Monday, January 28, 2013

Dolby B: Noise Reduction Alphabet For The Masses?

Famed for lifting a lowly dictation medium – the Philips’ compact cassette – to true high fidelity status, does the Dolby B noise reduction system deserve to be called the noise reduction alphabet for the masses?

By: Ringo Bones

The Dolby B type noise reduction system makes use of single chain signal processing. The filter is a fixed-gain variable bandwidth device. The resulting output of the Dolby B-type encoder is a boosted high frequency output response which gradually flattens out as the high-frequency output level rises to avoid tape saturation. The Dolby B circuit reduces noise above approximately 500 Hz and achieves about 8 to 10 dB of noise reduction. Despite becoming the ubiquitous noise reduction system of choice during the heyday of the compact cassette, the Dolby B type noise reduction system would not have existed at all without the help of Henry Kloss.

During his early work in radio astronomy, Ray Dolby got inspiration on how to make an effective noise reduction system for analog tape based recording form his own work in trying to extract very weak cosmic signals from background radiation noise, which later became the working principle behind Dolby A, a noise reduction system that then became in standard use in professional recording studios during the start of the Golden Age of Stereo near the end of the 1950s. Despite its effectiveness, Dolby A was deemed too complicated for domestic use that domestic hi-fi enthusiasts during the 1950s up to early 1960s had been doing their do-it-yourself audio recordings on their consumer-grade open reel tapes without the help of any form of noise reduction whatsoever.

In 1967, after hearing about Ray Dolby’s famed noise reduction system – the Dolby A type noise reduction system – which then became more or less the standard noise reduction system used in every professional recording studios, Henry Kloss tried to urge Ray Dolby to develop a much simplified version of Dolby A for domestic use. Dolby later on complied and developed a simplified consumer version of the Dolby A-type noise reduction system which we know today as Dolby B, which Kloss originally saw as a boon to home / domestic open reel tape users. Dolby B then became a runaway success in the domestic open-reel tape recording front that one of the first open-reel tape machines intended for home use that adopted the Dolby B type noise reduction system – the now famed Revox A77 open-reel tape deck – got a very favorable review on the August 1972 issue of Stereo review magazine that reviewer Julian Hirsch said “he cannot imagine how the sound quality of this machine could be improved in any way” when he tested the Revox with the Dolby B noise reduction engaged. By the way, Dolby B was also used on 8-Track tape systems back then. 

Somewhat later, Henry Kloss linked the Dolby B noise reduction system with a previously unsuccessful Du Pont product – the chromium-dioxide tape. And thanks to the magical consumer product midwifery that Kloss excels, these inventions helped make the Philips compact cassette tape – introduced primarily for office dictation purposes – become a viable musical storage medium that went on to surpass the vinyl LP in sales during the very end of the 1970s.

  For any type of Dolby noise reduction system to work properly, the level in playback must be matched to the level in recording. Without such “tracking”, treble frequencies can sound muffled or suppressed. Fortunately, the Dolby B type noise reduction system seems to function more or less adequately if there’s no gross mismatch in playback and recording levels. And what makes Dolby B very popular to consumers despite the next generation of Dolby noise reduction systems slated to replace it is that Dolby B also works adequately in cassette tape playback equipment that are not equipped with any Dolby noise reduction system whatsoever, thus gaining the approval of the masses who during the 1980s more than a half of them probably don’t own a cassette tape playback equipment equipped with any form of Dolby noise reduction system. 

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