Dolby B
Dolby B was developed after Dolby A and presented in 1968, as a single sliding band system providing about 9 dB noise reduction (A-weighted), primarily for cassettes. It was much simpler than Dolby A and therefore much less expensive to implement in consumer products. Dolby B recordings are acceptable when played back on equipment that does not possess a Dolby B decoder, such as most inexpensive cassette players. However, Dolby B provides less effective noise reduction than Dolby A, generally by a factor of more than 3 dB.
From the mid 1970s, Dolby B became standard on commercially prerecorded music cassettes in spite of the fact that some low-end equipment lacked decoding circuitry, although it allows for acceptable playback on such equipment. Most pre-recorded cassettes use this variant.
Read more about this topic: Dolby Noise-reduction System