Signal of HDD platter to controller

Question

HDD heads read the surface of HDD, giving a signal to the controller:

For a single signal, is it about one bit, one sector, or something else?
1. If it is one bit, how many kinds of signals are there? My naive guess is four:
  Fully magnetized, Not magnetized, Partially magnetized (rare), Unreadable(rare)
2. If it is not one bit, what are the possible causes of the signal?

Starting research: https://en.wikipedia.org/wiki/Modified_frequency_modulation — u1686_grawity, Jun 05 '20 at 11:51
@jluizsouzadev, I am just studying about the principle of HDD. — imida k, Jun 05 '20 at 12:29
See https://superuser.com/questions/427554/is-it-possible-to-detect-the-previous-byte-position-on-a-hard-drive-after-it-has/427611#427611 — sawdust, Jun 05 '20 at 22:26

score 2 · Answer 1 · edited Jun 05 '20 at 12:40

2

It is a magnetic media, so there are flux changes that are read by the head:

The signal is an analog wave that must be amplified, cleaned up, then converted to 0 or 1
The data stream is encoded (e.g. RLL\MFM, etc.) and contains ECC correction bits at the end of each sector (4k sectors are used on modern hard disks), which allows the decoder to detect and correct errors

edited Jun 05 '20 at 12:40

JW0914

answered Jun 05 '20 at 11:58

SSi

Thank you. If it is always one of 0 or 1, then which signal is regarded as a bad bit, or bad sector? – imida k Jun 05 '20 at 12:16
The magnetic flux that is read is not interpreted directly as `0` or a `1` bits. Rather the starting flux state (while reading in the gap) is considered a `0` bit, and then each flux reversal indicates a bit inversion. A bit clock synchronizes the sampling. The string of bits are shifted in a register, and then stored as bytes in controller memory. When the entire sector has been read, the ECC is used to verify (and correct) the data. – sawdust Jun 05 '20 at 22:21

score -1 · Answer 2 · answered Jun 05 '20 at 11:47

-1

Data stored on a hard disk is in a binary state, either being demagnetized or magnetized. That equates to a single bit, being a 0 or a 1.

answered Jun 05 '20 at 11:47

Keltari

2 Answers2