# Differential Pulse Code Modulation MCQ || Differential Pulse Code Modulation Questions and Answers

1.  In digital transmission, the modulation technique that requires the minimum bandwidth is:

1. PCM
2. PAM
3. DPCM
4. Delta modulation

Explanation

Differential pulse code modulation (DPCM) is a procedure of converting an analog into a digital signal in which an analog signal is sampled and then the difference between the actual sample value and its predicted value (predicted value is based on previous sample or samples) is quantized and then encoded forming a digital value.

• In PCM an analog signal is sampled and encoded into different levels before transmission
• The bandwidth of PCM depends on the number of levels
• If each sample is encoded into n bits, then the bandwidth of PCM is nfs
• The bandwidth of DPCM is almost the same as that of the PCM signal, the only difference between PCM and DPCM is that the dynamic range is reduced in the DPCM signal
• However, in the case of Delta modulation, each sample is sent using only 1 bit which is +Δ or -Δ. Hence there is bandwidth saving in Delta modulation.

2. (DPCM) is a procedure of converting an _____ to ______

1. Digital to Analog
2. Analog to Digital
3. Digital to Discrete
4. None of the above

Explanation

Differential pulse code modulation (DPCM) is a procedure of converting an analog into a digital signal in which an analog signal is sampled and then the difference between the actual sample value and its predicted value (predicted value is based on previous sample or samples) is quantized and then encoded forming a digital value.

3. DPCM compression depends on the ______ technique.

1. Sampling
2. Amplifying
3. Prediction
4. None of the above

Explanation

DPCM compression depends on the prediction technique, well-conducted prediction techniques lead to good compression rates, in other cases, DPCM could mean expansion compared to regular PCM encoding.

4. 1) DPCM requires less Bandwidth than PCM for transmitting the same signal

2) DPCM is used to decrease quantization noise.

3) The number of quantized levels is increased in DPCM

4) In DPCM the difference between successive samples is quantized

Which of the above statements are true regarding DPCM:

1. (4) and (3) only
2. (4) and (2) only
3. (4) and (1) only
4. (1), (2) and (4) only

Answer.4. (1), (2) and (4) only

Explanation

The major advantages of DPCM are

1. In DPCM. the difference between successive samples is quantized
2. DPCM is used to decrease the quantization noise
3. DPCM provides less bandwidth requirement.

5.  The Bandwidth requirement of DPCM is ______ than PCM.

1. Less
2. More
3. Equal
4. Any of the above

Explanation

In DPCM the difference signal is much less in amplitude than the actual sample and, hence, less number of quantization levels are needed. Therefore, the number of bits per code is reduced, resulting in a reduced bit- rate. Thus, the bandwidth required in this case is less than the one required in PCM.

The disadvantage of DPCM is that the modulator and demodulator circuits are more complicated than those in PCM.

A comparison of different modulation schemes is as shown in the table below:

 Parameter PCM Delta Modulation (DM) DPCM Number of bits It can use 4, 8 or 16 bits per sample It uses only one bit for one sample Bits can be more than one but are less than PCM Level/Step size Step size is fixed Step size is fixed and cannot be varied Fixed number of levels are used. Quantization error or Distortion Quantization error depends on the number of levels used Slope overload distortion and granular noise is present Slope overload distortion and quantization noise is present Bandwidth of the transmission channel Highest bandwidth is required since the number of bits are high Lowest bandwidth is required The bandwidth required is lower than PCM Signal to Noise ratio Good Poor Fair Area of Application Audio and Video Telephony Speech and images Speech and video

6. The Number of bit used in DPCM is ____ than PCM.

1. Less
2. More
3. Equal
4. Any of the above

Explanation

In DPCM the difference signal is much less in amplitude than the actual sample and, hence, less number of quantization levels are needed. Therefore, the number of bits per code is reduced, resulting in a reduced bit- rate. Thus, the bandwidth required in this case is less than the one required in PCM.

The disadvantage of DPCM is that the modulator and demodulator circuits are more complicated than those in PCM.

A comparison of different modulation schemes is as shown in the table below:

 Parameter PCM Delta Modulation (DM) DPCM Number of bits It can use 4, 8 or 16 bits per sample It uses only one bit for one sample Bits can be more than one but are less than PCM Level/Step size Step size is fixed Step size is fixed and cannot be varied Fixed number of levels are used. Quantization error or Distortion Quantization error depends on the number of levels used Slope overload distortion and granular noise is present Slope overload distortion and quantization noise is present Bandwidth of the transmission channel Highest bandwidth is required since the number of bits is high Lowest bandwidth is required The bandwidth required is lower than PCM Signal to Noise ratio Good Poor Fair Area of Application Audio and Video Telephony Speech and images Speech and video

7. Which one of the following statements about differential pulse code modulation (DPCM) is true?

1. The sum of message signal sample with its prediction is quantized
2. The message signal sample is directly quantized and its prediction is not used
3. The difference of message signal sample and a random signal is quantized
4. The difference of message signal sample with its prediction is quantized

Answer.4. The difference of message signal sample with its prediction is quantized

Explanation

Realization of basic concept (described above) is based on a technique in which we have to predict current sample value based upon previous samples (or sample) and we have to encode the difference between the actual value of the sample and predicted value (the difference between samples can be interpreted as a prediction error). Because it’s necessary to predict sample value DPCM is a form of predictive coding.

DPCM compression depends on the prediction technique, well-conducted prediction techniques lead to good compression rates, in other cases, DPCM could mean expansion compared to regular PCM encoding.

8.  What are the advantages of Delta Modulation?

1. Simpler circuit
2. Less Quantization level
3. Less Bandwidth requirement
4. All of the above

Explanation

➨ In Delta modulation, the electronic circuit requirement for modulation at the transmitter and for demodulation at the receiver is substantially simpler compare to PCM.

➨ In delta modulation, the amplitude of speech signal does not exceed maximum sinusoidal amplitude.

➨ PCM has a sampling rate higher than the Nyquist rate. The encoded signal contains redundant information. DPCM can efficiently remove this redundancy.

➨ DPCM needs less number of quantization levels and hence less number of bits are needed to represent them.

➨ The signaling rate and bandwidth of DPCM or delta modulation is less than the PCM technique.

9. What are the disadvantages of Delta Modulation?

1. Limited usage
3. High Bitrate
4. All of the above

Explanation

➨ If changes in signal are less than the step size, then the modulator no longer follows the signal. Thus produces a train of alternating positive and negative pulses.

➨ Modulator overloads when the slope of signal is too high.

➨ High bit rate.

➨ It requires a predictor circuit and hence it is very complex.

➨ Its practical usage is limited.

10. In DPCM ______ are transmitted per six samples.

1. Four
2. Six
3. Nine
4. One