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Digital systems dedicated to audio and speech processing usually require sample rate conversion units in order to adapt the sample rate from different signal flows: for instance 8 and 16 kHz for speech, 32 kHz for the broadcast rate, 44.1 kHz for CDs, and 48 kHz for studio work. The designer chooses the sample rate conversion (SRC) technology based on objective criteria, such as figures of complexity, development or integration cycle and of course performance characterization. The performances of the fractional SRC system include the in-band and the aliasing characterization plus its distortion behavior due to internal rounding errors. The paper shows the existence of a new compound time-invariant system made of multiple instances of the same SRC system. The characterization of the original SRC is obtained from the linear and distortion characteristics of this time-invariant system. Regular methods for characterizing time-invariant systems apply. The SRC system can be analyzed in black box conditions, either in batch processing or in real-time processing. Examples illustrate the capability of the method to fully recover characteristics and rounding noise behavior from actual SRC implementations.