Critical band spectrum

This spectrum is calculated by applying a critical band analysis. The filters are either Bark (see H. Hermansky, "Perceptual linear predictive (PLP) analysis of speech", JASA, Vol. 87 1990) or Mel. The dialog window enables the size of the signal window, the FFT order, the number of bands, and their distribution to be adjusted.

After the application of the critical band or Mel filters the spectral the user can apply a linear prediction to get a PLP spectrum, or a liftering to get a Mel cepstrally smoothed spectrum, or nothing. In the latter case the spectrum displayed is only the result of the critical band filtering.

 

In the case of the cepstral smoothing, a prior linear cepstral smoothing (before the application of critical band filters to the spectrum) can be applied in order to remove the effect of the fundamental frequency.

When the cepstrum liftering is applied, WinSnoori displays the Mel cepstrally smoothed spectrum and not the Mel cepstral coefficients. This spectrum thus is not the data fed into an automatic speech recognition system but gives a correct idea of the spectral information used in ASR.

When the PLP is used the user can set the prediction order and activate the intensity conversion, i.e. the application of .1/3 after the application of the critical band filters.

The distribution of bands can be linear in Hz "linear in frequency" to display a spectrum or a spectrogram or linear in Bark or Mel (called "ASR like"). The first choice should be used only with a reasonably high number of points because too small a number of points gives rise to a very poor representation in low frequencies and consequently display artefacts.

The dialog window checks the overall consistency of parameters.

Example: Here is the dialog window to apply a Mel cepstrum analysis with 24 bands and 12 Mel coefficients. The spectrum displayed is not the vector of the Mel coefficients but the Mel cepstrally smoothed spectrum, i.e. a IDCT has been applied after the DCT of the Mel cepstrum computation.