Re. hard drives storing information via two levels of voltage variation on a hard drive - wouldn't it be more efficient to store it in 3/4/n "steps" instead of just binary? I know that audio tapes work by an "infinite" (limited by SNR, i suppose) level of voltage variations, and audio is stored digitally by sampling the voltage of the source tapes at 44.1 khz, but with a precise enough read head and low SNR i guess you could store a really large amount of data but maybe i'm just overthinking it

i simplified how the data is stored. its more complicated than applying a high voltage to a cell to get a ‘1′ and a low one to get a ‘0′

the data is fundamentally stored stored as an induced magnetization of a cell. a cell is just a small discrete length/area of space on the magnetic surface of the platter. the disk head magnetizes these cells such that each cell is always either emitting a field to the left or a field to the right. it is not the case that a ‘left’ is interpreted as ‘0′ and right interpreted as a ‘1′:

image

a ‘1′ is encoded & interpeted according to the following sequence of events:

the read/write head begins to pass over cell ‘A’. it is right at the begining of the cell, not in the middle, not at the end. it samples the magnetic field at this point a sees that it is non-existent meaning the cell directly behind it is pointing the opposite direction than the one it is about to pass over

it passes over cell ‘A’ to the very end, the boundary between ‘A’ and ‘B’. it samples again, and feels no magnetic field. two sequential instances of “no magnetic field felt” indicates a ‘1′. the converse is how a ‘0′ is stored: a read of a “strong magnetic field” and then “no magnetic field” indicates a ‘0′

all of this logic is done by a controller inside the disk controller such that all data going from your computer -> disk or from disk -> your computer is digital

in the case of an audio tape, this controller doesn’t exist. you have an analog signal going in and out instead of a digital one. since with an audio player you have no real “computer” on board, the analog signal is just amplified and fed to your speakers. there is never any digitization occurring