To meet the growing demands for higher density storage, increased bandwidth, lower power, and lighter weight, magnetic recording engineers must constantly search for new techniques.

This paper discusses some of these techniques and their application by the Magnetic Recording Section of the "DEP Communications Systems Division", and describes specific equipment in which many state-of-the-art techniques are used.

To appreciate fully the advances made in magnetic recording in a relatively short period of time, note that magnetic recording was in its infancy at the end of World War II; at that time, it was difficult to achieve 15-kc audio at a tape speed of 15 ips (0.001-inch wavelength).

One of the first advancements resulted from the need to reduce the quantity of tape for a given amount of recorded information. Magnetic heads were developed with finer gaps capable of resolving 1/2-mil (0.0005-inch) wavelengths; this development resulted in a tape speed of 7.5 ips. The continuing need for a reduced quantity of tape for a given amount of information lead to the use of half-track audio recorders - that is, two tracks were recorded in the space of one.

RCA's continued development work resulted in another two-to-one reduction in tape speed (3.75 ips) and at the same time doubled the number of tracks (four tracks on 1/4 inch tape). Concurrent with the development of audio recorders was the development of tape stations for the storage of large volumes of data for computer use. Henceforth, a need for the storage of television pictures led to the development of magnetic tape recorders with a 4-Mc bandwidth.

In more recent times a need for both dual-channel wideband recording and high-density recording has arisen. Dual-channel wideband recording is essential in closely correlating the signal from two separate radar returns and in recording one wideband channel of analog signals along with a channel of high-bit-rate digital signals. In both cases the isolation between channels must be at least 36 db.

The importance of weight and size in airborne and spaceborne recorders necessitated the development of techniques for greatly reducing the amount of tape required for recording a given amount of daia. Also, because of the limited power in spacecraft, the recorders must have high operating efficiency.

To fullfill the above needs, magnetic recording engineers of the CSD have developed several novel techniques. To satisfy the demand for dual-channel wideband operation, the "octaplex scan system" was developed.

This technique allows the recording of two simultaneous wideband channels with 0.1 usec interchannel time displacement with isolation between channels of over 36 db. The high-density analog need was satisfied in some cases by narrow-track recording.

In other cases, recording on both sides of thin-base, double-coated tape was the answer. A combination of narrow-track recording and double-coated tape allowed a giant step to be taken in increasing the information per unit volume of tape.

The need for high-density digital recording was satisfied by the development of a modified diphase system1 in which 3,300 bits/inch per track can be recorded. This packing density is about three times greater than that currently available in computer applications.

The octaplex scanning system is based on the earlier development of transverse-scan recording in which a two-inch wide tape is moved longitudinally at 15 ips while being scanned transversely (at right angles to its direction of motion) by each of four recording heads equally spaced on the periphery of a 2-inch-diameter scanning wheel.

Fig. 1 shows a standard (system Ampex) quadruplex scanning assembly for television recording. With a rotational speed of 14,400 rpm and a longitudinal tape speed of 15 ips, 64 transverse scans per inch are made across the width of tape.

The octaplex scanning method allows the simultaneous recording of two channels of wideband information. In the development of the octaplex system, the primary objectives were minimum interchannel time displacement and low crosstalk. Earlier attempts to achieve the minimum interchannel time displacement by other companies resulted in the location of the heads of the A channel so that they are parallel and so that their gaps are in-line with the heads of the B channel. However, this close spacing results in excessive interchannel crosstalk.

The interchannel time displacement resulting from this arrangement is approximately 1 usec. Since the time relationship between pulses recorded simultaneously on two sequential heads in a single-channel operation can be held to a fraction of a microsecond, it was apparent that when another channel of information was recorded with another set of four heads located at 45° from the original set, the time error between the original set and the second set was reduced by a factor of 2; therefore, 0.1 usec interchannel time displacement could be achieved.

Since the heads of the A channel are located at 45° from the heads of the B channel, maximum separation is achieved; thus, in the octaplex system the crosstalk is greatly reduced. The octaplex tape recording format is shown in Fig. 2.

Fig. 3 shows the arrangement of eight heads equally spaced around the periphery of a 2-inch-diameter wheel. The signals to and from the heads of both channels are carried by a common slip-ring assembly, with a design that limits the crosstalk to approximately 40 db.

In an airborne system where the power must be conserved, special attention must be given to such items as the frictional drag of the slip rings in a headwheel assembly.

In the system shown in Fig. 3, the slip rings are clustered on a common shaft. Slip rings of 1/4 inch diameter are needed to allow sufficient separation of the leads to achieve the 40db isolation between the A and B channels.

However, for airborne applications the amount of power required to overcome the friction of 1/4 inch slip rings would be extremely high. In addition to the power problem, if wider bandwidth is needed, even greater separation of the leads is needed; thus, the design in Fig. 3 means even larger slip rings.

The solution to the problem of lower crosstalk at higher frequencies is shown in Fig. 4. The slip rings of channel A are on the "head-wheel end" of the motor shaft (Fig. 4a). The slip-rings of channel B are on the opposite end of the motor shaft (Fig. 4b). The leads of B extend from the heads through a small hole which is drilled through the entire length of the motor shaft. This technique of "isolation-by-distance" of the slip-rings of the A and B channels eliminates the requirement for large (1/4 inch) slip rings.

The slip-ring size in Fig. 2 is 0.090-inch, approximately 2.8 times smaller than the slip-rings shown in Fig. 3. Using this technique, motor power is greatly reduced and over 80db isolation between channels is achieved.

The next step in meeting the additional requirement of higher packing density of information was achieved by the reduction of both the recorded track width and the space between the recorded tracks.

Fig. 5a shows a normal 10-mil-wide track and the 5-mil separation used in the standard television recorders. Use of this format in an airborne recorder yields 66.6 transverse tracks per inch when the longitudinal speed of the tape is 24 ips and when 1,600 head trans-verses per second are made.

Fig. 5b shows the new tape-recording format in which both the recorded track width and the inner-track spacing have been cut in half. The track width is 5 mils and the spacing between the tracks is 2.5 mils. The reduction of the track width, however, does reduce the signal-to-noise ratio by 3db from that of the 10-mil track and 5-mil spacing.

The 3db track-width loss has been more than adequately regained by improvements achieved in head material and improvements in magnetic tape.

Using the new narrow-track format, twice the number of transverse tracks per inch can be recorded (2 x 66.6 = 133 transverse tracks per inch).
To summarize the developments at this point, the two-channel octaplex operation combined with the narrow-track technique allows two channels to be recorded in the original space of one channel; and for the single-channel quadraplex operation, twice the recording time is achieved. The density of information recorded on magnetic tape has thus been increased by a factor of two.

The next step was to develop a double-coated tape; this also uses the transverse scan method.

Fig. 6a shows a 1-mil base of polyester tape with coating thickness of 0.35 mil (a total thickness of 1.35 mils).

Fig. 6b shows the new tape with a base thickness of 0.5 mil, both sides of which have been coated with 0.2-mil oxide (total, 0.9-mil).

Two identical scanning assemblies are used, one for each side of the tape. As the tape moves forward, the first headwheel assembly records; as the tape moves in the opposite direction, the other assembly records. Considering only the double-coated aspect of this development, twice the amount of information per unit length of tape can be recorded.

But what about crosstalk from one side of the tape to the other? The crosstalk from one face of the tape to the scanning head on the opposite face of the tape can be illustrated by the separation loss curve in Fig. 7.

Note that when the separation of the magnetic head from the tape surface is equal to one recorded wavelength, the output is decreased 54 db. Consider the condition where a recording has been made on the A side of the tape and the tape is scanned by a magnetic head on the B side, and the recorded signal on the A side of the tape is an fm signal whose wavelength is approximately 0.3 mil.

Fig. 7 illustrates that the separation due to the base thickness plus the coating thickness is 0.7 mil, and the losses are much greater than 54 db. Thus, crosstalk from one side of the tape to the other presents no problem.

Contact printing describes the condition under which a recording is transferred from one face of the tape to the other face at high temperatures.

Since the "fm"-recording technique results in an extremely short recorded wavelength, azimuth of the magnetic head is extremely critical. If an azimuth error is judiciously chosen for the magnetic heads on each side of the tape, the contact printing will also have an azimuth error. An azimuth error of 3° on each side of the tape will result in a contact printing azimuth error of 6°. This total error reduces the contact printing crosstalk to greater than 60 db, even under the conditions of maximum contact printing.

To erase the B side of the tape after having recorded a signal on the A side, the fringing field of the erase head must be confined to a distance equal to the thickness of the B side of the tape plus the base thickness. In the case where the base thickness is 0.5 mil and the coating thickness is 0.2 mil, the fringing must be limited to a maximum of 0.7 mil. The effective fringing of an erase-head gap is approximately equal to the gap length; consequently, a head designed for this tape and coating thickness must have a gap in the order of 0.2 mil.

A new technique, called modified diphase, (*1) allows the recording of binary bits at a rate approximately equal to the bandwidth of the recorder system. If the information is in a return-to-zero (rz) format, it is converted to a continuous signal in which logical 1's and 0's change the phase of the continuously recorded signal. Using this technique, a rate of 3,300 bits/inch per track has been demonstrated with a dropout rate of 1 bit in 10G. The combination of modified diphase and transverse scan recording results in the achievement of recording and reproduction of a 10-Mc-bit-rate signal.

We now turn to the development of hardware which incorporates the techniques previously described.

A wideband recorder-reproducer was developed for recording data from two separate radar systems (see Fig. 8). The equipment uses the transverse scan technique and the octaplex head assembly. In addition to the recording of two 4-Mc video channels, it also records sweep- and antenna-position data on five auxiliary tracks. Packaged in a single rack, the recorder-reproducer can record 1 hour of two-channel information. Inter-channel crosstalk is greater than 36 db with a signal-to-noise ratio of 36 db.

The transport is shock-mounted for shipboard operation. The octaplex head-wheel design is an extension of the standard television headwheel panel design. An additional requirement of the equipment is operation in a television mode in which the octaplex headwheel panel may be replaced by a standard television headwheel panel. In the television mode, television signals may be recorded and reproduced in the same format as the RCA broadcast video tape recorders, TRT-1B, TR-2, and TR-22.2 . (*3) Thus, interchangeability of tapes between machines is achieved. Except for monitor tubes, the equipment features solid-state electronics throughout.

The first equipment of this type has been delivered to the Naval Electronics
Laboratory in San Diego, California and has recorded and reproduced two simultaneous radar presentations successfully.

Fig. 9 shows the recorder for airborne application; it is a record-only machine with the capability of recording two simultaneous 6-Mc bandwidth channels with two 25-kc-bandwidth audio channels. To achieve 50 minutes of recording, 3/4-mil double-coated tape is used. Recording of 25 minutes is done on one side of the tape with a transverse scan assembly, and an additional 25 minutes of recording is achieved with another scanning assembly as the tape passes in the reverse direction. This unit employs the octaplex controlled-erasing, and narrow-track recording techniques. The airborne unit is contained in 1.3 cubic feet. When inserted into a single-rack playback equipment, the airborne unit becomes a recorder-reproducer.

The SL-X00 was developed originally for the Gemini program. The modified diphase system is used to record 3,300 bits/inch per track. Using 1/4 inch tape, this unit can record up to seven tracks. Operating at 24 volts DC, the unit consumes 10 watts in either the record or playback modes. Because of its small size and ability to record and reproduce under vibration, it is ideally suited for airborne and spacecraft operations. A 2,300-foot reel of instrumentation tape (1/4 inch wide and 1 mil thick) allows 4 hours of recording at l 7/8 ips. A hysteresis speed converter allows playing the tape in the reverse direction at a higher speed, e.g., the recorder plays back 22 times faster than it records.

The PT-300 is the first truly portable wideband (4-Mc) recorder (Fig. 10).
The narrow-track (5-mil track, 2.5-mil spacing) technique allows 1 hour of recording at 4-Mc bandwidth. The recorder is designed in three separate packages: power supply and servo package (16" x 19" x 5") transport package (16" x 18" x 11") and the control panel (6" x 13" x 6").

The transport weight is 55 pounds; the power supply servo, 35 pounds; and the control panel, 6 pounds. Operating from a single 28-volt supply, the recorder requires 15 amps. Tapes recorded on the PT-300 can be played back on the RCA TRT-1B, TR-2, and TR-22 commercial television recorders. Although small and compact, the PT-300 uses a standard, narrow-track headwheel assembly.

There are occasions when the most critical subsystem in a major system is the magnetic tape recorder. Often, the basis on which a contractor is chosen for a
major system is the advanced techniques used in the magnetic recorder; that is to say, the magnetic recording techniques that are proposed determine the ultimate capability of the major system.

The proposal of a high-precision, 14-channel, broad-band recorder for the "Tradex" program was a major contributing element in the contract being awarded to RCA. This tape transport stores 15 channels of broad-band data at a tape speed of 1,000 ips, while maintaining a speed stability of two parts in 100,000. CSD designed and developed the breadboard model and the first prototype for the DEP Missile and Surface Radar Division, Moorestown.

The RCA sealed-cartridge approach to the "Multi-System Test Equipment Program" of the DEP Aeropsace Systems Division, allowed a reduction of weight and size of four to one over the competition (Fig. 11). This approach was a key element in the award of a contract to RCA.

In support of other RCA engineering activities, both in and out of CSD, the Magnetic Recording Equipment Section has developed and is supplying high-resolution heads for programs such as Nimbus, Orbiting Geophysical Observatory (OGO), Tiros, and Gemini. Such heads require extreme sensitivity and short-wavelength resolution.

The dynamic flow of the magnetic recording technology has been demonstrated -from need, to research and development, to hardware.

The future trends seem to be toward higher quality, better signal-to-noise ratios, smaller-size and lighter-weight equipment, recording at higher frequencies, and greater packing density. Packing density per unit volume of tape is the area of most rapid progress. RCA has combined narrow-track tape and the modified diphase and octaplex recording techniques to design, build, and deliver digital recorders with a packing density of 256,000 bits/in2 (square inches), at a 10-Mc bit rate.

As a comparison from the storage point of view, this packing-density capability is an order of magnitude greater than that of present computer storage devices. Through the development of thin-base, double-coated tape and narrow-track recording, we have increased analog packing density by a factor of six in the last four years. A factor-of-four increase in digital and analog packing density is envisioned for the next four years.

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Fig. 1-Closeup of quadruplex scanning assembly.
Fig. 2-Octaplex tape format.
Fig. 3-Octaplex headwheel-arrangement of the eight magnetic heads.
Fig. 4a-Airborne application of octaplex headwheel assembly.
Fig. 4b-Octaplex headwheel assembly for ground application.
Fig. 5-a) standard wide-track tape format; b) narrow-track format.
Fig. 6-a) standard tape; b) double coated tape.
Fig. 8-Rack-mounted wideband recorder-reproducer.
Fig. 7-Curve of separation / wavelength losses.
Fig. 9-Airborne video recorder (inset) and ground reproducer system.
Fig. 10-Portable video recorder.
Fig. 11-Sealed tape cartridge for multipurpose test equipment (MTE).


BIBLIOGRAPHY
1. A. Katz, "High Packing Density Digital Recorder for Project Gemini,"
2. A. H. Lind, "TV Tape Recording-A Review of Techniques and Equipment,"
3. B. F. Melchionni, "Magnetic Head Development and Design,"
4. J. M. Uritis and B. A. Cola, "A High-Speed Precision Tape Recorder,"
5. A. D. Burt, S. P. Clurman, and T. T. Wie, "The Design of Satellite Tape Recorders-After Tiros I,"
.