13 articles about "Magnetic Recording" - RCA (1964)
MAGNETIC HEAD DEVELOPMENT AND DESIGN (1964)
B. F. MELCHIONNI, Mgr. - Audio, Projector, Magnetic Head, and Scientific Equipment Engineering - Broadcast and Communications Products Division - Camdenf N.J.
B. F. MELCHIONNI
B. F. MELCHIONNI graduated from the Temple University Technical Evening School in 1951. Since 1747 he has been a member of the Broadcast Studio Engineering staff. During this period he has engaged in the development and design of broadcast transmitter equipment, television cameras, television film projectors, television and audio tape recorders and other television terminal equipment, in addition to his work in magnetic recording heads. He is currently Manager of the Audio, Projector, Magnetic Head, and Scientific Equipment Engineering group of the Broadcast and TV Equipment Division. He is a member of the SMPTE.
MAGNETIC HEAD in Electronic Data Processing
The Magnetic Head Engineering group of the Broadcast and Communications Products Division is responsible for the development, design, and product support for all the heads required for RCA's line of commercial video tape recorders. In addition, the group develops special heads for other RCA areas such as Applied Research, Communications Systems Division, and Missile and Surface Radar Division of DEP. It has in the past supplied designs to Electronic Data Processing for the present line of computer tape stations. Special skills and equipment are utilized to constantly improve the state of the art. Reviewed herein are some of the engineering considerations, materials, and techniques.
In any magnetic recording system, the focal point is the magnetic head; here, the information is transferred to the tape, disk or storage drum to be played back at some future time. On playback, the recording procedure is reversed and information on the magnetic storage medium excites the head and is played back through the recording system. Thus, the substantial design and development effort concentrated on the magnetic head is vital to both commercial and defense applications of tape recording systems.
EARLY ALL-METAL HEADS
When RCA entered into the data-processing field in the early fifties with the Bizmac computer, the Magnetic Head Engineering activity that is now part of the "Broadcast and Communications Products" Division (BCP) was formed to design the necessary heads for the tape transport. The magnetic head designed for the Bizmac recorder was one of the first all-metal heads in computer work. Other heads had been built where only metal contacted the tape but this was done by undercutting the plastic potting compound around the heads.
This is an interesting consideration in the design of a stationary head where the tape will be moving across it. Most data recording heads consist of a number of individual tracks in one basic head structure; some heads also require a read gap immediately following the write gap, doubling the number of gaps per track in a particular head assembly.
In the design of a multiple-head structure, the most common approach is to have the heads flush with the surrounding support structure. This type of head is simpler to fabricate in production and is adequate for most tape stations employing a properly designed pressure pad to keep the tape in contact with the head.
In the design of some tape systems, a more intimate contact than that described above is required; furthermore, it cannot be attained by simply increasing the force on the pressure pad because of the adverse effects on tape stability. This problem has been overcome in several of our head designs by making the pole pieces proud in the vicinity of the gap. This technique allows the tape to make most intimate contact with the gap and not the support structure. However, this intimate and at times necessary type of contact increases head wear.
CONSIDERATIONS OF HEAD WEAR
Head wear (= Kopf Verschleiß), ever present in contact recording systems, is likened to dragging a metal bar across the pavement; the rate of wear is a direct function of the pressure per unit area and of the distance the bar is dragged across the pavement.
In the most popular flush-head design, both the heads and the surrounding surfacets contact the tape. This results in more evenly distributed forces and, subsequently, less head wear per lineal foot of tape.
In the present proud head design, the tape contacts only the pole tips, and the pressure per unit area increases with a subsequently higher rate of wear. However, this greater head wear has been a willing sacrifice to obtain the higher performance desired.
With the use of harder pole-tip materials and assembly techniques we have been able to build heads that have achieved the intimate contact required without any loss in head life. Such a head was designed specifically for the DEP Missile and Surface Radar Division (MSR) High-Speed Precision Instrumentation Tape Recorder.
To solve the wear-vs-performance problem required the special skills and facilities of the Magnetic Head Group. Initially, a head using proud pole pieces (herausstehende Pole) with conventional laminations was tried by MSR. Although this head met all the original specifications when tested on a loop machine, it would not maintain such performance for any length of time when put on a reel-to-reel machine; this was due to the wearing of the pole tips at the relatively high speeds-thus, creating a smearing of the gaps. A complete new head design program was then undertaken by the BCP Magnetic Head Design Group using new and different head materials and configurations. The new heads were fabricated successfully and are now in use on this system with excellent results.
ADVANCED HEAD DESIGN FOR RCA ELECTRONIC DATA PROCESSING
The basic head designs for RCA "Electronic Data Processing" (EDP) product line of computed tape stations were done by the BCP Head Design Group for the RCA 501, 601, and 301 systems. The heads for the 501 and 301 systems were of relatively conventional designs with 16 and 7 tracks, respectively. The RCA 601 system, however, required a read and write gap in the same head displaced by 0.2 inch. This created additional problems of providing good shielding to assure minimum crosstalk between channels as well as between the read and write gap lines. Both "C" and "I" configurations were developed; the "C" sections with their windings are part of the main head halves and the "I" return sections; they are imbedded in the center section used to support the shielding between the read and write gaps.
In the design of magnetic heads for multichannel systems employing narrow tracks, the uniformity of output is quite critical from channel to channel. This is a problem in tape guiding as the tape wanders across the head differently on different machines; to minimize this effect and to reduce the requirements in the transport design, the recording heads have a slightly wider track than those of the playback heads.
Where high bit-densities and extremely accurate timing between tracks is required, it is necessary to hold the gap scatter very accurately; when this is coupled to a read-write head configuration with a relatively thin center section, special techniques must be employed. A head to meet such requirements was developed and built for an Advanced Development program in EDP. This special head employed a unique floating center section, which, when clamped to the head halves, would not deform the gap lines formed by the lapped faces of the two halves. Gap scatter requirements of about 25 micro-inches (0.000025 inch) has been met on heads using this technique.
MAGNETIC HEAD APPLIED RESEARCH
An integral part of any magnetic head development and design activity is the constant search for improved materials that will yield higher sensitivity and increased life. Thus, such research goes on constantly, on company-sponsored programs.
The results of this research are put into practical use as soon as possible and then the cycle starts again to push the state-of-the-art still further. Much of this work is applicable to fixed heads for longitudinal recording (such as the high-performance heads previously described) as well as to the more pressing problems confronted in rotating-head assemblies such as the video headwheel panel.
The first RCA video panels using an "A"-configuration head design (Fig. 4) were developed by H. R. Warren's group when he was a member of DEP Applied Research. Two pole tips of alfenol (an alloy of aluminum, iron, and nickel) were used with a cross bar of ferrite which carries the coil.
Such a configuration resulted in a very short flux path in the pole tips; this was necessary because of the high loss-characteristics of alfenol in magnetic circuits.
The sensitivity of this magnetic head was adequate and headwheel panels were used successfully for many years in RCA video tape recorders for commercial as well as military applications.
However, head wear seriously limited the useful life of the panel. With the high head-to-tape speeds (approximately 1,560 ips) employed in the viedo recorder and with the high head-to-tape pressures required to get the intimate contact necessary for high-quality performance, the only solution remaining was to improve the wear characteristics of the pole-tip material; at the same time, any possible improvement in sensitivity would surely be welcomed.
With these goals in mind, a major breakthrough was accomplished in 1960 when an aluminum-iron-silicon alloy was found. This was pursued by RCA and is now the alfecon pole tip material currently in use in all RCA video head-wheel panels produced for commercial use.
Many variations of the alfecon alloy were tested for wear and sensitivity before the present alloy was selected. When the final selection was made exhaustive life tests were carried on in the laboratory; accelerated life tests were all but meaningless and the tests had to be performed finally under actual operating condiitons. The new alloy proved to be capable of increasing the life (in actual practice) by a factor of about 3 or 4; this increase in life substantially reduces user costs to run the machine.
With this increase in life came a gratifying increase in sensitivity, which enables the heads to be driven less in recording; nevertheless, the proper signal level from the system was retained in playback with a slightly better signal-to-noise ratio. Since June 1962, the new head material has been used on all head-wheel panels. Needless to say, customer acceptance has been most gratifying.
As the state-of-the-art progressed with the new pole-tip material, industry was ready for a machine running at half the then sole standard speed of 15 ips. It was reasoned that with the increased life (approximately four times that of the previous material), a head could be made only half as wide-and although it would wear at approximately twice the rate of the standard head, it would provide the user a two-to-one advantage over the older design. This was true enough provided the loss in signal (because of the narrower track) would not become a problem; although no problem did arise, the shorter life seemed to be a step backward.
With this problem confronting us, another design program was based on improving the standard alfecon head. In January 1962, a new pole-tip configuration head-assembly was demonstrated successfully. This pole-tip configuration (Fig. 5) is similar to that of the standard head assembly used for 15 ips which lays down a 0.010-inch-wide track. The only difference is that pole tips are notched in the vicinity of the gap down to 0.005 inch wide for 7%-ips tape speed.
The Fig. 5 narrow-track configuration satisfied all new requirements of minimum loss in head-to-tape contact area and maintained substantially the same life for either type of head assembly.
The narrow-track headwheel was also adapted in July 1962 by the DEP Communications Systems Division for special applications such as octaplex head-wheel.
HIGH-SPEED, WIDEBAND RECORDING REQUIREMENTS
Specific design proposals are often based on use of an existing equipment but with much greater bandwidth requirements; such an application is the Wideband Recording System. This basic design is similar to the standard quadraplex panel; but, to meet bandwidth specifications, the headwheel panel must rotate at approximately 28,800 rpm, twice the speed of the standard panel.
A standard air-bearing panel was reworked to include a new headwheel assembly containing eight heads for a dual-channel system; this method required a new head with a shorter ferrite to improve efficiency of the head. Also, a different assembly structure was required to withstand the higher centrifugal forces developed at the higher speeds. To minimize crosstalk between sets of heads, the headwheel unit was constructed with a set of slip rings on either end of the shaft (Fig. 6) ; wires to the upper slip ring are fed through a hollow shaft which carries rotor and wheel assemblies.
A LOW-SPEED APPLICATION
Concurrent with the design of the highspeed panel for Applied Research, a low-speed panel based on our standard video headwheel panel was also designed (Fig. 7). This headwheel runs at a much lower speed, providing a 200:1 reduction in head-to-tape speed, and allowing for a 200:1 expansion in the time base of the signal.
Another program for DEP Applied Research was the development of a three-head wheel assembly for DEP's helical scan machine. The final unit employed a larger-diameter headwheel with three heads; head design was similar to the standard video arrangement in which heads are imbedded in the wheel. Assistance was given to Applied Research in the design of the wheel and the drum, specifically in the area of the aerodynamic problems associated with a slant-track recording transport. The headwheel shaft is a belt-driven, air-bearing assembly with relatively large bearing surfaces.
MAGNETIC HEAD PRODUCTION
Closely allied to the Magnetic Head Engineering Activity is the Magnetic Head Production Facility, which is part of the Broadcast and Communications Division manufacturing operation. This manufacturing facility has produced all of the heads for RCA's EDP tape transports designed by the Magnetic Head Engineering activities.
Additionally, this production facility manufactures the video headwheel panels for the RCA Television Tape Recorder. This panel is very exacting in its requirements and has necessitated a new approach in production and in the necessary engineering support. In conjunction with the factory, a constant evaluation and improvement program is carried on to improve the performance and reliability of the panel as well as to improve production techniques for these very precise assemblies.
The Magnetic Head Engineering Group is primarily a product design and development activity; but, due to the nature of the work, the advance development aspects and the production requirements must remain closely related. To this end, much advanced development work is carried on by this same activity.
Constant engineering effort to improve the state-of-the-art in magnetic head design assures a quality of performance necessary to support the magnetic recording industry in its constant quest for better performing, more economical tape systems. Major improvements in basic heads are promised in the future, based on work now underway in conjunction with the RCA Laboratories in the area of improved materials and head configurations.
1. J. M. Uritis and B. A. Cola, "A High-Speed Precision Instrumentation Tape Recorder," RCA
2. J. J. Worthington, "The RCA 601 Computer System," RCA
3. H. Kleinberg, "The RCA 301," RCA
4. J. D. Bick and F. M. Johnson, "Magnetic Heads for TV Tape Recording," RCA
5. H. R. Warren, "Advanced Magnetic Recording Techniques and Equipment," RCA
6. J. D. Rittenhouse and F. D. Kell, "Advanced Tape Equipment for
Instrumentation Recording," RCA
Table 1 Glossary of terms
Track Width; The width of the recorded track on the tape.
Head Gap: A physical break made by the insertion of non-magnetic material between the. pole pieces.
Gap Length: Sometimes referred to also as gap width. It is usually expressed in micro inches (10"" inch ) and is the controlling factor in determining the resolving characteristics of the head assembly. Reducing the size of the gap increases the resolution but. also lowers the output. Increasing the tape speed will increase resolution but also increase head wear. Optimum head design requires striking the best balance between these interrelated parameters.
is critical i
Proud Head: A head design where the pole piece is raised above the surrounding surface for maximum contact with the tape. In actual practice the tape is deformed and literally
flows over the head and gap,
Pole Pieces: The parts of a head structure that carry the magnetic flux from the core to the gap and tape.
Fig. 1-RCA 501 head.
Fig. 2-The TRADEX head.
Fig. 3-MH-582 read-write head for RCA 601 station.
Fig. 4-Photomicrograph enlargement of "A" head configuration; white circles in the enlargement are actually the cross section of the coil wires.
Fig. 5-View looking down on a narrow-track head.
Fig. 6-A high-speed, dual-channel, dual-speed panel.
Fig. 7-A low-speed, dual-channel, dual-speed panel.
Fig. 8-A three-head wheel arrangement for slant-track recording.
Fig. 9-A video headwheel panel.