Magnetic Recording MediaPart I: Care and Handling o[ Magnetic Tape

Part Ii : Principles and Current State of the Art

Part I : Care and Handling of Magnetic Tape

Norman C . Ritter3M CompanyMinneapolis, Minnesota

INTRODUCTIONMagnetic tape recording plays an important

role in many information storage applications .Those people involved with the recording andstorage of this type material should be aware ofthe conditions and procedures that will help en-sure the best overall results . The purpose of thissection is to give some of the guidelines to achievegood tape life and tape performance . Althoughthis information is aimed at the video tape record-ing field, it is basically applicable to othermagnetic recording areas .

BASIC INFORMATION

Magnetic recording has proven to be a veryviable, versatile, and reliable form of storing in-formation . The electromagnetic media used inrecording tapes are capable of retaining the signalfor indefinite periods of time. The recordedsignals will remain virtually unchanged fordecades unless they are intentionally erased .Those problems that have been encountered withmagnetic recording are mostly physical in nature,due to improper handling or storage conditions .These problems can be caused by malfunction ofmachines, improper operator care, or poor en-vironment in either the operations area or storagearea .

5.7

The following information is intended to helpin achieving optimum tape performances by hit-ting upon certain key points with as little rhetoricas possible to simplify the reader's future ref-erence to parts of this chapter .

The Recording and OperationsArea Environment

Foreign debris from the environment is one ofthe reasons for the cause of dropouts, scratches,machine parts wear, head life, and tape life . Thiswarrants important consideration today, and isbecoming more important as the recording den-sity on tape increases .

1 . The area in which tapes and machines areused should approach "clean room" condi-tions, such as those found in main framecomputer tape installations . This degree ofair filtration, though ideal, is not realistical-ly achievable in many video tape operations .It is recommended that at a minimum thefiltration used must at least meet the efficien-cy rating of 90 percent based on the NationalBureau of Standards Dust Spot EfficiencyTest-Atmospheric Dust. Qualified air con-ditioning and air filtration firms are awareof the requirements to satisfy this need . Theairflow system should be designed to main-tain a positive pressure in the recording area .

6.7-122

This prevents dust particles from floating in-to the room from other locations . In addi-tion the duct work sixes and placement,leading to this area, should allow thenecessary amount of air flow to maintain theproper temperature and humidity control,without having air velocities at a level highenough to blow settled dust and debris aroundthe room.

2 .

The temperature should be controlled at ap-proximately 70°F plus or minus 5 01o, and therelative humidity at 5001o plus or minus 20010 .Without this control the risk of head clog-ging, stiction, and higher headwear are in-creased . The old cliche is still true "what iscomfortable for humans is a good environ-ment for the tape."

3 .

To avoid inadvertent contamination of boththe tapes and machines, smoking, eating, anddrinking should be restricted to certain areasand preferrably out of the recording loca-tions,

4 . Special attention must be given to the floorsin or near the recording area, as they are asource of debris due to pedestrian traffic . Cement flooring must be scaled and tile floorsshould not be waxed. Both should be moppedclean on a routine basis . The use of indoor-outdoor type carpeting that contains anti-static material is acceptable and may be mostdesirable, depending on the needs of anoperation . The carpeting helps reduce roomnoise from equipment, and affords an at-mosphere conducive to good operator prac-tices . The use of industrial vacuum cleanersis recommended on carpeting or floors, andit should be done on a regular basis. Theexhaust from these units should be locatedoutside the recording area .

5 . To aid in the reduction of debris due topedestrian traffic, the recording area shouldbe located such that it is not a normal passageway to other parts of the operation . Thisnot only improves the cleanliness of the areabut avoids continual distraction of the tapemachine operator's function .

b .

During times of construction in any area ofthe facility, the frequency of cleaning shouldbe increased substantially . The area underconstruction should be scaled off with plasticsheeting as best as possible . Surprising as itmay seem, cement dust can get through filtersthat water will not .

7 .

The tops of equipment and other exposed sur-faces should be cleaned on a periodic basisto maintain the integrity of the operationsarea . This procedure will also give clues as

Section 6 : Production Facilities

to how clean the envrionment is in the area.The more dust and debris found during thisroutine is an indication of the amount of dustand debris that has found its way onto thetapes and into the recording equipment,

Operating Practice RecommendationsThe video tape operations in a facility are

usually involved with more than one type of tapemachine ; namely reel to reel type and cassettetype .The differences from a tape life or reliability

point of view are only the questions of who orwhat is handling the tape during thread up, howexposed is the tape when on or off the machine,and the tape path conditions . The cassette ap-proach virtually eliminates the human handlingof the tape, and reduces the exposure to at-mospheric contamination ; however, themechanical threading mechanisms require atten-tion as well as the condition of the tape path .The recommended environmental conditions app-ly to both type systems . Before proceeding, thenext comments may help put things in a betterperspective .

People involved in any video tape recordingoperation, at some point, may ask whether allthe emphasis put on cleaning, handling, and en-vironmental control really accomplishes anything ;that is, in other terms, "familiarity breeds con-tempt ." Fig . 1 gives a good picture of the relativesizes of what is being dealt with . Not only is thisdebris capable of causing large drop-outs in thearea they are located on the tape ; but some ofthese particles, when wound into a tape pack,may cause impressions into adjacent layers whichin turn will result in additional drop-out activity .

Wallace's law on signal loss due to head to tapespacing theoretically indicates that compacteddebris contained in a one pint jar, if equallydistributed, would be sufficient to put every videotape ever produced out of manufacturers' drop-out specifications .Good housekeeping and operation practices are

very important to a successful video tapeoperation .

Following are some of the operational recom-mendations that apply to the handling of reel toreel and cassette type video tape recording for-mats,I . The components in a video tape machine that

contact either the front or back side of a tape,should be cleaned before each tape is thread-ed to make a recording or playback. Therecommended cleaning fluids are Freon TF(a DuPont trademark), Genesolve D (anAllied Chemical Trademark), isopropolalcohol, or other cleaners recommended by

0 10

Chapter 7 : Magnetic Recording. Media

the machine manufacturer . The cleaningshould be accomplished by applying the liq-uid to a lint free cloth, such as Texwipe(trademark), and gently rubbing the tape pathsurfaces with the dampened material . Dur-ing this procedure extreme care should be ex-ercised in cleaning the video heads . They canbe easily broken or damaged with excessivepressure, particularly in the case where fer-rite materials are used . Ferrite is the headmaterial used in most all current day helicalmachines and in many rebuilt quadruplexheads . Since the video heads will not with-stand a strong scrubbing action, compared tothe other transport areas during cleaning, thestronger solvents such as isopropol alcohol,or machine manufacturers recommendedsolvents (usually Xylene type), are the bestchoices for removing debris build up on thevideo heads . They tend to help soften materialbuildup in addition to acting as a wash .

2 . The cleaning of the capstan and the capstanpinch roller are especially important for tworeasons . Any dirt or debris build up on theseareas will cause impressions in the tape oneach revolution that can cause drop-outsthrough a long period into the roll . Also theaccumulation of material in this area mayreduce the frictional pressures on the tapewhich are needed to maintain proper lineartape speed control . One word of caution : thematerial used in the pinch roller can beadversely effected by some cleaning solutions .

Fig . 1 . Debris perspective on 1" video tape .

6.7-123

The machine manufacturers recommenda-tions should be followed for cleaning thepinch roller .

3 . Cleaners, other than Freon TF or GenesolveD are relatively slow in evaporating, so giveample time for the transport to dry beforethreading the tape . This usually means about30 seconds .

4 . The video drum (scanner assembly) on helicalmachines have tape edge guiding surfaces thatmay accumulate debris . These surfaces needextra cleaning attention to ensure smooth tapehandling and good interchange with otherrecordings .

5 . The tape reel is specialty designed for trans-porting magnetic tape . The reel should alwaysbe handled by the hub, which is the strongestportion of the reel . (See Fig . 2 .) The reelflanges are designed to protect the tape edges,not to guide the tape . A reel should never becarried by the flanges . Handling the tape bythe reel flanges or dropping an unprotectedreel can bend the flanges . If the tape is rub-bing against the flange of the reel, either thereel flanges are bent, or the reel pedestal orguides require adjustment . Avoid squeezingthe flanges of a reel when putting on or tak-ing off the transport .

6 . The take-up reels should be cleaned at thestart of each day. The tape winding surfaceof the hub and the inside surfaces of the

Magnetic Recording MediaPart 1; Care and Handling of Magnetic Tape

Part 11 ; Principles and Current State of the Art

Part n: Principles and Current State of the Art

M. P . SharrockMemory Technologies Research Laboratory3M CompanySt. Paul, Minnesota

INTRODUCTION

The broadcast of audio and video program-ming is greatly facilitated by the ability to storeand recover information, and sometimes erase oralter it . Despite the availability of photographyand other means of storing images, sounds andsymbols, magnetic recording has enjoyed a longpre-eminence in these applications because of anumber of features :1 . Easy recording by inexpensive transducer

(head)2 . Stable long-term storage3 . Simple playback process with good signal-to-

noise ratio, possibly using the same head asfor recording

4 . Easy erasure and editing or updating, especi-ally important in broadcast studio environ-ment

5 . Relatively high surface information densityfi . Thin flexible media, which can be rolled up

(unlike phonograph record or video disk) forextremely high volume information density

7 . Inexpensive media-high-quality magneticvideo cassette tape is available at a lower costper square inch than the transparent tape usedto mend books!

NOTE : Superscript numbers refer to references at end of the chapter .

BASIC PRINCIPLES

5.7

This article will attempt to give a brief over-view of the basic physical principles involved inmagnetic recording, some of the leading types ofrecording materials, and the directions of currentresearch and development . More thorough dis-cussions of various aspects of recording theoryand practice are availablet,2 .

Audio and video signals are handled as elec-trical quantities (voltages and currents) ; some fun-damental physical laws that relate electrical andmagnetic phenomena, and therefore make mag-netic recording possible, are shown in Fig . 1 . Thechanging flow of electrical current in the record-ing head gives rise to a changing magnetic field,which imposes a magnetization pattern on therecording medium (e.g ., tape) . During playbackthe reverse process occurs, as the changingmagnetic field experienced by the head, due tothe passing recorded magnetic surface, inducesa signal voltage .

The inductive nature of the playback head dic-tates that only a change in the magnetic field ex-perienced by the head leads to an output signal .Further, a faster change produces a greater out-put amplitude . A consequence of this is a fre-quency dependence in the output amplitude . Fig .2 shows this dependence, for sinusoidal signalsrecorded at a constant depth and magnetic ampli-

5.1-12a

Signal Current, at Frequency f

Head (reco(ding)i

_ Magnetization

',

Non-Magnetic/Wavelength X

B

e Fllm

~i

Motion, -

li

induced Signal Voltage,Magnetic

at Frequency fCoating w

Head (playback)i

-- Magnetization

Nan-Magnet

Wave en

h X

Film

Motion,at Speed v

Fig. 1 . The elements of magnetic recording and therelationship between signal frequency, transport

speed, and recorded wavelength.

tude in the medium . The output is proportionalto frequency over a large range ; the fall-off athigh frequencies (high recording densities or shortrecorded wavelengths) is due to signal loss froma number of sources which will be discussed later .The frequency dependence of the output is largelycompensated for in many applications by the useof a frequency-dependent electronic gain ; thisprocess is called equalization .The performance of a magnetic recording sys-

tem depends, to a large degree, upon the mag-netic properties of the material used to make therecording medium. The most significant prop-erties are explained in Fig . 3, which shows a plot(called a hysteresis loop) obtained by placing asample of the material in a varying magnetic fieldH and measuring its magnetization intensity M.The points at which the plot crosses the verticalM axis define the quantity known as the rema-nent magnetization Mr , which describes howmuch magnetization intensity the material is ableto retain in zero field after being magnetized bya saturating field . The quantity usually specified(in the cgs system) is 4nMr , also designated asBr , which is called the retentivity . The ratio of

Log (OutputAmplitude)

Fig . 2. Logarithmic plot of output amplitude vs .signal frequency, for sinusoidal magnetic recordingat constant magnetization amplitude and depth . The

slope of the linear rise Is B dfi per octave .

Mr to the saturated magnetization Ms is calledthe squareness, it helps to define the shape of theloop and therefore the recording properties of thematerial . The points at which the loop crossesthe horizontal H axis define the intrinsic coer-civity, often referred to as simply the coercivityand designated by Hc . This is the field neededto reverse half of the magnetization after satura-tion in one direction, resulting in zero netmagnetization . The steepness of the plot as it goesfrom one direction of magnetization to the op-posite one relates to the ability of the materialto record a signal with sensitivity and precision .A quantitative measure of this steepness is theswitchingfield distribution (SFD), described inFig . 4 .The response of a magnetic recording material

to an applied field is clearly non-linear (see Fig .3) . In many applications, it is desirable to remove

Section 5 : Production Facilities

Log (Signal Frequency)

Fig . 3. Plot of magnetization M vs. applied magneticfield H, for a typical magnetic recording material.

Chapter 7 : Magnetic Recording Media

Fig. 4. Plate of magnetization M (broken line) andfirst derivative dMfdH (solid line) vs. applied field H .Width GH, used In the definition of the switching .

field distribution (SFD), Is measured athalf the peak height.

this non-linearity so that the recorded mag-netization can be made proportional to thesignal current . The needed linearization of theresponse is accomplished through the techniqueknown as ac-bias, in which the desired signal cur-rent is added to an alternating current of muchhigher frequency and amplitude before being ap-plied to the recording head . The process and theresult are diagrammed in Fig . 5 ; more completediscussions are available elsewherez,3 . The re-cording material is taken through a number ofprogressively smaller hysteresis loops as it leavesthe vicinity of the recording head gap and is leftin a state in which the remanent magnetizationis very nearly proportional to the signal . The useof ac-bias is important in applications where thewaveform read back must be an accurate repro-duction of the input waveform, particularly inaudio recording .

In some applications, the recorded informationis contained in the timing of the zero-crossingpoints of the signal, rather than in the shape oramplitude of its waveform . Two of these appli-cations, which do not usually utilize ac-bias, aredigital recording and frequency-modulation (FM)recording . Digital recording is very simple in con-cept, although the applictions can be verysophisticated . Digital recording seeks only to con-vey reliably the strings of binary digits (ab-breviated as "bits"), 0's and I's, that make upthe information used in computers and otherdata-handling devices . Fig . 6 shows a simplescheme for doing this . In practice, more complexprocedures are used; some additional magnetictransitions are used for timing purposes and someconvey information that is redundant but need-ed for the detection and correction of errors . Er-

Fig. 5 . The principle of ac-bles . As a specific smallarea on the tape moves sway from the recording

head gap, It is subject to a high-frequency bias fieldof decreasing amplitude. This causes Its magnetlu.tlon to vary as shown by the arrows, and to be left In

a final state proportional to the signal field uponwhich the high-frequency field Is superimposed .

Bits ofInformation

CorrespondingMagnetizationPattern

Magnetic Coating

5.7-129

Fig . a. A simple form of digital recording, In which1's are encoded as magnetic transitions and 0's as

the absence of transitions.

rors may occur through excessive random noiseor through defects in the surface of the tape ordisk .

Digital recording, in addition to its obviouscomputer applications, is increasingly being usedto replace analog recording techniques in audioand video applications4,5 . The original signal ismeasured at precise intervals and the measure-ments expressed as a series of numbers that arethen recorded digitally like any other data . Therecovery of the signal consists of reading backthe series of numbers (again, at precise time in-tervals), converting them into values of an analogquantity (e .g ., voltage), and smoothing thesediscrete values into a waveform .The first design consideration of a digital

recorder is the frequency of sampling, which mustbe at least twice the highest frequency present inthe signal to be recorded, according to the Ny-quist theorem. Thus, for a high-fidelity audio

49 0

6.7-130

signal containing frequencies up to a 20 kHz, 'sampling might be done at a rate between 40,000and 50,000 times per second . Next, the dynamicrange of the final reproduction is determined bythe precision of each conversion to digital form.If a final dynamic range of 60 dB (an amplituderatio of 1000) is required, then the binary codefor each sampling must be able to express integernumbers from 0 to 1000 . This requires ten bitsper sampled value, since 2'° = 1024 . Similarly,a dynamic range of 90 dB requires a minimumof 15 bits per sampled value . The dynamic rangeis essentially the signal-to-noise ratio of thereconstructed signal, since the steps between ad-jacent numerical values provide the only noise in-herent in the final output .The advantages of a properly designed digital

system include an excellent dynamic range, anessentially perfect time-base (absence of "wow"and "flutter"), and the ability to do repeatedediting or dubbing operations without signaldegradation . The tenth-generation audio repro-duction made on a studio digital recorder is vir-tually indistinguishable from the original . Thisresistance to degradation of the recorded signalis intrinsic to digital technology and results fromthe fact that the information is encoded only inthe timing of transitions . As long as these canbe reliably detected and are not shifted, substan-tial degradation of the amplitude or shape of thedigital waveform can occur with no effect on theresult .

Digital magnetic recording, while not requir-ing linear reproduction of a waveform, makesgreat demands upon the recording system becauseof the drastically increased bandwidth . If anaudio signal of 20 kHz bandwidth is to be repro-duced with the modest dynamic range of 60 dB,frequencies up to at least 225 kHz are needed toaccomodate its digitally encoded representation .An actual studio recorder might operate beyond600 kHz, depending upon sampling rate, dynamicrange, and the digital code used . Such frequen-cies dictate very short intervals between themagnetic transitions on the recording surface iftape usage is not to become prohibitive . On theother hand, the signal-to-noise ratio of the digitalwaveform that is read back need not be as greatas that required in an analog recording . Ingeneral, the lower the signal-to-noise ratio of thedigital waveform the higher the probability ofmis-read bits (bit errors), but these can bedetected and either corrected or concealedthrough the use of encoding schemes that involvea certain amount of redundancy in the recordedinformation . These schemes can overcome notonly isolated single-bit errors but also the effectsof recording surface defects that delete a numberof adjacent bits . (See, for example, reference 6) .

Signal

ModulatedCarrier

Section 6: Production Facilities

Fig. 7 . The principle of frequency modulation (FMS

Error correction techniques make possible the useof very high densities in digital recording, pro-vided that the heads and media can operate withthe required resolution,

Another mode of magnetic recording that doesnot require linear waveform reproduction isfrequency-modulation (FM) . This is primarilyused in video recording7 , but can also be appliedto audio recording and is in fact used to give high-fidelity sound in some video cassette recorders .The principle of FM is that a carrier wave isgenerated whose frequency varies with the valueo ¬ the signal to be transmitted (Fig . 7) . It is thisfrequency-modulated carrier that is recorded onthe magnetic medium . As in digital recording, theinformation is contained in the intervals betweenthe zero-crossings of the recorded waveform ; itsamplitude is not significant as long as the signal-to-noise ratio is adequate for reliable detection .Unlike digital recording, FM recording containsan analog relationship between frequency and thevalue being transmitted . The benefits of FM, ascompared to direct analog recording, are two-fold . One is the insensitivity to amplitude varia-tions in the recorded waveform . The second,which is crucial to magnetic video recording, isthat FM reduces the ratio of the highest to thelowest frequencies that need to be recorded ; thisis shown schematically in Fig . 8 . Suppose thatit is desired to record a video luminance (bright-ness) signal that contains frequencies from 30 Hzto 4 .5 MHz. The upper and lower frequencylimits have a ratio of 150,000 or 17 octaves, (Bycomparison, a full-fidelity audio system is re-quired only to reproduce frequencies from 20 Hzto 20 kHz, a ratio of 1000, or 10 octaves .) A fre-quency span of 17 octaves (ratio of 150,000) isimpractical in magnetic recording . First, the ratioof highest to lowest frequencies cannot exceed theratio of the physical length of the playback headpole-pieces to that of the gap (reference 1, pp .99-119) . This ratio cannot in practice be as great

Chapter 7 ; Magnetic Recording Media

Fig . 4 . Plot@ of magnetization M (broken line) andfirst derivative dMldM (solid Ilns) vs. applied field H .Width &H, used In the definition of the ewllching .

field distribution (SFO), Is measured athalf the peak height .

this non-linearity so that the recorded mag-netization can be made proportional to thesignal current . The needed linearization of theresponse is accomplished through the techniqueknown as ac-bias, in which the desired signal cur-rent is added to an alternating current of muchhigher frequency and amplitude before being ap-plied to the recording head . The process and theresult are diagrammed in Fig . 5 ; more completediscussions are available elsewhere2,3 . The re-cording material is taken through a number ofprogressively smaller hysteresis loops as it leavesthe vicinity of the recording head gap and is leftin a state in which the remanent magnetizationis very nearly proportional to the signal . The useof ac-bias is important in applications where thewaveform read back must be an accurate repro-duction of the input waveform, particularly inaudio recording .

In some applications, the recorded informationis contained in the timing of the zero-crossingpoints of the signal, rather than in the shape oramplitude of its waveform . Two of these appli-cations, which do not usually utilize ac-bias, aredigital recording and frequency-modulation (FM)recording . Digital recording is very simple in con-cept, although the applictions can be verysophisticated . Digital recording seeks only to con-vey reliably the strings of binary digits (ab-breviated as "bits"), 0's and 1's, that make upthe information used in computers and otherdata-handling devices . Fig . 6 shows a simplescheme for doing this . In practice, more complexprocedures are used ; some additional magnetictransitions are used for timing purposes and someconvey information that is redundant but need-ed for the detection and correction of errors, Er-

Magnetic Coating

5.7-129

Fig . 5 . The principle of aablas. As a specific smallarea on the tape moves away from the recording

head gap, It Is subject to a high-frequency bias fieldof decreasing amplitude . This causes Its magnetiza.tion to vary as shown by the arrows, and to be left Ine final state proportional to the signal field uponwhich the high-frequency field Is superimposed.

Fig . 6 . A simple form of digital recording, In whicht'a are encoded as magnetic transitions and O's as

the absence of transitions .

rors may occur through excessive random noiseor through defects in the surface of the tape ordisk .

Digital recording, in addition to its obviouscomputer applications, is increasingly being usedto replace analog recording techniques in audioand video applications 4 .s, The original signal ismeasured at precise intervals and the measure-ments expressed as a series of numbers that arethen recorded digitally like any other data . Therecovery of the signal consists of reading backthe series of numbers (again, at precise time in-tervals), converting them into values of an analogquantity (e.g ., voltage), and smoothing thesediscrete values into a waveform .The first design consideration of a digital

recorder is the frequency of sampling, which mustbe at least twice the highest frequency present inthe signal to be recorded, according to the Ny-quist theorem . Thus, for a high-fidelity audio

5.7-130

signal containing frequencies up to a 20 kHz,sampling might be done at a rate between 40,000and 50,000 times per second . Next, the dynamicrange of the final reproduction is determined bythe precision of each conversion to digital form.If a final dynamic range of 60 dB (an amplituderatio of 1000) is required, then the binary codefor each sampling must be able to express integernumbers from 0 to 10110, This requires ten bitsper sampled value, since 2'° = 1024, Similarly,a dynamic range of 90 dB requires a minimumof 15 bits per sampled value . The dynamic rangeis essentially the signal-to-noise ratio of thereconstructed signal, since the steps between ad-jacent numerical values provide the only noise in-herent in the final output .The advantages of a properly designed digital

system include an excellent dynamic range, anessentially perfect time-base (absence of "wow"and "flutter"), and the ability to do repeatedediting or dubbing operations without signaldegradation . The tenth-generation audio repro-duction made on a studio digital recorder is vir-tually indistinguishable from the original . Thisresistance to degradation of the recorded signalis intrinsic to digital technology and results fromthe fact that the information is encoded only inthe timing of transitions . As long as these canbe reliably detected and are not shifted, substan-tial degradation of the amplitude or shape of thedigital waveform can occur with no effect on theresult .

Digital magnetic recording, while not requir-ing linear reproduction of a waveform, makesgreat demands upon the recording system becauseof the drastically increased bandwidth . If anaudio signal of 20 kHz bandwidth is to be repro-duced with the modest dynamic range of 60 dB,frequencies up to at least 225 kHz are needed toaccomodate its digitally encoded representation .An actual studio recorder might operate beyond600 kHz, depending upon sampling rate, dynamicrange, and the digital code used . Such frequen-cies dictate very short intervals between themagnetic transitions on the recording surface iftape usage is not to become prohibitive, On theother hand, the signal-to-noise ratio of the digitalwaveform that is read back need not be as greatas that required in an analog recording . Ingeneral, the lower the signal-to-noise ratio of thedigital waveform the higher the probability ofmis-read bits (bit errors), but these can bedetected and either corrected or concealedthrough the use of encoding schemes that involvea certain amount of redundancy in the recordedinformation, These schemes can overcome notonly isolated single-bit errors but also the effectsof recording surface defects that delete a numberof adjacent bits . (See, for example, reference 6) .

Un-modulatedCarrier

Signal

Section 5 : Production Facilities

ModulatedCarrier

Fig. 7. The principle of frequency modulation (FM) .

Error correction techniques make possible the useof very high densities in digital recording, pro-vided that the heads and media can operate withthe required resolution .Another mode of magnetic recording that does

not require linear waveform reproduction isfrequency-modulation (FM) . This is primarilyused in video recording?, but can also be appliedto audio recording and is in fact used to give high-fidelity sound in some video cassette recorders .The principle of FM is that a carrier wave isgenerated whose frequency varies with the valueof the signal to be transmitted (Fig . 7) . It is thisfrequency-modulated carrier that is recorded onthe magnetic medium . As in digital recording, theinformation is contained in the intervals betweenthe zero-crossings of the recorded waveform ; itsamplitude is not significant as long as the signal-to-noise ratio is adequate for reliable detection .Unlike digital recording, FM recording containsan analog relationship between frequency and thevalue being transmitted . The benefits of FM, ascompared to direct analog recording, are two-fold . One is the insensitivity to amplitude varia-tions in the recorded waveform . The second,which is crucial to magnetic video recording, isthat FM reduces the ratio of the highest to thelowest frequencies that need to be recorded ; thisis shown schematically in Fig . 8 . Suppose thatit is desired to record a video luminance (bright-ness) signal that contains frequencies from 30 Hzto 4.5 MHz. The upper and lower frequencylimits have a ratio of 150,000 or 17 octaves . (Bycomparison, a full-fidelity audio system is re-quired only to reproduce frequencies from 20 Hzto 20 kHz, a ratio of 1000, or 10 octaves .) A fre-quency span of 17 octaves (ratio of 150,000) isimpractical in magnetic recording . First, the ratioof highest to lowest frequencies cannot exceed theratio of the physical length of the playback headpole-pieces to that of the gap (reference 1, pp .99-119) . This ratio cannot in practice be as great

Chapter 7 : Magnetic Recording Media

as 150,000 . Even if it could, the 6 dB per octavefrequency dependence (Fig . 2) would require adynamic range of more than 100 dB for directvideo recording, too much for successful equaliza-tion . Frequency modulation, as indicated in Fig .8, reduces the ratio of highest to lowest frequen-cy ; this ratio after modulation is as low as I oc-tave in some video recording systems .The frequencies involved in video reproduction

are much higher than those in audio . Since thereare practical lower limits on the recorded wave-length, this means that much higher head-to-tapespeeds are needed . To attain these speeds, allvideo systems use heads mounted on a spinningdrum. The tape is transported around part of thisdrum at a speed much lower than that at whichthe heads are moving across the tape surface . Thevarious systems in current use employ differentFM carrier frequencies as well as differentschemes for conveying the color information,which in practice has a lower resolution and there-fore requires a smaller bandwidth than that ofthe luminance information . In all systems the hueand saturation information is conveyed byamplitude and phase modulation of a special col-or sub-carrier (at 3 .58 MHz in the I1 .S.A.) . Inthe one-inch, open-reel tape format commonlyused in broadcasting, the modulated color sub-carrier is simply added to the luminance signaland the resulting sum is the signal used to

PowerIntensity

PowerIntensity

Signal,Before Frequency Modulation

Frequency

After Frequency Modulation

Frequency

Fig . 8. In frequency-modulation (FM) the InformationIs shifted to higher frequencies, but the ratio

between highest and lowest frequencies Is greatlyreduced . FM schemes actually used In video

recording do not use all of the sidebandfrequencies ; see pp . 396401 of reference 2 .

5 .7-131

frequency-modulate the video carrier . In 314-inchU-matic systems and in non-professional one-half-inch cassette systems, the color sub-carrier(with its amplitude and phase information) isheterodyned down to a lower frequency (about0.6 MHz) which is then recorded directly, usingthe frequency-modulated luminance carrier forac-bias . Some recently developed one-half-inchcassette systems for professional applicationsutilize FM recording of both luminance and col-or information with a separate head, tape track,and carrier frequency for each. This achievesbroadcast-quality reproduction using the samecassettes used in consumer systems, but at the costof much lower recording densities (higher tapeconsumption) .

MAGNETIC RECORDING MEDIA

Most magnetic recording media in use todayare made by dispersing small magnetic particlesin an organic binder and coating the resulting"paint" on a support material . See Fig . 9A. Eachparticle remains uniformly magnetized ; its mag-netization can change its direction but not itsmagnitude (this is a result of the very small size) .The particles are sufficiently small that there aremany of them in each magnetized region of therecorded pattern ; this provides the needed resolu-tion and signal-to-noise ratio . The particles canbe of various types, according to the intendedapplications-9 . In addition to the magnetic par-ticles, the coating contains polymers for strength,lubricants to reduce friction and wear, and sur-factants to aid in dispersing the particles . An ad-ditional particulate component is usually addedto impart a controlled amount of abrasivity forhead-cleaning purposes .Another possible construction for a magnetic

recording medium is shown in Fig . 9B . Like thedispersion-coated type, it also uses a non-magnetic base . The magnetic substance is in theform of a thin metallic film . Thin films offermagnetization intensities far surpassing thoseavailable in the coatings of dispersed particles,which compensates for the films' much lesserthickness . The thinness of the films is in itselfan advantage in that the magnetic material is allin very close proximity to the record or playbackhead, This intimate contact with the head is ex-tremely important to high-density recording andis further enhanced by the great smoothness possi-ble in the thin films . These attractive featuresmake thin-film recording media an exciting areaof research and development and possible can-didates for future products . At present, their ma-jor commercial applications are in video instant-replay memories and in advanced computer disks .

5.7-132

VHS cassette

C-formatMinimum Video Wavelengths

(A) Typical Coated

(B) TypicalParticulate Tape

Thin-Film Tape

Fig. 9 . Cross sections of a coated particulaterecording tape and a thin-film tape . The particles aredrawn approximately to scale but their spacing anduniformity have been exaggerated Only a few havebeen drawn, although they actually fill the magneticcoating layer, occupying about 40% of Its volume .

On the negative side, metallic thin films are sub-ject to some concerns regarding their chemicalstability, durability, and economical manufacture,Many of them are coated with a thin non-magnetic overlayer for protection and lubrication .This layer, of course, tends to diminish the close-ness of contact with the head . Metallic thin filmsare made by chemical plating or by some formof vacuum deposition and usually contain cobaltas a major constituent . The need for the specialequipment used to carry out these processes isan additional barrier to the widespread commer-cial use of thin films in recording media . Futher-more, the thin-film products must compete witha particulate technology that not only benefitsfrom more than 30 years of experience in manu-facturing and application but is also evolvingtoward greater capability .For the foreseeable future, economic and prac-

tical considerations dictate that coatings ofdispersed particles will be predominant in audioand video tapes, The earliest comercially availableaudio tapes, made in the late 1940's, used ironoxide particles . The most important oxide, y-FC,O,, has been greatly improved as to particleshape and size since its introduction, and it re-tains an important role in audio, video and data-recording tapes today . It is also the most com-

monly used material for the disks, both flexibleand rigid, used in digital data storage for com-puters . The relatively low coercivities (2504(10 Oe)of pure iron oxides, however, proved to be aserious limitation as recording densities increased .An explanation for this is found in the self-demagnetization effect that exists in magnetizedmaterials . Every magnet generates an internalfield, which tends to oppose the magnetizationthat gives rise to it . This effect becomes strongeras the magnet is made shorter along its directionof magnetization (Fig . 10), and will be enhancedby the presence of nearby magnets if similar polesare placed together . The result of these phe-nomena is that magnetic recording, conventional-ly visualized as having the magnetization parallelor antiparallel to the direction of motion of thehead with respect to the tape or disk (Fig . 1), suf-fers from a demagnetization effect that becomesmore severe as the recording density becomeshigher (that is, as the recorded wavelengthbecomes shorter) . The ability of a magneticmaterial to resist demagnetization is expressed byits coercivity .The requirement for higher coercivities was first

met in the mid-1960's with the introduction ofchromium dioxide (Cr0z) . Like iron oxide, thismaterial derives its coercivity and squarenessproperties, and therefore its ability to retain mag-netization, from the needle-like shape of the par-ticles . Chromium dioxide was first introduced fordigital computer tapes . It was soon applied inaudio cassettes and is today used in some videocassettes .At the present time, the most widely used

materials for applications requiring coercivitiesin excess of 400 Oe are iron oxide particlesmodified by the addition of cobalt . The interac-tion of the cobalt ions with the iron oxide struc-ture provides additional resistance to the switch-ing of the magnetization direction, and thus in-creases the coercivity, Values in the range of500-600 Oe for audio cassette tapes, 600-700 Oe

Section 5: Production Facilities

Short Magnet ;Strong Internal Field

Long Magnet:Weak Internal Field

Flip . 10 . Internal demagnetization fieldsfor magnets of different shapes .

Chapter 7: Magnetic Recording Media

for video tapes, and 800 Oe or above for futureapplications are readily achieved . Many processesexist for adding the cobalt to the particles . Mostin use today leave the cobalt segregated near thesurface of the particles . Such particles, called"cobalt-surface-doped", "cobalt-adsorbed","cobalt-epitaxial", or "cobalt-ferrite epitaxial"are now the predominant materials used in videocassettes and open-reel video tapes and are find-ing increased use in high-density data diskettes .Like undoped oxides and chromium dioxide, theyretain magnetization along the particle axis . Themanufacturing processes of tapes are usuallydesigned to orient the particles along the tapelength in order to increase the maximum rema-nent magnetization in this direction .Tapes made with particles of metal (iron or iron

alloys) have much higher remanent magnetizationvalues than are possible in tapes that use oxideparticles, and accordingly give much higher signallevels, This property is especially valuable in verycompact formats, and metal particles are used insome audio cassette tapes and also in 8-mm con-sumer video tapes . These products have general-ly had coercivity values in excess of 1000 Oe, re-quiring much higher head fields for recording anderasure than those needed for oxide tapes . Recent-ly, however, metal particles have become availablewith coercivities near 700 Oe, similar to those ofcobalt-modified oxides .Another particulate material currently being

studied for recording applications is barium fer-rite . The particles are formed in the shape ofsmall hexagonal plates, each of which has itseasiest axis of magnetization perpendicular to itsflat faces . If these plates are incorporated intoa tape coating so as to lie roughly in the planeof the tape surface, recording can be done pri-marily with perpendicular magnetization .

Fig . 11 shows the properties of various partic-ulate magnetic recording materials .Whatever the composition of the particles in

a recording tape or disk, their size is extremelyimportant, since it determines the number of par-ticles contained in each magnetized region of therecorded signal . The greater this number, thegreater will be the maximum signal-to-noise ratiothat the recording medium can provide, since eachparticle contributes a pulse of magnetization tothe playback headed . A great many small pulsesclearly will combine to make a more accurate,less noisy signal than a smaller number of largepulses (even though the total density of magneticmaterial present may be the same), and the ef-fect of random fluctuations in particle size orplacement will be less .One of the most important and difficult arts

in the manufacture of magnetic recording mediais the dispersion of particles in the coating . If ap-

CrQ,

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5.7-133

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Fig . 11 . Various particles used In magnetic recor-ding. The preferred axes of magnetization ere shownby broken lines . Some typical, approximate dimen.slons (In pm) and magnetic parameters are Indicated.

preciable amounts of the particles are clumpedtogether, the recording performance will suffer .The noise properties of the signal will bedominated to some extent by the clusters of par-ticles, which are of course much larger than theindividual particles themselves . A great deal ofattention must therefore be paid to the formula-tion and milling of the dispersion, and to itscoating and drying .

In the search for magnetic materials that givehigher performance, care must be taken to avoidvarious undesirable features . For example, thetrend to reduce particle size, and thus enhancesignal-to-noise ratios, must not be carried to thepoint where the particles are so small that theybegin to show the effects of thermal instability .In particular, the presence of excessively smallparticles can lead to the unwanted acquiring ofa signal in one layer of tape on a reel as a resultof the fields due to a strong signal on an adja-cent layer, This phenomenon is often referred toas "print-through" or "pre- and post-echo", andis important only in (ac-bias) audio recording . Insome applications, especially audio, thorougherasure is an important concern and one thatplaces an upper limit on the coercivity of therecording medium . Efficient recording anderasure also require that the material's switching-

0 0

5.7-134

field distribution (Fig . 4) be relatively narrow.This means that the fields required to reverse themagnetization of the individual particles shouldbe tightly clustered around the coercivity (whichis essentially the median switching field) .Although this discussion has focussed on

magnetic properties, one must not underestimatethe importance of the non-magnetic componentsof recording media . The base film, the binderpolymer, and the various lubricants and other ad-ditives in the coating are crucial to strength,durability, and freedom from undesirable levelsof friction . The manufacture of recording mediarequires at least as much capability in organicchemistry as in magnetic materials . The abilityto reliably produce coatings with smooth surfaces,substantially free of defects, is also an absolutenecessity .

PROGRESS TOWARDHIGHER RECORDING DENSITIES

The state of the art in magnetic recording hasclearly advanced in recent years . Fig . 12 shows

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Fig. 12 . Tape volume required per hour of playback,plotted against year of Introduction, for various

video formats . This plat was supplied byA.R . Moon, of 3M.

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Section 5 : Production Facilities

the great gains in practical recording density thathave occurred in the video area . Intensive researchand development is today aimed at continuing thistrend . Two major driving forces are the desirefor more compact consumer video products, es-pecially cameras, and the trend toward digital re-cording of both audio and video signals . A briefsurvey of current developments will be given here,with emphasis on digital video .As was mentioned earlier, the use of digital

techniques requires a frequency bandwidth thatis much increased over that needed for analogrepresentation . Consider, for example, a broad-cast-quality video signal that requires a bandwidthof roughly 10 MHz far adequate analog (FM)transmission . The analog recording system musthandle 2 x 107 magnetic transitions per second .A digital system designed to transmit this samesignal adequately would require nearly 3 x 10shits per seconds . Assuming that one bit of infor-mation corresponds to one magnetic transition(although this ratio varies somewhat with thedigital code used), the digital recording systemmust handle nearly 3 x lily magnetic transitionsper second, a fifteen-fold increase over the analograte . Clearly, if the use of digital rather thananalog technology is not to entail a large increasein tape consumption, the digital recorder mustuse a higher density of magnetic transitions persquare inch of tape . This density is the productof track density and the linear transition densityalong a track, Table I gives the values of theseparameters in some practical and hypotheticalvideo systems . The projected increases of record-ing density require the achievement of adequatesignal strength and signal-to-noise ratios despitethe use of much smaller magnetized regions inthe recording medium. The difficulty presentedby self-demagnetization at high densities wasdescribed earlier, as an explanation for the needto increase coercivities . However, a limit existsas to how much high-density performance can bebought by simply increasing the coercivity, sinceavailable heads must still be able to record, andalso to erase or over-write, the material . Anotherroute to avoiding or relaxing the density restric-tions imposed by demagnetization is to avoid thehead-to-head arrangement of magnetized regions .This can be done by using, to a greater or lesserextent, the perpendicular (sometimes tailed ver-tical) component of magnetization' 1,12 . See Fig .13 . The relative merits of the various recordingpatterns, and of various head designs for creatingand reading them, are currently under intensivestudy in many academic and industrial labora-tories . It is likely that recording at relatively highdensities has always used same degree of perpen-dicular magnetization, but new materials, in-cluding barium ferrite particles and some metallicthin films, are designed to accentuate this com-

Chapter 7; Magnetic Recording Media

aThis system embodies recording densities achieved in the laboratory . Digital systems closer to practical realizationuse somewhat smaller densities and achieve a tape usage closer to that of the analog C-formats

blncludes tape area not devoted to video information .

ponent . The multiaxial cobalt-doped oxide par-ticles (see Fig . 11) can be used to implementmixed-mode recording (Fig . 13) in a more or lessisotropic medium .Output losses at high densities can result from

a number of causes other than demagnetization .Some are related to head design, and it is veryclear that development of heads and recordingmedia must proceed together in the search forhigher-density recording, Fig . 14 shows somehead designs under current consideration . Thecritical dimension of the head used for playback,the gap width of a conventional ring head or thepole width of a perpendicular-recording head,must be smaller than the shortest wavelength to

Longitudinali

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demagnetization fields ; see Fig. 10.

TABLE I

be reproduced . The head used to record the wave-form can have larger dimensions; indeed, thismay be advantageous .Whatever the head design and the recording

medium composition, the closeness of the con-tact between the head and the surface of themedium is a factor of major importance in deter-mining the achievable recording density . This

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5.7-135

Fig. 14 . (A) Conventional ring-type head,

usbothlongitudinal and perpendicularrecordSingle-polehead, for perpendicular

recordiSingle-polehead, driven by auxiliary pole,

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madsuitabledimensions, be used for reading as

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0 6

5.7-136

follows from the fact that the resolution withwhich the head can record and read back deterio-rates rapidly as spacing increases . The effect onthe recording process has been estimated as a lossof 44 dB per wavelength of spacing, for a sinu-soidal signalt 3, and that on the playback processas a loss of 55 dB per wavelengthl 4 . This totalof nearly 100 dB per wavelength means, for in-stance, that for sinusoidal recording at 50,000transitions per inch (corresponding to a wave-length of 40 microinches or about one micro-meter) every microinch of spacing will causeabout 2 .5 dB of overall loss in the reproducedsignal . Obviously, variations in the spacing willamplitude-modulate the signal with the same sen-sitivity, and therefore add noise components tothe signal . Thus, media surface smoothness maywell be the ultimate limiting factor of magneticrecording density, although further developmentof heads and media are needed in order to reachthis limitts . In addition to smoothness, the free-dom from flaws and contaminants is crucial, ifinformation "drop-outs", or momentary lossesof signal, are to be minimized . As mentionedearlier, techniques of error detection and correc-tion (or concealment) allow satisfactory perfor-mance even if imperfections exist in the record-ing medium. Even a highly sophisticated error-correction system, however, can be overwhelmedby defects that are excessive in number or size .As the recording density increases, the size of theflaw needed to cause a given system failure (visi-ble video defect, audible audio defect) decreases .Thus, the development of advanced magneticrecording media entails the search for the meansof producing highly perfect surfaces, as well asideal magnetic properties .

Section 5 : Production Facilities

4 . B . Blesser, "Digital Recording", db, TheSound Engineering Magazine, a series begin-ning with July, 1980 . (The July, August, andSeptember issues give good introduction tothe basics .)

5 . J . P . Watney, "Technical Choices for aVideo Recorder", in Television Image Quali-ty, J . B . Friedman, Ed ., Society of MotionPicture and Television Engineers, Scarsdale,New York, 1984, pp . 127-136 .

6 . J . R . Watkinson, "Error Correction Tech-niques in Digital Audio," Fifth InternationalConference on Video and Data Recording,Inst . of Elect . and Radio Eng ., Proc . No .59, 1984 .

7 . H . Kybett, "Video Tape Recorders,"Howard W. Sams & Co ., Inc ., Indianapolis,1978 .

8 .

G. Bate, "Recent Developments in MagneticRecording Materials," Journal of AppliedPhysics, 52, pps . 2447-2452, March 1981 .

9 .

M. P. Sharrock and R. E . Bodnar, "Mag-netic Materials for Recording ; An Overviewwith Special Emphasis on Particles", 30thAnnual Conference on Magnetism andMagnetic Materials, to be published in theJournal of Applied Physics, 1985 .

10,

J . C. Mallinson, "Maximum Signal-to-NoiseRatio of a Tape Recorder," IEEE Trans .Magn., MAG-5, pp . 182-186, September1969 .

11 . S . Iwasaki, "Perpendicular Magnetic Re-cording", IEEE Trans. Magn., MAG-16,pp. 71-76, January 1980 .

Recording ACKNOWLEDGEMENT

12 . J . U . Lemke, "An Isotropic ParticulateMedium with Additive Hilbert and Fourier

Medium Field Components," Journal of Applied(Tape orDisk) The author wishes to thank J . T . McKinney, Physics, 53, pp . 2561-2566, March, 1982 .

A. R, Moore and N . C. Ritter for helpful infor- 13, H. N . Bertram and R, Niedermeyer, "Themation and discussion ; also L . J . Nelson, C. A. Effect of Spacing on Demagnetization inNewman, and R.J . Spangenberg for help in Magnetic Recording," IEEE Trans. Magn.,preparing the manuscript and illustrations . MAG-18, pp. 1206-1208, November 1982 .

14 . R . L. Wallace, Jr ., "The Reproduction ofREFERENCES Magnetically Recorded Signals," Bell System

Tech . Journal, 30, pp. 1145-1173, October1 . C . D . Mee, The Physics of Magnetic Re- 1951 .

cording, North-Holland Publishing Com-pany, Amsterdam, 1964 . 15 . M. P . Sharrock and D. P . Stubbs, "Perpen-

dicular Magnetic Recording Technology,"eful for 2 . F . Jorgensen, The Complete Handbook of in Television Image Quality, J . B . Friedman,ing . g) Magnetic Recording, TAB BOOKS, Inc ., Ed., Society of Motion Picture and Televi-ng . (C)r perpen .e with

Blue Ridge Summit, Pennsylvania, 1981 .3 . P . Vogelgesang, "Another Look at Record-

sion Engineers, Scarsdale, New York, 1984,pp . 137-151 ; and published in the SMPTE

well as

ltt t9

9ing Bias," Modern Recording & Music, pp. Journal, 93, pp . 1127-1133, December

arch and 36-43, December 1980 . 1984 .

System

CurrentC-format,one-inchopen-reelvideo

Currentsix-hour

VHS video

Projecteddigitalvideos

Magnetic Transitions per second (maximum) 2x107 10 , 3x10'Transitions per inch along track (maximum) 2 x 10` 4 x 104 10 ,Tracks per inch 1 .4 x 10= 1 .3 x 10' 103Transitions per square inch (maximum) 3 x 101, 5x10' 108Tape consumption, in square inches per second lab 0.2b 3

84-9811-0795-0(105 .3) R

"Reprinted with the permission of the National Association ofBroadcasters"

Copyright 0 1985 by the National Association of Broadcasters .

All Rights Reserved .

Copyright 1935, 1938, 1946, 1949, 1960, 1975 by the National Associationof Broadcasters . All rights reserved . Printed in the United States of America.No part of this book may be reproduced in any form or by any means withoutpermission in writing from the National Association of Broadcasters, 1771N Street, NW, Washington, DC 20036.

The content of manuscripts provided by individual authors to the NationalAssociation of Broadcasters for inclusion in this publication is solely the ef-fort of those individuals . The National Association of Broadcasters is notresponsible for and makes no claim as to the veracity of statements containedtherein or any omissions thereto.

Library of Congress Cataloging in Publication DataMain entry under title :

National Association of Broadcasters Engineering Handbook .

Includes bibliographies and index.1 . Radio-Transmitters and transmission . 2. Radio stations-Design and con-

struction. 3 . Television-Transmitters and transmission . 4. Television stations-Design and construction . I. Crutchfield, E.B. II . National Association ofBroadcasters .TK6561 .N3 1985 621 .3841 85-3023

ISBN 0-89324-000-1