Dolby was for TAPES, particularly Cassette Tapes, NOT LPs.
And, prior to improvements in tape formulations and tape mechanisms.
All Tapes make Hiss, and Dolby B was developed for the noisiest: small 1/8" wide Cassette Tapes, running at very slow 1-7/8" IPS.
The narrower the track (less magnetic material) and/or the slower the speed (also less magnetic material), the higher amount of noise i.e. Signal to Noise Ratio!!!
Master Example: 1" wide 30 IPS. Some used 35mm 1-3/8" wide.
Cassettes were originally made for dictation, they ran 1 mono track one direction at 1-7/8 IPS. Then manually reverse, 2 mono tracks at 1-7/8 IPS.
Next: 2 forward tracks: Stereo; and then 4 even narrower tracks on 1/8" wide tape two forward and two reverse, still only 1-7/8 IPS.
Next, auto reverse, and either a 2 track head that spun around, or separate forward and separate reverse heads.
A piece of crap format to begin with, made acceptable, and even better than that by all subsequent advances, and the ability to make your own tapes was a genuine treat. Dual decks, dubbing, OMG the freedom was intoxicating.
similarly 8 track cartridges, before cassettes, the ultimate piece of crap, were initially made for use in radio stations for advertisements (not much tape (i.e. less weight thus less force/abuse involved), And a moving head with serious alignment problems. But, their PORTABILITY was a revolution!
8 tracks were meant to be thrown away when that ad campaign was over). essentially same width track as cassettes (8 tracks on 1/4" wide tape), and twice the speed of cassettes, 3-3/4 IPs. Never advanced like Cassettes did.
The beloved consumer loved portability and like mp3, accepted reduced quality for quantity, reduced cost.
improvements along the way: better tape formulations, different bias types, more precise mechanisms, 6 heads AND Dolby or DBX noise reduction combined to yield ’better and better’ results from Cassettes
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So, regarding Dolby, as it relates to LPs, is not so easy to decipher.
Let’s not forget, early auto systems weren’t great either.