Wireless & Mobile Communications Multiple Choice Questions & Answers (MCQs) on “Speech Codecs”.
1. The choice of speech coder does not depend on cell size used.
a) True
b) False
Answer: b
Clarification: The choice of speech coder depends on the cell size used. When the cell size is sufficiently small such that high spectral efficiency is achieved through frequency reuse, it may be sufficient to use a simple high rate speech codec.
2. Which of the following is an important factor in determining spectral efficiency of the system?
a) Multiple access technique
b) Cell size
c) Modulation technique
d) Vocoder
Answer: a
Clarification: The type of multiple access technique used is an important factor in determining the spectral efficiency of the system. It strongly influences the choice of speech codec.
3. The type of modulation does not affect the choice of speech codec.
a) True
b) False
Answer: b
Clarification: The type of modulation employed has a considerable impact on the choice of speech codec. Using bandwidth efficient modulation scheme can lower the bit rate reduction requirements on the speech codec and vice versa.
4. Which of the following is the name of original speech coder used in the pan European digital cellular standard GSM?
a) Multipulse excited codec
b) Residual excited codec
c) Regular pulse excited long term prediction
d) Code excited codec
Answer: c
Clarification: The original speech coder used in the pan European digital cellular standard GSM goes by a rather grandiose name of regular pulse excited long term prediction (RPE-LTP) codec. This codec has a bit rate of 13 kbps.
5. Which of the following is true for baseband RELP codec?
a) Good quality of speech, low complexity
b) Good quality of speech, high complexity
c) Bad quality of speech, low complexity
d) Bad quality of speech, high complexity
Answer: a
Clarification: The advantage of baseband RELP codec is that it provides good quality speech at low complexity. The speech quality is sometimes limited due to tonal noise introduced by the process of high frequency generation.
6. Which of the following is true for MPE-LTP codec?
a) Good quality of speech, low complexity
b) Good quality of speech, high complexity
c) Bad quality of speech, low complexity
d) Bad quality of speech, high complexity
Answer: b
Clarification: The MPE-LTP technique produces excellent speech quality at high complexity. It is not much affected by bit errors present in the channel.
7. RPE-LTP codec combines the advantage of RELP codec and CELP codec.
a) True
b) False
Answer: b
Clarification: The RPE-LTP codec combines the advantages of the earlier French proposed RELP codec with those of the multipulse excited long term prediction (MPE-LTP) codec proposed by Germany.
8. Which of the following codec is used by IS-136?
a) Residual Excited Linear Predictive Coders
b) Multipulse Excited LPC
c) LPC Vocoders
d) Vector sum excited LPC
Answer: d
Clarification: The US digital cellular system, IS-136 uses a vector sum excited linear predictive coder (VSELP). This coder operates at a raw data rate of 7950 bits/s and a total data rate of 13 kbps after channel coding.
9. VSELP speech coder is a variant of ___________
a) CELP
b) MPE_LTP
c) RELP
d) RPE-LTP
Answer: a
Clarification: The VSELP speech coder is a variant of the CELP type vocoders. The code books in the VSELP encoder are organised with a predefined structure such that a brute-force search is avoided.
10. Which of the following is true for VSELP?
a) Low speech quality, modest computational complexity, robust to channel errors
b) Highest speech quality, low computational complexity, channel errors
c) Highest speech quality, high computational complexity, robust to channel errors
d) Highest speech quality, modest computational complexity, robust to channel errors
Answer: d
Clarification: VSELP speech coder is designed to accomplish the three goals of highest speech quality, modest computational complexity and robustness to channel errors. The code books used by VSELP impart high speech quality and increased robustness to channel errors.
11. What is DAM in speech coding system?
a) Diagnostic Acceptability Measure
b) Digital Acceptability Measure
c) Diagnostic Accessibility Measure
d) Digital Accessibility Measure
Answer: a
Clarification: The diagnostic acceptability measure is used in speech coding system. It is used for evaluation of acceptability of speech coding systems.
12. ________ exaggerates the bit errors originally received at the base station.
a) Non linear transformation
b) Tandem signalling
c) Large cell size
d) Complex vocoders
Answer: b
Clarification: Tandem signalling tends to exaggerate the bit errors originally received at the base station. Tandem signalling is difficult to protect against but is an important evaluation criterion in the evaluation of speech coders.
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