digital preservation issues · digital audio carriers • digital audio tape (r-dat) • compact...
TRANSCRIPT
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Digital Preservation Issues
Antony GordonBritish Library National Sound Archive
IAML Berkeley, 5th August 2002
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Summary• Not about digitization for preservation• Preservation issues in existing media• Carrier formats – change is continual• What is digital? Why is context important?• Obsolescence problems• Digital media degradation• Digital preservation and digital stores
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Digital is forever?
This is a dangerous illusion!
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Analogue audio carriers• cylinders (mostly wax) – fragile• coarsegroove discs (mostly shellac)• Metal/glass platters coated with a thin layer of
lacquer / cellulose nitrate – very fragile• microgroove discs (vinyl)• steel wire• flexible ferromagnetically-coated plastic strips
(tape)
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Digital audio carriers• Digital Audio Tape (R-DAT)• compact disc (CD)• “enhanced” CD• betamax/VHS tape cassettes (PCM-encoded)• minidisc• digital compact cassette (DCC)• DVD (digital versatile disc)• Where next? … 100Gb and upwards …
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Signal compression
• Lossy compression used for minidisc, DCC, DVD, mp3 – original can never be re-created
• Only lossless compression – or no compression at all – should ever be used for archiving of digital originals
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What do we actually mean by digital anyway?
• Definition– The representation of complex (analogue)
data (sound, graphical, etc.) by encoding in any two-state (binary) system.
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Sampling a waveform
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Case example “Red book” compact disc (16 bit resolution at 44.1kHz)
• this registers 65,536 different levels, generating data at 705,600 bits per second per channel = a little under 606 Mb per hour for stereo
• DVD Audio samples at 24 bit resolution and a rate of 192kHz (around 4Gb per hour for stereo)
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Data packets, redundancy and error checking
• Grouping of bits into: bytes (8 bits) or words (16, 20, 24, 32, 64 or 128 bits)
0001010010010100000101001101010011001101IAML
0001010010010100000101001101010011001101§†¶¶
• Error checking and data redundancy
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The issue of context1101001100101010000110101011110001110100110010101000011010101111000111010011001010100001101010111100011101001100101010000110101011110001
• text file (e.g. ASCII, RTF, Word…)• graphic image (e.g. BMP, GIF, TIFF, PNG)• sound file (e.g. WAV, RDF, AIFF)• moving image file (e.g. QTM, MPG)
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Hardware obsolescence• BBC microcomputer• Acorn Archimedes• Amiga• NeXT box• IBM-compatible PC (from 8086 to Pentium IV)• Apple machines (from 68000 to G4)• Case study: BBC Domesday Project
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Digital media formats
• 8" (200mm) diskette, or floppy disk• 5.25" (130mm) floppy disc• 3.5" (90mm) floppy disk• Zip disks, Jaz disks, portable hard
drives (SCSI /Firewire/USB 2)• Tape formats: DAT or DLT cartridges
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Software obsolescence• IBM-compatible PCs
– DOS – Windows 3.1– Windows 95 and 98, NT, 2000, Me and now XP
• Apple Macs– System 6– OS 7, 8 and 9 – OS X
• Connector changes can have an impact !
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Degradation of storage media
• Magnetic mediaDrop outsSubstrate problemsBinder problemsTape edits
• Optical mediaScratches to the playing surfaceOxidation of the reflectant layer
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Born-digital materials
• With digitized analogue materials we still have the original
• With born-digital materials we are conserving the original, or a digital clone
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Digital preservation
… the series of managed activities necessary to ensure continued access to digital materials for as long as necessary … beyond the limits of media failure and technological change.Digital Preservation Coalition handbook (Jones and Beagrie)
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Preservation strategies• Technology preservation• Data refreshing• Data migration• Emulation• Virtual envelopes• Data archaeology• Use of open standards
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Preservation strategies II
• Print it out !• Save in a simple common format• Not a viable option for audio• HD-ROM ??
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Digital stores
• Metadata– traditional catalogue data
– file format– hardware and software required for replay– origination/copying dates
• Data pertaining to media and file formats
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Trusted Digital Repositories
Defined as:"… one whose mission is to provide
reliable, long-term access to managed digital resources to its designated community, now and in the future." (RLG-OCLC report entitled ‘Trusted digital repositories: attributes and responsibilities’, May 2002 .)
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Conclusion
“It is an exercise in futility to try to pick the technological winner, for ‘the latest’ will be hopelessly outdated in ten years’ time. What is necessary is a permanent strategy for handling perpetual change.”
The archive as an ecosystem / Julia Martin and David Coleman (Journal of Electronic Publishing, April 2002)
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Links• Open Archival Information System
www.rlg.org/longterm/oais.html• Preserving Access to Digital Information
www.nla.gov.au/padi/• Digital Preservation Coalition handbook
www.jisc.ac.uk/dner/preservation/workbook/• BBC Domesday Project
www. atsf.co.uk/dottext/domesday.html