Introduction

The encoding format used by the ISO/IEC-7811 standard is known as “F2F” or “Aiken Biphase” encoding. The F2F encoding format allows the serial data to be self-clocking. Bits are encoded serially on the magnetic stripe using a series of magnetic flux transitions. Each bit of data on a track has a fixed physical length on the magnetic stripe. Flux transitions are located at the edge of each bit, and also in the center of each “1” bit. As the stripe passes the magnetic read head, the flux transitions on the stripe are converted into a series of alternating positive and negative pulses, as shown in Figure 1. After determining which flux transitions represent the edges of a bit, ones and zeros can be differentiated by the presence or absence of a pulse in the center of the bit.