Modified Frequency Modulation (MFM) -- Encoding for Magnetic Stripes

Contents
Review of F2F Encoding Technique
MFM Code For Doubling Data Density
Five MFM Encoding Rules
Definitions Applicable to MFM Encoding

This description for Modified Frequency Modulation recording of magnetic stripes was compiled by the Magnetic Stripe Committee of AIM, Inc.  Its purpose is to promote a standard methodology for MFM recording on magnetic stripe media by suppliers and users of magnetic stripe technology.

Purpose
The purpose of this paper is to describe a code that will double the data density on magnetic stripe cards without changing the shortest distance between flux transitions. Magnetic stripe composition and magnetic heads will not require change unless more than double density upgrade is desired.

Review Of F2F Encoding Technique

The technique known as two frequency recording allows for serial recording of self clocking data on each track. The encoding comprises data and clocking transitions together. A flux transition occurring between clocks signifies that the bit is a "one" the absence of a flux transition between clocking bits signifies that the bit is a "zero". Reference examples of F2F code are shown in figure 1.

MFM Code For Doubling Data Density

Modified frequency modulation (MFM) which is also referred to as MNRZb1, Miller Code and Delay Modulation (DM) is a code with the capability of doubling data density without increasing the recorded magnetic transition density. The code is governed by five basic encoding rules when it is applied to magnetic stripe cards. Reference examples of MFM code are found in figure 2.

Five MFM Encoding Rules

1) Flux transitions are never located at the mid point of a zero bit.

2) Flux transitions are always located at the mid-point of a one bit.

3) Flux transitions are never found at the starting point of a one bit or at the end point of a one bit.

4) The lead-in up to the first data bit shall be recorded with clocking bits (zeroes) and the space after the last data bit shall also be recorded with clocking bits (zeroes). Zeroes prior to 3,30 mm (0.130 in) or after 82,17 mm (3.235 in) from the right edge of the card when viewed from the back do not have to meet the specifications given herein.

5) The run length limit is two bit cells so that a flux reversal is always located between two adjacent zero bits.

The rules stated above limit the number of possible inter- reversal lengths to 1 bit cell, 1 1/2 bit cells and 2 bits cells. The length of a bit cell is determined by the average inter-reversal length of the starting zero bits. The measured length between transitions equals 1 bit cell for strings of zeros or ones. Inter- reversal length of 1 1/2 bit cells starts from the adjacent transition of three possible bit sequences which are {001}, {100} and {010}. Only the {101} bit sequence is possible whenever the inter-reversal length is two bit cells long.

The code can be made self clocking with an algorithm that continually processes the times in stride with the three predictable inter-reversal lengths.

Definitions Applicable to MFM Encoding

Inter-Reversal Length (IRL)  - IRL is the distance between flux transitions.

Run Length Limited (RLL) - MFM is a run length limited code with a maximum distance between flux transitions of two bit cells.

Bit Cell (BC)  - The averaged length between clocking zero bit transitions establishes the size of a bit cell. Every bit cell must contain a single bit of data.

This document was developed by AIM, Inc., the world wide trade association for manufacturers and providers of automatic identification products, services, and supplies.

AIM, its member companies, or individual officers assume no liability for the use of this document.

Published by:
AIM, Inc.
634 Alpha Drive
Pittsburgh, PA 15238-2802, USA
Phone: +1 412 963 8588
Fax: +1 412 963 8753
Email: aidc@aimglobal.org
Web:  www.aimglobal.org
Copyright © 1994 AIM, Inc.

All rights reserved. This document may be reproduced in any form without prior written permission provided the following conditions are met:
1. The document must be reproduced in its entirety including all references to AIM, Inc.
2. The document is not sold or remunerations received for the document.
3. The document is not altered or changed without prior written permission of AIM, Inc.

Published 11/94

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