Improved LSF estimation through anti-aliasing filtering

Abstract

Vocoders compress speech by estimating model parameters at a given transmission rate over an analysis window, assuming that speech is stationary within this window. In this paper, the limits of this assumption are explored with regard to the spectral envelope parameters in the form of Line Spectral Frequency (LSF) parameters. It is shown that all LSF parameters have considerable variations over time, regardless of LSF vector extraction and transmission rates. LSF track variations are investigated through oversampling and are shown to contain high frequency variations above the frequency corresponding to the LSF vector transmission rate. An anti-aliasing filter with cut-off frequency adequate for the chosen LSF vector transmission rate is proposed to alleviate possible spectral overlapping of the LSF parameter spectra. It is confirmed, through experiments, that the proposed method offers an advantage over the classic LSF extraction method with respect to quantisation shown by bit savings of typically 10 to 15%.

LSF Aliasing Removal for Improved Speaker Verification

“…This produces some aliasing noise in the extracted LSF parameters. In order to remove the high frequency variations observed in the LSF track spectra, a pre-processing stage was proposed in [6] that involves the use of low-pass filtering before LSF vector decimation. We employed this anti-aliasing filtering approach in the LSF parameter extraction section of the GSM-EFR.…”

“…This result is currently being investigated. Although this method requires extra computational cost and time delay, as shown in [6] the method improves the synthesised speech quality, while providing easier quantisation compared to the original LSF extraction methods. Initial experiments indicate that the NA-LSF parameter extraction method can be used with any speech coder that employs LP analysis in its structure.…”

“…Nevertheless, the results are also expected to be applicable to AMR [5], as its principles are similar to GSM-EFR. It has been shown in [6] that the LSF parameters contain high frequency variations, which cause some aliasing noise in the LSF parameters. These unwanted frequency components can be removed by employing an anti-aliasing filter.…”