| 摘要 |
<p>A multi-rate speech codec supports a plurality of encoding bit rate modes by adaptively selecting encoding bit rate modes to match communication channel restrictions. In higher bit rate encoding modes, an accurate representation of speech through CELP (code excited linear prediction) and other associated modeling parameters are generated for higher quality decoding and reproduction. To achieve high quality in lower bit rate encoding modes, the speech encoder departs from the strict waveform matching criteria of regular CELP coders and strives to identify significant perceptual features of the input signal. To support lower bit rate encoding modes, a variety of techniques are applied many of which involve the classification of the input signal. For each bit rate mode selected, pluralities of fixed or innovation subcodebooks are selected for use in generating innovation vectors. At lower encoding bit rates, a decoder utilizes adaptive compensation to attempt to correct for spectral variations in the weighted synthesized residual. Although many approaches are possible, a long asymmetric window is applied to the synthesized residual to generate a reflection coefficient that is smoothed, scaled and used in a first order filter. Because the content of the window varies over time, the coefficient and therefore the filter varies (or adapts) to remove at least a portion of the spectral tilt. As a result, the synthesized speech signal sounds brighter without having introduced signifiant coding noise.</p> |
