5.6.1.1 Since their introduction in 1982 replicated optical disc media have become the dominant technology for distribution of published audio recordings. Recordable optical disc formats, first made available in the late 1980s¹, play an increasingly significant role in distribution and storage of unpublished audio. Initially marketed as permanent, it has become clear that the usable life of the optical disc is finite and that steps will need to be taken to copy and preserve their data content. This is especially the case with recordable disc media, which are not only less reliable than their manufactured counterparts but are also more likely to contain unique material. Unless recorded and managed under specified conditions (see Section 6.6 Optical Discs: CD/DVD Recordables), recordable disc media constitute an unreasonable risk to collection material. This section of the Guidelines concerns itself with the accurate and efficient copying of CD and DVD optical disc media to more permanent storage systems. CD is the abbreviation for Compact Disc, DVD initially stood for Digital Video Disc, then Digital Versatile Disc but is now used without referring to a specific set of words.

5.6.1.2 The Audio CD family may include, in CD-DA format; CD manufactured, CD-R, CD-RW, and in this form are all characterised by 16 bit digital resolution, 44.1 kHz sampling frequency and 780nm wavelength read laser. DVD Audio includes SACD and DVD-A. Data formats such as .wav files and BWF files may be recorded as files on CD- ROM and DVD-ROM. DVD media are characterised by blue laser around 350 to 450nm for glass mastering and 635-650 nm playback, DVD+R (650 nm), DVD-R (both for authoring (635 nm laser)). Blu-Ray Discs (BD) are a high definition video and data format on the same diameter 12 cm disc as DVD and CD. Using a 405 nm blue laser BD is able to store 25 GB of data per layer.

5.6.1.3 Recordability, rewritability, erasability and accessibility:

5.6.1.3.1 CD and DVD (CD-, DVD-A, CD-ROM and DVD-ROM) discs are pre-recorded (pressed and moulded) read-only discs. They are neither recordable nor erasable.

5.6.1.3.2 CD-R, DVD-R and DVD+R discs are dye-based recordable (write once) discs, but not erasable.

5.6.1.3.3 CD-RW, DVD-RW and DVD+RW discs are phase-changed based repeatedly rewritable discs permitting erasure of earlier and recording new data in the same location on the disc.

5.6.1.3.4 DVD-RAM discs are phase-changed rewritable discs formatted for random access, much like a computer hard disc.

5.6.1.4 The table below (table 1 section 5.6) provides a listing of commercially available CD and DVD disc types.

Table 1 Section 5.6 Commercially available CD/DVD disc types
SS= Single-sided, SL=Single layer, DL=double-sided, DL=dual layer

5.6.1.5 Under optimum conditions digital discs can produce an unaltered copy of the recorded signal, however, in the case of audio specific recordings, any un-corrected errors in the replay process will be permanently recorded in the new copy, or sometimes unnecessary interpolations will be incorporated into the archived data, neither of which is desirable. Optimisation of the transfer process will ensure that the data transferred is most closely equivalent to the information on the original carrier. As a general principle, the originals should always be kept for possible future re-consultation, however, for two simple, practical reasons, any transfer should attempt to extract the optimal signal from the best source copy. Firstly, the original carrier may deteriorate, and future replay may not achieve the same quality, or may in fact become impossible, and secondly, signal extraction is such a time consuming effort that financial considerations call for an optimisation at the first attempt

1. The first working CD-R system,Yamaha’s PDS (Programmable Disc System), was launched in 1988

5.6.2.1 Compact Disc Standards:The standard for CD was originally a product of the companies Philips and Sony. The standards are named after a colour, the first being the Red Book: Philips-Sony Red Book CD Digital Audio, also includes CD Graphics, CD (Extended) Graphics, CD-TEXT, CD-MIDI, CD Single (8cm), CD Maxi-single (12cm) and CDV Single (12cm).Yellow Book standard specifies the CD as a data file carrier, the Green Book describes CD-I or interactive data, Blue Book describes Enhanced (multimedia) CD, and White Book specifies CD-V (video) characteristics. Orange Book is the standard that refers to Recordable and Rewritable CDs (and is described more fully in Chapter 6). The colour book standards, subject to certain limitations, may be ordered from the Philips web site at http://www.licensing.philips.com/. They are primarily intended for manufacturers. The ISO standards which describe CDs are purchasable from International Standards Organisation (ISO) Central Secretariat http://www.iso.org/. IEC 908:1987, Compact Disc Digital Audio System (CD-DA) (n.b. IEC 908:1987 and Philips-Sony Red Book are basically equivalent.) ISO 9660:1988,Volume and File Structure (CD-ROM) (ECMA-119) and ISO/IEC 10149:1995, Read-Only 120 mm Optical Data Discs (CD-ROM) (ECMA-130).

5.6.2.2 DVD Standards: There is an extensive range of ISO standards for DVD. However, similarly to CD, there are also proprietary versions of the standards. These standards are referred to by an alphabetical appellation: DVD-ROM, the basic data standard, is specified in Book A, DVD video is described in Book B, DVD- Audio in Book C, DVD-R in Book D, and DVD-RW in Book E. The ISO standards are purchasable from International Standards Organisation (ISO) Central Secretariat http://www.iso.org/ ISO 7779:1999/Amd 1:2003 Noise measurement specification for CD/DVD¡ROM drives. ISO/IEC 16448:2002 Information technology -- 120 mm DVD -- Read-only disc and ISO/IEC 16449:2002 Information technology -- 80 mm DVD -- Read-only disc.

5.6.3.1  Unlike copying analogue sound recordings, which results in inevitable loss of quality due to generational loss, different copying processes for digital recordings can have results ranging from degraded copies due to re-sampling or standards conversion, to identical “clones” which can be considered even better (due to error correction) than the original. In choosing the best source copy, consideration must be given to audio standards such as sampling and quantisation rate and other specifications including any embedded metadata. Also, data quality of stored copies may have degraded over time and may have to be confirmed by objective measurements. If there is only one copy in poor physical condition in a collection, it may be wise to contact other sound archives to determine whether it is possible to find a better preserved copy of the same item.

5.6.3.2  As a general rule, a source copy should be chosen which results in successful replay without errors, or with the least errors possible. Replicated discs are more stable than recordable media and would normally be preferred if a choice is available. Physical condition may provide an indication of quality, however the only certain method for choosing an error free disc is to institute routine error checking and reporting as part of the transfer process.Even with error checking and reporting,the extraction of best possible signal is problematic as the lack of standards with drives means that different players may produce different results on the same disc (see 8.1.5 Optical Discs – Standards). As with all digital to digital transfers,an error status report must be made and incorporated in the administrative metadata of the digital archive file, along with a record of the drive used.

5.6.4.1 The variety of standards and the manner in which they may be encoded makes selection of the correct replay equipment necessary. The domestic freestanding CD player, for instance, will most likely only play CD-Audio and its variants, whereas the CD-ROM drive in a computer will play all the formats, though it requires the appropriate software to determine the content. DVDs will not play in CD drives or players, although many DVD drives are compatible with CDs.

5.6.4.2 The tables below lay out the compatibility between certain drives and their appropriate media.

Table 2 Section 5.6 Read Write Compatibility; CD

Table 3 Section 5.6. Compatibility; DVD (Write Mode)

Table 4 Section 5.6. Compatibility; DVD (Read Mode).

5.6.5.1 CDs or DVDs do not require routine cleaning if carefully handled, but any surface contamination should be removed before replay or in preparation for storage. It is important when cleaning to avoid damaging the disc surface. Particulate contamination such as dust may scratch the disc surface when cleaning, or use of harsh solvents may dissolve or affect the transparency of the polycarbonate substrate.

5.6.5.2 Use an air puffer or compressed clean air to blow off dust, or for heavier contamination the disc may be rinsed with distilled water or water based lens cleaning solutions. Care should be taken as the label dyes in many CD-Rs are water soluble. Use a soft cotton or chamois cloth for a final wipe of the disc. Never wipe the disc around the circumference, only radially from the centre to the outside of the disc - this avoids the risk of a concentric scratch damaging long sections of sequential data. Avoid using paper cleaning products or abrasive cleaners on optical discs. For severe contamination isopropyl alcohol may be used if required.

5.6.5.3 It is preferable that no repairs or polishing is undertaken on archival optical discs as these processes irreversibly alter the disc itself. However, if the disc surface (reading side) shows scratches that produce high level errors, repairs which return the disc to a playable state may be allowed for the purposes of transfer. These may include wet polishing systems providing careful testing of the effect of these restoration systems have been undertaken before being applied to important carriers. This should be undertaken by testing an expendable disc, undertaking the restoration process, and retesting to determine the effect of restoration (for further details consult ISO 18925:2002,AES 28-1997, or ANSI/NAPM IT9.21 and ISO 18927:2002/AES 38- 2000). Though some initial testing of wet polishing indicates adequate results, the removal of surface material makes sound archivists reluctant to endorse this approach. Moreover wet polishing is only effective with small scratches; discs with deep scratches deliberately inflicted with, for example a knife or scissors, will not be returned to playability by wet polishing. Damages on the label side will not benefit from any repair measures described.

5.6.5.4 Before and after cleaning and/or repairing measures and prior to the reproduction it may be advisable to first measure the CD’s or DVD’s error rates, as a minimum:

5.6.5.4.1 Frame burst errors (FBE) or Burst Error length (BERL)
5.6.5.4.2 Block error rate (BLER)
5.6.5.4.3 Correctable errors (E11, E12, E21, E22, errors before interpolation)
5.6.5.4.4 Uncorrectable errors (E32)

And preferably:
5.6.5.4.5 Radial noise and tracking error signals (RN)
5.6.5.4.6 High frequency signals (HF)
5.6.5.4.7 Dropouts (DO) 5.6.5.4.8 Focusing errors (PLAN)

5.6.5.5 There are a range of error measuring devices available for CD and DVD of varying sophistication, accuracy, and cost. A reliable tester is, however, a necessary part of a digital disc collection to determine if critical error thresholds are exceeded (cf 8.1.5 Optical Discs – Standards and 8.1.11 Testing Equipment). If after cleaning and repair one or more of the error rates exceed these thresholds refer to 5.6.3 “Selection of Best Copy”.

5.6.6.1 There are two fundamentally different approaches to reproduction of audio CD and DVD sources: traditional replay using format-specific reproduction equipment; or digital audio extraction (DAE) using a general purpose CD-ROM or DVD-ROM drive, commonly referred to as “ripping” or “grabbing”. The chief advantage of the data capture or ripping method is greater speed, for while traditional reproduction requires transfer in real time, data capture or “ripping” utilising high speed drives can easily transfer audio data in less than one tenth of the actual audio running time.

5.6.6.2 Digital Audio Extraction: The chief disadvantage of DAE is in error handling. The simplest “ripping” software has no error correction capability at all. More sophisticated systems make some attempt at error management but do not have the functionality to fully implement the error checking, correction and concealment that is necessary for accurate transfer, and which is built into format specific equipment. Top end professional systems promise error handling equivalent to the format specific approach, yet few have accurately implemented it.

5.6.6.3 Reproduction at rates significantly faster than real time are desirable in that this reduces the resources required to transfer audio material to the target archival system. If the DAE system can be automated, this has the added advantage of freeing staff resources for the more human resource intensive tasks of converting analogue audio to digital. Automated systems can be used appropriately if there is no loss of accuracy in the transfer process. In fact, in the better systems, there is less danger of data inconsistencies, particularly those affecting metadata but also possibly affecting the content itself.

5.6.6.4 Reproduction of digital audio data should always be accompanied by an accurate error detection and recognition system that describes and identifies exactly the kind and number of CD-specific errors and associates them with the metadata specific to that audio file. This is all the more critical where automated, faster-than-real-time processes are used to acquire the audio data.

5.6.6.5 The reproduction of an audio CD is a unique process where a somewhat subjective decision needs to be made about the success, or otherwise, of the transfer process. Unlike the transfer of audio data files, this decision can only be made by considering the error protocol. Data formats, such as .wav or BWF, can be objectively checked by bit for bit comparison between the new and old files. CD audio is not a digital file, but a coded stream of audio data, a significant difference when it comes to managing the audio integrity.

5.6.6.6 Systems which guarantee error detection and recognition including error protocol in a faster- than¡real-time mode up to a maximum of 12 times, relative to real time audio replay, are available on the market and are generally specifically aimed at the archival market.

5.6.6.7 The minimum requirement for archival use of DAE is that the DAE system must detect and alert the operator to any digital audio errors.

5.6.6.8 Format Specific Replay Approach: To transfer a CD encoded in CD-A format a stand-alone CD player may be used. The required replay equipment is a CD player with digital output, permitting ingest of the digital audio stream via a sound card with digital input. Preferred interface standard for the digital audio stream is AES/EBU. Use of the SPDIF interface can provide the same results but cable runs must be kept short. Any conversion between AES/EBU and SPDIF needs to accommodate the differences between the two standards, notably the different use of status bits that carry emphasis and copyright flags (Rumsey and Watkinson 1993). The disadvantage with this real time replay approach is that it is very time consuming, and no record of error correction is maintained in the record metadata.

5.6.6.9 Sound cards for ingest of CD audio must accommodate two channels at 16 bit 44.1 kHz. Replay equipment should be of commercial quality. Care taken in ensuring a stable vibration free mounting for the player will ensure maximum reliability of replay.

5.6.6.10 The CD player must be in good replay condition. In particular, optimum laser power is mandatory, and the laser lens should be cleaned from time to time. Devices such as disc-tuners are of no use to any replay of a CD. It is advised against using protective foils (so called CDfenders/ DVDfenders) because they may come off from the disc and damage the drive².

2. CDs aus dem Kuhlschrank. Funkschau no. 23, 1994, p.36-39. The effect of improving the replay quality of CDs or DVDs by cooling them down in a refrigerator is so minute that though it was shown in theory (mathematically) it has never been shown in practice

5.6.7.1 DVD audio delivers 6 channels of audio at the 24 bit 96 kHz standard, and/or two channels at 24 bit 192 kHz, however digital outputs on most DVD players are limited to 16 bit 48 kHz resolution as a piracy control measure. The DVD forum has selected IEEE1394 (firewire) as the preferred digital interface for DVD Audio, using the “Audio and Music Data Transmission Protocol” (A&M protocol) (http://www.dvdforum.com/images/guideline1394V09R0_20011009c.pdf).

5.6.7.2 Decoding compressed formats such as MLP can be done by the player or at a later processing stage. Discs may include alternative versions or additional content including down mixing of surround signals to stereo, alternative tracks, accompanying video etc, requiring a policy decision as to whether all these versions are to be collected or if not which alternatives are required for the archive. It is also important that archive staff be aware that hybrid discs,such as those recorded in compliance with the Blue Book standard as Enhanced CDs, may contain other data. The extra graphical or textual data may be critical components of the work and are therefore necessary in acquiring and preserving the content.

5.6.8.1 The SACD format is based on Direct Stream Digital (DSD), a 1 bit sampling technique at 2.8 MHz sampling frequency which is not directly compatible with linear PCM. At the time of writing there are limited options for ingesting this type of signal into a digital audio storage system, as most SACD players do not provide either an SACD bitstream output or a high quality PCM signal derived from the bitstream.Sony has its proprietary I-Link interface using firewire,and some third party manufacturers have marketed proprietary interfaces that can handle SACD in its native format, but there is no widely accepted digital interface standard for this format. Indications are that a suitable open standard protocol for transmission of SACD over IEEE 1394 firewire though promised, may never eventuate.

5.6.8.2 Workstations developed for SACD mastering have capabilities for input, output and processing of DSD signals (http://www.merging.com/). It should be noted that even basic processing such as gain adjustment of DSD or SACD streams requires a completely different computational approach, and therefore very different algorithms to that of PCM, consequently, the restoration and re-use of audio encoded into such formats will be limited unless converted to PCM.

5.6.9.1  Time required for ingest of the audio data from optical disc in real time for conventional replay approaches a factor of two for every hour of audio. DAE approaches may reduce this by around a factor of 10, and an automated juke box system will load 60 or more CDs in a few hours without staff resources beyond the initial loading. Additional time must be allowed for selection of best copies, re-copying in the case of unacceptable errors, plus file and data management.

5.6.10.1 The original Minidisc (MiniDisk, MD) appeared in two forms: as a mass replicated disc, which works according to the principles of optical discs, and as a recordable, actually rewritable, disc, which is a magneto -optical recording medium (cf Section 8.2 Magneto Optical discs). Both sub-formats may be read by the same players. The discs are of 2.5” (64mm) diameter and housed in a cartridge. Minidisc recordings employ Adaptive Transform Acoustic Coding (ATRAC),a data reduction algorithm based on perceptual coding. Data reduced formats, although highly developed (at least in the later versions of ATRAC), not only omit data irretrievably that would otherwise be captured by a non-data reduced format, but also create artefacts in the time domain as well as in the spectral domain.Such artefacts can lead to misinterpretations of spectral components as well as of time-related components, especially when analysing the signal by means of a spectral tool. The artefacts of data reduction codecs cannot be recalculated or compensated for at a post processing stage, as they are dependant on the level, dynamics and frequency spectrum of the original signal. ATRAC is a proprietary format, with many versions and variations, and for archival purposes it is advisable to re-encode the resultant files of compressed recording formats as .wav files.

5.6.10.2 Many minidisc players have digital output which will allow the production of “pseudolinarised” data stream. The resultant file should comply with specifications laid out in chapter 2 Key digital principles and stored in accordance with that section. Metadata about the origin of such signals are imperative, as pseudolinearised signals cannot be distinguished from signals recorded without data reduction. This information would be recorded in the coding history of a BWF file, or be rendered as change history as per PREMIS recommendations (see Chapter 3 Metadata).

5.6.10.3 In 2004 the Hi-MD was marketed, and it incorporated changes to hardware which, with the new media, would record up to 1 GB of audio data.With Hi-MD it was possible to record several hours of data reduced signals, but more importantly, it was also capable of recording linear PCM signals. For archival purposes these recording should be treated like CD signals and transferred as a data stream to a suitable file storage system. Extracting audio data directly from HD-MD at higher transfer rates requires specific proprietary software, some of which is available from manufacturers’ websites. It is advisable to purchase dedicated replay equipment and software immediately as prolonged manufacturer’s support cannot be guaranteed.

5.6.10.4 The use of Minidisc as an original recording machine is not recommended (see section 5.7 Field Recording Technologies and Archival Approaches).