A method and device eradicate the occasional instability inherentin analysis-by-synthesis speech/audio codecs and caused in particular bychannel errors during transmission of highly periodic signals such ashigh-frequency sine waves. Analysis-by-synthesis techniques involveproduction, in response to the speech/audio signal and at regular timeintervals called frames, of (a) a set of spectral parameters for use indriving a synthesis filter in view of synthesizing the speech/audio signal,and (b) a pitch gain for constructing a past-excitation-signal componentfor supply to the synthesis filter. In accordance with the instabilityeradication method, a first step consists of detecting a set of conditionsincluding (i) a resonance condition assessed from the spectralparameters, (ii) a duration condition detected when the resonancecondition has prevailed for at least the M most recent frames, M being aninteger greater than 1, and (iii) a gain condition which evidencesconsistently-high values of the pitch gain in the N most recent frames, Nbeing an integer greater than 1. To eradicate the occasional instability,the pitch gain is reduced to a value lower than a given thresholdwhenever these three conditions are detected.
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