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The world today is fairly bursting at the seams with recording tape. Many of our normal business, social, and entertainment activities would grind to a halt without it. At least three different tape formats are currently in use for entertainment purposes alone, and each of these has its own particular areas of special competence. Little wonder, then, that all this activity has resulted in a proliferation of special-purpose tape types manufactured to do a particular job particularly well.

The types of audio tapes available to the home user go by such names as general purpose, low noise, low print, high-output/low-noise, and so on. Within each of these categories there is also likely to be a variety of thicknesses, lengths, and base materials available. Recently, moreover, some tapes have appeared that employ special coatings on their non-oxide surfaces that are designed to improve mechanical handling or storage characteristics. And, finally, there are the various types of tape used in cassettes, eight-track cartridges, and in open-reel machines, some of them specially treated to perform optimally in their special formats.

Despite the profusion of products, all tape types do resemble each other superficially, if one ignores occasional differences in color, surface shininess, or the use of a back coating. But these are physical qualities, not electromagnetic ones, and similar appearance does not necessarily mean similar performance. An examination of some of these common factors, however, as well as some of the differences, is helpful in understanding what makes one tape superior to another for a given recording task.

All magnetic tapes consist of a coating, or emul-sion,-permanently bonded to a plastic film, or base. The coating contains the magnetic material, the "active" ingredient that makes recording and reproduction possible. The base film, which determines the mechanical properties of the tape, acts as the physical support for the coating. Open-reel tapes are always wound with the coated side facing the hub. In cassettes and eight-track cartridges, the reverse is true  -  the coating faces out because of the tape-path arrangements in these formats.

In most cases, it is easy to distinguish the shiny base side from the relatively dull coated side. However, many modern tapes (particularly cassette tapes) have a polished coating that is nearly as shiny as the base. And recent developments, such as the use of dull black conductive back coatings applied to the base of the tape, have further confused the issue. This back coating helps in several ways: it eliminates static electricity, it affords the capstan and pinch roller a better grip, and it allows the tape to hold its "pack" better on its hub or reel. But unless care is exercised, the question of "dull" or "shiny" can sometimes lead to confusion; the shinier side may well be the side to record on.

The color of the magnetic coating has been undergoing some changes as well. The usual oxide-coating color is brown, because this is the normal color of the iron oxide used in the majority of tapes. Chromium dioxide, however, is a black powder-but that is not to say that all black tapes are made with chromium dioxide. Many iron-oxide tapes are black, too, or at least dark grey, because of black carbon particles added to reduce electrostatic-charge buildup on the tape (such charges can cause noise, and sometimes even jamming in cassettes). Widespread opinion to the contrary, the color of the coating has no necessary bearing on the tape's performance in the electromagnetic area.

The film that serves as the base for the magnetic coating is in most cases either cellulose acetate (often called just "acetate") or polyester (the best-known brand being Du Pont's Mylar). The buyer can tell them apart by simple inspection: acetate is translucent and polyester is opaque when a reel is looked at edgewise against a bright light.

A good base material must be strong enough for its intended use; it must also be flexible, smooth, and dimensionally stable. Both acetate and polyester have proved themselves capable of meeting these specifications, but polyester is the better of the two, especially if it is to be used or stored under extreme or varying conditions of temperature or humidity.

It is significantly stronger than acetate and chemically more stable (acetate, because of the slow loss of its plasticizer, becomes subject to brittleness and cracking with age). Further, tapes expand or contract on their reels under the effects of changing temperature and humidity, and these dimensional changes generate stresses within the tape pack that can cause a number of physical problems, among them radial deformation ("spoking"), curled edges, or "cupping." Tapes wound unevenly or at too high a tension are particularly vulnerable to such deformations, and although polyester and acetate can both be affected, the changes in polyester are much less severe.

Acetate-base tape does have two important advantages, however, and they have made it popular even in professional recording studios: it costs less than polyester, and it breaks cleanly when accidentally snapped, with little or no stretch at the point of the break.

Polyester, on the other hand, stretches as much as 100 per cent or more before breaking, often making it impossible to repair a recorded tape without losing critical material. To combat this shortcoming, polyester-base tape is made available in "tensilized" form, or T-polyester. Tensilizing is a prestretching process that increases both the break strength and the stretch resistance by a factor of nearly two. Long-playing cassettes with very thin tapes were made possible only through the use of T-polyester.
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The tape's magnetic coating (Beschichtung) consists of several ingredients that are carefully - uniformly - mixed and dispersed for maximum homogeneity in the finished product. A good coating must meet all the many physical and electromagnetic requirements for its intended use.

Since some of these requirements may be conflicting, best results often depend on a number of carefully chosen design compromises. For instance, the coating must be physically tough and durable to resist wear - but at the same time it must not be abrasive to the tape heads. It must also be dense and scratch-resistant, but not brittle or stiff, for these characteristics would eventually lead to edge damage and momentary signal loss (dropouts).

The basic ingredients of a typical magnetic tape coating are the magnetic material, the "binder", and various additives (plasticizers, lubricants, etc.). The key ingredient is, of course, the material that makes the tape magnetic. In most tapes, this ingredient is iron oxide, but other materials such as chromium dioxide and "cobalt-doped" iron oxides have been introduced for use in cassette tapes. Since these materials, in the view of some manufacturers, have certain shortcomings in addition to having magnetic properties that can benefit performance, they are now being challenged by the newest iron-oxide formulations. These latest tapes, whose improved qualities derive from better oxides and novel processing, have the additional advantage that they do not require special bias or equalization settings.

The typical oxide particle used for magnetic recording, whether of iron or some other substance, is a tiny needle-shaped crystal approximately six times as long as it is wide. The particles come in various sizes, but the length-to-diameter ratio remains substantially the same. So-called "standard" tapes use a relatively "large" particle about 25 millionths of an inch long by 4 millionths of an inch thick. The oxide particles used for low-noise tapes are several times smaller, the size reduction being responsible for the lower tape hiss.

High-output/ low-noise tapes may use still other iron oxides that, in addition to being smaller, are smoother, more uniform, and therefore capable of being more densely packed in the coating. The needle shape of these oxide particles makes them magnetically anisotropic, which is to say that they have different magnetic properties in different directions. Anisotropic oxide particles are much easier to magnetize and harder to demagnetize (important features of a good tape) in the long direction than in the short.

To take advantage of anisotropy, the oxide particles are physically rotated when the tape coating is still wet, so that their long dimensions line up with the length of the tape. As a result, the tape has better magnetic properties in the direction that the record and play heads are effective - i.e., along the tape length. Once the tape coating is dry, of course, the particles are fixed in place by the binder and cannot move around. They can, however, be magnetized in either of two directions or polarities, in accordance with the polarity of the external magnetic field applied by the recording head. The word "permanent," as used in connection with these tiny magnets, implies a resistance to change. Indeed, the particles do resist a change in their magnetic state: a minimum magnetic force (coercive force) is required to overcome this resistance.

The amount of coercive force, expressed in oersteds, is therefore one of the important specifications for a magnetic oxide - and for a type of tape, too, for that matter. Specifically, it is the minimum magnetic force which must be applied to the tape by the recording head before a recording is made. The same coercive force is required to erase a recording already made, since this also involves a change in the magnetic state of the tiny particles.

A high-coercive-force tape such as chromium dioxide requires more magnetic energy to record and erase than a tape with a lower coercive force. A second significant property of any permanent magnet is its ability to "store" magnetism. The quantity of magnetism which a tape can "take" and retain is called its magnetic induction, and the unit of measure is the gauss. It is obvious that the higher the induction the better, since the induction determines the amount of output signal a tape can produce in the playback head.
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Next comes the binder (übrigens ein deutsches Wort !!), the coating substance that fixes the oxide to the plastic base. Ideally, the binder also maintains an optimum separation between the individual oxide particles by providing a spacing coating surrounding each particle. Particles which are not properly separated will partially cancel each other's magnetic energy and thus reduce the tape's potential output.

The basic binder materials are various compositions of plastic resins. All the important physical properties of the coating - resistance to wear, friction, loss of oxide, and many others - are determined by the binder. Of the total coating volume, the oxide particles occupy only about 30 to 40 per cent. If, in an effort to achieve a greater signal output, more oxide is put in, a weaker cohesion of the coating could result. It is quite a technical feat to "build" a high-output / low-noise tape with superior recording performance without sacrificing some of the tape's desirable physical properties.

Depending on the use for which the tape is intended, the basic binder materials may be supplemented by additives which change or improve certain properties that affect physical performance. For example, some binders are rather stiff, especially when the coating is thick, and plasticizers added to the binder will give the coatings a necessary flexibility.

Different lubricants (there are many effective ones beyond the much-publicized silicone) may also be added to reduce friction between the tape, the heads, and the tape guides, and even between layers of tape in eight-track cartridges. And, of course, not every tape needs a lubricant; some use a low-friction binder that obviates the use of an additional ingredient.

Since tapes are composed principally of plastic, they are electrical non-conductors and therefore susceptible to the buildup of electrostatic charges. These charges attract dropout-producing dust; they may also produce popping noises during use, and even cause jamming in cassettes. To prevent this, electrically conductive agents such as carbon powder may be added to the binder to prevent the buildup of electrostatic charges.

A superior method of increasing a tape's conductivity is to use the carbon in a backcoating rather than in the binder. This not only eliminates static much more efficiently, but also improves the mechanical performance of the tape. Furthermore, the removal of the carbon from the coating leaves more room for oxide, thus raising the output level.

There are many other binder additives such as wetting agents, stabilizers, fungicides, etc., and each performs a specific function. A well-designed tape, however, will have as few additives as possible, for each additional ingredient must be integrated into the coating without displaying any short- or long-term tendency to migrate or undergo other undesirable changes.
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Combining all the elements that make up a quality finished tape requires strict control of materials and procedures throughout the manufacturing processes.

Each step must be right the first time or the tape will be defective. There is no opportunity for later adjustment or correction. Quality control begins with the first inspection of the incoming raw materials. Since all materials have some minor variations in either physical or chemical properties, the inspection must determine which materials fall within the established tolerances.

The oxide material, for instance, is tested for its magnetic properties using such sophisticated equipment as vibrating sample magnetometers and hysteresis-loop tracers. Electron microscopes are employed to examine the particles visually under a magnification of many thousands of diameters. Properties such as particle size, size distribution, and imperfections can then be readily observed.

All manufacturing-process chemicals and solvents undergo chemical analysis with such equipment as gas chromatographs, infrared spectrometers, and other instruments which analyze their composition in minute detail, detecting the smallest trace of impurity. Even base film is examined for thickness uniformity, cleanliness, physical stresses, etc. Base materials also require special ambient conditioning before use to assure that they are "relaxed," wrinkle-free, and without contaminants.

Much effort and expense is devoted to all these preliminaries, since no chances can be taken by a supplier of first-class tape. Critical users rightly expect that not only will the product be good, but that it will be consistently good, reel after reel, over the years.
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The first manufacturing step is the milling - the mixing/interdispersing of all the coating ingredients. The most familiar type of machine used in this step is the ball mill (die Kugelmühe), a large rotating drum partly filled with small steel balls. (Perhaps needless to say, in this as in other areas of tape manufacture, different companies have developed their own techniques and hardware.)

The various ingredients are loaded into the mill and a solvent for the plastic binder material is added. The entire composition is then thoroughly mixed by the mill until it is homogeneously uniform and smooth. The ultimate purpose of the process is to have each oxide particle wetted, coated with binder, and isolated from its neighbors. When the milling is finished, the result is a thickish liquid, of paint-like consistency, called slurry.

Milling is an extremely critical operation because either too little or too much is harmful to the quality of the finished slurry. Insufficient milling may result in undispersed groups of oxide particles which cause hiss, noise bursts, lower output, poor amplitude uniformity, and coating weak spots which will eventually turn into dropouts.

On the other hand, overmilling breaks the particles down to too small a size, causing loss of high frequencies, increase in print-through, and other problems. Milling time can range from several minutes duration all the way up to two weeks. Magnetic and physical tests are performed on the liquid slurry to determine the precise end point of the milling process for a desired result.
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After filtering and several other conditioning steps, the slurry is fed to the coater, a giant machine that resembles a rotary printing press. The minimum number of separate processing stages built into the coater machine are called the take off, the coating zone, orientation, drying, and the take up - the taking off and on referring, of course, to reeling the raw base material off and then, when coated, onto a spool, and orientation having to do with the magnetic aspect of the coating. Some coaters perform more individual functions, but these five operations are essential to even the simplest machine.

A typical high-speed coating process begins with a large roll of base film, 15,000 feet (or more) long and several feet wide, which is loaded on the input side of the machine. The film is threaded through the entire complex of continuous operations in the coater, the whole is started up, and in due time the coated film issues from the take-up end, many hundreds of feet away from the start. The process is designed to be continuous to the degree tiat there is even a method of supplying new rolls of base film automatically as the previous rolls are used up.

The base film goes first through a base treatment and conditioning section until the coating zone is reached, at which point the slurry is applied in a precise and uniform thickness. Many coating methods are used. One of them resembles, in principle, the spreading of soft butter on bread with a huge knife.

Another could be likened to the operation of a precision paint roller, and a third imitates the inking roller used in printing magazines such as the one you are now reading. And there are perhaps a dozen other methods capable of doing the job right.

The coating thickness on typical consumer tapes ranges from 70 to 650 microinches; some backcoatings are as thin as 20 microinches (compare with the 4,000-microinch thickness of a dollar bill). During the coating operation, thickness uniformity is monitored and controlled continuously by measuring the coating's absorption of either X-rays or of a radioactive source. This control is vital because variations in thickness will cause corresponding variations in low-frequency output; it can also change optimum operating points for bias or recording level, disturb amplitude stability, and generate other havoc.

Thin coatings make particularly strenuous demands on the coating equipment. A cassette tape, for instance, with an average coating thickness of 200 microinches, may require a thickness control of ±5 microinches to maintain a ±2.5 per cent thickness tolerance.

A good coating requires more than thickness uniformity. It must also be extremely smooth, completely free of streaks, voids, or even microscopic blemishes. Perfection of this kind is costly and difficult to achieve. It is not always necessary for home recording, but it is of paramount importance in computer tapes and in recording-studio mastering tapes.

The tape manufacturers who successfully produce such tapes naturally have the capability of achieving the desired degree of perfection for the home user as well.

After coating, the oxide particles in the slurry are oriented by passing the already coated base, still wet, through a powerful electromagnetic field that lines the particles up parallel to the long dimension of the tape. Then the tape moves into the drying tunnel, where heated air evaporates the solvent from the coating at a carefully controlled rate.

Drying which is too fast can cause some solvents to evaporate too rapidly, leaving the coating with pits and pinholes that cause noise and dropouts. Conversely, an incompletely dried tape may stick to itself or gum up (ankleben) the recorder's heads. Both of these problems can occur if temperature, air volume, and air velocity are not closely adjusted for the specific tape being manufactured.

The coated tape coming out of the drying tunnel - it is still in the form of a sheet of film several feet wide - is wound with carefully controlled tension onto large-diameter cores to make up jumbo rolls.

The large cores reduce the number of layers in the rolls and thus minimize stresses in the tape. When good high-frequency response is an important requirement, quality tape may undergo polishing of its freshly coated surface. Intimate contact with the heads is necessary if high-frequency losses are to be avoided.

For example, if a 10,000-Hz signal is recorded on a cassette tape running at 1 7/8 ips, a -6 dB output loss will take place if there is only a 10-microinch gap between tape and head. Any roughness of the tape coating will tend to cause such gaps, since only the high points on the surface will come into contact with the heads.
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The polishing treatment is accomplished in several ways. Brushing, burnishing, and even rubbing tape surfaces against each other have all been used with reasonably good results.

In one popular method, the tape is passed between two or more highly polished heated rollers that exert a very high pressure on the tape. A mirror-like smoothness is obtainable with this technique.

(Obvious differences in the visual shininess of two coatings can sometimes indicate the one with the better high-frequency performance, but the method is not foolproof, since there are many other invisible factors which can also influence high-frequency response.)

Slitting the wide rolls of tape to the widths (Das badn auf Maß schneiden) in which they will be used is the final manufacturing step. Cassette tapes have a width of 0.149 inch; all other audio tapes for home use are slit to the 1/4-inch width - more precisely, 0.248 inch.

Slitting is done by rotary cutters running at high speeds. In most cases, the entire roll width is slit simultaneously. But, despite the mass-production nature of the process, slitting has to be a precision operation because of the critical demands that will be made of the end product.

First, tape width must be very accurate - the width tolerance on a cassette tape is ±1 mil, for instance. Tape which is too wide will stick in the recorder guides and suffer edge damage through folding or scraping; tape which is too narrow may weave as it passes through the tape transport.

Second, finished tape must not exhibit skew or "snakiness," which occurs if the tape is not slit in a perfectly straight line, for it causes the tape to move past the heads at a constantly changing angle with respect to the head gap and creates a continuously varying azimuth misadjustment with resultant variations in high-frequency output.

And finally, the slit edges must be cut cleanly. A poorly slit edge will generate dirt and dropouts and affect the sound quality on the track closest to the edge. On the other hand, it should be noted that the edges can never be as smooth as the coating, and a slight polishing action consequently takes place as the tape edges rub against the guides and reel flanges. The material thus rubbed off the edges is frequently deposited on rubber pinch rollers or even on the heads, appearing as two thin lines of oxide just a tape-width apart. This occurrence is not abnormal providing it is not excessive, and is one of the reasons periodic cleaning of your tape recorder is necessary.

Such things as the application of back-coatings, attaching of leaders and auto-reverse switching foils, preparation of tape for loading into cartridges and cassettes, and more are manufacturing steps that may or may not be performed, depending on the tape's ultimate application. Most open-reel tapes, at least, are completed at slitting, at which time they are tested, wound on reels, demagnetized, and packaged for shipment.
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Laboratory quality-control tests of the finished products are carefully performed using various procedures. For example, the important quality of surface smoothness could be evaluated by eye or by running a frequency-response test. To secure quantitative measures, however, a precision surface analyzer is used that produces a chart of the actual physical profile of the tape - with a sensitivity of 1 microinch !

The basic quality-control tests performed are:

• 1. Physical - dimensions, strength, smoothness, life, head wear, temperature-humidity stability, etc.
• 2. Magnetic - coercive force, induction, and other purely magnetic properties.
• 3. Recording performance - bias characteristics, frequency response at various speeds, distortion, uniformity, noise, dropouts, print-through, and others.

Quality control may even extend to checking boxed tape after it has arrived for warehousing or on dealers' shelves. Tapes are examined critically from the customer's point of view: cartons of tape may be shipped back and forth across the country by various means, for example, to see how well the tape and its packaging stand up under typical shipping and storage conditions.

As might be gathered from the foregoing discussion, it takes a great deal of effort, much experience, and extensive manufacturing and testing facilities to produce a tape that does all the things it should do, reliably and consistently, reel after reel, year after year. But it is all in a good cause: better and better tape recordings!
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