Details of the speech spectral analysis. (a) The speech spectrogram is analyzed separately at each time instant. Each spectrogram slice is filtered through a bandpass filter H
) parameterized by Ω
. The ∗ operator signifies the filtering operation. Four such filtering operations yield four views of the same spectral slice; each view highlights different details about the spectrum, notably formant peaks and harmonic structure. (b) Cortical features for clean and noisy versions of one phoneme ∖ow∖. The plots show magnitude as a function of frequency and scale. For visualization, the discrete image points have been interpolated in MATLAB using a bicubic interpolation routine. Notice the consistency of formant peaks around 1 and 4 KHz and of harmonic energies at 2 CPO and 4 CPO despite the additive noise distortion. (c) Cortical features for different types of additive noise. Note that the patterns exhibited are quite different. Subtle peaks due to harmonicity and formant structure of human speech can be seen in the left panel (babble noise).