COEN 180
Magnetic Recording
Magnetic Recording Physics
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Leaves patterns of
remanent
magnetization on a
track within the
surface of magnetic
media that sits on
top of a physical
substrate.
Magnetic Recording Physics
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Track formed by head passing over it.
We say that the head flies over the
track, i.e. we assume the view point of
the head.
Magnetic Recording Physics
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Three principal orientations of
magnetization with respect to a track:
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Longitudinal, Perpendicular, Lateral.
Magnetic Recording Physics
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Longitudinal recording:
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Transducer is ring-shaped electromagnet with a
gap at the surface facing the media.
If head is fed with current, the fringing field from
the gap magnetizes the magnetic media.
Media moves at constant velocity under the head.
Temporal changes in the current leave spatial
variations in the remanent magnetization along
the length of the track.
Magnetic Recording Physics
Magnetic WriteHead Schematics:
Functioning of Gap.
Magnetic Recording Physics
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Remanent magnetization pattern:
Magnetic Recording Physics
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Read head used to be the same as
write head.
Passing the gap head over the track
would let the magnetization pattern
cause an induced read current.
Magnetic Recording Physics
Writing and Reading with a Gap Head: From top to bottom: Write Current,
Magnetization Pattern, Read Current.
Magnetic Recording Physics
The read current is a (deformed) derivative of the write current. The
deformation results from the length of the gap.
Magnetic Recording Physics
The read current is a (deformed)
derivative of the write current.
The deformation results from the
length of the gap.
Magnetic Recording Physics
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Perpendicular Recording
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Uses a Probe Head.
Has the potential for better magnetization
retention.
MEMS
Magnetic Recording Physics
Probe Device:
Remanent Magnetization is
in the same direction as the
probe.
Magnetic Recording Physics
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Hard drives currently use exclusively
longitudinal magnetization.
Switch to perpendicular is expected in
the near future.
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Better retention  Higher Areal Densities.
Lateral never used.
Magnetic Recording Physics
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Magneto-Resistive Effect (MR)
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GMR
Standard read head.
Magnetic Recording Physics
MR-Effect: Magnetic field (red) moves electron flow in the
sense current (yellow) up by an angle of . The magnetoresistive material (blue) has different resistance based on
the angle .
Magnetic Recording Physics
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MR head directly reads the magnetic
flux.
Gap head reads the changes in
magnetic flux.
MR head can adjust the sense current.
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Better sensitivity.
Data Storage on Rigid Disks
Data Storage on Rigid Disks
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Single platter or stack of platters
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Magnetic recording heads mounted on arms record
data on all surfaces.
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Heads moved across the disk surface by a high speed
actuator.
Circular tracks.
Cylinder
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Thin magnetic coating
Rotate at high speeds.
Formed by the tracks on all surfaces by same actuator
position.
The tracks are broken up into sectors (or disk blocks).
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The old format of 512B per block still remains in effect.
Data Storage on Rigid Disks
Data Storage on Rigid Disks
Data Storage on Rigid Disks
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Hard drives rotate at constant angular
speed.
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Constant linear velocity impractical.
Heads see more track in the outer layers.
Nr. of sectors per track varies.
Remains constant in a “band”.
Data density increases in a band as we move
to the inside.
Data Storage on Rigid Disks
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The platter consists of a rigid aluminium
or glass platter, coated with various
coats.
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Rigid platter
Magnetizable thin film that actually stores
the data.
Overcoat
Lubricant
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Protects (somewhat) against head crashes
Data Storage on Rigid Disks
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Use surrounding air pressure to maintain the
proper distance between head and the surface
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The spacing controls the focus of the head; if the
head is further away from the surface, then it will
read from and write to a wider area.
To increase data densities, the head - surface spacing
has decreased dramatically.
The head can no longer be parked on the surface
during power down (when the rotation ceases, the
head will actually land).
Special landing area.
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Surface is treated to allow air to get between the head
and the surface.
When head flies again, move over the data tracks.
Data Storage on Rigid Disks
Data Storage on Rigid Disks
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Data Access:
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Seek
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Place head over right track.
Servo: Find the right track.
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Rotational Delay
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Used to be done with a special servo-surface on
one of the platters.
No servo data is embedded in the sector gaps.
On average half the time of a disk revolution.
AKA latency.
Transfer Time
Data Storage on Rigid Disks
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Performance Parameters:
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Capacity / Data Density
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Disks with smaller form factors have become
popular in niche applications.
Trend towards smaller disk, that can rotate faster.
Data density is a two-dimensional value:
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tpi: Tracks per inch: How far do tracks have to be
separated.
bpi: bits per inch: How many sectors on a single
track.
Data Storage on Rigid Disks
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Operations on adjacent
tracks can interfere
with each other:
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Track misregistration.
During read
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Too much noise.
During write
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Data written can be
unreadable.
Data on next track can
become unreadable.
Data Storage on Rigid Disks
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Data Density:
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Limited by the ability to distinguish distinct
magnetization patterns.
Pulse superimposition theory:
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Flux from nearby magnetization patterns
influences reads.
Data Storage on Rigid Disks
Read current picked up by a
magnetic gap head.
Red line: Read current in absence of the
other change.
Green line: Resulting read current.
Top: No interference.
Middle: Peak shifts to the outside.
Bottom: Peak shift much more
pronounced.
Data Storage on Rigid Disks
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Seek time:
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Determined by the speed of the actuator.
Determined by the capacity of the servo
mechanism.
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If the actuator moves very fast, then there is more
of a settling time.
Data Storage on Rigid Disks
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Latency:
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Solely determined by rotational speed.
Rotational speed limited by the aerodynamics
of the platter.
Larger platters cannot be rotated as fast as
smaller ones.
Data Storage on Rigid Disks
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Access Time:
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Random Access
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Seek
Latency
Transfer
Stream (block after block)
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Only first seek, only first latency.
Zero Latency Disk
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Starts reading whenever data needed appears under the
head.
Others wait for the first block of the stream.
Occasional track to neighboring track seeks.
Data Storage on Rigid Disks
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Errors
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Disks are not intended for error-free
operations.
Soft error
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Error cannot be repeated.
Hard error
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Cannot do the operation.
Data Storage on Rigid Disks
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Interference
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Cross-talk between different channels or
through feedthrough.
Track Misregistration.
Imperfect Overwrites / Incomplete Erasures.
Side fringing
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when the head picks up flux changes from an
adjacent track.
Bit loss due to Intersymbol Interference.
Data Storage on Rigid Disks
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Noise
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Media noise
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Defects or random media properties
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Electronic Noise
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Spot on the surface does not retain magnetization because of a
manufacturing problem or because of a previous head crash.
A modern disk drive has spare sectors on each track and complete
spare tracks to substitute for sectors that have these defects.
Even without an outright defect, the magnetic properties of the
medium vary.
caused by random fluctuations typically in the first stage
amplifier in the reproducing circuit.
Head Noise:
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The magnetic flux in both write and read heads is subject to
thermally induced fluctuations in time.
Data Storage on Rigid Disks
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Error rate is controlled through the use
of Error Control Codes.
In addition, each sector has a checksum
to prevent false data from being read.
Data Storage on Rigid Disks
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Reliability
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Device failure
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SMART (UCSD MRC) can predict 50% failures
based on higher rate of soft errors.
Block failure: bit rot
Data corruption: bit rot that is undetected.
Data Storage on Rigid Disks
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Power Use
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Major problems for laptops.
Major problems for very large disk-based
storage centers.
Various proposals of spinning up / down
strategies:
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MAID: Massive Arrays of Idle Disks.
System Interface:
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SCSI vs. IDE.
Magnetic Codes
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Magnetic codes bind the bit stream to
magnetization patterns.
Direction of write current determines
the direction of magnetization
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Easiest: NRZ code
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No Return to Zero Code.
Needs clocking.
Magnetic Codes
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NRZ Code: Vertical lines are clock ticks.
They define a window.
Write current in one direction is a zero, in other is a
one bit.
We detect magnetization changes (Peak detection).
We miss one, we reverse the rest of the string.
Magnetic Codes
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NRZI
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No Return on Zero Inverted
Switch magnetization pattern = 1
No switch during window = 0.
Has difficulties of counting with long
strings of zeroes.
Magnetic Codes
NRZ (top) and NRZI (below)
Magnetic Codes
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Phase encoding:
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Transition up for a one in window
Transition down for a zero in window
Two or more zeroes / ones in a row:
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Additional transition in the middle.
Self-clocking
Magnetic Codes
Top to bottom:
PE
FM
MFM
Magnetic Code
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Self-clocking:
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Transitions are never spaced out.
Easy to synchronize clock to transitions.
Magnetic Codes
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Problem with PM:
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Up to twice as many flux changes than
transitions.
Limits bit density because flux changes too
close together leads to noisy signal.
Magnetic Codes
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FM
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Frequency Modulation
Transition in the middle of the cell defines
a one bit
Absence means a zero bit.
Magnetic Codes
Top to bottom:
PE
FM
MFM
Magnetic Codes
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FM still has potentially up to twice as
many flux changes than bits.
Self clocking.
Magnetic Codes
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MFM
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Delay Modulation / Miller Code
Transition in the middle of the cell for a one.
No transition in the middle of the cell for a zero
bit.
Additional transition on the window boundary
between two zeroes.
Number of flux changes equals the number of
bits.
Magnetic Codes
Top to bottom:
PE
FM
MFM
Magnetic Codes
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Generate MFM by a state
diagram.
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Data bits determine transition.
Bits in state our output when
state is reached.
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First bit for the clock window.
Second bit for the transition /
lack of transition within the
window.
Magnetic Codes
Top to bottom:
PE
FM
MFM
Magnetic Codes
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Modulation Codes
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Transform data bit string into a magnetic code.
Written on magnetic medium as an NRZI waveform.
3 Parameters:
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d = minimum of zeroes between consecutive ones.
k = maximum of zeroes between consecutive ones.
Data density: ratio of x data bits over y magnetic code bits.
Important for capacity:
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Large values of d are important for data density:
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Flux transitions are spaced out.
Lower values of k indicate ease of synchronizing clocks.
Magnetic Codes
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½(2,7) code
Data
Code Word
10
0100
11
1000
000
000100
010
100100
011
001000
0010
00100100
0011
00001000
Magnetic Codes
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PRML channel
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Uses maximum likelihood decoding (ML)
Partial response:
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Readback pulses from adjacent transitions are allowed to
interfere with each other.
ML decoding unravels the results of interference.
Write Precompensation
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Predistorting the write data before they are sent
to write driver
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transitions are correctly placed when read.
Disk Defects
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Channel impairments
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Intersymbol interference
Off-track interference
Amplifier noise
Disk defects
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Random noise associated with the random
nature of the disk surface without defects.
Media defect.
Error Correcting Code
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Disks use error detection and error
correction
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Reed Solomon code example:
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38 bytes added to 512 data field
Probability of uncorrectable error moves from
10-7 per bit to 8.8*10-16.