Dynamic Noise Reduction

Dynamic Noise Reduction (DNR) is an audio noise reduction system, introduced by National Semiconductor to reduce noise levels on long-distance telephony. First sold in 1981, DNR is frequently confused with the far more common Dolby noise reduction system. However, unlike Dolby and dbx Type I & Type II noise reduction systems, DNR is a playback-only signal processing system that does not require the source material to first be encoded, and it can be used together with other forms of noise reduction. It was a development of the unpatented Philips Dynamic Noise Limiter (DNL) system, introduced in 1971, with the circuitry on a single chip.
Because DNR is non-complementary, meaning it does not require encoded source material, it can be used to remove background noise from any audio signal, including magnetic tape recordings and FM radio broadcasts, reducing noise by as much as 10 dB. It can be used in conjunction with other noise reduction systems, provided that they are used prior to applying DNR, to prevent the other noise reduction system from mistracking.
One of DNR's first widespread applications was in the high-end GM Delco Bose car stereo systems in U.S. GM cars (later added to lower-end Delco-manufactured car stereos in GM vehicles as well), introduced in 1984. It was also used in factory car stereos in Jeep vehicles in the 1980s, such as the Cherokee XJ. Today, DNR, DNL, and similar systems are most commonly encountered as a noise reduction system in microphone systems.

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