Speech Processing by Computer





This lab session demonstrates how a rough spectral analysis of a signal may be obtained through the use of a bank of filters.  The combination of the spectral analysis with some information about the excitation allows the recreation of the signal with some interesting modifications.


1.         Low-pass/High-pass and Band-pass filtering


            (i)         Acquire a speech signal at 20000 samples/second.

            (ii)        Low-pass filter the signal at 1000Hz and replay the result.

            (iii)       High-pass filter the original signal at 900Hz and replay the result.

            (iv)       Band-pass filter the original signal between 900 and 1000Hz and replay the result.

            (v)        Compare spectrograms of the four signals.


2.         Filterbank analysis


            (i)         Acquire a speech signal and a laryngograph signal at 20000 samples/second.

            (ii)        Use the ‘19-channel auditory filterbank’ program to save the energies in 19 frequency bands from the speech signal.

            (iii)       Use the ‘make grey scale’ option to turn the energies into a grey-level picture.  Dsiplay the grey-level picture of the filterbank outputs alongside a spectrogram of the original signal.


3.         Filterbank synthesis


            (i)         Convert the laryngograph waveform to Tx pulses.

            (ii)        Convert the Tx data into a fundamental frequency contour.

            (iii)       Use the '19 channel synthesizer' program to resynthesize the original signal from the filterbank analysis and the fundamental frequency contour.

            (iv)       Use the '19 channel synthesizer' program with the 'Whispered’ switch to produce a whispered version of the utterance.  What excitation is being used here?


4.         Synthesis with alternative excitation.


            (i)         Acquire into the same file as the original speech, a piece of orchestral music at 10,000 samples/sec.

            (ii)        Using the '19 channel synthesizer' program with the 'alternative excitation' switch (and the music item number), resynthesize the speech signal using an orchestral excitation.  Replay the result.  Compare spectrograms between the different resynthesized versions and the original signal.