 |
Task Force to establish Selection Criteria of Analogue and Digital Audio Contents for Transfer to Data Formats for Preservation Purposes |
3. Technical Selection Criteria
3.1 Technical aspects relevant for prioritisation of transfer projects
Technical aspects play a prominent role in
establishing a hierarchy of transfer for carriers that have been selected
on the basis of other criteria.
In its current version, IASA-TC 03 (IASA
Technical Committee, The Safeguarding of the Audio
Heritage: Ethics, Principles and Preservation Strategy, Version 2,
2001) considers digital technology within
the general context of audio archiving philosophy. The document specifies
the principles of digital archiving, specifically the stringent quality
control of the digital holdings, and the continuous further monitoring of
data integrity, before migration becomes imperative. It also deals with
the transfer of analogue holdings into the digital domain, the problems
related to signal extraction, i.e. the adequate retrieval of the content
from their original analogue carriers, the choice of digital resolution
and target formats, and the principle of unmodified and linear (i.e.
uncompressed) transfer to digital. As transfer technology still improves,
and ever higher digital resolutions become available at ever decreasing
prices, IASA-TC 03 recommends an unhurried strategy for the digitisation
of analogue documents. Priority should be given, however, to those
analogue and digital documents which are at immediate risk, and/or which
are in regular demand. The document includes a list of inherently unstable
carriers.
This chapter builds upon IASA-TC 03, explaining the relevance of
technical aspects for prioritisation of transfer projects, and assisting
archivists to assess their collections accordingly, specifically their
degree of degradation, taking format obsolescence into account.
3.2 Original versus replicated carriers
Originals of all kinds of carriers have higher priority than replicated items. While originals are irreplaceable, there is always a chance that other, even better preserved copies are held by collections elsewhere. The chances may be low for antique replicated cylinders, but very high for recent LPs and CDs. It is recommended that every effort is made to find the best copies available. In the case of the routine transfer of entire collections of originals produced by the phonographic industry, co-operation should be considered between audiovisual archives in order to avoid unnecessary expenses by the multiple, labour intensive transfer of such collections into DMSSs. It may be cheaper to arrange a meaningful division of labour, and to exchange digitised copies, observing of course the related legal aspects including the payment of licence fees, wherever applicable.
3.3 Degradation of Carriers and its Assessment
The most important, and the most difficult assessment is to determine the state of deterioration of a given carrier, or group of carriers in order to arrange for a timely transfer. This section attempts to give a broad overview of the specific situation of various types of carriers, in order to assist archivists in drafting a meaningful transfer strategy. It should be stated, however, that such risk assessment requires a high degree of expertise. Moreover, in many cases it is still impossible (even for experts) to assess the life expectancy of a given carrier, because no valid methods for their evaluation have yet been developed.
Original cylinders consist of wax, replicated cylinders either of wax or of a celluloid (nitrate cellulose) tube covering a plaster core. Both types of materials are considered highly endangered, as wax is extremely fragile and specifically prone to fungus growth under humid storage conditions, while celluloid is endangered by increasing embrittlement with age. Therefore, because of the risk of further deterioration, cylinders should always be of top priority in digitisation programmes.
3.3.2 Coarse grooved discs (78rpm records)
There
are significant differences in the risk of deterioration between
replicated coarse grooved discs, so called shellac records, and so called
"instantaneous", or “direct-cut” discs, original recordings, which may
consist of manifold materials.
Replicated shellac records consist mainly of mineral substances bound
together by an organic binder. Unless stored under irregular conditions,
they have proved to be fairly stable over many decades. Seen from that
perspective they are on the low end of the priority scale.
Pre WW I replicated discs, however, may consist of more critical
materials, which suggest a higher ranking for prioritisation.
Instantaneous discs are much more unstable. The most widely used
variety is the so called "lacquer" disc. It consist of a glass or metal
core, covered by a layer of lacquer, carrying the information, mainly
consisting of nitrate cellulose, sometimes acetate cellulose, which gave
this group of records the generic name "acetate discs". Mainly due to high
levels of humidity and temperature, these lacquer coatings shrink, and
become brittle with age until they crackle, without significant previous
warning. Once the surface is crackled, generally the signal cannot be
retrieved. Therefore, all lacquer discs should be transferred without
delay, even if they seem in excellent condition - because of this
susceptibility to sudden deterioration.
All other kinds of instantaneous disc should be accorded a high
priority for transfer because of uncertainty about their stability (and
because of their unique content).
3.3.3 Microgroove discs (“vinyls”, LPs)
Replicated microgroove discs, as produced since around 1948, consist of a PVC/PVA (polyvinyl chloride - polyvinyl acetate) co-polymer. Generally, they have proved stable so far. According to present knowledge, no systemic degradation of these materials is expected in the near future. It is necessary however, to inspect collections for the possible plasticizer exudation of earlier records, and of plasticizer migration which sometimes happens when such discs have been kept in inadequate plastic materials. Such items should be transferred immediately, but the great majority of vinyl discs can be ranked at the lower end of the priority list.
Magnetic tape mainly consists
of two layers, the base film and the magnetic layer which carries the
information. The base film consists, in historical sequence, of acetate
cellulose (produced until the mid-1960s), PVC (produced mainly in Germany
between 1943 and 1972) and polyester terephtalate, generally called
polyester, which has been in use since the late 1950s. While PVC and
polyester have proved stable materials and deterioration is not
anticipated in the near future, acetate cellulose is severely at risk. As
already explained for the lacquer coatings of instantaneous discs, this
material shrinks and becomes brittle with age. The degradation is caused
by hydrolysis, a chemical decomposition process under the influence of
water, omnipresent in the form of humidity in the air. The higher the
levels of humidity and temperature have been, the higher the
deterioration. Additionally, acetic acid is produced in the course of
hydrolysis, which reacts as a catalyst that accelerates further
deterioration. This process mainly affects stocks of films made from
acetate cellulose, and is known as the "vinegar syndrome" which sometimes
render films irretrievable. With audio tape such disastrous consequences
have not yet been observed. However, acetate tape stocks should be
considered at risk and should be ranked at the upper end of priority.
Acetate tapes can be identified by their brand and type (list
forthcoming from the IASA TC), but also by relative simple investigation.
All standard play, most long play, and some double play acetate tapes have
a translucent wind when held against a light source. This is not the case
with PVC or polyester tapes. Acetate tapes are also fairly stiff,
sometimes brittle, and break without previously stretching when
mechanically stressed. Aged acetate tapes can also be identified by their
warped edges or polygonal appearance when hanging down from a reel.
Generally, only standard play tapes open reel tapes (SP, 52 µm total
thickness) should be trusted to be mechanically stable. Long play tapes
(LP, 35 µm thickness), double play tapes (DP, 26 µm thickness) and the
rarely used triple play tapes (TP, 18 µm thickness) are vulnerable at
increasing degree. Audio cassette tapes are mechanically even more
vulnerable: C 60, C 90 and C 120 cassettes have a thickness of 18, 12 and
9 µm respectively. R-Dat tapes have a thickness of 12 µm. The lesser the
mechanical stability, the greater the chance that the tape suffers from
inadequate winding, which is one of the most underrated risks for magnetic
tapes. Open reels and cassette tape are equally affected, and prolonged
storage of badly wound tapes causes irreversible deformations, which may
lead to severe replay problems, specifically with thin tapes and high
density recordings, e.g. R-Dat.
The magnetic layer usually consists
of two parts [or components]: the magnetic pigment itself, that takes up
and holds the information, and the binder that glues the magnetic
particles together and onto the substrate. The following magnetic
particles are in use: g-Fe2 03, used for all audio open reel tapes, and
for IEC type I audio cassettes; CrO2, and chromium doped particles, as
used for IEC type II audio cassettes and the earlier video cassettes, some
of which are also used for digital audio recording, e.g. U-matic (PCM
1600/10/30), Betamax (PCM F1), VHS (ADAT). More recent magnetic pigments
known as metal particle (MP) have a core of pure, metallic (non oxidised)
iron with an inert ceramic or mineral layer to protect them from
oxidisation. These are used in IEC type IV audio cassettes, R-DAT
cassettes, and video cassettes used for digital audio formats, e.g. DTRS.
The stability of g-Fe2 03 magnetic particles is not questioned, although
CrO2 and chromium doped particles are less stable magnetically. Some MP
tapes are threatened by oxidation, or corrosion of the particles. The MP
coatings at greatest risk are those manufactured in the late 1980s and
early 1990s prior to improvements in passivation techniques. A more recent
development is the use of thin, vapour-deposited metal coatings known as
metal-evaporative (ME). Some ME tapes suffered catastrophic failure as a
result of the metal layer flaking away from the polymer base-film.
The greatest problem with magnetic tapes is the material which binds
the magnetic pigments to the substrate. Generally, traditional binding
materials have a good to fair reputation of stability. From the mid-1970s
onward, however, new polyester polyurethane binders (PEU) have been used,
which, to various degree, are prone to hydrolysis. Water present in
humidity of the air reacts with the binder, which leads to its chemical
transformation, accompanied by a different physical performance. Binders
loose their binding properties, which lead to a loss of pigments. In the
course of the replay process, these pigment particles are deposited on
tapes guides and replay heads swiftly impairing the quality of the
replayed signal. This phenomenon is called “Sticky tape/sticky shed
syndrome” and is often accompanied by a squeal in the replay process,
caused by undue friction of affected tapes in the tape guides. In severe
cases this friction may even lead to the break down of the tape travel.
Sometimes, massive oxide shedding and even a total peeling-off of the
magnetic layers can be observed.
There are only a few types of tape with conventional chemical binder
technology which have not been affected to date by this unfavourable
process, mainly broadcast studio tapes of German origin. A list of these
tapes is under preparation by the IASA Technical Committee.
It is yet unclear whether binder degradation is the problem of a
limited number of ill-designed or ill-produced tapes, or whether sooner or
later all magnetic tapes will affected by this phenomenon. The development
of methods to predict life expectancy (LE) of magnetic particle binder is
in its infancy, and considerable research is needed before a valid
methodology will be available.
Consequently, most of the tapes produced after the mid-1970s should be
suspected of being inherently unstable. Before efficient and easily
applicable LE tests become available, utmost vigilance is necessary to
find potentially affected stocks by labour intensive individual tape
inspections. The IASA Technical Committee hopes that the problem will be
solved by a continuation of the systematic co-operation between sound
archives and manufacturers that has recently been established under the
auspices of UNESCO. The disclosure of potentially risky types of tapes by
the manufacturers would be a substantial help.
Apart from the potential effects on the LE of magnetic tapes arising
from the specific parameters described above, the storage history of
archival holdings has also to be taken into account. Heavily used
materials, as well as irregular storage condition over the lifetime of
specific holdings, both call for a higher ranking in transfer
projects.
In summary, only general and indirect conclusions can be drawn from
parameters related to the physical and chemical conditions of magnetic
tape carriers as to the inherent risk of their becoming irretrievable.
Exceptionally, the digital R-Dat format does allow for an objective
assessment of its state of preservation. Based on the availability of
specific replay equipment in association with dedicated software, the
monitoring of data integrity of a given tape can be assessed objectively.
Monitoring in regular intervals allows assessment of the slope of
degradation of a given carrier, this in turn allows for a timely transfer
onto a new digital carrier before uncorrectable errors occur as a
consequence of further deterioration (cf. also the respective notes on CD
integrity).
Compact Discs are the most
widely used optical carriers. Replicated CDs were first introduced in
1982. These consist of a polycarbonate body which carries the information
on its upper surface in form of a helical track of “pits” and “lands”
embossed in the process of production by injection moulding. This upper
surface is covered with a reflective layer, generally of aluminium. The
reflective layer is protected by a layer of varnish, also carrying the
label information.
Before the stability of the various components is discussed, it is
important to remember that data integrity of CDs, like all other digital
media, is objectively measurable by special CD players and suitable
software, which should be standard equipment of all sound archives holding
and generating CDs. According to digital archival principles (cf. IASA-TC 03, § 11), every CD must be free of uncorrectable errors. It is important
to understand, that the CD-Audio format (CD-A) provides for error
concealment (interpolation), if the level of (true) error correction is
surpassed. The CD still produces an audible signal in such cases, which,
however, contains interpolations and is thus not exactly the original
signal. Consequently, it is advisable that newly acquired replicated CDs
as well as internally generated CD-Rs are tested for full error
correction, and such tests be repeated in regular intervals. Transfers
onto a new medium becomes imperative well before the threshold for full
error correction is surpassed. It must be noted that under likewise
identical conditions CD-Rs which start with a low level of correctable
errors will have a longer lifespan before they fail.
Regarding stability, the polycarbonate substrate of the CD has proved
to be fairly stable. Considerable problems have been observed, however,
with its predecessors, the analogue video discs that are composed of the
same materials. Instances of crazing have occurred, rendering the
polycarbonate opaque to the reading laser beam. Whether this problem has
been totally overcome with current CDs, or only retarded, is unknown. The
reflective layer, generally of aluminium, is prone to oxidation.
Therefore, this layer is covered by a protective layer of varnish. This
varnish is the most delicate part of a CD. Instability in this layer,
which was frequently observed in the early years of the CD, can cause the
reflective layer to deteriorate, rendering discs unreadable. Deterioration
in the protective layer can result from mechanical scratches, the use of
inappropriate, "bleeding" dyes for the label information, and by chemical
degradation due to ageing or improper storage. The migration of adhesives
from labels traditionally used in libraries to identify the object and its
owner, can have destructive effects. For all these reasons, specifically
older parts of CD stocks, and all labelled CDs must be considered at risk.
Recordable CDs also consist of a polycarbonate substrate, carrying on
its upper surface a helical groove filled with an organic dye that carries
the information content. Above this is the reflective layer, originally of
gold, more recently of silver. This is in turn covered by a protective
layer of varnish which is generally of more sturdy character than that of
replicated discs, allowing the use of felt pens and also the application
of printed labels with specials adhesives for identification. Various dyes
have been used so far, to which various degrees of stability have been
ascribed. While some manufacturers claim a life time of their products for
100 years and more, experience tells that many CD-Rs have failed only
after few years.. Beyond the chemical and physical decay processes, an
important element of LE is the rate of correctable errors produced during
recording. As explained above, under otherwise equal conditions, the lower
the error rate of CD-Rs, the longer the LE; the higher the error rate, the
shorter the LE. Currently, however, CD-Rs are undergoing further
development in order to comply with endeavours for ever increased
recording speeds, leading to problems in the compatibility between disc
writers and blank discs. Such incompatibility inevitably leads to
increased error rates, negatively influencing the LE of these CDs from the
outset. Whenever CD-Rs are used, compatibility between writers and blanks
has to be carefully explored, the error status of each CD-R has to
assessed and recorded for further monitoring.
Consequently, in assessing the state of preservation especially of CD-R
stocks, the measurement of data integrity is imperative. Transfer must be
organised well before uncorrectable errors occur. It is difficult to
recommend a certain level which must not be surpassed. A flat gradient of
deterioration may allow for a higher level of correctable errors before
taking action, while a steeper gradient may call for earlier or even
immediate action.
DVDs, replicated as well as recordable, are of the same construction as
CDs. Because of their higher data density (by factor of 7 as compared to
CDs), the factors determining the stability of CDs will have a relatively
more aggravating influence on DVD stability. As with CDs, a serious risk
assessment can only be based on frequent monitoring of the data integrity
of these media.
Replicated MiniDiscs (MDs) also function like CDs. Recordable MiniDics
function on the basis of magneto-optical recording, which has been
successfully used for data recording in various formats, and has proved
fairly stable, at least in the medium term.
In concluding the section on carrier degradation it should be
emphasised that substantial enhancement of preservation activity will be a
key element in the establishment of successful transfer strategies.
Prioritisation of transfer projects should be complemented by strategies
to prolong the life time of lower ranked parts of the collection by
optimising storage conditions.
3.4. Obsolescence of Replay Equipment and Associated Software
As mentioned earlier, all audio carriers are machine readable documents. A meaningful strategy in the ranking of transfer projects has also to take the availability of replay equipment into account, specifically the threat of sophisticated equipment becoming obsolete resulting in major problems with the future retrieval of content, even where carriers are still in good condition.
3.4.1 Formats already obsolete
All mechanical
carriers formats are already obsolete. However, this does not constitute a
major threat to their continuing playback. Replay equipment designed by
experts is available for the playback of cylinders and coarse grooved
discs.
Another group of obsolete audio formats are early digital audio formats
employing analogue video formats as target carriers. The most prominent of
these are: Sony PCM F1 (701, 601 and 501) using Betamax recorders, and
Sony PCM 1600/10/30, using U-matic recorders. Betamax was widely used by
smaller recording studios as well as research and heritage institutions,
and U-matic was the standard for CD-mastering. Betamax players can only be
found on the second hand market. U-matic machines are out of production by
now, but spare parts continue to be available for the time being.
It may also be noted that most early digital formats of the DASH and PD
families are practically dead. These have been employed by the recording
industry and it may be assumed that sufficient numbers of replay machines,
including spare parts, are available in these institutions.
3.4.2 Formats about to become obsolete
The quarter inch analogue magnetic tape format is currently about to become obsolete through the progressive withdrawal of manufacturers from the production of new equipment. It is of utmost importance that sound archives immediately assess their need of new machines, and spare parts for their existing equipment, in order to assure the future orderly and complete transfer of available stocks. It should also be noted that replay equipment must match the recordings in terms of speed, track format, and equalisation. This poses considerable problems for many heritage collections, as no modern replay machines are available for low speeds such as 2.38 and 4.76 cm/s.
R-Dat has been widely utilised
in smaller studios and radio stations. It has also been widely used as a
digital target format in digitisation projects of endangered analogue
carriers. It remains the format of choice for location recording and field
work across all scholarly disciplines for the time being. However, because
of the spread of digital audio workstations, and the increased use of
CD-Rs or computer tape like DLT as target formats, R-Dat has recently lost
its predominant position. Consequently, it is a wise precaution to
consider the forthcoming unavailability of new equipment and spare parts
and to act accordingly.
With the spread of multi-channel digital workstations recording formats
such as ADAT or DTRS may become obsolete very soon.
With format or platform obsolescence, service engineers may also become
obsolete; another important factor to be taken into account when
establishing a hierarchy of transfer projects. Unlike obsolete mechanical
formats, obsolescence of all other formats is associated with a severe
problem regarding service and repair of modern equipment, specifically
digital cassette formats. Expert knowledge and skill is required, often
associated with specific reference tools and dedicated equipment.
Presently, the MiniDisc as such seems not to be threatened by
obsolescence. It is a data reduced format, however, and its data reduction
process (ATRAC) is proprietary and unpublished. Since the introduction of
MDs, ATRAC has been further developed and is presently in its fourth
version. It is unknown, whether it will be further developed and whether
future versions will be fully backwards compatible, capable of replaying
contents encoded with earlier versions with full original quality, or of
replaying at all.
Given this situation it is recommended that unique contents from
recordable Minidiscs should be transferred without delay in a
pseudo-linearised version as recommended by IASA-TC 03, § 10. It should be
noted, however, that the many replicated MiniDiscs are parallel editions
to CDs, and therefore are not necessarily items for which early
preservation transfer is a high priority.
Obsolescence is not foreseen
in the nearer future for the (analogue) compact cassette, the CD and its
subformats, and, of course, the DVD.
In establishing a meaningful hierarchy of transfer projects it is necessary to take into account the nature of the document (unique or replicated), the threat of carrier degradation, and the threats of obsolescence. It will often be very difficult to make a proper quantitative assessment while properly balancing degradation against obsolescence. Because of new experiences and results of research related to carrier degradation on the one hand, and the development of the market which determines obsolescence on the other, sound archives need to be on constant alert and prepared to adapt their strategy to new situations as they arise.