Noise Reduction - Important Concepts in Signal Processing, Image Processing and Data Compression - page 16

well as for live broadcast applications. Single-ended surface noise reduction (such as
CEDAR and the earlier SAE 5000A and Burwen TNE 7000) is applied to the playback of
phonograph records to attenuate the sound of scratches, pops, and surface non-linearities.
Dual-ended systems (such as Dolby NR and dbx Type I and II) have a pre-emphasis
process applied during recording and then a de-emphasis process applied at playback.
Dolby and dbx noise reduction system
While there are dozens of different kinds of noise reduction, the first widely used audio
noise reduction technique was developed by Ray Dolby in 1966. Intended for
professional use, Dolby Type A was an encode/decode system in which the amplitude of
frequencies in four bands was increased during recording (encoding), then decreased
proportionately during playback (decoding). The Dolby B system (developed in
conjunction with Henry Kloss) was a single band system designed for consumer
products. In particular, when recording quiet parts of an audio signal, the frequencies
above 1 kHz would be boosted. This had the effect of increasing the signal to noise ratio
on tape up to 10dB depending on the initial signal volume. When it was played back, the
decoder reversed the process, in effect reducing the noise level by up to 10dB. The Dolby
B system, while not as effective as Dolby A, had the advantage of remaining listenable
on playback systems without a decoder.
Dbx was the competing analog noise reduction system developed by dbx laboratories. It
used a root-mean-squared (RMS) encode/decode algorithm with the noise-prone high
frequencies boosted, and the entire signal fed through a 2:1 compander. Dbx operated
across the entire audible bandwidth and unlike Dolby B was unusable as an open ended
system. However it could achieve up to 30 dB of noise reduction. Since Analog video
recordings use frequency modulation for the luminance part (composite video signal in
direct colour systems), which keeps the tape at saturation level, audio style noise
reduction is unnecessary.
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