1. In digital transmission, the modulation technique that requires the minimum bandwidth is:
PCM
PAM
DPCM
Delta modulation
Answer.3. DPCM
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 ______
Digital to Analog
Analog to Digital
Digital to Discrete
None of the above
Answer.1. Digital to Analog
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.
Sampling
Amplifying
Prediction
None of the above
Answer.3. Prediction
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:
(4) and (3) only
(4) and (2) only
(4) and (1) only
(1), (2) and (4) only
Answer.4. (1), (2) and (4) only
Explanation
The major advantages of DPCM are
In DPCM. the difference between successive samples is quantized
DPCM is used to decrease the quantization noise
DPCM provides less bandwidth requirement.
5. The Bandwidth requirement of DPCM is ______ than PCM.
Less
More
Equal
Any of the above
Answer.1. Less
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.
Less
More
Equal
Any of the above
Answer.1. Less
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?
The sum of message signal sample with its prediction is quantized
The message signal sample is directly quantized and its prediction is not used
The difference of message signal sample and a random signal is quantized
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?
Simpler circuit
Less Quantization level
Less Bandwidth requirement
All of the above
Answer.4. All of the above
Explanation
Advantages of Delta Modulation:
➨ 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?
Limited usage
Modulator Overload
High Bitrate
All of the above
Answer.4. All of the above
Explanation
Disadvantages of Delta Modulation:
➨ 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.
Four
Six
Nine
One
Answer.1. Four
Explanation
In DPCM, four bits are transmitted per six samples.
In the original 32-Kbps DPCM method, the difference between the predicted and actual sample value is coded with four bits, that is, into 15 quantum levels, and the data rate is half that of conventional PCM.
If several subsequent samples vary widely, the quantizing steps are adapted to that change so that four bits are enough for prediction error. If prediction errors tend to increase, quantizing steps is increased and vice versa.