Magnetic Stripe Performance Parameters

by Dr. Brad Paulson, Thor Engineering, ICMA Standards Representative

Magnetic stripes were introduced on credit cards in the 1960s. The technology of the time was low coercivity media, typically constructed from gamma ferric (iron) oxide particles, giving rise to the ISO standard, 7811-2, which was developed using a 300 +/- 30 oersted reference material. To remedy accidental erasure problems, the ISO high coercivity magnetic stripe was introduced, typically composed of barium ferrite or strontium ferrite; magnetic media of 2750 +/- 100 oersted was used as the reference material during development of the ISO standard 7811-6, which was published in 1995.

The fundamental characteristic of the magnetic stripe, and the foundation for magnetic stripe encoding, is the ability of the particles in the card stripe to become magnetized when subjected to a magnetic field. A coil of wire, wrapped around an iron rod, will produce a magnetic field when a DC (Direct Current) current flows through the wire, creating an electromagnet. A magnetic recording head is an electromagnet having a magnetizable core shaped much like a “horse shoe” magnet with the poles close together, shown in first figure. When electric current flows, lines of magnetic flux form at the gap between the North and South Poles. Consequently, encoding heads are designed so that the magnetic stripe runs against this narrow gap. When the power source changes the direction of the electric current, the magnetic field lines also change directions. This change of magnetic field direction produces a flux transition on the magnetic tape, much like laying small bar magnets end to end along the magnetic stripe, which is illustrated in the second figure.

Fundamentally, the magnetic read head is constructed in the same way as an encode head – a coil of wire around a “horse shoe” shaped magnetizable core. However, instead of a current being sent through the coil by a power source, a voltage is created in the coil as the gap of the read head passes over the flux transitions in the encoded magnetic stripe, generating an electric current that is the measured read back signal, as illustrated at the bottom of the second figure. This read back signal forms the basis for the measurement and decoding of the encoded data.

Now that we have the basics, by changing the polarity of current applied to applied to the encode head as it passes over a length of magnetic tape at a rate of 200 times per inch, 200 flux transitions per inch (200ftpi) are encoded on the magnetic media. When a magnetic tape is recorded at 200ftpi with increasing current, as shown in the third figure, the resulting signal amplitude peaks can be read and the peak values plotted as a function of the encode current level. This plot, shown in the fourth figure, is known as the magnetic saturation characteristic of the tape. Notice that the signal amplitude increases linearly as the encoding current increases until a maximum value is reached, after which it slowly decreases with ever increasing encode current. The maximum value of signal amplitude on this plot is called the saturation point of the magnetic tape, denoted Uref, and the current when the curve is at 80% of peak amplitude is called Iref. Once Iref is found, Imin and Imax are determined by definitions in parts 2 and 6 of ISO/IEC 7811. These represent the boundaries of the ISO “box” of the magnetic stripe requirements, shown as the shaded area in the fourth figure.

As a point of interest, audio recordings take place in the region before saturation of the magnetic tape. That is, audio recorders limit the amount of recording current so that when loud passages are recorded, the playback signal amplitude is the greatest, while quiet passages generate small currents and yield small levels of signal amplitude. As shown in the location of the ISO box, digital recording utilizes the region after the saturation point where deviations in encoding current have much smaller impact on the read signal amplitude.

The only requirement for ISO compliance identifiable directly from the saturation curve is that the curve must fall continuously between the boundaries provided by Imin and Imax. However, additional parameters specified in parts 2 and 6 of ISO/IEC 7811, which are obtained by the magnetization and demagnetization of the magnetic stripe while generating the saturation curve, are intended to ensure

• The encoded magnetic flux is within specified tolerances, indicating the stripe can be written satisfactorily with normal encoding heads.
• The voltage of the read-back pulses is sufficient for satisfactory detection, but not so high as to risk overloading the electronics of the read system.
• Pulse amplitudes are sufficiently uniform, to reduce the risk of detection errors.
• Pulses can be placed sufficiently close together without interfering with each other.
• Once encoded, the media can be erased and re-encoded without retaining a “memory” of the first magnetization.
• The magnetization is not destroyed or distorted too easily by stray magnetic fields.
• Read pulses are reasonably distortion free, without secondary peaks, to reduce detection errors.

In summary, encoding and reading magnetic stripes involves many technical parameters that can seem complicated and confusing. However, the material characteristics specified in the magnetic stripe standards, parts 2 and 6 of ISO/IEC 7811, were developed as minimum requirements to ensure compatibility of the magnetic stripes with the commercial encoding and reading equipment.

Figures used with permission of Barnes International Ltd., 29-30 Highcroft Industrial Estate, Enterprise Road, Horndean, Waterlooville, Hants PO8 0BT UK and Eclipse Laboratories, Inc., 7732 West 78th Street, Bloomington, Minnesota 55439 USA

Brad Paulson, Ph.D., is the ICMA Official Standards Representative and concurrently serves as principal and founder of Thor Engineering, a consulting company he started in 2001 to test and evaluate media and materials to determine failure modes and recommend material and process improvements. Views expressed here are his own. Questions? Contact Brad at Tpaulson@rconnect.com.