# Digital Communications Questions and Answers – Correlative Coding

This set of Digital Communications Multiple Choice Questions & Answers (MCQs) focuses on “Correlative Coding”.

1. The method in which small amount of controlled ISI is introduced into the data stream rather than trying to eliminate it completely is called as
a) Correlative coding
b) Duobinary signalling
c) Partial response signalling
d) All of the mentioned

Explanation: The interference at the detector can be cancelled out using these methods in which some controlled amount of ISI is introduced into the data stream.

2. From digital filter we will get the output pulse as the _______ of the current and the previous pulse.
a) Summation
b) Difference
c) Product
d) Ratio

Explanation: The digital filter incorporates one digit delay and thus it adds the incoming pulse with the value of the previous pulse.

3. In duobinary signalling method, for M-ary transmission, the number of output obtained is
a) 2M
b) 2M+1
c) 2M-1
d) M2

Explanation: In duobinary coding, the number of output obtained for M-ary transmission is 2M-1.

4. The method using which the error propagation in dubinary signalling can be avoided is
a) Filtering
b) Precoding
c) Postcoding
d) None of the mentioned

Explanation: In duobinary signalling method if one error occurs it repeats everywhere through out the next steps. To avoid this precoding method can be used.

5. In precoding technique, the binary sequence is _____ with the previous precoded bit.
a) And-ed
b) Or-ed
c) EXOR-ed

Explanation: To avoid error propogation precoding method is used. In this each bit is encoded individually without having any effect due to its prior bit or decisions.

6. The duobinary filter, He (f) is called as
a) Sine filter
b) Cosine filter
c) Raised cosine filter
d) None of the mentioned

Explanation: The transfer function is 2T cos(πfT) which is called as cosine filter.

7. The method which has greater bandwidth efficiency is called as
a) Duobinary signalling
b) Polybinary signalling
c) Correlative coding
d) All of the mentioned

Explanation: If more than three levels are introduced in duobinary signalling technique the bandwidth efficiency increases This method is called as polybinary signalling.

8. In polybinary signalling method the present bit of binary sequence is algebraically added with ______ number of previous bits.
a) j
b) 2j
c) j+2
d) j-2

Explanation: In polybinary signalling method the present binary digit of the sequence is formed from the modulo-2 addition of the j-2 preceding digits of the sequence and the present digit.

9. The primary advantage of this method is
a) redistribution of spectral density
b) to favor low frequencies
c) redistribution of spectral density & to favor low frequencies
d) none of the mentioned

Explanation: Each bit can be independently detected in-spite of strong correlation and this provides redistribution of spectral density and also favors low frequencies.

10. Source encoding procedure does
a) Sampling
b) Quantization
c) Compression
d) All of the mentioned

Explanation: Source encoding includes a sampling of continuous time signals, quantization of continuous valued signals and compression of those sources.

Sanfoundry Global Education & Learning Series – Digital Communications.

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