Tape Articles- Storage Media
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This is the third installment of a series on magnetic devices
and media used in computer systems. In this article we will look at
the storage media, primarily tape and disk, and how signals are
represented on that media. To better understand this I will compare
the differences in audio tape, as most people are most familiar
with that, and digital tape.
In the first part I mentioned the bar magnet - a piece of iron
with one end having a magnetic north pole and the other end being
the south pole.
The media for recording is made of very small pieces of
magnetizable particles, and for the most part these pieces are just
like small versions of the bar magnet. Somewhat rounded - sort of
cigar shaped. The term for describing these is 'acicular' - which
means 'needle shaped.
A transducer is the broad name for a device which transforms
energy from one form to another. A microphone transforms
[transduces] a sound wave [acoustic energy] into mechanical energy
[the movement of the diaphragm in a microphone] and thence into
electric energy. We will discuss magnetic heads later, but first we
are going to look at the media, the tape or disc which stores the
magnetic energy, and how it is designed and made to enable us to be
able to store data.
We need this form because we have to have something which is can
be magnetized and have a size and shape which is readily
magnetizable, and one whose state can readily be determined. We use
these particles to induce a magnetic field into a transducer -
which in this case is a magnetic head.
Magnetic tape first made it appearance in Germany in the
mid-1930s as developed by BASF. In the US much work was done at
Armour Research - later IIT - Illinois Institute of Technology -
and one of the most significant people there was Marvin Camras -
who helped develop the particles and held over 500 patents in the
[For anyone who wants to learn almost anything you could ever
imagine about magnetic recording, in all formats, audio, video, and
data, I can unhesitating recommend the "Magnetic Recording
Handbook" by Marvin Camras. Published in 1988 by Van Norstrand
Reinhold - out of print - technically oriented - but should be
available through some larger libraries.]
To describe it crudely, magnetic tape is nothing more than rust
on tape. Magnetic tape consists of a base - the carrier for the
'rust'. Today we see different forms of plastic material for the
base. However the base could be any flexible material capable of
being magnetized. Some of the early experiments were used long
In the US the first tape seen by the general public was a paper
base with a magnetite coating. Then came plastic and the ferric
oxides [rust] became the standard. Over the years tape technology
has evolved to a very high level - but at the heart it is really
just a long band of flexible magnetizable material.
The characteristics we rely on in the magnetic media are
coercivity and retentivity. The first - coercivity - is how much
energy we have to use to force a change in the magnetic state. [You
might remember that from the word 'coerce' - meaning 'to force. It
would be the magnetic equivalent of The Godfather making it an
offer it could not refuse"]
A low coercivity material is known as being soft magnetically,
while a high coercivity is considered to be hard. A low coercivity
material can have it state changed by quite low magnetic fields.
Early floppy disks could even be erased if you set a telephone on
top of the disk and the phone rang. The magnetic field generated by
the telephone bell caused this. [For the younger readers old phones
had real bells driven by electromagnets to cause the clapper to
vibrate. A disk with low coercivity could be erased by the field
generated by the bell].
The other property is retentivity - how much magnetism remains
after the magnetizing force is removed.
Most people are familiar with the tapes in their audio
cassettes. While data tape and audio tapes are both forms of
magnetic material their design is quite different. I'm going to try
to explain the difference because that should make the entire
process easier to understand.
Remember that the shape of the particle is needle-like or bar
shaped. Also recall that to generate a current in a wire we can
move a magnet past a wire [or in tape playback past a head] to
generate a current. It doesn't matter whether which pole goes first
[north or south] but we do have to have it change in order to
generate a current. [Remember that a constant north or south pole
would generate a short burst when it first started and then would
fall to nothing].
In our magnetic 'paint' these particle are aligned in random
directions. That means that some of the particle are going to pass
under the head parallel to the head gap and since both the north
and south poles pass under the head as the same time no current
will be generated by the parallel particles.
The random order doesn't matter that much in data as we are
recording the the domains to their maximum [saturation], and by
magnetizing enough particles we will be able to generate a current.
However orientation matters greatly in audio tape. the major reason
is the signal we are recording and reproducing.
Analog recording places a magnetic representation of the audio
wave form on the tape by varying the current and direction of the
current going to the recording head. The highest output will come
from particles that pass longitudinally under the head. We also
will be magnetizing these at various levels so that when we pass
the tape past the head an image of the original signal will be
generated. [This is where the name analog recording comes
Particles oriented at any other direction than perpendicular to
the tape head will not contribute as much energy. What this means
is that the output will be lower than it would if all the particles
were oriented along the path of the tape.
The effect of this is a poorer signal to noise ratio with
resulting hiss which you are no doubt familiar with in cassette
recordings. In audio we also have to apply the voltage between
certain levels - a minimum and a maximum - in order for an increase
in the recording level will be at the same relative level on
In analog tape, while the coating [slurry] is still wet, the
tape is passed through a magnetic field so that most of the
particles will be oriented in a given direction. This is why just
because a tape is the same size/width you can't always use it in
any devices. Old video tape was 2" wide and used rotating heads
that moved across the tape - so the tape was 'aligned' vertically.
Using this in an audio 2" machine would give poor performance
because most particles were oriented almost 90 degrees from
In digital we don't worry about this. If you think about this in
relation ship to floppy disks - since these are 'pancakes' cut from
large sheets of tape - random orientation is the only method which
If you've ever had to push a car to move it, you know it takes
more force to get it moving, but once moving you can get going
faster and faster up to a point. It takes more force to get it
moving, and once you get to a certain speed no matter how much
harder you try you aren't going to get it going any faster.
Magnetic media works in much the same way - a little current and
nothing happens, and as the current increases the amount of
magnetization increases until it reaches a point where no matter
how much you force it, no further changes will occur.
There are points just after the lowest point and before the
maximum where the changes occur in a straight line. We call this
linear response. If we try to record at too high a level the signal
becomes compressed as we have reached the maximum output. We have
most often distorted the signal by overdriving the amplifiers also.
To low a recording level and we just get the noise from randomly
What we are trying to do in audio is to make the tape itself as
transparent as possible so we hear only the recording and not the
tape. Recording at too high a level increases the distortion and
good audio should always be under 1% and maximum should be 3%.
Signal to noise ratio should be at least 60db.
However in data we don't really care about distortion, and we
want to record as 'loud' as possible, but within limits. We call
this saturation recording.
In digital we can accurately reproduce the original signal with
distortion well past 30% and signal to noise ratio in the order of
20db. You would NOT want to listen to an audio reproduction at that
rate. All we have to do is determine if there is a pulse there or
not. We may send the pulse to the head in a nice square fashion but
we don't care what it looks like when we observe it on a display
tube because all we want to know is if there is a pulse there or
not. It it looks like there is a pulse we can rebuild one that is
perfectly square if we wish to.
The ruggedness of data signals - whether there is a pulse or not
- is why morse code was used in early message transmission. Even if
the noise and static on a radio receiver was so bad you couldn't
understand the voice, you could hear the tones that made up the
dots and dashes. Other methods turned the transmitter on and off
which was also readable where other means were not.
Now that you understand how the signals are stored lets look a
bit more at the coercivity/force in regards to this.
If the coercivity is lower then it is easy to change the state.
It's easy to magnetize, easy to erase. But there are other problems
that low coercivity brings about.
If you put too much signal into the recording head you can erase
part of the signal you just recorded so you have to space the
pulses far enough apart so you don't effect 'self-erasure'. The
lower the coercivity the easier it is to self erase. This limits
the amount of signal we can store.
If you try to put the data pulses too close together the second
pulse while being recorded can help erase the first pulse because
while the magnetic field is concentrated at the gap it exists
outside this are.
Another property of magnets - that I neglected to mention in the
first part - is their interaction. A north pole of a magnet will
attract a south pole of another. Opposites attract, and likes
repel. One north pole will try to repel another north pole.
In the original disk formats where the data stream was kept
constant - the bits on the inner tracks were closer together than
the outer tracks. Bit-shifting would occur on the inner tracks, but
the data patterns which cause this were predictable so that when
you reached a certain diameter of the disk you would change the
writing pattern to compensate. For those who have been around disks
a long time method - called write precompensation - was a standard
parameter you would enter when formatting a disk. Thankfully we
don't have to worry about that in today's world.
This gives rise to a phenomenon known as 'bit-shifting'. If two
like fields are placed close together they will tend to move away
from each other. In a medium where the bit-density is constant - as
in tape this defines how much data can be placed in a given area.
In low-coercivity medium the bits much be further apart and less
data can be saved.
Therefore the amount of data we can put on tape or disk is a
function of the coercivity and the size of the particle. Think of
the difference between a wall built with 8" cement blocks and one
built with bricks - with the blocks representing a single magnetic
If the particles [bricks] are made smaller they can be closer
together. We have to use a higher coercivity to as not to erase the
adjacent particle and we also have to have a higher retentivity -
more remaining magnetism - because the particles are now smaller
and have to do more work.
We can help this along by maximizing the amount of magnetic
material by a process called calendaring. This is accomplished by
forcing the tape through very high pressure rollers so that if we
compare it to paint we have more pigment and less binder. This also
give the tape a highly polished surface.
This is needed to ensure high data density also. If the tape
were not smooth the peaks in the oxide could be high enough so that
the valley in the oxide would be further away from the head-gap
than the width of the headcap - and the field changes would be
effectively invisible to the reproducing head.
We continue this process of making particles smaller and smaller
until we reach a limits. One limit is because the particles are
ferric oxide. That is they are part metal and part oxygen. If we
could get rid of the oxygen and have only the metal remaining we
would be better off.
This was the step to metal particle tape. This is tape where the
domains are made of particles of pure-metal instead of an oxide.
Now we can make data pulses smaller as we can get more output from
pure-metal particles than we could with particles that are not
As we try to get more and more data onto the medium we see that
now that we have metal particles the only way we can get more
material to magnetize is to put the particles as close together as
possible, and a way to do this is to get rid of the binder - the
part of the tape paint that holds things together just as the
carrier in paint holds the pigments.
That brings us to the media which we use today - that is called
metal tape. It really is just metal particles clinging to the tape
in the same way metal is plated on plastic toys to make them look
like metal. Still small domains but with no binder gluing them
together so that essentially everything can be magnetized
Now the only limits are how small we can make the magnetic
domains. These particles become rather small. So small in fact that
even though they are metal they can break. Different manufacturers
approached solving this problem in different ways. Sony coated
these small particles in ceramic which gave them strength and also
protected against oxidation/corrosion. Oxidation [rust] was an
early problem with metal tapes - and if it rusted we'd be back to
the original tapes.
That wraps up this overview of the magnetic material, the next
articles will deal with how we put all this together to store and
An editorial observation at this point:
During the process of developing metal and metal particle tape -
manufacturers such as Sony and Fuji were having problems making
this technology work as it should. But the did not abandon their
work. While this was going on many American based tape
manufacturers were pushing the current tape technology to the
limits but changing their formulations of the oxide - a method
often called doping - by adding various other elements to the
As with any technology there are usually finite limits and the
manufacturers were more comfortable pushing the current technology
than embracing a new technology - eg exchanging oxide technology
for metal technology.
Once the metal medium was perfected the traditional
manufacturers were left far behind. Manufacturers such as 3M - who
revolutionized the world with the standard Scotch 111 audio tape in
the early 1950 from the orignal Armour research have now all exited
that field. Ampex also exited the field as did many others who
stayed with 'traditional' methods.
The old saw 'innovate or die' seems to apply here.
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