A significant amount of research has been conducted in the development of a test methodology for evaluating small impairments in audio codecs. Specifically, ITU-R Recommendation BS.1116 provides a full description of the accepted methodology, which has been used extensively in the development and evaluation of high-quality audio codecs. This methodology has proven effective at generating consistent results in subjective tests and can provide a high degree of resolution by discriminating between codecs. With the recent trend toward very low bit-rate audio codecs and the corresponding lower quality, a need has arisen for a subjective test methodology that will allow the performance of these codecs to be evaluated and compared in a rigorous fashion. The method described in BS.1116 is not entirely appropriate for this purpose; therefore, a new method has been developed for evaluating audio codecs with larger impairments. The new method strives to maintain those aspects of BS.1116 that have proven most effective, while extending it to address the particular difficulties encountered when evaluating large impairments. This paper describes the methodologies for evaluating small and large impairments and includes results from formal subjective tests.
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