Dolby B-type noise reduction, providing about 10 dB of noise reduction at high frequencies, was a simplification of A-type that was introduced in 1968. It worked by boosting high frequencies on recording them and then reducing them (and recorded noise) upon playback. It extended the use of Dolby technology into the consumer environment, giving consumer electronic companies the ability to make cassette tapes and players, which gave the consumer quiet recordings. Dolby makes professional B-type encoding equipment that is sold to tape duplicators, allowing them to make consumer B-type encoded cassettes. In addition, the technology is licensed to consumer hardware manufacturers, giving them the ability to purchase Dolby B-type integrated circuits (ICs) from various semiconductor manufacturers and then build cassette decks, portable players, car units, etc., which can record and/or playback cassettes with B-type encoding.
Dolby B has become a maligned form of noise reduction in the past 20 years because so many users have reported a loss of high frequency content on playback with it. While there are more sophisticated forms of noise reduction available today, Dolby B does have a bit of a bad rap. Most users who had problems with it did so because they recorded to close to the saturation level of their tape. The tape then had no available headroom in the high frequency range where the boost occurred and thus, upon playback the highs, which were cut as if they had been boosted, ended up being rolled off too much and the resulting tapes sounded dull. A common work around for this problem was to record tapes with Dolby B and play them back without it. Of course this defeated the noise reduction aspect of it, but many users liked the high-end boost as well as the smooth high frequency tape saturation on their tapes. When used properly Dolby B actually works quite well.
