13 articles about "Magnetic Recording" - RCA (1964)
HIGH-PACKING-DENSITY DIGITAL RECORDER
FOR THE GEMINI SPACECRAFT (1964)
from A. S. KATZ - Magnetic Recording Equipment Engineering Communications Systems Division, DEP, Camden, NJ.
A. S. KATZ
A. S. KATZ graduated from the Drexel Institute of Technology, Philadelphia, in 1957 with a BSEE; he received his MSEE in 1963. He joined RCA Airborne Systems Division upon graduation in 1957. Mr. Katz was responsible for design and development of the missile auxiliary electronics for the ASTRA program and an Electrical Panel Simulator for the Army. He has two patents pending on special pulse measurement equipment for a missile check-out system. He has also participated in the design of digital logic circuitry and complex sweep generators for an Optical Character Recognition System. He joined the Magnetic Recording Equipment Engineering Department of the Surface Communications Division in early I960. At that time, he was given the responsibility for the design of a new method of recording and playing back digital information for which he has a patent pending. Mr. Katz was responsible for developing new techniques and circuitry to increase the packing density of digital information on magnetic tape, beyond the present state-of-the-art; this program resulted in the diphase recording technique for which he has a patent pending. Mr. Katz is currently the Design Project Engineer for the GEMINI recorder program.
About data transfer from an orbiting spacecraft
In instrumented, orbiting spacecraft, much data must be continually recorded and then very quickly read out during the short period of the orbit when it is in telemetry contact with a ground station. This lightweight (14.5-pound), low-power (12.5-watt) digital recorder-reproducer for the GEMINI two-man spacecraft records two channels of pulse-code-modulated information simultaneously at 5,120 bits/sec for 4 hours at a tape speed of 1 7/8 ips. Packing density is 2,730 bits per linear inch on each track, with reproduction errors of less than 1 in 10 ^5 bits. Complete read-out is at 4M/4 ips in a total of 10.9 minutes.
Manned spacecraft missions have brought about numerous advances in digital telemetry technology. A vital link in such telemetry systems is the digital pcm (pulse code modulation1) recorder-reproducer. Such digital recorders are needed in spacecraft telemetry systems for continuous montioring and recovery of instrumentation data generated by the spacecraft systems. Since the spacecraft can communicate directly with ground stations via a transmission link for only a small portion of its mission, a recorder-reproducer system must continuously record vital data and then reproduce that data in the shortest possible time when the spacecraft is in contact with a ground station.
Sophisticated digital magnetic recording techniques had to be developed to meet this requirement.
SPACECRAFT RECORDER REQUIREMENTS
A digital telemetry recorder-reproducer for manned spacecraft must be extremely compact and light in weight, and it must consume minimum power. The amount of data storable by such a recorder and the accuracy of reproduction must be maximized. The recorder must also perform reliably throughout the rugged environment of the orbital and re-entry phases of the mission while remaining within the severe constraints of space vehicles.
The pcm recorder-reproducer, developed by the Magnetic Recording Engineering section of RCA's Communications Systeme Division for the Gemini two-man space flight, has been designed to meet such requirements (Figs. 1-3).
GEMINI RECORDER DESIGN FEATURES
The Gemini recorder-reproducer is contained within 431 cubic inches, weighs 14.5 pounds, and consumes only 12.5 watts of power. It can continuously record two channels of pcm information simultaneously at 5,120 bits/sec, for 4 hours at a tape speed of 1 7/8 ips. On command, such information is reproduced at 22 times the recorded speed (41 1/4 ips) in 10.9 minutes.
The use of an advanced recording technique permits the recording of the digital information on magnetic tape at a packing density of 2,730 bits per linear inch of tape on each track. Information is reproduced with error rates of less than 1 in 10 ^5 bits. To accomplish this accuracy over the entire mission, specific consideration was given to the tape transport design and to the selection of recording and reproduction techniques.
Compactness a Necessity
It was necessary to design a mechanical system (within the allowable size and weight) which was structurally sufficient not only to withstand the vibration and shock of the launch and re-entry phases of the mission, but also to impart a minimum of spurious head-to-tape motion, thus minimizing jitter of the reproduced data. Since relative head-to-tape separation (on the order of 0.1 mil; i.e., 0.0001 inch) with resultant signal amplitude variations can be expected during the high-vibration periods of launch and re-entry, the recording technique had to be (for accurate detection) independent of the amplitude of the reproduced signal.
Magnesium For Low Weight
The tape transport (Fig. 4) consists of a stuctural magnesium laminated motor board on which the two coaxial reels, capstan drive system, 24-volt-DC-to-400-cps two-phase power inverter, and electronics are fastened. For minimum weight and maximum strength, magnesium was used extensively.
Hysteresis Speed Converter For Low Jitter
Since a coaxial reel system is used, it is necessary to transfer tape from one reel to another by executing two 90° twists in the tape. The twists in the tape set up differential stresses across the width of the tape which cause it to seek a natural path through the capstan and head assembly. Dynamic skew measured across the tape indicates a total skew of less than 50 microinches (0.000050 inch).
The requirement for a drive mechanism capable of imparting a dual tape speed with minimum jitter, while conforming to the overall requirement for minimum size, weight and power, resulted in the design of a hysteresis speed converter. This drive mechanism employs no mechanical contacting parts and is operated in a manner similar to that of a hysteresis motor. The speed converter operates in a choice of two speed modes by energizing either of two stationary coils, thus magnetically coupling the capstan shaft to the motor via the selected polyester belt system.
The high-frequency oscillatory components of the motor are not transmitted (as jitter) to the capstan, since the polyester belt system is essentially a low-frequency bandpass filter, filtering out the high-frequency jitter components.
The use of a precision 400-cps two-phase frequency source in conjunction with a hysteresis synchronous motor and belt drive results in a total rms low frequency capstan speed variation of 1 part in 1,000.
Constant Torque - Constant Tension
The tape reel assembly consists of twc coaxial reels and two dynamically balanced, constant-tension spring assem blies. A total of 2,300 feet of polyester base tape (0.25 inch wide and 0.83 mil thick) is stored and exchanged between the two coaxial reels. Tape tension is maintained by the essentially constant-torque, constant-tension spring system which torques one reel against the other and eliminates the need for reel motors. The reel hub-assembly can be attached to an adapter for playback of the recorded tape on a standard (National Association of Broadcasters) hub.
Low Magnetic Head Maintenance
The magnetic heads are of all-metal construction in keeping with the design requirements for long life and minimum maintenance. The magnetic heads require no maintenance for 750 hours of continuous use. At the end of that period, the heads are cleaned and the tape changed as a preventive maintenance precaution.
Heads of similar construction have endured up to 3,000 hours of sustained use with less than 3db loss in playback signal amplitude. Both record and playback heads have a gap length of 90 micro-inches, which provides the required definition of the record and playback signals. Full-width intertrack shielding is used to minimize cochannel interference. The geometry of the magnetic tape track conforms to IRIG (Inter-Range Instrumentation Group) telemetry standards for 0.25-inch-wide tape.
Modified Diphase Technique
The recording and accurate reproduction of digital information on magnetic tape at a packing density of 2,730 bits/ inch on each track necessitated the development of a signal-processing technique for digital recording.
Conventional techniques for the recording of digital information are severely limited at packing densities in excess of 1,000 bits/inch.
Inadequate pulse resolution, pulse crowding, and tape skew are among the many limiting factors. The signal processing technique, which is the RCA modified diphase system, encodes the digital information prior to recording and, on playback, decodes and reconverts the reproduced signal into standard nrz (non-return-to-zero) form.
The application of the RCA modified diphase system to the recording art has been based on a thorough analysis of the problems inherent in the design of a high-density digital recording system. Eliminated are the serious problems of pulse crowding and tape skew which arise when conventional nrz recording techniques are extended to high packing densities.
The system is also insensitive to extreme variations in playback signal amplitude due to tape inperfections and head-to-tape separation. The diphase technique is essentially a phase-modulated carrier process. RCA and others have used phase-modulation technique extensively for data communications because of its high performance in narrow-bandwidth channels. Since a magnetic tape recording system at high packing densities is quite similar in behavior to a normal communications channel, phase modulation is a natural candidate.
SPACECRAFT RECORDER OPERATION
A simplified block diagram and associated waveform chart are shown in Figs. 5 and 6. The incoming clock and inverted rz (return-to-zero) data are each fed to separate trigger inputs of a binary flip-flop. The flip-flop undergoes a transition on the negative-going edge of both the clock and the inverted rz data signal.
By defining a bit cell as shown on the waveform chart, it may be seen that a transition occurs in the center of bit cell each time a logical 1 is received, and no transition occurs in the center of bit cell when a logical 0 is received.
The output of the flip-flop is termed the diphase signal, since the digital information is inherent in the change in phase (or lack of change in phase) in the center of a bit cell. The diphase signal is then fed to the record amplifier. During the reproduce mode the signal picked up by the magnetic head is fed to the playback amplifier. Here it is amplified approximately 60db, filtered, and equalized in order to compensate for the effects of the head-to-tape system.
The equalized signal is then fed to an input coupler where approximately 40db of hard limiting is provided, with the result that the system is insensitive to an amplitude variation up to 40db in the reproduced signal.
The squared-up diphase signal is fed simultaneously to the timing extractor and decoder. The timing extractor consists of a precision one-shot, monostable multivibrator whose period is set to a 3/4 bit cell. The one-shot is triggered from the leading edge of each diphase bit. In this manner, timing pulses are accurately positioned in the last 3/4-bit cell on a bit-by-bit basis. In the decoder, the timing pulses are used to interrogate the phase of the squared-up diphase signal. The phase of the diphase signal is interrogated with consecutive timing pulses; thus, a 0 is detected whenever a change has occurred in the polarity of the diphase signal between any two consecutive timing pulses.
If, however, the polarity of the diphase signal is identical at the sampling times of two consecutive timing pulses, a 1 has occurred.
Essentially, the detection of a 1 or a 0 is dependent on the change (or lack of change) in polarity of the diphase signal at the sampling times of any two consecutive timing pulses. The detected output in nrz form is fed to a transmitter via a pcm filter for transmission to a ground station.
The Gemini recorder-reproducer described here is presently undergoing extensive testing to determine its performance in an environment similar to that which it will encounter in space flight. It is scheduled for use in the first Project Gemini two-man space flight.
1. H. M. Straube, "Pulse Code Modulation," RCA
Fig. 1-The GEMINI recorder.
Fig. 2-A. Witchey (left) and H. Z. Weaver adjust the front plate of the capstan drive system. The recorder is assembled in a controlled environment which along with 100% parts inspection are among the many precautions taken to insure high reliability. (Also see front cover, this issue.
Fig. 3-G. S. Newcomb (left), and E. R. Ware conduct an environmental test in the Camden Environmental Test Facility to check performance under extensive vibration, equivalent to that of the spacecraft re-entry phase.
Fig. 4-Tape transport assembly; all parts are mounted on a magnesium motor board.
1. Flanged reel outer
2. Disk-upper reel ass'y
3. Spur gear pinion
4. Retaining ring, external
5. Retaining ring, internal
6. Ball bearing (s)
9. Post, reel bearing
10. Torque tube bearing
11. Bearing retainer
12. Motor board
13. Guide roller block
14. End of tape switch
16. Capstan drive ass'y
17. Magnetic head
18. Magnetic head
20. Connector, micro miniature
22. Bracket, connector mounting
25. Spacer, heat seat
26. Spur, gear drivtm
27. Ball bearing
28. Spooi, negator
29. Spring, negator
Fig. 5-Simplified diphase system block diagram showing the record and playback modes.
Fig. 6-Diphase system waveforms.