Adaptive tilt compensation for synthesized speech residual

• US 6,385,573 B1
• Filed: 09/18/1998
• Issued: 05/07/2002
• Est. Priority Date: 08/24/1998
• Status: Expired due to Term

• Current Assignee: Samsung Electronics Co. Ltd.
• Original Assignee: Conexant Systems Incorporated (Synaptics Incorporated)
• Inventors: Gao, Yang, Su, Huan-Yu
• Primary Examiner(s): Tsang, Fan
• Assistant Examiner(s): Opsasnick, Michael N.
• Application Number:
  US09/156,826
• Time in Patent Office: 1,327 Days
• Field of Search: 704/220, 704/219
• US Class Current: 704/220
• CPC Class Codes: G10L 19/002 Dynamic bit allocation for perceptual audio coders G10L19/032
• CPC Class Codes: G10L 19/005 Correction of errors induced by the transmission channel, if related to the coding algorithm
• CPC Class Codes: G10L 19/012 Comfort noise or silence coding
• CPC Class Codes: G10L 19/08 Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters
• CPC Class Codes: G10L 19/083 the excitation function being an excitation gain G10L25/90 takes precedence
• CPC Class Codes: G10L 19/09 Long term prediction, i.e. removing periodical redundancies, e.g. by using adaptive codebook or pitch predictor
• CPC Class Codes: G10L 19/10 the excitation function being a multipulse excitation
• CPC Class Codes: G10L 19/12 the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
• CPC Class Codes: G10L 19/125 Pitch excitation, e.g. pitch synchronous innovation CELP [PSI-CELP]
• CPC Class Codes: G10L 19/18 Vocoders using multiple modes
• CPC Class Codes: G10L 19/265 Pre-filtering, e.g. high frequency emphasis prior to encoding
• CPC Class Codes: G10L 2019/0005 Multi-stage vector quantisation
• CPC Class Codes: G10L 2019/0007 Codebook element generation
• CPC Class Codes: G10L 2019/0011 Long term prediction filters, i.e. pitch estimation
• CPC Class Codes: G10L 21/0364 for improving intelligibility

Abstract

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 significant coding noise.

Claims

1. A speech system using an analysis by synthesis approach on a speech signal, the speech system comprising:

at least one codebook containing at least one code vector;

processing circuitry that generates a synthesized residual signal using the at least one codebook; and

the processing circuitry applying adaptive tilt compensation to the synthesized residual signal based in part on an encoding bit rate of the speech system and a flatness of the synthesized residual signal.

2. The speech system of claim 1 wherein the processing circuitry comprises an encoder processing circuit that generates the synthesized residual signal, and a decoder processing circuit that applies the adaptive tilt compensation.

3. The speech system of claim 1 wherein the synthesized residual signal comprises a weighted synthesized residual signal.

4. The speech system of claim 1 wherein the adaptive tilt compensation comprises identifying a filter coefficient for use in a compensating filter.

5. The speech system of claim 4 wherein the compensating filter comprises a first order filter.

6. The speech system of claim 4 wherein the identification of the filter coefficient comprises application of a window to the synthesized residual.