Session II of the 1975 Lake Arrowhead
Workshop on "Advances in Storage for Minis
and Micros" was devoted to low-cost tape
devices. This article presents a summary of the
discussions in that session. No attempt is made
to present an all-inclusive coverage of low-cost
tape devices, nor necessarily to present a
balanced picture, but rather to present the
actual material discussed at the workshop. As
a result, several types of low-cost tape devices
presently on the market are not covered.
However, the unique gathering of individual
practitioners in the field made this session a
lively one, and the article that grew out of it
represents a collection of insights that are not
likely to be found elsewhere.
Low-Cost
Tape Devices
L.C. Hobbs
Hobbs Associates, Inc.
Introduction
As the price of minicomputers and microprocessors
continues to decline, many new applications for
computer technology are developing. However, in
many cases the use of minicomputers or microprocessors in a particular application has been prohibited
by the cost of the necessary peripheral equipment.
Many applications that can benefit from the use of
computer technology cannot justify the cost of the
necessary input, output, and storage equipment,
even if the minicomputer and microprocessor were
free. Hence, there has been a continuing pressure to
reduce the cost of peripheral devices.
Perhaps the greatest cost improvements have
been made in on-line and off-line secondary storage.
The major contributors to enhanced storage capacity
and reduced price have been low-cost reel-to-reel
industry-compatible tape drives, magnetic tape
cassette drives, magnetic cartridge drives, and floppy
disks. Session III of this workshop was devoted to
low-cost rotating memories, including floppy disk
March 1976
drives; hence, they are not covered in detail here.
However, it is important to note that in many applications floppy disk drives are a viable alternative
and are directly competitive with tape devices in
minicomputer and microprocessor systems and
applications.
The terminology in this area is not completely
consistent and logical, but in most cases the term
"reel-to-reel" is used to identify tape devices that
handle a half-inch magnetic tape reel with start/stop
characteristics and read/write electronics designed
to provide industry compatibility. The term "cassette" is used almost exclusively to refer to the low
cost Philips type cassette originally developed for
audio applications and later modified and enhanced
for digital recording. The term "cartridge" is generally used to refer to anything that does not fall
into one of the two preceding classifications, particularly a device wherein the tape media is enclosed
in some type of container or cartridge which also
includes the two take-up reels. Until recently, cartridges were generally larger in both tape width
21
and tape length than cassettes, but last year a
smaller version of the 3M cartridge was introduced.
Reel-to-reel tape drives are used where larger capacity and industry compatibility is required, cassettes
where low cost is the primary consideration, and
cartridges where a compromise between the advantages and disadvantage of reel-to-reel devices and
cassette devices is needed. Reel-to-reel devices are
more expensive, whereas the lower-cost cassettes
provide lower capacity and lower reliability (at least
in the past).
The list of possible applications for low-cost tape
storage is almost as large as that for minicomputers and microprocessors themselves. The major
categories of applications include minicomputer
systems, terminals, instrument and test systems,
data collection and data acquisition systems, data
entry systems, educational systems, and small
business systems.
Of these applications, cassette devices are more
likely to be used in terminals, data collection systems,
data entry systems, and small business systems.
Reel-to-reel devices are more likely to be used in
minicomputer systems, instrumentation and test
systems, and data acquisition systems. Cartridge
Low-Cost Reel-to-Reel Tape Devices
This discussion of reel-to-reel tape drives deals
primarily with half-inch IBM format compatible tape
drives applicable to minicomputers. It is limited to
drives that are 19 inches x 24 inches or smaller, and
excludes the "refrigerator" size units built to replace
the IBM 2400 and 3400 series. This is certainly an
arbitrary limitation, and even some of the faster
19-inch x 24-inch drives may not be applicable to
minis. Cassettes, cartridges, and flexible disks are
compared in a later table. (Flexible disks are properly
the domain of tape engineers since they use contact
recording.)
The broad line of reel-to-reel products ranges from
7 inch reel tension arm 12.5 ips units to 10.5 inch
reel vacuum column units up to 125 ips, some with
self threading, all with PE or NRZI recording. Formatters are available for PE or NRZI with or without
buffers, and incremental tape drives are available
for special applications where a buffered, synchronous
drive is not justified.
Table 1 shows the typical characteristics of available tension arm units. The storage capacity listed
is unformatted-that is, without allowance for inter-
Table 1. Half-inch Compatible Tape Drive-Tension Arm
REEL SIZE
DECK SIZE
7 in.
7 in.
Min Interface
8.75 x 19
8.5 in.
12.5 x 19
10.5 in.
19 x 14
MAX DATA RATE
CAPACITY
ACCESS TIME
(UNFORMATTED)
40
20
15 MS
11.5 MBYTE
60
10 MS
KBYTES/SEC
72 KB/Sec @ 45 IPS
(120 KB/Sec @75 IPS)
.7
7.50
23 MBYTE
.8
8.50
8.33 MS
(5 MS @ 75 IPS)
46 MBYTE
1
11.00
8.5 in.
Incremental
12.5 x 19
Read 1000 Ch/Sec
Write 500 Ch/Sec
IRG Time 550 MS
(800 BPI)
11.5 MBTYE
10.5 in.
Incremental
19 x 24
Read 1000 Ch/Sec
Write 500 Ch/Sec
IRG Time 550 MS
(800 BPI)
23 MBYTE
devices offer an alternative in any of these applications depending upon the relative importance of
cost versus capacity and reliability.
In the following discussions, contributed by individual participants in the workshop session (see
acknowledgements), a general overview is given of
reel-to-reel and cassette devices, but the presentation on cartridge devices is restricted to a specific
new type of cartridge device that offers enhancements and performance characteristics over previously available cartridges. However, it is important
to note that two or three other cartridge devices
using different types of cartridges are also in production and are in widespread use in the industry.
22
MEDIA
COST
FACTOR COST
(.5)
8.50
11.00
record gaps. The efficiency is probably around 60%
but subject to record length. Access time is really
start time to the next record since the use of tape
assumes random access is not a requirement. Data
rate is the product of the maximum speed available
and the maximum recording density-i.e., 1600 BPI.
Defining a cost factor is always difficult, but in this
case cost is normalized to 1 for the basic 10.5 inch
reel tension arm tape drive. The purpose is to show
the effect of reel size on cost. An 8.5 inch reel unit
costs 80% of a 10.5 inch reel unit, and so on.
Two special cases are shown in the table. One is a
low-cost 7 inch reel "mini" at a cost factor of 0.5.
This shows the cost difference for a unit with a read/
COMPUTER
write head (as opposed to read while write) with
NRZI and minimum frills. The 0.7 cost factor is a
fully equipped mini with maximum speed, dual stack
head, and PE data electronics as are the other units.
The other special case is a 75-ips tension arm unit
with 10.5 inch reels. This is a relatively low usage
product designed for the application where high
data rate is necessary, but with low standby power,
low acoustic noise, etc. This is not the most costeffective way to get 75-ips operation, but it serves
well for special applications where power and/or noise
restrictions are especially severe.
All of the units are available as 7-track or 9-track,
with 9-track offered as NRZI or PE (1600-bpi Manchester code) or both. One other standard option is
a read-only package, meaning, really, read anything7-track or 9-track; 200-, 556- or 800 bpi NRZI; or
9-track PE. This option is useful for computer output
microfilm, off-line print stations, etc.
All of the units may be obtained with formatter
for full data recovery with NRZI, PE, or both, with
various size buffers if necessary. "Industry standard"
interface really exists so that formatters from one
vendor work nicely with drives from another.
are too short for reliable operation above the 75-ips
range. The next higher category of drive offers a
more complex tape path with longer storage columns,
the possibility of autothread, and compatibility with
easy load cartridges. These drives cost considerably
more than the simpler drives, even if used at lower
speeds, but are very cost effective at 125 ips compared to the larger "refrigerator" size drives that
are really 200-ips drives slowed down. Group code
recording (GCR) is discussed later but is shown in
Table 2 to illustrate what happens to access time,
data rate, and st6rage capacity. These vacuum
column drives are also offered with NRZI, PE, both,
read only, appropriate formatters, etc.
One surprise is the relatively low usage of the
PE format, even now. Only about 50% of the sales of
compatible tape drives are PE, even though the
format with its advantages of single track error
correction and freedom from skew problems has
been around for almost 10 years! One explanation is
the complexity of data recovery as compared to
NRZI, but the formatter to handle this is readily
available and is well understood.
One other explanation may be that skew (the inac,curate guiding of tape across the head) hasn't really
Table 2. Half-inch Compatible Tape Drive-Vacuum Column
SPEED RANGE
MAX DATA RATE
KBYTE/SEC
ACCESS TIME
CAPACITY
(UNFORMATTED)
COST FACTOR
MEDIA COST
7.5 MS
46 MBYTE
1
11.00
120
5 MS
46 MBYTE
1.1
11.00
75-125 IPS
200
3 MS
46 MBYTES
1.5
11.00
125 IPS w/GCR
780
1.2 MS
180 MBYTES
1.7
11.00
25-50 IPS
80
50-75 IPS
Most of the drives shown are available with 48
VDC primary power for telephone equipment application, as well as the usual 56/60 cycle AC.
Table 2 shows the available ranges of competitive
tape drives using vacuum column storage. The cost
factor is again normalized to 1 for a 10.5 inch reel
tension arm unit-that is, the cost factor of 1 here is
the same as in Table 1.
This shows that vacuum column tape units cost
about the same as tension arm units in the 45-ips
range, and less than tension arms at 75 ips, assuming
no extra frills. The 75-ips vacuum column unit with
the simplified tape path (i.e., no autothread) is one of
the most cost-effective devices available. There is a
break in the cost performance above the 75-ips area
where costs increase more rapidly, as indicated in
the chart.
The lowest cost vacuum column tape path arrangement is not "autothread," and the storage columns
March 1976
been such a problem after all. The manufacturers of
mini drives have almost totally held to the head
guide geometry of the 2401 series IBM drives. In so
doing, one of the major interchange problems that
faced the large mainframe manufacturers in the 60's,
when "trough" guide machines from one supplier
were asked to interchange with pc ' guide units from
another supplier, is minimized.
A detailed discussion of this interchange skew
problem is more involved than can be discussed
here, but Juan Rodrigues of Storage Technology
Corporation has covered it nicely in the August
1975 Proceedings of the IEEE.
Another surprise is the very low usage (5% to 10%)
of hard faced heads though they have been around
for several years, too. These heads offer three to five
times the wear life of ordinary metal heads at initial
cost increases of about 20% of a new head, not
counting the installation expense. These are a good
deal, and users are missing out!
23
Table 3. Small Storage Device Comparison
DEVICE TYPE
Half-Inch Compatible
Tape w/7-Inch Reel
(Min Configuration)
RECORDING TRANSFER RATE
CAPACITY
KBITS/SEC
(UNFORMATTED) DENSITY
SOFT ERROR
RATE
RELATIVE
COST
MEDIA
COST
100 MBITS
1000 BPI
360 (25 IPS)
10-9
1
7.50
22 MBITS
1600 BPI
160 (25 IPS)
10-8
.4
13.00
Philips Cassette
7 MBITS
800 BPI
19 (12 IPS)
10-7
.2
4.00
Flexible Disk
3 MBITS
3248 BPI
10-9
.25
8.00
3M Cartridge
Table 3 compares the lowest half-inch compatible
drive to other contact recording systems. Each type
of device has its place, although the future of the
Philips cassette seems questionable when dual
density flexible disk is compared. This would mean
about the same cost per bit stored for cassette and
flexible disk, but with access and reliability somewhat better for flexible disk.
Our best cost reduction efforts this past
the line against
inflation. The cost of heads, the biggest
single item in tape units, is up substantially,
partially due to the loss of production in
Portugal.
year have not even held
But before considering where we are going, let's
consider where are we not going. Much reduced
prices for the half-inch tape drives appear unlikely.
Most factors that determine price indicate the price
should go up, even though the cost is already high
compared to the cost of the processor in some applications. Our best cost reduction efforts this past
year have not even held the line against inflation.
The cost of heads, the biggest single item in tape
units, is up substantially, partially due to the loss of
production in Portugal. The recent changes to the UL
requirements for plastics have also increased costs
about 25% for many parts. This is a good standard
and we fully support it, but the cost goes with the
benefit. The cost of most materials is up; the new
semiconductors are the only bright spot other than
just trying harder.
One popular idea that's been around awhile is
that of a $1000 mini-i.e., a 7-inch reel half-inch
IBM-compatible tape unit with the usual features to
sell for $1000 in small OEM quantities. We would
like very much to offer such a device, but we seem to
be getting further away-not closer. Brushless DC
motors don't seem to offer much since the brush
types have very acceptable life.
Major use of custom LSI devices in the product
seems unlikely, though standard and special purpose
devices look attractive. The production quantities
24
250
The control logic on a typical tape drive only
costs about $9 now, so there isn't much room
to move. The money is in heads, motors, and
other mechanical parts.
simply not high enough to justify custom LSI,
since in most possible applications only one device
of a given type per drive would be needed. The
control logic on a typical tape drive only costs about
$9 now, so there isn't much room to move. The money
is in heads, motors, and other mechanical parts.
Then what can we expect? One area where LSI
looks good is in formatters. A single, small formatter
for NRZI/PE from 12.5 to 125 ips mounted integrally
to the tape unit is now possible. The potential savings
in power supply and a package make this an attractive approach.
Another activity that absorbs considerable effort
is compliance with new standards for safety, RFI,
acoustics, etc. More work will be done in these areas,
just as in the automobile industry. Table 4 shows a
partial list of standards and/or requirements that
are becoming more important. As mentioned earlier,
the result here is good, but there is a price associated
with benefit.
We expect to be borrowing from the "big boys" in
a few areas and passing the improvements in performance along on lower cost drives. Group Code
Recording (6250 bpi) is an obvious addition. The
changes in storage and transfer rate were shown
previously on the vacuum column chart (Table 2).
IBM-compatible GCR on vacuum column units at
speeds from 25 to 125 ips will be offered soon. The
associated formatter will be considerably more complex than current units, but for those who need it,
the benefits will be available with minimum system
impact other than data rate-i.e., a 25-ips GCR
unit could be used where 100 ips 1600 bpi was
required with no system impact.
Along with GCR, it is reasonable to expect changes
to tape cleaners for more effective cleaning, capstan
systems that use digital feedback devices rather than
analog, more use of LED's, and more built-in test
equipment. Emphasis will be on durability and
serviceability-i.e., lower cost of ownership.
are
COMPUTER
Table 4. Applicable Standards
SAFETY STANDARDS
U.L. 478 (10/1/75) .
.
.
.
.
.
.
.
.
U.L. 114 . . . . . . . . . . . .
C.S.A. C22.2 #154 . . . . . . . . .
National Fire Protection Association . . .
International Electrotechnical Commission .
British Standards Institute . . . . . .
International Commission on Rules for the
Approval of Electrical Equipment . . .
VDE (Association of German Elec Engrs)
-0804 . . . . . . . . . . . .
-0730 . . . . . . . . . . . .
RFI
. . . . . . . .
VDE 0875/7.71
CISPR (International Committee on Radio
Interference . . . . . . . . .
FCC . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Electronic Data Processing Equipment
Office Appliances and Business Equipment
Data Processing Equipment, 1975
National Electric Code, 1975
IEC435
B.S. 3861
.
.
C.E.E.10, Part II
.
.
.
.
Regulations for Telecommunication Devices
Motor Operated Appliances
.
.
Regulation for RFI Suppression, Conducted and Radiated
.
Publication 1, 2 Specification for CISPR - RFI Measuring Apparatus
(150 KHz to 300 MHz)
"Coming Up"
.
.
.
.
.
.
...
.
.
.
ACOUSTICS
ISO 1996.
Assessment of Noise with Respect to Community Response
Lastly, there appears to be a need for a new ma
storage device possibly based on one of the wi
tape cartridges. It's too early to say what form su
a device would take, but the availability of a ni
media opens up several possibilities, one of whi
will probably be made available for mini systems.
A New Cartridge Tape Device
Tape cartridges and drives have been availal
for several years from 3M, IBM, Tridata, a
others; but the search for an improved cartrid
system has continued.
The Emerson Tape Pac (see Figure 1) is a ni
magnetic tape storage unit which provides the rel
bility, interchangeability, capacity, and versatili
to meet the needs of a wide variety of compul
For an instrumentation tape recorder
manufacturer, the bane of any application is
tape threading-a highly sensitive precision
instrument is exposed to the human operator
where all sorts of horrible things can happen.
The obvious need is an instrumentation cartridge.
applications. A brief description of the background
of the development will be of interest and will put the
Tape Pac and our views on the future of storage in
perspective.
For an instrunentation tape recorder manufacturer,
the bane of any application is tape threading-a
highly sensitive precision instrument is exposed to
the human operator where all sorts of horrible things
March 1976
Figure 1. Emerson's Model 2000 Series Tape Pac System (photo
courtesy of Emerson Electric)
can happen. The obvious need is an instrumentation
cartridge. In general, either self-threading or tworeel containers are highly impractical. High-density
recording techniques are required to reduce the tape
capacity needed to meet mission data requirements.
At the same time, until a cartridge design could be
found which provided the reliability, interchangeability, and precise tape handling (i.e., low wow and
flutter, low time base error) instrumentation cartridges were not practical.
A similar condition can be stated for tape storage
for computers when the broad spectrum of computer
tape applications are considered. A practical broadbase application of cartridges to computer storage
requires the following:
(a) high-density recording;
(b) a design of sufficient precision and simplicity
to yield reliability and interchangeability;
25
(c) a design which can be produced at low cost; and
(d) a large enough capacity to meet most computer
applications.
A summary of the status of high density recording in both digital instrumentation and commercial
computer applications will help establish the background for the development of the Emerson Tape
Pac. In digital instrumentation applications, 16.6
kilobits/in. is commonplace, and 30 to 50 kilobits/in.
systems are being evaluated. These higher densities
are not ready for commercial applications because
the tape needs improvement and the heads and
electronics are too expensive. There has been much
discussion on encoding techniques for optimum
performance, and the balance between acceptable
dropout performance and overhead for error correcting coding is still being explored.
In commercial computer applications, 1600 BPI
(9 track parallel) is routine. Although 6250 BPI
(average-approximately 9000 BPI on tape) is
available, it puts excessive demand on present tape
quality and requires excessive overhead in error
correcting. Hence, 3200 to 4000 BPI is the most
practical operating density range. It is consistent
with tape quality, saturation recording remains
practical, and simple electronic recording and reproducing techniques are applicable.
The Emerson Tape Pac was developed in this
environment. This device uses a simple reel-to-reel
configuration as shown in Figure 2. The C-2002
Tape Pac contains 600 feet of half-inch magnetic
tape. Because of the precision die-cast housing construction, the use of ball bearings, and the simple
two-guide design, tape transfer between reels is very
uniform with low wow and flutter and small instantaneous speed variation. The accuracy and rigidity
also provide reliability and interchangeability which
are most important to any application.
A brake is also provided internal to the Pac to
prevent tape slacking in handling. The brake is designed to tighten the Pac when exposed to vibration. A spring-loaded hinged cover is also provided
to protect the tape from damage during handling
and exposure to the environment.
The Model 2004 Tape Pac is driven peripherally
by the twin-capstan Model 2005 Pac Drive, as
illustrated in Figure 3. When in place, each capstan
engages a reel. Each capstan in turn is driven by a
motor. The relative speed control of each can be
programmed to yield the proper tape speed and
also precise control of tape tension.
From end to end, acceleration or steady state, the
tape tension can be maintained to 6 ± 1/2 oz. This
results in long life and precision speed control. Fivethousand passes are readily obtained. At this juncture, the design has been tested at speeds from 15/1,
ips through 240 ips. At 240 ips, or 1200 feet per
niinute, the traverse time of the Tape Pac is 30
seconds. In some applications in which IBM compatibility is desired, start/stop times become an
important consideration. The Tape Pac has no
vacuum columns or compliance arms, but IBMcompatible recording can be produced up to speeds
of 60 ips.
26
The characteristics provided by the Model 2004
Tape Pac and the Model 2005 Pac Drive are summarized below:
* 600 feet-1 mil-half-inch tape (1000 feet-1/2 mil)
* die cast housing
* ball bearings
* two tape guides
* brake
BALL BEARING MOUNTED
REEL HUBS STORES
6DO FT OF 1 MIL X 1/2 INCH
WIDE TAPE
SPRING LOADED
DUST DOORS
PRECISION
TAPE GUIDES
NO OXIDE CONTACT
WITH TAPE GUIDES
TAPE PACK SIZE:
DEPTH
5.0 INCHES
8.6 INCHES
WIDTH
HEIGHT 1.1 INCHES
2 LBS
WE!GHT
BRAKE MAINTAINS TAPE TENSION
WHEN TAPE PACK IS OUT Of THE
TRANSPORT
Figure 2. Exploded view of Tape Pac (figure courtesy of
Emerson Electric)
Figure 3. Tape drive concept (figure courtesy of Emerson
Electric)
COMPUTER
.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
hinged
covers
low wow and flutter or low instantaneous speed
variation
reliable
interchangeable
good handling
average tape speed-control-0.25% (a function
of phase lock oscillator only)
ISV-0.5% p-p at 30 ips
tape tension 6 oz + 1/2-oz-end to end-accel/
decel
tape passes-5000 minimum
tape speeds-15/16 ips-240 ips
start/stop times-35 milliseconds at 60 ips
twin capstans
individual reel peripheral drive
phase locked tapeup reel control
supply reel drive slaved to take up reel control
acceldecel ramp control
It can record IBM-compatible tape (7-track or 9track) at any density desired with NRZ-I or Phase
Encoded quality consistent with existing designs. It
can be used in a serial format up to 3400 bpi, 8 tracks,
180 megabits without redundancy or error correcting.
Less than 1 in 108 permanent error rate has been
observed after 5000 passes. A 10.8 KB/inch, 8-track
serial design has been committed and 16.6 KB/inch
densities definitely feasible. It is capable of time
expansion or contraction over a wide speed range,
and it eliminates tape handling problems inherent in
conventional tape transport designs. Its inherent
simplicity of design yields greater reliability, and
the totally enclosed environment provides ruggedness and other requirements of hostile environment
applications.
Digital Cassette Tape Drives
The Philips Cassette is a small, convenient, lowcost, reusable, non-volatile magnetic storage medium
that can easily store lOOK bytes, or as many as
550K bytes in an ANSI 2-track format, or as many
as 2 megabytes in a special 4-track configuration.
One of the advantages of a cassette is that it comes
in many lengths: standards are 150, 300, and 450 feet;
noncertified cassettes contain as much as 600 feet.
The Philips Cassette is the most widely used medium
since paper tape and punched cards. There is a digital
machine population of well over 500,000 units at
the present time, and this number is growing at a
rate of 150,000 units per year. This rate is predicted
to continue for several years.
On the basis of storage capacity, data rate, access
time, cost (both entry costs and media costs), and
media reliability, the comparison of the cassette
with other storage devices is as follows:
Storage Capacity (see Figure 4). At the high end,
special cassette units can equal the storage capacity
of a 3M cartridge unit using 4 tracks. ANSI-compatible
units are about double the capacity of present day
floppy disk units when two tracks are used, and lowMarch 1976
performance cassettes are roughly equivalent to
1000 feet of paper tape, or 1300 80-column punched
cards.
Data Rate. Data rates vary from very slow asynchronous, by character or bit rates, up to 6K bytes/
sec. However, typical transfer rates are generally in
the range of 1K to 2K bytes/sec. Therefore, on the
high end, this puts the cassette on a par with a 3M
cartridge unit reading one track at a time. However,
they are both an order of magnitude slower than a
flexible disk unit which typically handles data at
31.25K bytes/sec. It is important to point out that
data rates are not generally a major consideration
when selecting a tape drive. Rates can easily be
changed using buffering techniques, which are frequently required anyway; moreover there are other
tape positioning times which can be much longer
than the block transfer time.
Access Time. There are two kinds of access time:
the response time to a command when the unit is in
position to read or write, and the block search time.
Response Time. Typical numbers are 15 ms to 20
ms for intermediate tape speeds, but they can be as
high as 50 ms to 100 ms at higher speeds (20 ips or
higher).
Search Time. The average for 300 feet of tape,
assuming random block distribution, may be as low
as 25 seconds, but on low performance units can be
100 seconds or more.
Summary. The access times for cassettes are
equivalent to the 3M cartridge units, but are over
an order of magnitude slower than flexible disk units
which average about 0.6 seconds for completely
random data distribution (which usually isn't the
case for cassettes or disks).
Cost. Two kinds of cost can be considered: entry
cost and media cost (see Figure 4).
Entry costs are heavily dependent on application
details, so a general comparison cannot be made.
However, all pertinent system costs should be
included when making such comparisons. For
example, interface circuitry, data formatting circuitry, data buffers, software, and program storage
requirements, if any, should all be included.
Data stored on cassettes typically costs around
1.5 to 2.0 millicents/byte, but can be as low as
9.5 millicents/byte. This puts cassettes on a par
with the 3M cartridge a\t the low end, and still half
the cost/byte of a floppy disk at the high end.
Media Reliability. In order to compare media reliability, one must be specific about application details
such as hardware, duty cycle, and environment.
There are occasional claims, usually from cassette
competitors, that cassettes are only good for 300
passes or less. In some applications this is true, but
typical numbers using certified cassettes, in a good
transport, in an office environment, are 2000 to 5000
end-to-end passes, and when re-reading a short record,
they are capable of 10K to 20K error-free passes.
Present day digital cassette drives come in two
basic versions: capstan drive units with as many as
four motors, and hub drive units with up to two motors.
Capstan Drive Units. This type of drive comes in
two versions: dual and single capstan models.
27
CAPACITY
(BYTES)
30M
3
10 M
I
REEL TO REEL
(HALF-INCH, 2400 FEET)
COST*
(10-4 d/BYTE)
40
+
FLOPPY DISK
LOW PERFORMANCE CASSETTE
20 - (PAPER TAPE)
15 - - ANSI CASSETTE
1013M CARTRIDGE
5- HIGH PERFORMANCE CASSETTE
3M CARTRIDGE (4T)
2 M- HIGH-PERFORMANCE CASSETTE
iM
1-
550 K ± ANSI CASSETTE (2T)
0.5 ± REEL TO REEL
250 K - FLOPPY (ONE SIDE)
L LOW-PERFORMANCE CASSETTE
100 K -JI- (PAPER TAPE)
0.1
_L
*Assuming complete utilization
Figure 4. Tape cassette capacities and costs
Dual capstan drives usually have two counterrotating capstans in order to "pull" the tape across
the head in both directions. They use from one to four
motors, but typically they have three motors (one
capstan and two reel motors), two pressure roller
mechanisms, and a head retraction mechanism. This
makes this type of drive the most mechanically
complex of the types that will be discussed. They
generally use a high-inertia capstan system which
rotates all the time, and therefore uses a "banging"
pressure roller to start the tape. This is relatively
hard on the tape and therefore reduces the media
reliability. However, this type of unit has the lowest
speed variation and time base error. Also, the control
electronics is simple (especially on units with an AC
motor), and the read/write circuitry can be of "fixed
window" variety, which uses the fewest number of
components.
Single-capstan drives reduce the mechanical complexity somewhat by eliminating a capstan and pressure roller mechanism, and they generally use a lowinertia capstan which is capable of accelerating
and decelerating the tape without the need for a
"banging" pressure roller. Therefore, this type of
drive is easier on the tape-and somewhat more reliable,
but they either cannot run in the reverse direction,
under capstan control, or they must defeat the pressure pad in the cassette. This is generally accomplished either by using cassettes Without pressure
pa4ds, or by pushing the pad back so that it does
28
not drag on the tape. This type of unit has good
speed accuracy; however, the T.B.E. (time base error)
is generally worse due to the low inertia drive. The
control logic and circuitry is usually more complex
due to the servo; however, the read/write circuitry
generally can still be of the "fixed window" variety.
Hub Drive Units. These come in 2 versions also:
constant tape speed units, and constant hub speed
units.
Constant tape velocity units generally have two
motors which drive each reel. They don't use pressure
rollers or capstan shafts, so the mechanical complexity is further reduced. They achieve constant, or
nearly constant, tape velocity by using either a second
track with a timing signal pre-recorded on it, or
some type of motor EMF or tachometer differencing
scheme for both hubs, or they use a programmed
velocity profile which corrects the speed depending
on the distance from the beginning of the tape. The
units using timing tracks have good speed accuracy,
but T.B.E. is worse than most capstan drive units.
The other two types of units have relatively poor
speed accuracy (from ± 5 to ± 1 0%), and are adversely affected by packing irregularities, tape thickness variation, and length of tape in the cassette.
However, because there are no pressure rollers, this
type of unit is easier on the tape than capstan drive
units. The control logic and circuitry is probably the
most complex because of the two servos and the
motor coordination that is required. The read/write
COMPUTER
circuitry uses either a two-track or a single-track
speed-tolerant decoding technique. Because of this,
data density on tape is usually reduced and the
circuit complexity is increased.
Constant hub velocity units use either one or two
motors and have the simplest mechanism of all
the types of drives discussed. Since one of the reel
speeds is constant (generally the takeup reel), the
tape velocity varies with the radius of the tape pack,
which is about 2.5 to 1 for a 300-foot cassette. However, the speed profile repeats from pass to pass so
that fairly constant data rates can still be achieved.
The control logic and circuitry are generally very
simple, but the read/write circuitry is the most complex because it must be of the speed-tolerant variety
in order to provide unit-to-unit interchange. Average
data density on the tape is low because the maximum
packing density is achieved only at the lowest tape
speed. In addition to this, when two-track read/write
schemes are employed, the cassette storage capacity
is typically about 25% of a comparable capstan
drive unit.
Hub drive units reduce mechanical complexity, at
the cost of electrical complexity and reduced storage
capacity; however, the overall reliability is somewhat
improved. On the other hand, capstan drive units
still offer good reliability, improved edit and rewrite
capability due to more precise tape handling, and
good media life (though somewhat lower than hub
drive units, it is still in the range of 2000 to 5000
end-to-end passes on a well-designed transport with
a certified cassette).
Cassettes will continue to be used in even greater
numbers in the low-cost, low-performance applications but entry costs must be reduced and reliability
must be improved. The use of high-performance
cassette drives will tail off because there are fewer
applications and because of competition from other
types of memory devices. Because of a more liberalized attitude throughout the industry, new types of
competitive devices for the cassette may appear on
the scene. This is already apparent because of several
new products which have been introduced, or are
about to be introduced such as the "Reelette " which
American Videonetics Corp. has introduced, and the
Mini-3M cartridge which H-P has introduced in
some of their equipment. Interdyne is very interested
in this low-cost, high-volume OEM market, and has
some new products in the works.
Conclusion
Throughout the history of the computer industry,
electromechanical peripheral equipments have been
hard pressed to keep pace with advances in internal
logic and memory. However, continued development
of simpler mechanisms and increases in bit density
on magnetic storage media have tended to provide
lower-cost, higher-capacity secondary storage media
to complement lower-cost processors. In most cases,
a reasonable system balance is possible now for minicomputers; but microprocessors (including several
March 1976
thousand bytes of storage) priced in the few-hundreddollar range put new pressures on peripheral equipment development. In many cases the total cost of
secondary storage devices is the major criterion
rather than the cost per bit. Hence, very low cost
storage devices such as magnetic tape cassettes and
floppy disks take on added importance for smaller
equipments and systems.
Although speed and capacity can be compromised
to achieve a lower price, in most applications reliability cannot be sacrificed. Hence, there is a continuing need for cost reductions in storage devices which
must be achieved without sacrificing reliability. Since
no single device is likely to provide the optimum characteristics of low cost, high speed, high capacity,
industry compatibility, and random access, we will
continue to see requirements for and enhancements
in the cost/performance ratio of all types of secondary
storage devices, including reel-to-reel magnetic tape
drives, magnetic tape cassette drives, magnetic tape
cartridge drives, and floppy disk drives. .
Acknowledgements
The information presented in this article is the
direct contribution of the participants in Session II
of the 1975 Lake Arrowhead Workshop, and full
credit is due them for the material presented herein.
The material on low cost reel-to-reel tape drives was
contributed by Mr. Robert Tullos of Pertec, the
material on the new cartridge tape device was contributed by Mr. John Coolidge of Emerson Electric,
and the material on digital cassette tape drives was
contributed by Mr. Richard Lewis of Interdyne.
Linder Charlie Hobbs has worked in the
design, analysis, and evaluation of electronic
systems, information processing systems,
computers, displays, terminals, memories,
and peripheral equipments since 1948. He
held technical and supervisory positions at
RCA and UNIVAC and was manager of Data
Processing Engineering at the Aeronutronic
Division of Ford Motor Company. Since
_ 1962, he has been president of Hobbs AssociNl a m
ates, Inc., which provides computer timesharing and consulting
services. Recent work includes evaluation and forecasting of
technology and markets for computer equipment and systems
for industrial and governmental organizations in the US,
Canada, and Japan. He has published many papers on computer
technology, has organized several panel discussions and workshop sessions, has edited a book, Parallel Processor Systems, has
taught computer systems courses at UCLA since 1957, and
holds thirteen patents. He has a strong interest in conferences,
publications, and other means of disseminating technical information.
Active in IEEE and AFIPS, Hobbs has served as chairman of
the IRE Computer Standards Committee, chairman of the
Philadelphia and Orange County Chapters of the Computer
Society, chairman of the Orange County Section of IEEE,
chairman of the 1967 FJCC, chairman of the Computer Group
Administrative Committee, and as a member of the Board of
Directors of AFIPS and the IEEE.
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