Current operational capabilities of tape recording for Very Long Baseline Interferometry (VLBI) at the Haystack Observatory allow 0.7 terabytes (12 hours at 128 Mb/s) of data to be stored in a 128 cu. inch volume. On-going efforts are aimed at full time 1 Gb/s operation with two 36-channel headstacks. Applications for linear digital tape recording, with suitable development of thin-film head arrays, suggest a volume density exceeding 1 TB/cu. inch to be achievable in the future

Hinteregger, H. F.

English

NASA Technical Reports Server

N95- 24118
High Data Rate Recorder Development
at MIT Haystack Observatory
H.F. Hinteregger
MIT Haystack Observatory,
Westford, MA 01886-1299
508-692-4764
hfh@wells.haystack.edu
Abstract
Current operational capabilities of tape recording for Very Long Baseline Interferometry (VLBI) at
the Haystack Observatory allow 0.7 terabytes (12 hours at 128 Mb/s) of data to be stored in a
128 in 3 volume. On-going efforts are aimed at full time 1 Gb/s operation with two 36-channel
headstacks. Applications for linear digital tape recording, with suitable development of thin-film
head arrays, suggest a volume density exceeding 1 TB/in 3 to be achievable in the future.
I. Introduction
The sensitivity of Very Long Baseline Interferometry [VLBI] observations is proportional to only
the square root of the number of bits recorded and processed. This fact, and the lack of affordable
alternate means of sustained high-rate data transmission, continues to spur development of ever
higher rate and density tape recording systems for VLBI.
The VLBI recorders developed at Haystack for VLBI operation employ the linear [longitudinal]
Metrum, formerly Honeywell, Model 96 instrumentation drive, which was selected in 1975 for its
excellent tracking repeatability. These drives reliably handle 14" reels of inch-wide tape, now at
speeds up to 10 m/s [420 ips], as well as with tape tension as low as 5/8 and as high as 5 N.
VLBI-qualified tapes (3M5345 and SonyD 1K) can be shuttled for thousands of passes under these
conditions without any noticeable edge-damage [as evidenced by the formation of a bumpy pack].
An understanding of the damage mechanisms and tape-path modifications were developed to make
the drive safe for thin tape at high speed. The 15.2 mm thin, 5500 m long, SVHS-similar Co-
Fe203, PET-based tapes, which we have qualified for recent VLBI systems, each store 0.7
terabytes [12 hours at 128 Mb/s] in a 128 in 3 volume.
A total of 576 [38 mm wide] tracks, for example 16 passes with 36 channels at a time, are written
at a linear density of 2.25 transitions/mm [2 bits/mm with an 8/9 channel code]. A new version of
the recorder (MklV) operating at 8 m/s is capable of 1 Gb/s operation with two 36-channel
headstacks and 2 Gb/s with four headstacks. The growth of the data rate and volume density in
VLBI recording is illustrated in Figure 1.
Haystack's narrow-track ferrite read-or-write headstack design, illustrated in Figure 2, has been
used since its introduction in 1985. It has been a 'product' manufactured by Metrum since 1989,
unfortunately only in the small volumes required by its single VLBI application, which makes it
increasingly less affordable. More recent recorder applications all use the same sub-mm resolution
positioner [piezoelectric Inchworm actuator and LVDT sensor], one for each headstack. This
positioner was developed to support narrow-track serpentine recording.. It will need no significant
improvement to support much narrower, even sub-ram, trackwidths since actuator resolution is
about 4 nm.
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hz the 25-year history of VLBI, data rate has grown by more than three orders of magnitude to
one gigabit/second in the Mark IV Stlstem. A further hundred-fold improvement in bandwidth
and density is now a technically-realistic goal.
Figure 1
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II. Goals for Future Developments
The more recent version of our VLBI tape recorder developed for the NRAO VLBA system is
designed to operate full-time at 128 Mb/s at 2 m/s. With a 10-station array, the VLBA collects
14 TB/day and could process 56 TB/day with its 20-station 4 m/s correlator. A new correlator
(MklV) under development at Haystack will be configurable to keep up with 4 Gb/s per station;
another iteration could easily raise that to 4 GB/s. Processing hardware will therefore soon cease
to be the chief limit on VLBI bandwidth, hence sensitivity.
Thus there is a need to provide affordable full-time VLBI recording soon at Gb/s, and not much
later at GB/s, rates. The first goal, achieving Gb/s rates, requires only new thin-film head-arrays.
The second goal, achieving GB/s rates, requires new tape as well. Both these goals are discussed
below.
The highly repeatable tracking of the Model 96 drive has been modeled and is now well
understood. It is good enough to permit trackwidth reduction from 38 to 7-10 mm without, and
probably less than 2 mm with, possibly only minor modifications. Short-term nonrepeatability is
about 1 mm p/p and is due to wobble of the capstan, which is in close proximity to the head. The
'perfect' edge-guide [bearing point] in the slanted-wall vacuum pocket is much further away. The
desired modifications under development should have the effect of bringing head and edge-guide
very much closer together. They must also increase the along-tape distance from head to capstan,
and/or isolate it with another sliding edge-guide between head and capstan.
The principle of tilted-wall vacuum-pocket edge-guiding can be applied to vacuum-less guides
such as tilted posts or flexible conical-foil air-bearings. Long-term nonrepeatability and sensitivity
to pack imperfections which produce a reel once-around 'bump' signature is also eliminated by
bringing head and edge guide together. Much more robust, pack-imperfection-tolerant passive
tracking should result.
a. Thin-Film Head-Arrays
Haystack is seeking support to collaborate with Seagate Tape Technology Inc. to further develop
thin-film head-arrays for linear multichannel drives. Thin-film head-arrays, with inductive write
(IW) and magnetoresistive (MR) read elements, are key to achieving both high data rate and
density at the lowest cost. Sensitive MR heads will allow a 4-fold increase in track density with
the present tapes. A read-width of 9 mm and write width of 11 mm can therefore now be
targeted. More conservative specifications for VLBI thin-film head-arrays proposed in 1993 (not
funded) are given in Table 1.
Current processes for thin-film IW-MR disk heads allow increasing channel density from 36 to at
least 144 per inch. Channel densities of 1000 per inch or so are not far off for MR and 1-2 turn
IW heads. With a mature high-yield process, if chips are of comparable size, the cost of a dense
head-array should eventually become comparable to that of a single-channel disk slider. This will
happen only when and if a comparably large linear tape head market develops. Commonality at the
wafer level with a high volume PC/consumer product is therefore highly desirable. The
manufacturability of dense arrays is the central issue for this kind of product.
b. The Promise of Advanced Tapes
Advanced high-SNR, high-resolution, super-smooth, ultrathin (3-7 mm) metal-evaporated, metal-
particle, and barium ferrite tapes promise further 4-fold density increases in each dimension and
continue to be evaluated. TB/in 3 volume density is technically within reach with products like
WVHS and Hi8Master tape.
Efforts to guarantee more nearly ideally robust edges for ultrathin tape must be pursued with the
118
Specifications for VLBI Head Array Prototypes
Read-Only Array
Technology Thin Film
Track Width
Head Pitch
Number of Heads in Array
Equivalent Spacing
MR
19 Microns
349.25 microns
72
< 0.05 microns
Write/Read-Verify Array
Technology Thin Film
Track Width
Head Pitch
Number of Heads in Array
Equivalent Spacing
Inductive
22 microns
349.25 microns
72
< 0.1 microns
For Both Arrays
Minimum Record Density
Tape Speeds Up To
Media
56,000 kfci
8 m/s (320 IPS)
3M 5345 (16-micron thick)
Similar to S-VHS)
Table 1
119
collaborationof manufacturers.A degreeof roundingof the corners of order 10% of thickness
and comparably small edge asperities are needed to guarantee that plastic deformation of these
asperities does not lead to edge-thickening which results in a bumpy pack.
Since the stiffness of tape goes as thickness cubed there may be a more fundamental problem with
winding much thinner than 15 mm tapes on large self-packing reels. These were invented for
VLBI in 1978 and the design was improved in 1994 explicitly to suppress scatter-wind so as to
make the tape directly shippable. Self-packing reels intentionally use close spacing and curvature
of the flanges to guide the tape into a smooth-faced pack. This does not damage the edge even
under extreme high-speed [8 m/s], high-tension [4.4 N], long-term [1000 pass] running
conditions. Both 3M5345 and SonyD1K typically pass such 'torture' tests with no evidence of
damage [a bumpy pack when wound at 2 m/s]. With 9-mm tape samples similar to 3M5354 the
tape edge tends occasionally to hang up slightly on the flange and voids are formed because
subsequent turns yield too easily. Even without this problem, unreasonable thickness uniformity
requirements may be placed on ultrathin tape wound into a 14" diameter pack. Ultrathin tapes are
likely to be much better suited for formats where the pack diameter is kept under about 3" [VHS,
3480, DLT, QIC, etc.].
III. Longer Term Potential of Tape Recording Applications
Tape recording is and for the foreseeable future will remain by far the densest and most economical
form of data storage. Applications that must sustain high data rates in the 0.1 to 10 Gb/s
neighborhood or that require convenient access to enormous [of order 10"* 15 bytes (petabyte)]
data libraries will continue to rely on tape.
The overwhelming volume density and capacity advantage of tape is due entirely to its high
relative thickness-density. The areal density of tape recording can keep up with the advances in
disk recording. Though track densities on tape [especially in linear systems] have traditionally
been lower than on disk, simple passive edge-guides can provide essentially perfectly repeatable
tracking; active track-following should not be needed to at least 10,000 tpi.
The two fundamental advantages of linear [compared to helical-scan] tape drives are: (1) its
mechanical simplicity, and (2) its ability to support many parallel channels, hence very high
aggregate data rates, without added complexity.
A low-cost high-rate linear digital VCR is possible in the future. Given the 4 in 3 capacity of a
VHS cassette or 3480 cartridge for example, a 250 Mb/s machine could operate for two hours at a
volume density of only 1/16 terabyte/in 3, a density that can be achieved with some of today's tape
and head components. The 'effective density' of the Hi8 analog consumer video tape recording
format in long play mode is about 1/20 TB/in 3 for example. This density is already so high that
the use of so-called data-compression [redundancy reduction, especially the lossy varieties used
for video] for tape-storage seems counter-productive, an unnecessa_ added special-purpose
complexity. Rather, with more than an order of magnitude higher TB/in density on the horizon,
the highest-volume market for such high capacity storage should be cultivated. There should be no
doubt that the all-purpose high-rate capability of the linear recorder can be made attractive to the
consumer/PCuser.
The key prerequisite for this development is the availability of mass-production processes for (1)
dense thin-film narrow-track head-arrays as discussed above, and (2) simple, low-power, multi-
channel ICs.
The thin-film head and integrated channel technologies are ones in which the U.S. maintains a
120
strongleadership.Theseprovide a majoropportunity for theU.S. storageindustry to takethe
initiative to reenterthe consumermarketwith nativedrive, head,channel,and tapecomponent
technologies,eachof whichhassubstantialtechnicaland/ormanufacturabilityadvantages.
Author's VLBI-Related Work Background
Hans F. Hinteregger received B.A. and Ph.D. degrees in physics from MIT in 1964 and 1972
respectively and has been at Haystack Observatory since 1967 at the time of the first very long
baseline interferometry (VLBI) experiments. He pioneered the geodetidastrometric applications of
VLBI by introducing means to accurately measure group delay (by coherently sampling a wide
spanned bandwidth). Since 1975 his work within the Haystack VLBI Group has focused on the
development of the extreme wideband digital tape systems required by VLBI. The latest version of
this system (Mark IV) has demonstrated 1 Gb/s operation with sixty-four 16 Mb/s channels [using
two headstacks of the author's 1985 design].
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