Double-ended Noise Reduction refers to a class of noise reduction systems that depend on two distinct and complementary processes: one at the recording or transmission stage and another at the playback or reception stage. Unlike single-ended systems, which attempt to clean a noisy signal only at the end of the chain, this method requires the audio to be pre-conditioned or encoded before being exposed to the noisy medium, such as magnetic tape. During playback, a decoding circuit reverses the process exactly, restoring the original audio dynamics while drastically attenuating the accumulated background noise. Dolby and dbx systems are the most classic examples of this technology, operating on the principle of companding (compression and expansion).