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von P. C. COLDMARK, C. D. MEE, J. D. GOODELL/ W. P. CUCKENBURC
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As part of a long range development program in the field of magnetic recording which CBS Laboratories undertook on behalf of Minnesota Mining and Manufacturing Company, recorded tape systems for the home have been under study over a period of several years.

In order that recorded tape can take an important place in the field of home entertainment, one must take into account a great many requirements, some of which are not easily met.
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• 1. The tape must be contained in a compact cartridge in such a way that no part of the tape is exposed.
• 2. The amount of tape must be small and the cost of the cartridge low in order that the price of the final product can approach that of the disc record.
• 3. The sound should be stereophonic with provision for three tracks for maximum flexibility.
• 4. A complete musical composition should be played without interruptions; that is without reversing the cartridge or tape.
• 5. The quality of sound should be at least as good as the best of existing recorded media.
• 6. The durability of the tape and cartridge must be high enough so that after several hundred plays, the sound remains unchanged.
• 7. It should be possible to place a number of cartridges on a tape machine equipped with a changer-type mechanism so that one can provide music for several hours.

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Here we will report on the outcome of these studies and subsequent developments which we believe will satisfy the preceding conditions and requirements.

It was clear from the outset that one was dealing with "a system" rather than just "a few components". Thus intensive development work over a period of several years progressed simultaneously in such areas as methods of signal recording, magnetic transducers and playback heads, design of cartridges and tape transport mechanisms. The Laboratories' system work, in close cooperation with 3M, also included the development of a new tape with characteristics that provided optimum matching into the overall performance.

Late last fall (herbst 1959) the new recorded system was in a sufficiently advanced stage to demonstrate it to many members of this industry.

3M had, at that time, stated that the Zenith Radio Corporation had joined this effort and entered the design of commercial equipment based on these developments.

Some of the important features and parameters of the new tape cartridge system are as follows:
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• 1. Tape speed is 1 7/8 ips. The width of the tape is 150 mils; the thickness 1 mil, and there is provision for three tracks. Each track is 40 mils wide.
• 2. The cartridge is approximately 3 1/2" square and 5/16" thick. The cartridge contains sufficient tape to play continuously for 64 minutes, and thus will carry more than 98% of the music compositions available without interruptions. The space occupied by the cartridge in its container is approximately 4 cu.in. as compared with an LP record in its envelope with approximately 20 cu.in.
• 3. The tape machine can take five cartridges and play them automatically one after the other. A cartridge can be rejected during any part of its play similar to a record changer. The production versions of this machine now under development by Zenith will have fast forward and reverse speeds. The same instruments will also serve as a home recorder using the new cartridges with blank tape.

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Earlier reference was made to a third track which is located in the center of the 150-mil tape. Extended studies have been undertaken in the Laboratories to determine the optimum acoustic conditions desired by the listener in the average home while playing recorded music. Conventional stereophonic music, as now recorded, provides only a portion of the sounds that are perceived by the listener sitting in a concert hall.

A large percentage of the total acoustic energy which reaches the listener's ears is reverberated and delayed sound which is considerably depleted of its original stereophonic character.

Experiments in the Laboratories have shown that in a space simulating the average living room, a much more exciting and realistic sound can be produced giving an illusion of "being there". Thus, it is intended to record on the third track as an optional feature on the new recorded tape system, the stereophonic sum signal delayed and reverberated to an optimum degree.

The new medium will provide maximum flexibility and a new dimension in sound. The reproducing instruments (die Wiedergabegeräte) can be manufactured for two or for three tracks.

Later some of the electrical and magnetic characteristics of the new system will be discussed. The data and curves shown are already based on the newly developed tape and represent the overall behavior of the entire system, that is, recording, tape, and playback.

The new tape is now in pilot production at 3M, but the cartridges played in current demonstrations still use the older tape on which these programs were recorded last fall (Herbst 1959).
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Following the section dealing with the magnetic aspects of the new system, some of the mechanical problems and their solutions as encountered will be described.

A comparison of the new tape system with the original 15-ips tape master from which both the stereo records as well as the new tape cartridges have been derived, has been demonstrated with success. For this purpose, some sections of music were alternately transcribed from the original master and the 1 7/8-ips narrow track version onto a 15-ips halftrack tape.

In order to achieve an adequate signal-to-noise ratio, frequency response, and dynamic range at a tape speed of 1 7/8 ips, significant developments of most components used in magnetic recording are required. For instance, due to the shorter wavelengths encountered, developments have been aimed at reducing wavelength-dependent losses.

Among the losses in reproduction which have been minimized in the system at hand are those attending

• (1) separation of head and tape surface,
• (2) azimuth alignment of head and tape, and
• (3) playback-head efficiency.

Losses minimized in the recording process are

• (1) tape-thickness loss,
• (2) recording-field configuration loss, and
• (3) loss caused by non uniformity of tape particles.

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1. There is an exponential reduction of the playback-head flux with decreasing recorded wavelength due to the finite separation between the surface of the tape and the playback-head pole pieces.

At 15,000 cps and 1 7/8 ips, this loss is almost 0.5db per micro-inch separation.

2. Another important loss is associated with the azimuth alignment between the playback-head gap and the line of constant recorded magnetization across the track width.

For a conventional 90mil track a loss of 6db occurs at 15,000 cps and 1 7/8 ips. for a misalignment angle of 3 minutes.

3. The proportion of playback-head flux shunted by the gap will increase when using the narrow gaps necessary to resolve the shortest wavelengths recorded at a tape speed of 1 7/8 ips. In order to maintain a high efficiency it is necessary to compensate for a reduction in gap length by a corresponding reduction in gap depth.
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1. A separation loss of the type described for reproduction occurs during recording due to the finite coating thickness.

Those particles remote from the tape surface will thereby give an attenuated contribution to the tape-surface flux and so will contribute less to the playback-head flux.

2. The magnetization of a recorded tape will not be uniform throughout the coating thickness since it depends on the rate of extinction and the direction of the recording field when the critical value for recording is reached after the tape has passed the recording gap.
In addition to this, a further loss can occur due to change in phase of the recorded signal through the coating thickness caused by the vertical curvature of the effective recording plane of the recording-head field.

3. For high resolution of the effective recording plane a sharp cutoff of the recording field must be accompanied by a high uniformity in the magnetization characteristics of the individual particles of the tape. Elimination of particles with low critical fields for switching will also reduce self-demagnetization effects. The separation loss has one advantage in slow speed tapes for audio, since, due to the shorter wavelengths involved, print through is correspondingly reduced allowing new thin tape backing materials to be used with safety.

One of the central problems in recorded tape systems is the design of the tape packaging. Obviously, it is necessary to satisfy requirements of convenience as well as to provide adequate protection for the tape. Naturally high-quality performance with respect to music reproduction is a prerequisite.

In order to popularize recorded tape, it is essential to eliminate the process of manual threading between the reels.

This requirement is dictated by the need for avoiding manual threading and also by the requirement to make the cartridge compatible with a practical automatic changer mechanism.

On first examination the notion (die Idee) of threading the tape permanently between two side-by-side reels contained in the cartridge is attractive. However, every practical design incorporating both the supply and take-up reels in the cartridge requires that sections of the tape be exposed through openings in the cartridge walls with consequent dangers of damage.

Even in a single cartridge player there are many difficulties involved in coupling the tape of a dual-reel cartridge to the drive system and the heads, but when the design of an automated changer is considered, these problems increase rapidly in number and magnitude.

A basic consideration in any type of cartridge is the need for relatively high speed transport in so-called "search" operations. If flanges are used on the reels inside the cartridge, the bulk is considerably increased and many problems of stability are encountered. Thus, high-speed winding without flanges requires some method of maintaining a separation between the tape and the cartridge walls.

The three dimensional geometry of the reeled tape, the driving spindle in the transport mechanism, the walls of the cartridge and other components call for strictly orthogonal relationships or some automatic dynamic adjustment and an accurate system of tape guidance.

Otherwise, the cumulative errors in repetitive reeling of the tape, even on the same machine, will lead to telescoping or angular displacement of the tape reel with respect to the cartridge walls. In brief laboratory experiments these problems may not be evident but in long-term field use the increasing friction produces instabilities in the tape speed and eventually may completely block the reel from rotating.

The problem of smooth reeling without any flanges was solved by introducing a novel guiding member in the cartridge with adequate compliance to insure a smooth rewind cycle.

This arrangement allows a tape with an hour of playing time to be rewound in twenty seconds. (A five-second rewind time has been achieved in the laboratory.)

Threading of the tape (das Band einziehen) is accomplished by means of a leader (Zugband) permanently attached to the takeup reel (Aufwickelspule) in the mechanism. The end of the rewind cycle leaves the permanent leader in the threading path of the machine.

A very simple and economic solution was used for the design of the coupling between the reeled tape and the permanent leader.
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• Anmerkung : Dieses Zugband-System wurde erst 1984 bei der Firma DEC perfektioniert. Vorher hatte es nur mittelmäßig funktioniert. Die Elektronik war 1960 noch lange nicht so weit. Mehr steht bei den DLT Laufwerken im Magnetbandmuseum.

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The slipping clutches, brakes, speed-changing idlers, and the like are operated from the three-dimensional surfaces of a single complex cam which programs all pressure-pad, pressure-roller, and escapement operations.

It is necessary to provide a number of mechanical and a few electrical interlocks to prevent improper manual interference with machine operations, but these are relatively simple and straightforward in design.
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Obviously, there must be some means for sensing the end of the tape and various other portions of the operating cycle. In this machine these results are obtained by means of a simple analog computing linkage that cannot be disclosed in detail at this time. However, the method is independent of the length of the tape in a given cartridge and has displayed a very high degree of reliability.

The authors wish to express their appreciation for the advice and assistance during the course of this work by

B. B. Bauer, A. A. Goldberg, J. C. Jeschke, H. R. Sherman, E. L. Torick, and J. C. Wistrand of CBS Laboratories and Barbara Ivins, formerly with CBS Laboratories. We also wish to acknowledge whole-hearted cooperation of Dr. W. W. Wetzel and his associates of 3M's Magnetic Products Division.

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