Speech Processing
by Computer
LAB 4
SPECTRAL
ANALYSIS AND MODELLING
This lab session
demonstrates the use of spectral analysis applied to vowel sounds. Spectral analyses with different degrees of
frequency resolution are compared.
All-pole models of vowel spectra are calculated using linear
prediction. A demonstration is made of
the inversion of the linear predicted filter to provide an estimate of the
glottal input to the vocal tract.
1. Frequency
resolution of spectral analysis
(i) Acquire a steady vowel at 20000
samples/second.
(ii)
Using the 'Esection' program, display and print spectral cross sections
and LPC modelled spectral cross-sections corresponding to time windows of 1ms,
3ms, 10ms, 30ms and 100ms. You may need
to adjust the analysis window size for the longer windows.
(iii)
Estimate the accuracy to which a frequency component may be identified
in each case and for each type of analysis.
(iv)
Explain why different resolutions may be required to determine fundamental
frequency as opposed to formant frequency.
(v)
Contrast the LPC modelled spectra with DFT spectra. What are the useful characteristics of the modelled spectra?
2. Inverse-filtered
waveforms
(i) Acquire a monophthongal vowel on a
falling pitch at 10,000 samples/sec.
(ii) Use the 'Eswin' program to copy about
0.1 seconds of the vowel region to a new file.
(iii) Use the 'invfilt' program to (a)
calculate a vocal tract filter from this signal, (b) to invert this filter, (c)
to filter the original signal back through the inverted filter, and (d) to save
the glottal pressure wave and two successive integrations of the glottal
pressure wave.
(iv) Display the waveform and a spectrogram of
the original and inverse filtered signals.
Explain what you see. Replay the original and inverse filtered signal -
can the quality of the vowel still be identified?
(v) Repeat steps (i) to (iv) with a
different vowel.
3. Inverse-filtered
spectra
(i)
Use the ‘Esection’ program to acquire some different vowels at 10,000
samples/sec.
(ii)
Use the ‘view source spectrum’ option to study the inverse filtered
spectrum for the different vowels.
(iii)
How well is the inverse filtering able to remove the effects of the
filter?