Limits and future possibilities of information storage

Journal of Magnetism and Magnetic Materials 99 (1991) 335-355 North-Holland

Review paper

Limits and future possibilities of information storage H.

VSlz ’

Zentralinstitut

fti

Kybernetik

und Infomationsprozesse,

O-1199 Berlin, Germany

Received 22 November 1990; in revised form 8 May 1991

The present paper derives universal basic elements from physical principles. This leads to systematic classifications and limitations of storage equipment. A comparison with the quantities implemented so far shows that there are still considerable reserves for further development. Possible trends can be derived from universal possibilities of application.

1. What is storage?

In the first two cases the physical

Storage takes place in order to preserve material foods, energy or information for future use. In the case of information this preservation concerns current facts, states and events. So one has to distinguish between three cases here (fig. 1):

nonlinearity -

requirement is:

physical irreversibility.

More than one stable, discretely distinguishable storage bit state values can occur only if there

a) Storage of static facts/data,

b) c)

e.g., addresses, telephone numbers, measured values, texts; storage of instants from dynamical processes, e.g., photography, signal scarming; storage of processes in time, e.g., sound and video recording; as a result of digital recording technique, this is becoming increasingly similar to b).

In all cases there exist static storage states after completion of the storing operation (recording). These states preserve the “data” for the future, i.e., for subsequent reproduction (retrieval). They may have different features and use different energies or structures. The features of a storage bit state are:

I

analog

amplifier

for reproduction

a) irreversible (ROM character); b) reversible (diskette character); c) dynamic (DRAM character).

’ Permanent Germany.

address:

0304-8853/91/%03.50

Kopperstrasse

59,

O-1017

Berlin, Fig. 1. Comparison of analog and digital storage.

0 1991 - Elsevier Science Publishers B.V. All rights reserved

I

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H. V6l.z / Limits and future possibilities

exist nonlinearities in the energy or state curves. Therefore an equivalent requirement is: metastability energy threshold e thermal energy). Hence, in the requirement for irreversibility, the stress is on the thermodynamical energy (threshold). It must not lead to a random change of the stored state. Hysteresis-type curves represent special cases.

2. Applications of storage Storage was and is prerequisite to a great variety of processes in evolution and generally in the realm of life. The preservation of species is inconceivable without genetic storage. Consequently in this case life would not exist. Without neuronal storage the living beings would not be able to adapt themselves to the variable environmental conditions; the human being would be unable to learn. In the past few years the disturbance of the stored self-recognition of the immune system became tragically apparent. Other complex processes are influenced by the hormonal systems. Finally, life is provided with a storage that exists only in a collective form. It becomes particularly evident with the colony-forming insects and of course in a much more pronounced way in the human society. In a production line usually there are several “specialized” workers whose know-how is hardly replaceable for a trouble-free production process. Technical storage will be referred to separately in greater detail [l]. For the moment, table 1 shows a

of information storage

Table 2 Concise historical survey of technical storage 1445 1822 1875 1900 1905 1925 1930 1935 1940 1943 1951 1956 1962 1965 1970 1973 1982 1982 1987

Gutenberg: letterpress printing Niepce: photography Edison: phonograph Poulson: wire type magnetic storage Odeon gramophone record Pfleumer: magnetic tape Schiiller: ring head Kodak/Agfa: colour film Braunmtihl/Weber: high-frequency biasing Dirks: drum storage long-play record Ampex: video tape storage Philips: compact cassette IBM: changeable disk diskette storage IBM: Winchester storage compact disk Sony: Mavica optical disks

survey of the most important general applications of storage. In technical storage it is reasonable to start from its fields of application. Here the cultural history starts with the ancient rock picturesldrawn about 50000 years ago. These are followed by script used for the storage of language. The technical methods of storage in the proper sense, including letterpress printing, photography, film, gramophone record, came into being at much later times, and finally, about one hundred years ago, the principles of electronic storage were developed. They are now more and more gaining efficiency

Table 1 List of the four most important stages in the evolution of storages

‘be

Storage medium

Purpose

genetic

DNS in chromosomes in each cell neurons and synapses in the brain distributed over several or many individuals technically selected materials

preservation of species and life of the living being possibility of behaviour and learning joint work or division of labour

neuronal collective

technical

storage of knowledge outside of man

H. Viilz / Limits and future possibilities

and perfection. Table 2 shows a very concise historical review. Today it appears reasonable to start from the following classification. a) Directly readable media, e.g., books, drawings and photographs. This field can also be described by the fact that in principle technical storage operations are not absolutely necessary. Yet there exists technical equipment such as scanners, document readers and bar code readers. b) Audio-uisual media, where the analog signal recording still prevails (conventional record, magnetic tape and cassette, video recorder). With the compact disk, however, digital sound recording has already made its way from the studios into the general public domain. On the other hand, photography with electronic single-picture recording on diskettes has just made its entry into this area. c) Digital storage media. They still have their vastly preferred field of application in computer engineering, where they cover an entire spectrum of semiconductor (SRAM, DRAM, CMOS-RAM, EPROM, ROM, etc.) and magneto-mechanical storage equipment (magnetic tape, cassette, diskette, hard disk and moving head disk storages), and where recently also the optical disk storages have gained importance. A question of significance is whether any new principles of storage are still to be expected for the future, maybe generally on an optical or superconductive basis. Such conclusions and consequences will be referred to again at the end of this paper.

3. Energies in storage The kinds of energy to be considered for a state of a storage medium are [2]: 1) Static energy forms a) mechanical: structures, deformation, material deposition and removal, crystalline t) amorphous, mechanical, stresses, etc.; b) electric (charges --, capacitor, electret);

of information storage

331

c) magnetic (permanent magnet); d) superconductive (?); e) chemical (biology, e.g., genetics). 2) Dynamic energy forms a) flowing current; b) electromagnetic wave, such as light; c) acoustic waves; d) thermal energy; e) chemical energy (e.g., neuronal). These energies must be variable for each bit which occupies an exactly defined volume in the storage medium. (Holography is an exception.) Storage processes using static energy do not require any additional energy supply in order to preserve the stored information, whereas storage processes using dynamic energy forms involve a continuous energy consumption. In the latter storage processes the data are preserved only as long as the required amount of energy is supplied. There is a continuous loss of this energy and a transformation into heat. This will be illustrated by some examples. Magnetic hysteresis has obviously made an essential contribution to the great importance of magnetic storage. Now the phenomenon of electric hysteresis is used in electronic storages. A functionally similar electric behaviour occurs, for instance, in flip-flops or Schmitt triggers. So these devices form the basis of the most important electronic storages. In contrast to magnetism, however, they require an additional continuous energy supply for maintaining the respective storage bit state. For CMOS storages this energy supply can be reduced to a relatively low value. As soon as no automatic self-preservation of the storage bit state is required, a nonlinearity is no longer necessary. This is exemplified by the l-transistor cell. But in this case the state must be maintained by periodic refreshing, which likewise requires extra energy. Thus, taken altogether, one has to distinguish between two energy components in storage: #

a functionally required threshold distance for distinguishing between the information states (the value of which will be calculated here), and

338

#

H. Viilz / Limits and future possibilities

an extra energy that is functionally independent of the information states implemented, being required in addition to the first-mentioned energy.

The light signal is absorbed in the plate material, where it produces a considerable increase in temperature. By applying a magnetic field during cooling, the information is written into the medium as a magnetic storage bit state when the Curie point is passed (compensation-point-recording; electric-optical-thermal-magnetic energy transformation). In- video recording, a special “single-sideband” frequency modulation is used. Many Winchestertype drives today use special RLL codes (runlength-limited-code). The CD uses an SSB FM code, a variety of error codes and a complex signal spreading. In the storage process in most cases there is a transformation between position and space coordinates. This can be extended into a higher level systematics according to the order and number of physical dimensions. It is possible to distinguish three spatial and one time coordinates. Their combination provides eight possibilities of existing sets of coordinates in information engineering [4]:

4. Three storage functions A storage process consists of three functional parts (fig. 2) [3]:

4 Recording operation

b)

It generates the required storage bit states from the data and/or signals. Hence a corresponding amount of energy must act on the storage medium in order to overcome the energy threshold at the respective bit location. Storage bit state It is diverse in nature, and must, for example, - remain invariably stable over the desired period (frequently over many years, and in archeology over millions of years); _ but for reversible storages (of diskette character) it must be exchangeable if desired. Reproduction operation It must retrieve the data/signals from the storage bit states. Usually it corresponds to a high-sensitivity measuring operation.

coordinate-free (e.g., simple values such as numbers); one space coordinate (e.g., a distance along a

c>

The recording as well as the reproduction ation may additionally include

of information storage

path); two space coordinates (on a surface); three space coordinates (in a volume); time-dependent values (e.g., a signal vs. time curve); one time and one space coordinate (e.g., traffic load on a road); one time and two space coordinates (e.g., a TV picture); one time and three space coordinates (e.g., a theatrical performance).

oper-

- multiple energy transformations, - encoding of data/modulation of signals. In the case of the magneto-optical disk the multiple energy transformations take place in the following steps: by means of a laser, a modulated light signal is generated from the electric signal.

T

recordmg

reproduction

process

storage

process

state

Fig. 2. Recording and reproduction processes and storage state.

H. Vijlz / Limits and future possibilities

All possible information processes - which also include storage - can now be described by a matrix with the eight variants as inputs and outputs. This yields the process matrix as shown in table 3 [5]. This matrix corresponds to Zwicky’s morphological scheme. Among other things, it enables still unknown processes to be revealed by means of the free boxes. But yet the electronic still camera had not been forecasted.

Table 3 Survey of elementary Off F(O)

F(x)

F(x,

F(O)

read-only memory

data storage

symbols figures letters

F(x)

reading pointer instrument

magnetic tape rerecor-

graphical representation of an equation video file reproduction

screened printing block

a

ding heredity video file recording

F(x, Y, 2)

F(t)

storing computer signals

signal recording

F(x, 2) F(x, Y, t)

F(x,

Y, 2,

video record

t)

5. Converters

339

for recording and reproduction

Recording and reproduction in the storage process is carried out using special converters. Examples are the magnetic heads and the laser arrangements in optical disk storages. The recording converters transform the (electric) signals into a highly concentrated energy used to change one storage (cell) state each. The reproduction con-

volumes

On

F(x, Y)

of information storage

Y)

reprography printing

photograph map with contour lines holograms oscillography image reproduction sonogram path-time diagram moving picture drawing snapshot moving picture flying spot scanning

F(x,

Y, 2)

hologram reproduction

J=‘(t)

JTx, t)

F(x, Y, t)

measured value and digital reproduction reproduction process of storage

linear transport problems

reproduction a stored video-record

image scanning and reproduction

tracing a circuit a figure

film reproduction tracing

signal transmission

phototelegraph teletypewriter

TV image reproduction

F(x, Y> z, t)

construction of a building act. to sketches

holography

film scanning

twaphy TV transmission

oral report

true stereo film

340

H. Viilz / Limits and future possibilities

verters must derive equivalent signals from the individual storage bit states, in the sense of a measuring process (reproduction process). For the relation between the individual storage bit states and the associated converters one has to distinguish between three functional extremes:

4 Serial converter:

b)

c)

It accesses the respective states in the storage medium individually and consecutively by means of mechanical motion, beam deflection, etc. Examples are the magnetic heads. Parallel converter: It accesses (almost) all bit positions simultaneously (in parallel). An example is the optical system in photography. Multiple converter: Here (almost) each storage bit state is provided with its own converter. Examples are the very old semiconductor storage devices.

Especially the cases a) and c) differ from each other in the number of converters used on the one hand and by the serial and parallel information transfer, respectively, on the other hand.

of information storage

two states of a bit (e.g., 0 and 1) should be separated at least by this energy difference. This can be implemented, for example, by two quantum states. This also implies that a different estimate can be obtained via the thermodynamic entropy, as follows. With the probability p and Boltzmann’s constant k, for a state one has S=k

In(p).

Let us consider a system with two states of equal probability, i.e., a system suitable for storing one bit. Then S, = k ln(0.5)

= -k

ln(2).

After a measurement (of the reproduction) actual state is known, which implies that S, = k In(l)

the

= 0.

From the entropy difference and the operating temperature T it is possible to calculate the required energy: dQ=dS

T=kT

ln(2).

This is the same value as in the theory of information.

6. Energy in the storage bit state 7. Volume of the bit cell The above analysis showed the great importance of energy in the state of a storage cell. Therefore let us consider it more closely. For the sake of simplicity, suppose that digital signals are used. From the theory of information it follows that for each bit a stable state exists only at a minimum energy, such that E,,

= kT

ln( 2))

where k is Boltzmann’s constant and T is the absolute temperature. But at this minimum energy the signal-to-noise ratio is zero, and hence an error-free transmission requires an infinite decoding time. Therefore in practice an adequate signalto-noise ratio is required. Thus the energy used should be higher by at least a factor of three. At an ambient temperature of appr. 300 K the energy then amounts to about 10p2’ J. For storage the

Apart from the energy, the size of the bit cell is essential, too. Each information must be stored somewhere in space, and must eventually also be filed. Hence, for practical use, as much information as possible should be accommodated in a volume that is confined in each case. Therefore the volume of a bit should be as small as possible (or the packing density as high as possible). The bit volume must then accommodate the corresponding energy per bit (that is, the functionally

Table 4 Examples of some energy densities, in J/cm3 Magnetic tape Classical physics Chemical fuels Explosives Nuclear fuels

1o-4 0.5 102 104 109

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H. Vijlz / Limits and future possibilities of information storage

indiwduol llmlts

limiting cell

combined llmlt

Fig. 3. Matrix of the information processes.

required energy and possibly an extra amount of energy). Therefore one should attempt to achieve as high an energy density as possible. According to the current and the foreseeable state of the art in storage technology it makes sense to use storage states that obey the laws of classical physics. So in the following considerations quantum effects and the like will be disregarded. Starting from these conditions, a systematic investigation of all possible energy forms and methods showed that there is an upper limit of the energy density of appr. 0.5 Ws/cm3 [6]. Table 4 shows a survey of energy densities available in various storage media. It is seen that there are extremely high reserves for the future. But the previous development already shows how difficult it is to achieve even a small progress, and in what a complex way a variety of influences interact in this case. Considering the abovementioned values of in-

formation theory one thus finds that the minimum theoretical bit volume is about lo-20 cm3, or the maximum volume storage density is about 10” bits/cm3. Fig. 3 gives a survey of the minimum information cells with respect to energy, volume and time. The figure also indicates the interdependence of the respective quantities as well as limiting dependence on typical physical relations. In practice, however, a volume cannot be filled up completely with storage cells. A certain amount of redundancy is always required: The actual structure of a semiconductor circuit is always essentially located at its surface. Then the chip has to be accommodated in a casing. Several chips are arranged on a PCB, several PCB’s are inserted in a casing. Many equipment units must be so arranged that they are accessible for operation and maintenance. Each level requires considerable unusable volumes. Thus finally it is not even a few percent of the volume of an equipment unit that are used

Table 5 Typical volumes with rough estimates of the actually active volume fractions (in machine building there exist some peculiarities) Type

Volume

Utilization

Reference

component hand-held unit floor unit building

cm3 dm3 m3 10’ m’

100% 10% xl% 105

manufacture easily portable by man maintenance and operation passable

342

H. Vijlz / Limits and future possibilitiesof informationstorage

by active structures. In this way it is possible to distinguish between three basic types of volume that exhibit typical degrees of utilization. These types are shown in table 5; which indicates that maximum storage capacities can at best be referred to 1 m3. Thus on a long-term basis there should also be an upper limit of storage capacity. It should be reached long before all of the appr. log5 electrons of the universe would be used exclusively for storage.

8. Recording process and energy While in the state of a storage cell the amount of information is determined by the energy, the recording process requires energy for transforming the the bit volume into the desired energy (bit) state. For this it is necessary that at least a threshold energy is supplied. If the energy is not completely absorbed by the bit volume, then even a correspondingly higher amount of energy is required. Moreover it is necessary that practically no energy acts upon the neighbouring bit cells, because this would possibly cause an undesirable change of these cells. Moreover, to determine the necessary power values from the energy, the input time has to be taken into account. Even with a switching time of 10 ns and complete absorption, a power of as little as 5 x lo-l3 W must be concentrated to the correspondingly small volume. In practice the bit volume is much larger on the one hand, and the amount of the bit energy used is smaller on the other hand. The consequence is that in most cases higher powers are required. Yet it is clear that in this case no practicable limits are exceeded.

9. Reproduction process and energy In the reproduction process the bit volume must be analyzed for its energy state. This is a measuring process that always requires an energy coupling. In this case the maximum usable energy that can be recovered for the reproducing amplifier is that contained in the storage state of the bit cell. Optimally this is accomplished by a measur-

ing energy acting upon the cell, which is mfluenced, i.e., changed, by the storage state. Therefore the useful energy recovered in this way can only be a fraction of the energy of the storage state. On the other hand, in this process the-storage cell must not absorb an amount of energy that causes it to reach or exceed the state threshold. For in this case the reproduction process would change the state of the storage cell - possibly even in an uncontrolled manner. These are the two reasons why - even under ideal conditions - the maximum reproduction energy available is that of the storage state. But this energy is further reduced by the efficiency of interaction between the bit cell and the reproducing transducer. For magnetic heads it is about 50%, for magneto-optical methods it is as low as 10P3. Finally these considerations presuppose that the reproducing transducer can use the energy in a loss- and trouble-free manner. But the corresponding adaption problems can in most cases be solved in an efficient way.

10. Consequences,

relations and limits

Summarizing the above results, the following statements can be made with respect to energy: # Storage cell state The energy density and the operating temperature of the storage medium determine the minimum possible bit volume and hence the packing density. # Recording process The energy concentration must least to the energy density of the recording transducer must provide concentration to the bit volume, neighbouring cells are changed.

correspond at medium. The for an energy such that no

# Reproduction process The measuring energy acting upon the storage state must be less than the energy density of the storage medium. The reproducing transducer should achieve as

H. V&

/ Limits and future possibilities

high a reproduction energy as possible by interaction with the storage state. In magnetic storage the recording head is in most cases a ring head with a gap width of not less than 0.5 pm. Its magnetic field penetrates the storage medium down to this depth. From the application point of view its track width should be much greater (at least up to now). Otherwise there will be problems in positioning the head o the track. Therefore a track width of at least 10 urn is necessary. Thus the minimum bit volume achievable is about 2.5 l.~’ or about 2.5 X lo-l2 cm3. Hence in the long run the packing density that can be recorded by magnetic heads should hardly exceed 4 X 1O’l bits/cm3. In practice, however, it currently reaches a maximum of about 10”. What is of disadvantage in this connection is that all suitable techniques require a much thicker and mostly redundant base for the storage layer. In the case of magnetic tapes its thickness can be reduced to about 10 urn. For diskettes it is 70 urn thick, and as much as 2 mm for hard disks. So the two last-mentioned storing techniques achieve a packing density of as little as lo8 bits/cm3. In optical storage it is possible to reduce the bit volume down to the diffraction limit [7]. The latter is in essence determined by the numerical aperture A,, which is derived from the refractive index n and the aperture angle /I (beam boundary) according to A, = n sin(p). For systems without immersion the maximum aperture is 1. At a wavelength h it is possible to achieve a focal spot diameter of about D, I A/A,. Thus, roughly speaking, a bit area of A2 is achievable. Furthermore the focal spot has a length of about X/A,. This means that a minimum volume of X3 is possible. Even if the light penetrates the medium, the energy density is much lower at all other points. Thus, by chasing sufficiently short wavelengths - and provided that sufficiently monochromatic and coherent radiation can be technically generated and focussed by means of arrangements - it is possible to achieve

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343

arbitrarily high packing densities in the recording process by optical means, i.e., even higher than possible from the aspect of the storage cell state. Now suppose that a single photon can change the storage cell state. With f being the light frequency, h the Planck constant and c the velocity of light, one then has the following relation for the energy: E=hf=hc/X. For the energy density in the focal volume V it follows that w = E/V

= he/X4.

For the wavelength this gives A=?_. At 0.5 J/cm3 one thus obtains a wavelength of 25 nm. So highly ultraviolet light should be used for recording. Light of longer wavelengths enables only lower packing and/or energy densities to be achieved. Hence the upper limit of the packing density is 1016 bits/cm3. This is also the upper limit of the packing density achievable by classical means. Owing to the low efficiency of reproduction - e.g. lop3 in the case of magneto-optical techniques - the practically possible values are lower. In magneto-optical storage the maximum possible value would be 1013 bits/cm3. A third consideration refers to the solid state components. In this case recording and reproduction always require a special reproducing amplifier. It can be used in two extreme variants of those mentioned above: In the single-transducer variant there is only one transducer (amplifier) for all bit cells. Therefore it can be neglected with respect to the volume. This corresponds to optical or magnetic storage in an analogous way, while in semiconductor storages it is implemented only for about one hundred cells each. Thus one day perhaps a bit volume in the form of a cube of 1 km edge length will be achievable. This will give a packing density of 1012 bits/cm3. In the multitransducer variant each storage cell is provided with its individual transducer. Such transducer/storage cell combinations require a

H. V6l.z / Limits and future possibilities

344

much larger volume per bit. The achievable edge length should hardly be less than 10 pm. This will result in packing densities of lo9 bits/cm3.

11. General and minimum storage So far our considerations were based on the general case of storage as shown schematically in fig. 2. Strictly speaking, this principle is only applicable to the read-write storage of a RAM or diskette character. In the case of ROM-type storages, however, the situation is different. There is no longer a recording process in the application of this storage. For the electronic mask-programmed ROM as well as for the gramophone record this process is implemented only during the manufacture (production) of these storages. Let us characterize these two extremes as follows. a) General storages can be written into and read from at any time desired. b) Minimum storages have a constant memory contents, hence being readable only. These two cases are illustrated in fig. 4 [7].

general

of injormafion storage

The general storage has signal inputs and outputs as well as a control input for calling the recording and reproduction operations. Releasing the recording operation is of importance especially when discrete samples are to be taken from a process continuous in time. The minimum storage has no signal inputs. This brings it into close proximity to many other circuits, and arrangements. From the control input to the signal output it thus behaves like a combinational circuit. On the other hand the information “impressed” upon it means a great reduction of flexibility as compared with that of the general storage. Generalizing this fact, any fixed structure can in principle be regarded or even used as a minimum storage. Conversely, this also implies that any controllable structure enables a general storage to be established. The search for new physical storage principles carried out on a large scale in the sixties obviously (though unconsciously) followed these lines. On the other hand the interrelation and the mutual replaceability of the different elementary electronic circuits can be understood in a new manner on this basis. This is summarized in fig. 5.

minimum

storage

storage

is toggled

I

I recording SIgnal

I reproduction SIgnal

Fig. 4. General and minimum storage.

reproduchon SIgnal

H. VXz / Limits and future possibilities

of information storage

345

12. Types of packing density The above considerations as well as the practice show that four different types of packing density are of importance: # Linear packing density along a recording track. It is usually stated in bits/mm or bpi (bits per inch). # Track density, i.e., the center-to-center distance between two adjacent tracks. It is stated in tracks per mm or tpi (tracks per inch). # Area1 packing density, as referred to a surface. It is usually defined as the product of the two abovementioned packing densities. # Volume packing density. The first two data are used above all for technical/technological comparisons. The third measure is a quantity derived from the former two, which has a relatively general meaning. The fourth measure describes the storage capacity achievable for a filing volume, being of high value especially for theoretical considerations [8]. Figs. 6 and 7 show comparisons between implemented values and the theoretical limits. Fig. 6 shows the values of the first three measures. Letterpress printing, paper pictures, semi-

combinational

circuits

Fig. 6. Survey of surface packing densities.

conductor storages, the compact disk (CD) and microfilm are located on the symmetry line, with equal resolution in both directions. All magnetomotoric (magneto-mechanical) storage techniques as well q the gramophone record exhibit a high anisotropy: owing to the necessary track guiding, the track density is much lower than the linear packing density. The distance from the symmetry

feedback

memory

sequential e.g. counter, ALU,

feedback read - only memory

clrcults regtster

CPU, automata

Fig. 5. On the interchangeability

of typical structures.

memory

346

H. V&

/ Limits and future possibilities

,

Id -

Id thlcknesrloyer

Fig. 7. Survey of volume packing densities.

of information storage

line is a measure of this anisotropy. Thus the surface packing density could be increased considerably by means of high-quality guiding devices, e.g., automatically controlled ones. Fig. 7 shows the situation with respect to the filing volume as a function of the surface packing density and the thickness of the material. In this case too there is a line of symmetry. It is applicable to the case of equal resolution in all three directions. The four theoretical limiting cases of storage as mentioned above are located on this symmetry line. By coincidence, however, the audio cassette and the paper picture are also located on this line. For three coordinates there are simply several possibilities for the symmetry line. But also in this case it is remarkable that the majority of practical techniques require by far thicker bases. This appears most clearly in the cases of CD and hard disk, both of which require a thickness of as much as 2 mm to ensure mechanical stability. Hence, in this respect the odds are by far on the side of the diskette. This applies all the more to the rather thin magnetic tape of appr. 10 pm. In addition, table 6 shows another comparison with the theoretical values.

Table 6 Standard dimensions of some storage media, in pm Storage medium

Base

Track

Length

video tape audio cassette CD hard disk diskette semiconductor (no casing) paper picture microfilm gramophone record

15 10 2000 2000 70

7 500 1.5 1.5 125

0.3 1 1.5 1 2

(old) genetics brain Limits: single-transducer principle multitransducer principle optical

Area

Volume

2 500 2 15 500

30 5000 4000 30000 35000

200 50 70

5 100 2

5 100 2

25 IO000 5

5000 500000 350

2000

100

5

500

1000000

2 10

1 10 0.025

0.001 10

1 10 0.025

0.001 10

1 10 0.025

10-h 100

1 100 0.001

1o-6 1000

1 1000 0.00003

H. Viilz / Limits and future possibilities

13. Technical parameters

#

Storages are described by a great variety of parameters. However, there are some parameters of a rather general meaning.

#

13.1. Storage capacity This quantity indicates the limit of the amount of data that can be stored. For the storage devices of computer technology - or, to put it more universally, for the digital storages - data capacities are stated in bits or bytes. For analog storage devices - above all for audio and visual storages however, the playback time is of much higher interest. Basically, of course an infinite capacity is desired as the ideal case. But, as was already shown above, there are always practical (as well as theoretical) limitations. Assuming a net volume of 1 m3 for storage cells, the following three limiting cases can be derived from the above considerations [9]. Single-transducer storages Multitransducer storages Optoelectronic storages

lOi bits 1018 bits lo*’ bits

Till now the capacity implemented has been, and still is, a factor restricting the efficiency of the technology, in part even considerably. This is amazing, the more so because it is the storage capacities where there are always the highest growth rates. It appears that with increasing capacities at the same time the demands made upon them are further increased. As it will be shown below, there might be a change in this respect to come by the turn of the millennium. 13.2. Access time This quantity indicates the average time required until the desired information is available in a usable form. This definition is fuzzy for several reasons, because it is greatly influenced by several factors: # #

Capacity of the storage device; Organization of the storage process, e.g., linear, matrix-type, hierarchical, associative, etc.;

341

of information storage

Technical implementation, e.g., electronic, optical; The desired information itself.

mechanical,

The specification as a mean value is likewise relatively indeterminate. Indeed an access operation consists of several different partial operations. In a disk storage first of all the page, the cylinder and the sector must be electronically computed. Then the heads have to be moved towards the cylinder. From the data read out, the desired sector must now be determined, and not until this has been done the information can be read out. With the information being identical otherwise, this process may take rather different times1 depending above all on the respective preceding positions of the magnetic heads. Notwithstanding the great variety of influences there is a rough, typical limit for each principle of storage. Its approximate values are, respectively, # # # #

Human access Mechanical principles Electronic principles Optical & lowtemperature principles

seconds to minutes ca. 1 ms ca. 1 ns about 1 ps

13.3. Volume It should be as small as possible. However, what is of importance here are the classifications given already in table 5. Additionally, in computer technology there are the different standardized shape factors, e.g., of diskette and hard disk technology, but also of the standard plug-in units. 13.4. Price It should be as low as possible. 13.5. Acceptance It is extremely difficult to describe and has raised problems again and again, above all in connection with the individual audio and visual storage devices. But also in the case of the computer storage devices it should not be underestimated. Very probably it is in this field that the introduction of the optical disk storage raises, or

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H. Vijlz / Limits and future possibilities

raised, problems. The occasionally used argument of an excessive capacity is hardly acceptable. There is an interaction between storage capacity and access time, and it is in this respect where the specific possibilities of certain storage techniques take effect. Thus each technique covers a relatively sharply defined area in a capacity us. access time diagram. Obviously the respective technique is efficient and economically implementable only in this limited area. A first diagram of this type dates back to 1967 (fig. 8) [5]. About five years later the areas had already changed considerably, and moreover the abovementioned limits of access time were clearly observable (fig. 9) [7]. The present state of the art appears as shown in fig. 10 [lo]. The possibilities of different storage organizations likewise highly depend on the principle of storage used. For the semiconductor storage devices it is usual to calculate the information via addresses. With a sufficient address width it is thus possible to access any information desired in about the same time. Here it is of advantage that the necessary number of address lines increases

Fig. 8. Diagram of storage capacity vs. access time, of 1967.

of information storage

‘-0

10

‘-5 10

-1

10

10 ‘0

10 ‘3

acces-tImeIs

Fig. 9, Diagram of storage capacity vs. access time, of 1972, indicating different limitations.

only logarithmically with the storage capacity. From this aspect, in the long run it appears probable that always approximately the economically utilizable capacities should have to be managed. Yet there are problems with the speed, because high-speed semiconductor memories are technologically more demanding and hence more expensive. This is why the principle of the cache memory arranged in a hierarchical configuration with the main memory is used. The next step in the storage hierarchy then concerns the secondary storage devices, at present represented above all by diskette and hard disk storages. Their organization is much more complex and hence more timeconsuming, even if virtual addressing is used. The access operation is further considerably slowed down when additional secondary storages, e.g., tape libraries or even printed matter, are made use of. Furthermore it should be taken into account that, according to different estimates, it is not more than a few percent at all of human knowl. edge that is recorded m writing. In this connection

H. Mlz / Limits and future possibilities

1W

l!JS

lms

IS

349

of information storage

1000s

lo65

IO95 aces

-

trme

Fig. 10. Currently applicable diagram of storage capacity vs. access time, including nontechnical storages.

a great progress is (rightly?) expected from the expert systems. Finally let us consider the problems of searching for information in large databases. The course of such a search process in time can be optimized if it can be carried out in uniformly nested binary trees. In most cases, however, a stock of information cannot be ordered in this way. A different method was introduced by IBM in about 1955 with scattered storage (today usually called hash coding). In this case the address is computed via a function directly from the searched term or word. However, this is efficiently possible only up to an about 80% utilization of the storage capacity. Entirely new possibilities would result if an efficient associative storage would become possible [ll].

14. Magnetic tape storage With the rapid development of microelectronics, till now there have been in essence two principles of storage that proved dominant. These are the semiconductor-electronic and the magnetomo-

toric (magneto-mechanical) techniques. We shall not consider the semiconductors here. They have been dealt with in detail in electronics and are beyond the author’s special field. The oldest one of the magnetomotoric principles uses magnetic tapes and has (seemingly) lost importance in computer technology in the past few years - quite in contrast to audio and video engineering. It is now applied only on a small scale in home computers and for special tasks in personal computers and mainframes: # #

Data saving in the sense of backup and Individual data security against unauthorized access and misuse of data.

From both aspects the filing volume is of special importance. Therefore a high volume packing density is required in this case. This requirement is met very well by two specific features of tape-like materials: #

Tape can be made with an extremely thin base that increases the active volume by a comparatively very small amount.

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#

H. V6l.z / Limits

andfuture possibilitiesof information storage

Tape can be accommodated in a small volume with a large useful surface when wound into a tape reel.

In the future, however, yet higher storage capacities or/and smaller volumes will be required. There are three possibilities to meet such requirements: # # #

Increasing the longitudinal packing density; increasing the track density; decreasing the base thickness.

Probably there are the least possibilities with respect to the base. It has been possible so far to continuously improve the longitudinal packing density. In the past few years the experience gained in vertical storage proved extremely efficient, and there might still be reserves for a factor of 2 to 10. In this connection there exist three principal problems: # # #

Effect of the tape-to-head distance; narrow and stable head gaps; high saturation magnetization of the head material.

The second point is in essence a purely technological problem and, above all, is solved increasingly better by means of vacuum technologies. Maybe the possible limits are at 0.1 pm here. The distance effect involves an exponential damping for short wavelengths, following the function exp( - d/X). Here d means the distance between the tape and the head and X is the recording wavelength, being reciprocal to the longitudinal packing density. This effect equally occurs in recording and reproduction. As an additional effect in recording, the effective magnetic field decreases according to the same function towards the interior of the storage medium:

materials with a high saturation magnetization that are rare and exhibit additional specific disadvantages: # #

They are mechanically soft, subject to high abrasion and hence wear out rapidly; they have a high conductivity, with high eddy current losses.

Further it should be mentioned that at present there are no sufficiently clear findings concerning the causes of the head distance effect. The only thing certain is that in this connection the surface quality of the magnetic tape plays an essential part. Moreover there may be “non-magnetic” deposits or transformations (e.g., recrystallizations) at the surface of the magnetic head, perhaps also at the surface of the magnetic tape. In each case the selection and the further development of special head materials is of great importance for the progress of the technology. A review of current and possible future materials is shown in table 7. As regards an increased track density, above all there are two essential effects: #

Mechanically

accurate track guiding and find-

#

ing, adequate signal-to-noise duction).

ratio (energy of repro-

In principle, track widths of as little as a few urn are controlled already today (at least in video storage techniques). But in this case a special method involving no intertrack spaces is employed, using two head gaps slightly angled to each other. With the use of control engineering means (also in combination with optical methods as in the Floptical) it should be quite possible to achieve 1 urn. But here energy problems of the

H=Kexp(-d/X). This implies that at short wavelengths the recording field strength within the medium becomes too low. This is unfavourable especially for media with a high packing density. Since, however, it is just these media that, as was pointed out in the earlier sections, require a high energy product, i.e., a high saturation magnetization, this effect is particularly annoying. Add to this that it is just head

Table 7 Survey of magnetic head materials for possibly preferred use -1995 -1980 -1990 1978-1995 1980-2000 1988-

soft magnetic NiFeJAl) NiZn ferrites MnZn ferrites Sendust amorphous materials microcrystalline materials, novel materials

H. Viilz / Limits and future possibilities of information storage Table 8 Review of the development of tape storage materials 1950 1960 1970 1980 1985 1989

gamma-Fe,O,, later on with a variety of dopants chromium dioxide superpure iron particles metallic thin layer cobalt-chromium Ba-ferrite?

storage medium will then occur. Add to this that for thick layers of the storage medium a magnetization throughout will no longer be possible (distance effect). Thus, in contrast and contradiction to the heads, high-energy materials are required for the storage layers. Therefore the thin metallic films should be most favourable in the long run. A review of the development of storage layers is shown in table 8. All in all it can thus be concluded that the packing density in magnetic tape storage can further be increased by at least one decimal power by the turn of the millennium. Whether or not the vertical storage technique will be made use of in this case is not the crucial point. It should not be excluded either that there might be entirely new possibilities with optical tapes, where the distance effect does not occur and the track guiding problem should be easier to solve. The favourable proportion of a small volume to a large surface becomes even more efficient here.

15. Hard disk storage In the field of rotating secondary storages, worldwide production rates of more than 25 billion dollars are expected for 1990, for well over 20 million hard disk storage units, 55 million diskette storage units and 4 billion diskettes. For the Winchester type storage units the annual rates are slightly above 20%. Owing to the high reliability achieved the progress in the Winchester technology now above all concentrates on increasing the packing density. Here the problems encountered are analogous to those found with the tape storages. In addition, however, the head distance is

351

essentially determined by aerodynamics. Thus it could be drastically reduced in the course of time and the current minimum values are already below 0.1 urn. Moreover, the glider determining the aerodynamics is much larger than the corresponding magnetic head. Scaling up the glider to the size of a Jumbo jet, it would have to fly over the runway at supersonic speed, constantly at a millimeter distance. Nevertheless further improvements appear possible and with respect to the exchangeable media it has even been possible to dispense with clean room conditions. In the Winchester technology, too, the greatest reserve for the packing density lies in the track density. While the currently used track densities are about 20-30 tracks per mm, a density between 200 and 500 should be possible on the basis of the signal-to-noise ratio. But this will likewise require new principles of positioning. The currently employed track guiding over a master surface will hardly allow further improvements. Probably a coarse positioning with a superimposed autoadjusting high-precision control might be reasonable. Unfortunately, notwithstanding its many advantages, the Winchester storage has also disadvantages. These include * the risk of a head crash with a loss of all data; and * the fact that its medium is not exchangeable and hence not suitable for archive filing purposes nor protected against unauthorized access. Both facts require regular backup copies on tape, if necessary on diskettes. Thus the Winchester storage, notwithstanding its otherwise outstanding parameters, is not the last stage in the hierarchy of storages.

16. Optical disk storage The term of “optical meaning:

storage” has a multiple

a) Storage of signals connected with light, such as in photography, movie film, microfilm and video storage;

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H. Vijlz / Limits and future possibilities of information storage

Storage of any kind of signals using optical methods. This further includes all storage processes where light is involved in some way or other. Let us mention some extreme cases as examples: Photolithography, letterpress printing (e.g., especially bar codes), compact disk (CD) and video disk. In the introduction it had been stressed that it is most reasonable to employ the storage state for characterizing the method of storage. Obviously this is not the case in all of these methods. Rather one speaks of optical methods if light is essentially involved in the recording and/or reproduction. This means that at present there are (still) no optical storages in the proper sense. The currently existing optical disk storages, however, distinguish themselves by some interesting features as compared with the other secondary storages: # # # #

Contactless reproduction without wear; Possibility of archive filing and data protection; New possibilities of track retrieval; Extremely high packing density.

The first point on the one hand avoids the distance effect that greatly restricts the packing density and on the other hand makes the head crash irrelevant, a feared trouble in hard disk storage units. The second point, too, is of great importance with respect to disk storages, but here the development of the exchangeable Winchester disks must be taken into consideration. The new optical methods of track retrieval likewise are already being introduced into the diskette technology (Floptical) and probably also into the disk storage technology in the near future. But it remains to be seen whether such hybrid techniques will prove successful. So far they have automatically been integrated with the write and read mechanisms only in the optical disk storage. As regards the packing density, it might be only for the present that the odds are on the side of the optical storages. For on a long-term basis it is generally estimated that both the magnetic and the optical disk will have about equal surface packing densities. Nevertheless there are consider-

able differences with respect to the possibilities and the application data resulting from the latter. This is due to the following facts: #

#

In optical storage, the limitation of the packing density is determined by the diffraction limit depending on the aperture and the wavelength. Hence the bit distance and the track distance are always approximately equal. This further implies that increasing the packing density will be possible only by the use of shorter wavelengths and optical systems adapted to them. Here at best a factor of four is to be expected for the next few years. In the case of the magnetic disk storage, presumably there will always be a highly anisotropic gap and hence the longitudinal density will always be much higher than the track density. While the longitudinal density alone can be increased by a factor of 2-4 in the next decade, there are far higher reserves in the track density.

The anisotropy encountered in the magnetic disk storage implies that much more information can be stored in one track. This means that under otherwise equal conditions an optical disk storage has to change tracks much more frequently and, on top of it, with a greater mass. The CD, for instance, has 20000 tracks as compared with about one thousand of the hard disk. This should just be the reason why all optical disk storages have 3 to 10 times longer access times. This fact can easily be substantiated by a rough estimation. Suppose that the maximum energy E available for both systems be approximately equal. It is to be fully utilized in a bang bang control. With the maximum positive acceleration, followed by the fully negative one (for braking), the desired track is exactly reached. Then, to a first approximation, the following relation holds for the time required: t = SQR( xm/E ), where x is the track distance to be travelled and m is the mass to be moved. From this it can be seen that also on a long-term basis an optical storage must remain slower by a factor of 3 to 10, because the abovementioned time required for the

H. Viffz / Limits and future possibilities of information storage

greater number of access operations adds to the above value. At present (and for ever?) there are three specific variants of the optical storages: #

CD ROM with an information permanently impressed during manufacture; # WORM, i.e., a write once read mostly disk; # ERA, i.e., an erasable and recordable storage. It was exactly in this order that the individual types were introduced and are currently ranking in their ranges of application. Among these types only the C D ROM has found a somewhat wider application (based on the compact disk technology). It should also be further extended as cheaper drives will become available. Add to this that meanwhile the program installations simply require too many diskettes and are not just cut out for archive filing. The W O R M (write once read mostly) storage approximately corresponds to the P R O M units of the semiconductor storages, being almost in its introduction stage yet. Its optimum fields of application have not yet taken shape clearly enough. At present the erasable digital/optical storage is still subject to some problems, which are not very clearly defined. Owing to the high temperature effects (appr. 200 ° C) only a limited number of write cycles should be possible. Lowering the Curie or recrystallization temperature might raise stability problems in tropical regions. An additional problem with the magneto-optical variant is that it can first be erased and then written into only in successive cycles. The auxiliary magnetic field cannot be changed over quickly enough. Nevertheless the development is in a state of rapid flux. So the scene may be different already tomorrow. In 1990 it was attempted to analyze the development of the three optical disk storage types on the basis of the available trend data of the last four years. With a considerable uncertainty resulting from the inconsistency of these data, it was only possible to draw the following conclusions: # The CD ROMs always fell short of the expectations of the preceding year by a factor of 2 to 4.

353

# The W O R M storages developed slightly better than expected. They are obviously used to a greater extent for archive filing. # There are still relatively few data available for the ERA storages. These data indicate a growth exceeding the predicted values by about a factor of 2.

17. Attempt at a long term trend analysis Let the horizon of the forecast be about 1995. Then there should be semiconductor memory chips of at least 64 Mbits. This would enable main memory capacities of many Mbytes up to Gbytes to be implemented, even for smaller computers, In contrast to the currently existing ones, such computers would no longer require any built-in external storages. Rather the contents of the main memory will be backed up at the beginning and the end of each session - possibly also overnight or data and programs will automatically be read in and out, respectively. For that purpose the data transfer rate will gain a far higher importance. It might become even more important than a fast access and a rate of 10 Mbps should then be a very low requirement. The use of the external storage (portable but hardly permanently connected to the computer) should then be restricted to two principal features: # #

Data filing for backup operations and Individual data security against unauthorized access and misuse.

In view of the small size desired and the high data transfer rate, the odds should largely be on the side of the tape-like medial where it is quite possible that, apart from magnetic recording, among other techniques, optical recording may also be favourable. The following features might be essential: #

#

Parallel tracks, rotating heads or the deflection of ligh t complemented by a high tape speed make it easy to achieve extremely high data transfer rates. The tape reel ensures a high surface-to-volume ratio.

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H. Vijlz / Limits and future possibilities

of information storage

Fig. 11. Principle of extremely high frequency recording using a special cable. (a) Basic arrangement; (b) characteristics.

In this case as well as otherwise, both light deflection and holographic methods might prove useful. Tests of this kind were carried out already in the sixties. At that time they were postponed for various technological reasons. But meanwhile considerable progress has been achieved in this field. The change to such solutions may also be accomplished step by step. Already the use of beam splitting makes it possible to access serve1 tracks in parallel. This can be utilized to increase the data transfer rate and/or to reduce the access time. Furthermore the change to integrated optical systems should be taken into consideration. This may drastically reduce the moving mass of the currently existing systems. Besides the thermally conveyed interactions of light and the alteration of material properties, there are also direct interactions that affect the molecular structure, electron bonding, etc. These effects are in part much faster as well as completely reversible. The colour centers represent one example. With the use of such media it is probable that reversible media can be employed in a much better way. Further let us add that unexpected changes, above all qualitative ones, may also occur in the semiconductors. As an example of an entirely unfamiliar variant, let us refer to a paper published in 1965 [3] which, strictly speaking, makes sense only now, especially because of the high data transfer rates required. For it works only at extremely high frequencies. The dielectric of a cable is at the same time ferromagnetic, showing a high hysteresis. A wave enters this cable

as a signal, being magnetically frozen by means of a short e.h.f. pulse applied at a certain time. This is just the principle of magnetic signal recording. This wave can easily be recovered by means of a corresponding reproduction principle. On the basis of discrete components this principle was tested with success at that time (see fig. 11).

References

111Kleines

Lexikon der Speichertechnik ,(Concise Dictionary of Storage Engineering), vol. 224 of the series Automatisierung-stechnik (Automation engineering) (VEB Verlag Technik, Berlin, 1987/90). PI Allgemeine Systematik und Grenzen der Speichenmg (General Systematics and Limitations of Storage), Die Technik 34 (1979) 658. 131 Versuch einer systematischen und perspektivischen Analyse der Speicherung von Informationen (Attempt at a Systematic and Perspective Analysis of Information Storage), Die Technik 20 (1965) 650. 141 Eine mogliche Grdnung alIer Informationsprozesse (A possible Order of alI Information Processes), EIK 7 (1971) 447. 151 Betrachtungen zum Susammenhang von Speicherdichte und Zugriffszeit (Considerations on the Relationship between Packing Density and Access Time), Wiss. Z. f. Elektrotechnik 9 (1967) 95. 161 Zum Zusammenhang van Energie- und Speicherdichte in der Informationsspeicherung (On the Relationship between Energy and Packing Density in Information Storage), Intern. Elektron. Rundschau 16 (1967) 41. (71 Aussagen zum minimalen Informationsspeicher (Remarks on the Minimum Information Storage), J. Signal-AM 4 (1976) 227. 181 Prognose der Speichenmg von Daten und Programmen

H. V&

/ Limits and future

ftir die Rechentechnik (Prognosis of the Storage of Data and Programs for Computer Engineering), Nachrichtentechnik-Elektronik 38 (1988) 208. [9] Zur Perspektive magnetomotorischer Speicher (On the Perspective of Magnetomotoric Storages), J. Signal-AM 9 (1981) 333. [lo] Zur Entwickhmg digitaler Speicher (On the Develoment of Digital Storages), J. Inf. Rec. Mater. 17 (1989) 171. [ll] MQlichkeiten und Grenzen optischer Speicher (Possibilities and Limitations of Optical Storages), Radio Femsehen Elektronik 39 (1990) 160.

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of information storage

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[12] GrundIagen der magnet&hen Signalspeichenmg (Fundamentals of Magnetic Signal Storage), ~01s. l-6 (Akademie-Verlag, Berlin, 1968-1975). [13] Information I + II (Akademie-Verlag, Berlin 1982/83). [14] Abschltzung der KanaIkapazitB;t ftir die Magnetbandaufzeichnung (Estimating the Channel capacity for Magnetic Tape Recording), Elektron. Rundschau 13 (1959) 210. [15] Stand der Anwendung optischer Speicher - insbesondere auf rotierende Medien (State of the application - specifically to rotating media - of optical storages), Nachrichtentechnik-Elektronik 37 (1987) 164 (see also 3.119).