Project Cyclops, A Design... - Department of Earth and Planetary ...

Weproposeinsteadthat the databe storedon
magneticdisksequippedwith flying(noncontacting)
heads.With absolutefilteringof the air and with
constant operation (noshutdowns) diskwearisvirtually
absent, sincetheheadsdonotcontact thedisksurfaces.
Suchdiskscanrecorddatain eitheranalogor digital
form.Theformertypeisoftenusedforinstant replayof
TV pictureswhilethelatteris widelyusedfor data
storage in thecomputer field.Theusualdiskis14in.in
diameterandcanstoreon theorderof 250adjacent
tracksof datawithdensities ashighas4000bits]in,for
digitaldataand4000Nyquistintervalsper inchof
analogdata.Thus, each disk surface has a storage
capacity of some 40 million bits of digital data or
samples of analog data. Disks are built with moving
heads that can be positioned in 10 to 20 msec to pick up
or record a particular track, or with fLxed heads, one per
track. In the larger sizes the cost of moving head systems
is about 0.007 cents per bit while for fixed head systems
the cost is about 0,03 cents/bit. Head per track systems
are thus about four times as expensive per data bit, but
they offer the advantages of providing instantaneous
switching between tracks and of having fewer moving
parts. In a head-per-track system there is essentially only
one moving part: a simple spindle with its payload of
disks rigidly attached. Maintenance costs should be very
low.
In soma respects our application places less severe
requirements on the recording system than other applications
do. If we used digital recording we would need no
more than 4 bits to describe the power spectrum
amplitude. Similarly for analog recording we need no
more than about a 20 dB signal-to-noise ratio. (Our
signal is mostly noise anyway, and adding 1% to the
noise power raises the system temperature only that
amount.) Since we will be adding 100 or more tracks
together to get the final signals, a defect in a particular
track is of little importance. This is in contrast to
computer applications where a dropped bit can be
disastrous.
The various alternatives should be studied carefully
before a decision is made among them for the final
design. For the present, in order to get a rough cost
estimate we shall assume analog recording on head-pertrack
disks.
Our data rate is fixed by the two 100 MHz IF
channels, which produce 4× 10s samples of analog data
per second. Assuming this is recorded with fixed head
disks at a cost of 0.03 cents per sample the cost of the
data storage system will be
132
C = 4× 10s X 3× 10--4 = $120,000
per second of observing time. This figure is independent
of how narrow our resolvable frequency intervals are or
how many optical analyzers are needed. Thus, for 1000
seconds of observing time per star the cost of the data
storage system will be about $120 million.
Digital Storage Systems
If the optical spectrum analyzers have rectangular
gates, the video signal to be stored will be sharply band
limited at about the bandwidth of the IF signal supplied
to the spectrum analyzer. Thus, only slightly more than
2 samples/sec of spectrum must be taken per Hz of IF
bandwidth. If 4 bits per sample are used to encode the
signal, we will need about five times as many bits to
store the data digitally as we need samples to store it in
analog form. Thus, it appears that digital storage would
be much more expensive at present.
The economics could change overnight, however.
Magnetic "bubble" memories are being actively pursued
in several laboratories, notably at Bell Telephone Labs.
Conceivably these memories could ultimately bring the
cost of digital storage down to 10--4 cents]bit or less.
Bubble memories are suitable for digital storage only.
They lend themselves most naturally to the construction
of exceedingly long shift registers: registers with over
one million bits per square inch of magnetic material.
About 2X 106 wafers could then store 1000 sec of data
from both IF channels. At a cost of $10/wafer this
would represent $20 million worth of storage capacity.
Since they have no mechanical moving parts, bubble
memories should have low maintenance and would be
ideal for Cyclops.
Forming the Composite Signals
To process the stored data we propose that all
samples of the power spectrum be played back simultaneously
using the same heads that were used for the
recording process. This eliminates all the major sources
of timing errors and assures that the data corresponding
to a given frequency in the power spectrum will appear
simultaneously in each playback amplifier output. To
permit summing the spectra with different relative
displacements it is proposed to send the reproduced
signals down video delay lines equipped with appropriate
taps. Figure 1I-8 illustrates the delay lines and tap
arrangement required to synthesize 17 composite signals
from 5 samples of the power spectrum, each composite
signal representing an assumed drift rate. The pickup
lines are merely indicated symbolically; actually, they
would be coaxial or strip lines driven by current sources
at each tap. At the center where all lines are driven by
the same tap, a bridging amplifier with 17 independent