .of bits used to represent .one sample .value are also reduced

compared to PCM.

 

Adaptive delta modulation

 

In adaptive delta modulation, the step size is adjusted as per the slope of the input signal. Step size is made high if slope of the input signal is high. This avoids slope overload distortion.

 

Granular noise:

 

It occurs due to large step size and very small amplitude variation in the input signal.

 

Delta modulation

 

1.           Delta modulation transmits only one bit for one sample. Thus the signalling rate and transmission channel bandwidth is quite small for delta modulation.

 

2.           The transmitter .and .receiver .implementation is very much simple for delta modulation. There is no analog to digital converter involved in delta modulation.

 

Adaptive coding

 

In adaptive coding, the quantization step size and prediction filter coefficients are changed as per properties of input signal. This reduces the quantization error and number of bits to represent the sample value. Adaptive coding is used for speech coding at low bits rates.

 

 

Adaptive quantizer

 

Adaptive quantizer changes its step size according variance of .the .input signal. Hence quantization error is significantly reduced due to the adaptive quantization. ADPCM uses adaptive quantization. The bit rate of such schemes is reduced due to adaptive quantization.

 

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