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.
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.
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.
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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