# Wireless & Mobile Communications Questions & Answers – Linear Predictive Coders

This set of Wireless & Mobile Communications Multiple Choice Questions & Answers (MCQs) focuses on “Linear Predictive Coders”.

1. Linear predictive coders belong to _______ domain class of vocoders.
a) Time
b) Frequency
c) Direct
d) Indirect

Explanation: Linear predictive vocoders belong to the time domain class of vocoders. This class of vocoders attempts to extract the significant features of the speech from the time waveform.

2. Linear predictive coders are computationally simple.
a) True
b) False

Explanation: Linear predictive coders are computationally intensive. But, they are the most popular among the class of low bit vocoders. With LPC, it is possible to transmit good quality voice at 4.8 kbps and poorer quality voice at even lower rates.

3. Linear predictive coding system models the vocal tract as __________ linear filter.
a) Pole and zero
b) All zero
c) All pole
d) No pole

Explanation: The linear predictive coding system models the vocal tract as an all pole linear filter. The excitation to this filter is either a pulse at the pitch frequency or random white noise depending on whether the speech segment is voiced or unvoiced.

4. Linear predictive vocoders use __________ to estimate present sample.
a) Weighted sum of past samples
b) Multiplication of past samples
c) One past sample
d) Do not use past samples

Explanation: The linear predictive coder uses a weighted sum of p past samples. Using this technique, the current sample can be written as linear sum of the immediately precoding samples.

5. Which of the following LPC uses code book?
a) Multiple excited LPC
b) Residual excited LPC
c) LPC Vocoders
d) Code excited LPC

Explanation: Code excited LPC uses code book. In this method, the coder and decoder have a predetermined code book of stochastic (zero mean white Gaussian) excitation signals.
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6. How many past samples are used by linear predictive coders to estimate present sample?
a) 100-150
b) 10-15
c) 1
d) 1000-1100

Explanation: LPCs uses weighted sum of past p samples to estimate the present samples. The number of past samples used by linear predictive coders ranges from 10 to 15.

7. Which of the non-linear transform is generally used to improve the coding of reflection coefficient?
a) Long area ratio transform
b) Mutual information
c) Least square
d) Interpolation

Explanation: Long area ratio (LAR) transform is generally used to improve the coding of reflection coefficient. This non linear transformation reduces the sensitivity of reflection coefficients to quantization errors. LAR performs an inverse hyperbolic tangent mapping of reflection coefficients.

8. Which of the following LPC uses two sources at the receiver?
a) Multiple excited LPC
b) Residual excited LPC
c) LPC Vocoders
d) Code excited LPC

Explanation: LPC vocoder uses two sources at the receiver, one of white noise and the other with a series of pulses at the current pitch rate. The selection of either of these excitation methods is based on voiced/unvoiced decision made at the transmitter.

9. Which of the following LPC produces a buzzy twang in the synthesized speech?
a) Multiple excited LPC
b) Residual excited LPC
c) LPC Vocoders
d) Code excited LPC

Explanation: LPC vocoder requires that the transmitter extract pitch frequency information which is often very difficult. Moreover, the phase coherence between the harmonic components of the excitation pulse tends to produce a buzzy twang in the synthesized speech.

10. The problem of buzzy twang in synthesized speech is mitigated by multipulse excited LPC or code excited LPC.
a) True
b) False

Explanation: LPC vocoder produces buzzy twang in the synthesized speech due to phase coherence between the harmonic components of the excitation pulses. This problem is mitigated by multipulse excited or code excited LPC.

11. Multipulse excited LPC requires pitch detection.
a) True
b) False

Explanation: Multipulse excited LPC does not require pitch detection and the prediction residual is better approximated by several pulses per pitch period. This is the reason for better speech quality.

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