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Magnetic Stripe Technology

 

Magnetic stripe

Magnetic stripe technology is everywhere. We use cards with magnetic stripes on them everyday without even thinking about it. The technology has been with us for many years, but there are still many new things going on in the industry.

The first use of magnetic stripes on cards was in the early 1960’s. London Transit Authority installed a magnetic stripe system in the London Underground (UK). By the late 1960’s BART (Bay Area Rapid Transit) (USA) had installed a paper based ticket the same size as the credit cards we use today. This system used a stored value on the magnetic stripe which was read and rewritten every time the card was used.

Credit cards were first issued in 1951, but it wasn’t until the establishment of standards in 1970 that the magnetic stripe became a factor in the use of the cards. Today financial cards all follow the ISO standards to ensure read reliability world wide and along with transit cards constitute the largest users of magnetic stripe cards.

With the advent of new technologies many people have predicted the demise of the magnetic stripe. However, with the investment in the current infrastructure this is not likely to be any time soon. Magnetic stripe technology provides the ideal solution to many aspects of our life. It is very inexpensive and readily adaptable to many functions. The standardization of high coercivity for the financial markets has provided the industry with a new lease on life. This coupled with the advent of the security techniques now available means that many applications can expect to be using magnetic stripe technology for the next ten to twenty years.

What is a magnetic stripe?

A magnetic stripe is the black or brown stripe that you see on your credit card, or maybe the back of your airline ticket or transit card. The stripe is made up of tiny magnetic particles in a resin. The particles are either applied directly to the card or made into a stripe on a plastic backing which is applied to the card.

The material used to make the particles defines the Coercivity (see below) of the stripe. Standard low coercivity stripes use iron oxide as the material to make the particles, high coercivity stripes are made from other materials like barium ferrite. These materials are mixed with a resin to form a uniform slurry which is then coated onto a substrate. In the case of a credit card or similar application the slurry is usually coated onto a wide plastic sheet and dried. The coating is very thin and the plastic allows the coating to be handled. It is then sliced into stripe widths and applied to the card during the card manufacturing process. The methods of application include lamination (where the stripe and backing is laminated into the card), hot-stamp (where a heated die is used to transfer the oxide stripe from the backing onto the card after the card is cut to size), and cold-peel (where the oxide stripe is peeled from the backing, and then laminated into the card). Each of the methods have their own advantages and are largely irrelevant to the user of the card.

Another method of putting a stripe on a card is direct coating. In this case, the oxide slurry is coated onto the card (usually paper or card rather than plastic) during the manufacturing process for the card. There can be some manufacturing cost reductions by using this technique, though there may also be some quality trade off.

Once the slurry is coated onto the substrate (plastic backing or direct to card stock) the particles in the slurry are aligned to give a good signal to noise ratio. This is the equivalent of eliminating those pops and bangs you hear on old tape recordings. The tape with the wet slurry is passed through a magnetic field to align all the particles. With the iron oxide particles this is relatively easy for two reasons. The particles are low coercivity so do not need a large magnetic field to orient them, and the particles are acicular (needle shaped) with an aspect ratio of approximately six to one. The acicular particles have an easy axis of magnetization along the length of the particle which makes the alignment an easy process. This process is not so easy with the high coercivity materials. The particles used in most of the high coercivity materials are not acicular, they are platelets. These platelets have an easy axis of magnetization through the plate, which means the alignment field has to stand the particles on edge and they have to stay that way to get the best performance from the stripe. Obviously the particles want to fall over as soon as the field is removed from the stripe so part of the skill in making a high quality stripe lies in designing a process that can keep those particles on their side until the slurry sets.

Unfortunately, the lack of alignment can cause some major problems in the read and encode process of the magnetic stripe. The waveshape of the read process can be distorted by the lack of alignment. This distortion can cause significant problems for some read systems.

In all of the above processes, the final card has the familiar brown or black stripe on it. The stripe can be encoded because the particles (like iron filings) can be magnetized in either a north or south pole direction. By changing the direction of the encoding along the length of the stripe this allows information to be written on the stripe. This information can be read back and then changed if required as easily as the first encoding.

How does the magnetic stripe work?

The end-user defines the requirements for the magnetic stripe including the signal amplitude expected, the coercivity of the stripe, the encoding method and the bit density. The card manufacturer uses the first two points to select the type of magnetic material to use. The system designer is concerned with all four of the parameters.

As explained above, the stripe is made from many small particles bound together in a resin. The density of the particles in the resin is one of the controlling factors for the signal amplitude. The more particles there are, the higher the signal amplitude. The density (or loading) combined with the thickness give a method for controlling the amplitude. Signal amplitude is important because it defines the design of the readers for the cards. Standards exist (ISO/IEC 7811) which define the signal amplitude for cards that are used in the interchange environment (such as banking). By conforming to these standards, a user ensures that the magnetic stripe can be read in any financial terminal world wide.

The bit density of the information is selected based on the user requirement. The ISO/IEC standards (7811) give requirements for bit density for cards used in the interchange environment. These standards define tracks one and three as 210 bits per inch and track two as 75 bits per inch. The bit density in conjunction with the data format (see below) dictate how much data is encoded on each track.

How is information encoded on the magnetic stripe?

Each character that is encoded on the stripe is made of a number of bits. The polarity of the magnetic particles in the stripe are changed to define each bit. Several schemes exist to determine whether each bit is a one or a zero, the most commonly used schemes are F2F (or Aiken BiPhase) and MFM (Modified Frequency Modulation).

The ISO/IEC 7811 standards specify F2F encoding. In this encoding, each bit has the same physical length on the stripe. The presence or absence of a polarity change in the middle of the bit dictates whether it is a one or a zero. The width of a single bit always remains the same but some bits have an extra polarity change in the middle and these are called ones.

MFM encoding is more complicated. This type of encoding allows twice as much data to be encoded with the same number of flux reversals (edges). For more details on MFM the reader is referred to the AIM Inc. publication "Modified Frequency Modulation (MFM) for Magnetic Stripes" available on the AIM Inc. World Wide Web site.

The choice of encoding scheme is determined by the application and the user. If the application is one where conformance with ISO/IEC 7811 is necessary then F2F encoding is the choice. For applications where large amounts of data must be encoded, MFM may be a more suitable choice.

Once the encodation scheme is chosen, the format of the data must be selected. ISO/IEC 7811 specifies two different schemes for use on interchange cards. These are four bits plus parity and six bits plus parity. The four bits allow only the encoding of numbers plus some control characters, the use of six bits allows the full alpha numeric set to be encoded. The parity bit is used to help determine if an error occurred in the reading of the data. The total number of "one" bits in a character is added up, in odd parity this must equal an odd number. If the total is odd, the parity bit is set to a zero, if the total is even the parity bit is set to a one.

Although the encodation schemes are defined in ISO/IEC 7811, it is only necessary to follow them if the application requires conformance with 7811. Some applications depart from this scheme by allowing different bit density/encoding scheme combinations, others depart significantly by using "proprietary" schemes down to the bit level. As an example, an identification card may use two bits to determine eye color (00 = blue, 01 = brown, 10 = green, 11 = other). This is much more efficient in encoding space, but means the data cannot be read in a standard interchange terminal. For some applications this is not important and the extra space available is very important.

 

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