Wireless & Mobile Communications Multiple Choice Questions on “Vocoders”.
1. Vocoders analyse the speech signals at ______
a) Transmitter
b) Receiver
c) Channel
d) IF Filter
Answer: a
Clarification: Vocoders are a class of speech coding systems. They analyse the speech signal at the transmitter. And then transmit the parameters derived from the analysis.
2. Vocoders __________ the voice at the receiver.
a) Analyse
b) Synthesize
c) Modulate
d) Evaluate
Answer: b
Clarification: Vocoders synthesize the voice at the receiver. All vocoder systems attempt to model the speech generation process as a dynamic system and try to quantify certain physical constraints of the system.
3. Vocoders are simple than the waveform coders.
a) True
b) False
Answer: b
Clarification: Vocoders are much more complex than the waveform coders. They can achieve very high economy in transmission bit rate but are less robust.
4. Which of the following is not a vocoding system?
a) Linear predictive coder
b) Channel vocoder
c) Waveform coder
d) Formant vocoder
Answer: c
Clarification: Waveform coder is not a vocoding system. LPC (linear predictive coding) is the most popular vocoding system. Other vocoding systems are channel vocoder, formant vocoder, cepstrum vocoder etc.
5. Which of the following pronunciations lead to voiced sound?
a) ‘f’
b) ‘s’
c) ‘sh’
d) ‘m’
Answer: d
Clarification: Voiced sounds are ‘m’, ‘n’ and ‘v’ pronounciations. They are a result of quasiperiodic vibrations of the vocal chord.
6. Speech signal can be categorised in _____ and ______
a) Voiced, unvoiced
b) Active, passive
c) Direct, indirect
d) Balanced, unbalanced
Answer: a
Clarification: Speech signal is of two types, voiced and unvoiced. Voiced sound is a result of quasiperiodic vibrations of the vocal chord. Unvoiced signals are fricatives produced by turbulent air flow through a constriction.
7. Channel vocoders are the time domain vocoders.
a) True
b) False
Answer: b
Clarification: Channel vocoders are frequency domain vocoders. They determine the envelope of the speech signal for a number of frequency bands and then sample, encode and multiplex these samples with the encoded outputs of the other filters.
8. ________ is often called the formant of the speech signal.
a) Pitch frequency
b) Voice pitch
c) Pole frequency
d) Central frequency
Answer: c
Clarification: The pole frequencies correspond to the resonant frequencies of the vocal tract. They are often called the formants of the speech signal. For adult speakers, the formants are centered around 500 Hz, 1500 Hz, 2500 Hz and 3500 Hz.
9. Formant vocoders use large number of control signals.
a) True
b) False
Answer: b
Clarification: Formant vocoders use fewer control signals. Therefore, formant vocoders can operate at lower bit rates than the channel vocoder. Instead of transmitting the power spectrum envelope, formant vocoders attempt to transmit the position of peaks of spectral envelope.
10. Cepstrum vocoder uses __________
a) Wavelet transform
b) Inverse wavelet transform
c) Cosine transform
d) Inverse Fourier transform
Answer: d
Clarification: Cepstrum vocoders use inverse Fourier transform. It separates the excitation and vocal tract spectrum by Fourier transforming spectrum to produce the cepstrum of the signal.