magnetic strip

Card Technology

Q. How does the magnetic stripe work? A. 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.

Q. How is information encoded on the magnetic stripe? A. 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 AIM publication "Modified Frequency Modulation (MFM) for Magnetic Stripes". 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.