A high data rate recorder for astronomy

A magnetic tape recorder developed for the special requirements of radio astronomy and geodesy is described. These requirements include a high bit packing density and long record times. The current version of this longitudinal recorder used by the Very Long Baseline Array (VLBA) records 5.5 Terabits on a 14-in diameter reel of inch-wide tape. A maximum record rate of 256 Mb/s is achieved in the VLBA configuration with one recorder operating at 4 ms and utilizing 32 of the heads in a single stack. The VLBA recorders have been tested using a longitudinal density of 2.25 fr/μm (57.15 kfrpi); 448 data +56 system tracks are recorded in 14 passes, each lasting 50 min, for a total record time (at 128 Mb/s) of 12 h on 14-in diameter reel of inch-wide 13-μ-thick D1-equivalent tape     

Theory of linear superposition in tape recording

The conditions under which linear superposition (LSP) of isolated pulses is valid for the synthesis of multibit waveforms are examined theoretically. LSP is found to be valid subject to three conditions; first, that all the processes following the write process be linear operations on the tape magnetization; second, that the write field rise time be less than the bit interval; and third, that each change in magnetization, occurring during the write process, be a function only of the field causing that change. Additionally, the validity of LSP at 15 000 bit/in is demonstrated for a high resolution tape recorder using standard γ-Fe2O3tape.     

High-Density PCM Tape Recording

The search for a reliable encoding and decoding process for pulse-code modulation (PCM) data streams is discussed with application in both the serial and parallel mode employing standard IRIG wide-band (2.0 MHz at 120 in/s) longitudinal magnetic tape recorder/reproducer. Serial and parallel PCM data at 4.0 and 3.0 Mbit/s/track, respectively, may be handled by the Sangamo Electric Company instrumentation systems with PCM channels in both portable and laboratory configurations. Not only was a method identified to utilze the maximum bandwidth available in conjunction with acceptable data error rates, but definite advantages and disadvantages were verified for all the standard PCM formats.     

New high performance SAW convolvers used in high bit rate and wideband spread spectrum CDMA communications system

New surface acoustic wave (SAW) convolver structures with high conversion efficiency and self-temperature compensation characteristics have been developed. Strong piezoelectric substrates, regardless of temperature coefficients of delay (TCD), can be used in these convolvers. New demodulation techniques using the developed SAW convolver for high bit rate and wideband spread spectrum code division multiple access (CDMA) communications have also been developed. I- and Q-channel demodulation data can be derived directly from binary phase shift keying (BPSK) or quadri-phase shift keying (QPSK) CDMA signals. In an experiment using a 128 degrees YX-LiNbO(3) substrate, CDMA signals of 9 Mbps (megabits per second) with 60 Mcps (megachips per second) spread by 13-chip Barker code and 11 Mbps with 140 Mcps spread by 25-chip Shiba's code were clearly demodulated, demonstrating the effectiveness of these techniques for use in future CDMA communications.     

A low-cost, high-speed direct access device

This paper will deal with the novel approach taken to produce a low-cost, high-capacity, high-speed direct access tape drive. The drive utilizes a removable one-half inch magnetic tape cartridge to compliment industry trends. The device will initially store 80 Mbytes per cartridge with an initial data rate of 500 Kbytes per second. The cartridge format is made up of 615 blocks of 131 Kbytes per second. Within each block the tape is motionless, and the drive operates as a random access device with two active heads and sixteen cylinders. A stepping motor, in conjunction with a peripheral drive, is used to access the tape from block-to-block.     

Synchronization of Clocks by Very-Long-Baseline Interferometry

Two hydrogen-maser clocks, one at the Haystack Observatory in Massachusetts and one at the National Radio Astronomy Observatory in West Virginia, were synchronized by means of very-long-baseline interferometry (VLBI) observations of several extragalactic radio sources on March 28, and again on September 23, 1977. Observations were made sequentially in eight 360-kHz bands distributed between about 8.4 and 8.5 GHz with spacings designed to enable the group-delay difference between the signals received at the two observatories from a given source to be estimated unambiguously, within an uncertainty of less than 1 ns set by receiver noise. The epoch and the rate differences between the observatories' clocks for each experiment were estimated by analysis of observations that spanned several hours. The application of corrections for the contributions to the delays of the antennas, feeds, receiver systems, and recorders, yielded absolute determinations of the clock epoch differences. During each experiment, portable cesium clocks were flown from the U.S. Naval Observatory in Washington, DC, to the observatories and back. The traveling-clock data, analyzed in each case after the VLBI synchronization had been completed, confirmed the VLBI results to within 18 and 14 us for the first and second experiments, respectively.     

Progress in VLBIS at Shanghai Observatory

An overview is presented of recent developments on very-long-baseline interferometry (VLBI) at the Shanghai Observatory. The Shanghai Radio Station, which is equipped with a 25-m antenna, and other facilities that became operational in 1988, are described. The VLBI data-processing system is discussed. It includes an S-2 VLBI data processor and a series of postcorrelation processing programs for astronomy as well as geodesy. There are now two VLBI groups in Shanghai Observatory working on astronomy and geodynamics, and some current research is outlined     

The evolution of the high density 1/4" cartridge tape drive

A new cartridge tape recorder was designed to provide both a backup for fixed disc drives and an interchangeable media recorder for program loading and data logging. The drive employs a new format designed to use the streaming method of tape handling. Formatted capacities of 10 and 20 megabytes, with nominal backup times in the at-12 minute range, are included in the product family.     

Extensible, Low-Chromatic-Sensitivity, All-diffractive-Optics Relay for Interconnecting Optoelectronic Device Arrays

For free-space optical interconnections between optoelectronic chips to reach commercial realization, the technology must provide high-density optical channels in a simple, inexpensive, and easily aligned package. Although point-to-point connections with microlens pairs can provide densities of several thousand channels per square centimeter, the Gaussian nature of the beams limits the connection range to a few millimeters. We propose an arrangement of microlens pairs with an intermediate relay lens that significantly increases the connection distance. This basic setup can be tiled laterally across large chips to form extensible arrays. The optical design is constructed entirely with diffractive elements because of the low chromatic sensitivity over a range of approximately ?10% around the design wavelength. We derive the lateral positioning error at the image by using a simple ray trace, and we show the effect of Gaussian beams. We experimentally demonstrate the low chromatic sensitivity for a system with an interconnection distance of 64 mm. Finally, we demonstrate the interconnection of two linear arrays of multimode fibers with two adjacent channels operating at data rates of hundreds of megabits per second.     

Data acquisition system for the 8π spectrometer: A multidetector array for γ-ray spectroscopy

The data acquisition system for the 8π spectrometer, a multidetector array for γ-ray spectroscopy, is described. It is based on CAMAC modules equipped with a fast auxiliary bus in ECL logic. The system has been designed to handle a high rate of events, each with large information content. To offer maximum flexibility for the many types of experiments envisaged, the logic of the data stream to magnetic tape has been made user selectable. This is achieved through selective hardware triggers, hardwired computations and programmed instructions in a fast microprocessor, which condense the information from each event by extracting its essential features.     

Design Considerations for a Microprocessor-Based Tape Transport System

The use of a microprocessor to control an instrumentation tape recorder is discussed. New features including simplified calibration, automatic calibration verification, identification of data channels, enhanced shuttle operating modes, and selective track recording increase versatility and ease of operation. New possibilities of remote control via the IEEE-48-1975 bus or the RS232C/RS449 interface are also described.     

Capacitive system for three-dimensional imaging of fluidized-beddensity

The U.S. Department of Energy's Morgantown Energy Technology Center has been developing a capacitance imaging system (CIS) to support its fluidized-bed research programs. A second-generation system for capacitively imaging a cold, laboratory-scale, 15.24-cm diameter fluidized bed is described. The CIS acquires interelectrode, bed-crossing displacement current data to provide 193-pixel density values at four 2.54-cm vertically spaced levels and presents a three-dimensional density display at a rate of 30 maps per second in real time. The CIS also stores displacement current data at a rate of 60 density maps per level per second for post-run analysis. Different means of data processing are described that produce one method for real-time display and two methods for post-run analysis of data. The results of calibration and fluidization tests are presented, together with the errors associated with each of the methods for the known pixel densities in the calibration tests. Improvement in the calibration procedure to reduce these errors is proposed. Test results indicate the CIS would be a useful tool for research and monitoring operations in two-phase systems     

The helical-scan magnetic tape recorder as a digital communication channel

The properties of a magnetic tape recorder are viewed in terms of a digital magnetic recording/playback channel which exhibits fading (reduction of playback-signal level) and nonlinear behavior. A method is presented whereby channel nonlinearity may be quantified in a format useful for signal and receiver design. Measurements show this nonlinearity to be relatively small for symmetric two-level signals. Deep fades (dropouts) are the most significant source of errors in digital tape recordings. Fading is considered as multiplicative noise on an essentially linear channel, and measurements are made of the fade probability distribution and an associated additional time dispersion. While the fading process appears to occur relatively slowly (compared with the bit period), neither its probability distribution nor its associated dispersion encourage the use of a receiver which is able to adapt to the changing channel characteristics. Finally an attempt is made to ascribe the fading process to repeatable variations in head-tape separation, and a corresponding probability distribution for this separation is obtained. A helical-scan video recorder was used throughout the measurements because of its low cost and its potential as a high-density storage facility.     

An investigation of high-frequency bias-induced tape noise

Bias-induced tape noise remains a major limitation of the SNR in audio magnetic tape recording systems. Defined as the increment in system noise incurred when the bias oscillator is turned on, the noise can originate from a number of different causes; namely, bias oscillator harmonic distortion, magnetized heads, the earth's magnetic field, and an intrinsic noise source. The latter noise source is our primary concern here. Such record system parameters as head-to-tape spacing, gap length, bias current, and bias frequency were investigated with regard to their influence on this intrinsic bias noise source. Two models of the mechanism of intrinsic bias noise are examined. The first, the "amplitude modulation model." proposes that bias noise is generated by amplitude modulation of the recorded bias signal by the physical and magnetic variations of the head-tape system. In this model, bias noise is merely the lower AM sidebands of the recorded bias signal. The second model relates bias noise to the interaction fields in erased tape. This model proposes that these fields behave similarly to normal recording fields and can be "re-recorded" on the tape at an enhanced level. The two proposed mechanisms are examined in the light of the experimental data. The amplitude modulation model is shown to agree with all the observed data with the exception of the existence of bias noise at bias wavelengths smaller than the particle size. The second model, which does not incorporate a wavelength dependency of bias noise, is in qualitative agreement with the observed data. Methods are discussed for reducing the bias noise without materially affecting the system performance.     

A High-Accuracy Computer-Aided Profile-Measuring Machine

This paper describes a system for the high-precision, rapid measurement and evaluation of rotatable object profiles such as cam's. The data are acquired off-line by sampling with a high-resolution optical encoder and are recorded on magnetic tape via an incremental recorder. The recorded data are subsequently analyzed by a computer program to provide the required parameters. The system is characterized by ease of operation, high precision, repeatability, and relative simplicity. It makes available an automated measurement capability that was previously only possible by manual methods on an optical lathe or a measuring engine.     

A monitoring system for contact fatigue crack testing

A new monitoring system is developed to detect crack initiation and propagation in the surface layer during contact fatigue running. The magnetic head of a commercial tape recorder is used as a probe. A combination of leakage and eddy current methods is ussed for the contact roller specimen on the JPM-1 machine. Experimental results show that this system is very sensitive to artificial defects and can distinguish defects on the surface from those beneath the surface. The defect signals agree well with their positions. Testing of an ion-nitrided specimen shows that a tiny subsurface crack (∼ 1 μm), which can only be examined under a microscope with 200 × magnification, is observable with this system.     

The next decade in magnetic recording

Experimental data are given on the recording performance of several modern magnetic recording materials. On the basis that the signal to reproduce head noise will prove to be the eventual limit, estimates of the attainable recording densities are made. Areal densities of several hundred megabits per square inch are forecast.     

Research on resistance characteristics of YBCO tape under short-time DC large current impact

Research of the resistance characteristics of YBCO tape under short-time DC large current impact is the foundation of the developing DC superconducting fault current limiter (SFCL) for voltage source converter-based high voltage direct current system (VSC-HVDC), which is one of the valid approaches to solve the problems of renewable energy integration. SFCL can limit DC short-circuit and enhance the interrupting capabilities of DC circuit breakers. In this paper, under short-time DC large current impacts, the resistance features of naked tape of YBCO tape are studied to find the resistance – temperature change rule and the maximum impact current. The influence of insulation for the resistance – temperature characteristics of YBCO tape is studied by comparison tests with naked tape and insulating tape in 77 K. The influence of operating temperature on the tape is also studied under subcooled liquid nitrogen condition. For the current impact security of YBCO tape, the critical current degradation and top temperature are analyzed and worked as judgment standards. The testing results is helpful for in developing SFCL in VSC-HVDC.     

Maximum signal-to-noise ratio of a tape recorder

Using the Wiener autocorrelation theorem, the noise power spectrum of the pole strength in a thin lamina of an erased tape is shown to be approximately white. The noise power spectrum of the reproduce head voltage is calculated for a thick tape and compared with the signal power. The wide-band signal-to-noise ratio of a tape recorder equalized flat is deduced and expressed in very simple forms, which are inversely dependent upon the square of a bandwidth. Notably, in this special case the wide-band result is independent of reproduce head-to-tape spacing. Numerical examples demonstrate that this simple theory yields results in excellent agreement with practice.     

Automated Precision Polarimeter for the HF-VHF Range

An automated system for Stokes parameter-polarization analysis over the HF-VHF range is described. Axial ratio, orientation angle, polarization fraction, and polarization sense are determined by amplitude measurements using a conventional fieldintensity receiver. Six amplitude measurements from four crossed nonresonant dipoles, including quadrature sum and difference, eliminate the requirement for phase measurement. The antenna does not use active components and is adaptable for mobile or stationary operation. VSWR measurements on the antenna output cables show less than 1.2:1 (50 ohms) over the 2-70 MHz range. The antenna aperture increases from 1 × 10-5 square meters at 2.0 MHz to 0.019 square meters at 70 MHz. A solid-state sequencer processes each amplitude measurement separately through the receiver and digital conversion circuits (providing BCD output) to an incremental tape recorder. The Stokes parameter analysis is performed by an off-line digital computer using the magnetic tape data. This analysis permits computation of total received power from either set of orthogonal element measurements. When combined with the measured antenna aperture, power density (or field strength) also can be derived. Polarization fraction measurements for locally controlled signals show a mean of 1.02 as compared to a theoretical value of 1.00 (standard deviation of 0.1) over the 2-70 MHz range and polarization results consistent with propagation predictions.