Modified Frequency Modulation (MFM)
Encoding for Magnetic Stripes
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: [email protected]
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
Reprinted with permission from AIM, Inc.
www.aimglobal.org
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