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B ROADCAST E NGINEERING AND ACOUSTICSIntroduction To Magnetic Recording

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I NTRODUCTION T O M AGNETIC R ECORDINGHave you ever wondered how a whole album of yourfavorite music got onto one of those little cassette tapes?Have you ever wondered how floppy disks can hold 180 ormore pages of typed text?

How can recording your favorite television show on a videocassette recorder be possible?How can you be able to get money from an automated bankteller machine at two oclock in the morning?

MagneticRecording

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I NTRODUCTION T O M AGNETIC R ECORDING

History of Magnetic Recordersy 1888 Oberlin Smith originated the idea of using

permanent magnetic impressions to record sounds.y

1900 Vladeniar Poulsen brought Mr. Smithsdream to reality.Telegraphone a device that recorded sounds onto a

steel wire- the magnetism generated during the

recording process didnt vary directly to the currentwhich caused it.

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y 1907 Mr. Poulsen discovered dc bias- if a fixed dc voltage were added to the input

signal, it moved the input signal away from the stepin the magnetism curve.

- the output signal was weak compared to thenatural hiss of the unmagnetized tape passing theplayback head

- poor signal-to-noise ratio

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y 1925 electronic amplifiers were developedNRL ( U.S. Naval Research Laboratory) W.L.

Carlson and G.W. Carpenter found out that

adding an ac bias voltage to the input signalinstead of a fixed dc bias voltage wouldreproduce a stronger output signal

Greatly improve the signal-to noise ratiogreatly reduce the natural tape hiss that was so

common with dc bias

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y 1935 Fritz Pfleumer demonstrated hisMagnetophone at the German Annual RadioExposition in Berlin

Magnetophone a cellulose acetate tape coated withsoft iron powder (paper tapes)

y 1947 3M Company introduced the first plastic-based magnetic tape

y 1956 IBM introduced the next major contributionto magnetic recording the hard disk drive

Disk was a 24-inch solid metal platter and stored4.4 Megabytes of information

y 1963 IBM reduced the platter size and introduceda 14-inch hard disk drive.

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y 1966 IBM introduced the first removable-packhard disk drive.

y 1971 magnetic tape became popular again whenthe 3M Company introduced the first 14-inchmagnetic tape cartridge and tape drive.

y 1971 IBM invented the 8-inch floppy disk and diskdrive

- it used a flexible 8-inch platter of the samematerial as magnetic tape

- its main goal was to replace punched cards asa program-loading device.

y 1980 Seagate Technology invented the 5-1/4- inchfloppy disk drive.

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Prerequisites for Magnetic Recordingy Input Signal can come from a microphone, a radio

receiver, or any other source that is capable of producing a recordable signal.

y Recording Medium any material that has theability to become magnetized, in varying amounts, insmall sections along its entire length.

y Magnetic Heads the heart of the magneticrecording process.

- transducers that convert theelectrical variations of the input signal into themagnetic variations that are stored on a recordingmedium.

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3 Functions of a Magnetic Heady transfer or recordy recover or reproducey remove or erase

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Magnetic Head Construction- a magnetic core wrapped with a coil of very

thin wire.

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Magnetic Head Operation- when an electric current passes through

the coil of a magnetic head, magnetic field lines

associated with the electric current follow pathsthrough the core material.- when the magnetic fields get to the head

gap, some of them spread outside the core to forma fringing field.

- when a recoding medium is passed throughthis fringing field, it is magnetized in relation tothe electric current.

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Physical Properties of MagneticTape

y

Base Mater ial is made up of eitherplastic or metal.y Oxide Coating oxide particles that

can be magnetizedy Glue used to bond the oxide particles

to the base material

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Types of Magnetic RecordingTape

y

Analog Magnetic Tape used torecord, store, and reproduce audio andinstrumentation type signals.

y Digital Magnetic Tape is used to

record, store, and reproduce computerprograms and data

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Tape Errors and Their Effectsy Dropout Errors a temporary, sharp drop (50% or

more) in signal strength caused by eithercontaminants on the magnetic tape or by missingoxide coating on part of the tape.

y Noise Errors unwanted signals that appear whenno signal should appear . They are usually caused bya cut or a scratch on the magnetic tape.

y Skew Errors only occur on multi-track magnetictape recorders. It is the time differences that occurbetween individual tracks of a single magnetic headwhen the multi-track tape isnt properly aligned withthe magnetic head.

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Fixed Skew happens when properly aligned magnetictape passes an improperly aligned magnetic headDynamic Skew happens when misaligned tape passes a

properly aligned head

y Level Errors happen when the actual outputsignal level either drops or rises to a level outside theexpected range.

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Causes of Magnetic Tape Failurey Normal Wear occurs because the tape must come

in contact with fixed surfaces, such as a recordersmagnetic heads, rollers and guides.

y Accidental Damage any damage that wouldntnormally occur under ideal operating and handlingconditions.

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y Environmental Damage - magnetic tape is veryflexible and can be used in a wide range of environmental conditions ( 2 to 130 F, and in a relativehumidity range of about 10 to 95%).

Tape DeformationOxide SheddingHead-to-Tape StickingLayer-to-Layer Adhesion

Dirt Build-up

Excessive Tape and Head Wear

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y Winding Errors they happen when improperwinding practices create an excessive or uneven forceas the tape is being wound onto a tape reel.

Cinching happens when a tape reel is stopped too

quicklyPack Slip happens when the tape is loosely wound on

the reel and is exposed to excessive vibration or too muchheat.Spoking happens when the magnetic tape is wound onto

the tape reel with the tension increasing toward the end of

the winding Windowing voids or see-through air gaps in the tapewinding.

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Tape Reels and Tape Cartridges- types of magnetic tape carriers

y Tape Reels used on magnetic recorders that use a

manually loaded tape loaded supply reel and aseparate take-up reel.- reels purpose is to protect the magnetic

tape from damage and contamination.- can be made of plastic, metal, or glass

- has two parts, the hub and the flanges- designed to hold magnetic tape on its

hub without letting the magnetic tape touch the sidesof the flanges.

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y Tape Cartridges hold a spool of magnetic tape inthe same way as the tape reels, except that the insideof the cartridge contains both the supply reel and thetake-up reel.

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