All About Degaussers and Erasure of Magnetic Media

Foreword By David Partridge,

The purpose of this document is to provide a basic understanding of magnetic media degaussers, or erasers, what they do for you, why it needs doing and how it does it It will also explain how you should approach specifying the type of degausser, its operation and the procurement considerations.

This information will be of value to a wide range of operators which will include end users, professional media manufacturers and software duplicators Such operators can be from the video, audio, computer and broadcasting industries all of whom can benefit from the application of media degaussers to their respective operations, such benefits include significant improvements in the quality of their output together with sizeable savings in operating costs

Introduction

Degaussing takes its meaning from Johann Gauss (1777-1855) a mathematician who studied and worked on electro-magnetic fields It is used today to describe the process of erasing magnetic media, removing from that media remnants of previously recorded signals This process provides a number of advantages and this booklet will explain these This publication is not intended to be fully comprehensive but provides an overview into some of the issues that are important, fuller information is of course available and can be provided by contacting Eyecote International Limited.

To the uninitiated the default state is to reduce the risk of using questionable media by discarding it and by using new material This is wasteful and unnecessary

About Magnetic Media

The term magnetic media covers a vast range of material from audio and video tapes and cassettes to computer diskettes and reels, supplied in a wide choice of sizes and complexities All of them however, perform in the same basic way Today there is the widest variety of formats for magnetic media, many new types are appearing, some are defined as being high density, these newer formats have significantly differing characteristics in performance and in operation and will require different erasure processes

The media consists of material that is coated with minute metallic particles that react to magnetic influences applied to them, such influences fall into two types The first is the intended influence or signal that provides the desired "play-back" from the original recording. Ideally the replay should replicate the original recording, whether this is from an audio, video or computer data source. The second influence is the unwanted signal, which can take the form of distortion, bias, corruption or interference and can produce undesirable effects such as poor quality in audio or video media or software glitches in data material Clearly any process which enhances the first and eliminates the second is desirable, degaussing magnetic media can achieve this.

About Degaussers

The process of degaussing is achieved by passing the magnetic media through a powerful magnet field to rearrange the metallic particles, completely removing any resemblance of the previously recorded signal. Although this process is of course simple in theory, in practice the vast variation of media formats and magnetic densities makes the correct degaussing process quite different to achieve. The degausser is constructed in such a way as to enable the generated magnetic field to be available to the media when it is transported through it, either by physically holding the media by hand and moving it through the field or automatically conveyed by a belt transporter.

Different media demands varying magnetic field strengths therefore the coils that generate the magnetic field will also vary depending on this requirement Generally speaking a coil in a degausser should have three to four times the energy rating of the material being degaussed, this rating is measured in Oersteds after Hans Christian Oersted (1777-1851) who discovered the magnetic qualities of electricity. Further degaussing efficiency can be achieved by using more than one coil in multi-axial orientation, this produces a more effective degaussing field. Even better performance can be achieved by rotating the coils during the degaussing process.

Operation of Degaussers

The operation of degaussers will of course vary depending on the type of media, quantity, speed and degree of erasure required When erasing tapes, either the cassette must move through the magnetic field or you must move a magnetic field over the cassettes In either case, the consistency of the motion, the strength of the field and the distribution of that field over the entire media is what determines the quality of the erasure Most professionals and engineers agree that a conveyor transport degausser, which allows the user to place the magnetic media on a small belt, which in turn, passes the media through or over the degaussing coils at a constant speed, assures the most uniform degaussing Custom designed degaussers can assist with the specialist needs of bulk operators with belt feed conveyors and collection hoppers available to deal with tapes, disks and reels, all helping in making the operation effective and efficient.

Effective degaussing is very much a performance factor of the machine and generally speaking passing the media through the field twice does not improve the effectiveness of the degaussing, however if the media is rotated by 9O� then some improvement can be achieved.
With the ever increasing demand for more and more data on smaller media there is now a requirement that can be more important than absolute erasure That is consistency of erasure It is widely understood that data at a high packing density stored on magnetic media is easier to erase than low density data The data signal to noise ratios, bit-t~bit phase relationships, amplitude variations, all become more critical and require more sophisticated electronics to ensure valid error free data recovery Variations of magnetic flux, remaining from a poor erasure cycle will make data recovery more difficult resulting in increased errors.

Savings

Considerable savings can be achieved by the controlled application of degaussers to the processes involved in production and operation of magnet media For a start many operators, who do not currently degauss, simply throw away suspect media that with a careful process of degaussing applied to it would give a considerably extended life Some operators claim to benefit by up to twice the useful life of some types of media. But further direct and indirect savings can be achieved Quality of performance of the media has to have a value, although it would be difficult to quantify, however, this often is the only differentiation between competitive products! Indirectly, there are additional, quantifiable savings to producers and to end users due to a considerable reduction of "down time" of computers and data processing apparatus due to faulty or imperfect magnetic media being used Diskette duplicators claim to gain upwards of 25% better production yields directly attributable to degaussing their bulk blank diskettes, prior to processing.

Procurement

It can be seen that there are a considerable number of complex decisions that have to be made before placing an order for degaussing equipment that you require There are erasers designed to run continuously for high volume duplication operations They perform reliably and last for years in the hea~y demands of the high production environment However such degaussing models are expensive and may not provide the most cost efficient solution to the particular needs of your operation It is important that due consideration is given to the practical considerations that are detailed in the following section -

1. Media Types
Variety of material - do you have a range of different formats?
Will your requirements change?

2. Throughput Volume
Quantity of material to be degaussed - are you involved in long runs?
Is the speed of your operation critical?

3. Depth of Erasure
Is the material that you wish to be degaussed rated as being high density?
If so - is complete erasure vital?

4. Type of Operation
Do you require semi-automatic operation or would manual feed of the media to be degaussed be acceptable?

5. Operational Environment
where will the degausser be located?
Is there enough room to house and operate it?
Can its weight be supported safely?
Can the heat it generates be dissipated effectively?

Health and Safety Considerations

As with all electrical equipment connected to the mains electricity supply it is vital to follow the customary safety precautions, including, ensuring that the equipment is properly earthen and regularly inspected by a competent, qualified electrician Degaussers generate heat when they are in operation, particular care should be taken to ensure that adequate ventilation is available and that any heat generated can be safely dissipated. Research into the effects of magnetic fields to humans has failed to show any link with any form of injury in the short or long term.

GLOSSARY OF TERMS

1f - The lower operating frequency of a rotating memory device~ In a single density systems it corresponds to all clock and not data bits.

2f - The higher operating frequency of a rotating memory device. In a double density system it corresponds to clock and data bits in each bit cell.

bit - As applied in magnetic recording, it represents one recorded information cell ("bit" is a contraction of "binary" and "digit" to define a unit of information.

bit cell time - The average bit duration during the process of recording at continuous maximum flux reversal rates.

bit density - Bits per unit length, area or volume of the recording medium, for example, the number of bits per square centimeter of magnetic tape.

bit error rate (BER) - The fraction of bits that are in error. BER = e/N where e is the number of errors and N is the total number of bits (correct bits plus erroneous bits).

bit shift - Also called peak shift. A shift in the detected signal peak caused by the influence of a neighboring flux transition.

BPS - Bits per second.

certification - The process during manufacture of detecting and removing from the diskette defects which may cause data errors.

coercivity or coercive force - The field strength required to bring the flux density to zero in a magnetic material.

cookie - The coated circular diskette that has bee punched to its final shape, but has not been sealed in its jacket.

decibel - A dimensionless unit for expressing the ratio of two powers, or more usually voltages or currents, on a logarithmic scale. If A and B represent two voltages or currents the ratio A/B corresponds to 20 log A/B decibels. One decibel represents a ratio of approximately 1.1 to 1 between A and B.

degauss - To return the magnetization in a media coating or in a head to a zero state by applying a decaying and alternating magnetic field.

diskette - A flexible, circular substrate made of polyester, coated on one or both sides with magnetic oxide, and designed to receive and store electrically coded information for later retrieval.

EP Extra Pulse - Caused by minor media defects or contaminants on a disk when DC biased.

FCI - Flux changes per inch.

flippy - A diskette which has been certified on both sides to be 100 percent error-free and thus able to be "flipped" over for use on either side.

floppy - A term used to describe a diskette's characteristic flexibility when free of its protective jacket.

flux - Lines of magnetic force.

formatted capacity - The maximum amount of data bytes that can be stored on the media.

hard error - An error that is repeatable in the same track and sector location.

high energy tape - Magnetic tape having coercivity higher than 600 Oersteds.

kBPS - Kilobits per second.

magnetic flux - The magnetic lines of force produced by a magnet for electric current.

magnetic field strength - The magnitude of a magnetic field vector, usually expressed in oersteds or ampere-turns per meter.

MMF magnetomotive force - The magnetic analogue of electromotive force, which, when due to a current in a coil, is proportional to the product of current in amperes and the number of turns.

MP missing pulse - Caused by minor media defects or contaminants on the surface of a disk.

Maxwell - A unit of magnetic flux.

Oersted - A unit of magnetic field strength .

overwrite - Overwrite of an FM (digital) recording with the two frequencies if and 2f is defined as the ratio of the original if ( or half band edge signal) to that remaining after being overwritten with a 2f (or band edge signal) Overwrite of 30 dB is considered adequate for proper system performance retentivity - The maximum value of the residual flux density corresponding to saturation flux density.

TPI - Tracks per inch.

track - That portion of a recording surface available to one read/write head at each access position, Also An area of tape surface that coincides with the location of the recorded magnetization produced by one record gap.

What Will A Degausser Cost

Whatever the cost of the investment the potential savings gained by the re-use of previously discarded material, plus the increased efficiencies resulting from better quality all add up to a conclusion that such an investment in a degausser is most worthwhile.

If you would like to receive quotation by return for the Degausser that suits your requirements click HERE , for our assistance page. - we are always happy to help.

 
 
HOME     back

For further information contact
Eyecote International Limited
Corporate Office
Singleton Court, Monmouth,
NP25 5JA. United Kingdom.
Tel: +44 1600 772433
Fax: +44 1600 715749
E-mail:- sales@eyecote.com
Eyecote Media Services
USA Sales Office
26241 Enterprise Court,
Lake Forest, CA 92630
Tel:+ 949-598-9522
Fax:+ 949-340-7146
E-mail:-usa-sales@eyecote.com