The Dynamic Noise Limiter, or DNL for short, is a

noise reduction Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an u ...

system introduced by

Philips Koninklijke Philips N.V. (), simply branded Philips, is a Dutch multinational health technology company that was founded in Eindhoven in 1891. Since 1997, its world headquarters have been situated in Amsterdam, though the Benelux headquarter ...

in the late 1960s to improve the quality of audio recordings on

cassette tape The Compact Cassette, also commonly called a cassette tape, audio cassette, or simply tape or cassette, is an analog audio, analog magnetic tape recording format for Sound recording and reproduction, audio recording and playback. Invented by L ...

. It is used on playback only, and does not require encoding the original audio signal. It was not widely used, as Dolby noise reduction became very popular. Third-party implementations called it Dynamic Noise Reduction, or DNR. As Dolby may be initialized DNR, there is some confusion between the two. The small physical size of cassette tapes, compared to

reel-to-reel Reel-to-reel audio tape recording, also called open-reel recording, is magnetic tape audio recording in which the recording tape is spooled between reels. To prepare for use, the ''supply reel'' (or ''feed reel'') containing the tape is plac ...

tapes, means they must use a slow tape speed to allow practical recording times. Their smaller width also limits the

dynamic range Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...

of the signal that can be recorded to it. This results in much higher amounts of audible

tape hiss Tape hiss is the high frequency noise present on analogue magnetic tape recordings caused by the size of the magnetic particles used to make the tape. Effectively it is the noise floor of the recording medium. It can be reduced by the use of fine ...

compared to reel-to-reel systems. Tape hiss occurs on all

magnetic tape Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use magnetic ...

systems, as an ever-present low-volume, high-frequency signal. On reel-to-reel systems the higher dynamic range and faster tape speeds allowed the signal to be recorded at volume levels that overwhelm the hiss. On cassettes, there is simply not enough dynamic range to do this, especially in quiet passages, and the hiss becomes very audible. DNL addresses this issue by noting that the hiss is only audible when the signal is low. At higher volumes, the low-volume hiss is simply not as obvious. Philips took advantage of this by having a circuit that would mute high-frequency signals only when the overall volume was low. Although this also muted any high-frequency components of the audio, the hiss would be masking these anyway. If the high-frequency components were high volume, they would overwhelm the hiss and the circuit would not mute it. Philips released the circuit design royalty-free, which led to sample implementations being published in ''

Wireless World ''Electronics World'' (''Wireless World'', founded in 1913, and in October 1983 renamed ''Electronics & Wireless World'') is a technical magazine published by Datateam Business Media Ltd that covers electronics and RF engineering and is aimed at ...

'' in 1971 and ''

Elektor :''Elektor (ἠλέκτωρ) is also an ancient Greek name or epithet of the Sun, see Helios.'' ''Elektor'', also known as ''Elektor Magazine'', is a monthly magazine about all aspects of electronics, originally published in the Netherlands as ''E ...

'' in 1976 and 1984. DNL can be found e.g. in the 8000 cassette recorder series from

BASF BASF SE (), an initialism of its original name , is a European Multinational corporation, multinational company and the List of largest chemical producers, largest chemical producer in the world. Its headquarters are located in Ludwigshafen, Ge ...

, which were sold in the 1970s.

National Semiconductor National Semiconductor Corporation was an United States of America, American Semiconductor manufacturing, semiconductor manufacturer, which specialized in analogue electronics, analog devices and subsystems, formerly headquartered in Santa Clara, ...

further developed the process and offered it in the form of the IC LM1894.

References

* Gustav Büscher, Alfred Wiegelmann: ''Kleines ABC der Elektroakustik'' (= ''Radio-Praktiker-Bücherei.'' Bd. 29/30a). 6., völlig neu bearbeitete und erweiterte Auflage. Franzis, München 1972, ISBN 3-7723-0296-3. * Helmut Röder, Heinz Ruckriegel, Heinz Häberle: ''Nachrichtenelektronik. Rundfunk- und Fernsehelektronik'' (= ''Elektronik.'' Bd. 3). 5. Auflage. Verlag Europa-Lehrmittel, Wuppertal 1980, ISBN 3-8085-3225-4. * * {{cite magazine , magazine=Elector , url=https://www.elektormagazine.com/magazine/elektor-197603/57596 , title=dynamic noise limiter Noise reduction