5.5.1.1 Under optimum conditions digital tapes can produce an unaltered copy of the recorded signal, however any uncorrected 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 most closely equates 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 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.

5.5.1.2 Magnetic tape carriers of digital information have been used in the data industry since the 1960s, however, their use as an audio carrier did not become common until the early 1980s. Systems reliant on encoding audio data and recording onto video tapes were first used for two track recording or as master tapes in the production of Compact Discs (CD). Many of these carriers are old in technical terms and in critical need of being transferred to more stable storage systems.

5.5.1.3 A crucial recommendation of all transfers of digital audio data is to carry out the entire process in the digital domain without recourse to conversion to analogue. This is relatively straightforward with later technologies which incorporate standardised interfaces for exchanging audio data, such as AES/ EBU or S/PDIF standards. Earlier technologies may require modification to achieve this ideal.

5.5.2.1 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. As a general rule a source copy should be chosen which results in successful replay without errors, or with the least errors possible.

5.5.2.2 Unique Recordings: Original source materials such as multi-track sessions, field recordings, logging tapes, home recordings, sound for film or video, or master tapes, may include unique content in whole or in part. Un-edited material may be less or more useful than the edited final product, depending on the purpose of the archived material. Curatorial decisions must be made to ensure that the most appropriate or complete duplicate is selected. Truly unique recordings do not present any choice to the archivist. In the case where content is uniquely held on a single copy within a collection it is worth considering whether alternative copies might exist elsewhere. It may be possible to save both time and trouble if other copies exist which are in better condition, or on a more convenient format.

5.5.2.3 Recordings with Multiple Copies: Preservation principles indicate that copies of digital tape should ideally be a perfect record of the media content and any associated metadata as recorded on the original digital document. Any digital copy meeting this standard is a valid source for migration of the essence to new digital preservation systems.

5.5.2.4 In reality, effects of standards conversion, re-sampling or error concealment or interpolation¹ may result in data loss or distortion in copies, and deterioration over time degrades the quality of original recordings and subsequent copies. As a result, copying outcomes may differ depending on the choice of source material. Cost can also vary depending on the physical format or condition of the source material.

5.5.2.5 Determining the best source copy requires consideration of recording standards used to create copies, quality of equipment and processes used, and the current physical condition and data quality of available copies. Ideally this information is documented and readily available. If this is not so then decisions must be based on understanding of the purpose and history of various copies.

5.5.2.6 Duplicates on Similar Media: Best source material in this case will be that copy with the best data quality. First choice will usually be the most recently made unaltered digital copy. Earlier generations of unaltered digital copies may represent an alternative if the newer copies are inadequate due to deterioration or improper copying.

5.5.2.7 Copies Differing in Media or Standard: Production or preservation processes may result in availability of multiple copies on differing digital tape formats. The best source material should be identical to the original in standard, have the best available data quality, and be recorded on the most convenient format for reproduction. Judgment is called for if any of these conditions cannot be met.

5.5.2.8 If the digital recordings are only duplicates of analogue recordings, and where the analogue originals still exist, re-digitisation is an option to consider if those digital copies are inferior in standard, quality or condition.

1. Error concealment or interpolation is an estimation of the original signal when data corruption prevents accurate re-construction of the signal.

5.5.3.1 Magnetic digital tapes are similar in materials and construction to other magnetic tapes, and suffer from similar physical and chemical problems. Digital tapes achieve high data densities through the use of thin tapes, small magnetic tracks and ongoing reductions in the size of the magnetised domains which can be written and read. Consequently even minor damage or contamination can have major impacts on signal retrievability. All tape degradation,damage or contamination will appear as increased errors. Carrier restoration problems and techniques are similar for all magnetic tapes, but since base, binder and magnetic materials are subject to ongoing development any restoration processes must be tested and proven for specific media.

5.5.3.2 Commercial cleaning machines are available for open reel magnetic tapes and for most videotape formats commonly used to carry digital audio signals and are effective for moderately degraded or contaminated tapes.Vacuum or hand cleaning may be indicated for tapes with higher levels of contamination or of greater fragility, but requires conservatorial care to avoid damaging delicate tapes and intricate cassette mechanisms. Any cleaning process has potential to cause damage and should be applied with appropriate caution.

5.5.3.3 Jigs can aid in manipulating tapes and cassette housings, and are commercially available for some formats. Purpose-built jigs for other formats can be manufactured in a moderately well equipped mechanical workshop.

5.5.3.4 Digital tapes with polyester urethane binders have the potential to suffer from hydrolysis in the same way as analogue magnetic tapes. Any rejuvenation of digital magnetic tape will require tight process control, and should only be attempted in a purpose-built environmental chamber or vacuum oven² (see Section 5.4.3 Cleaning and Carrier Restoration). This may be even more critical with digital recordings as they will often have been made on thinner based tapes housed in complex cassette mechanisms.

5.5.3.5 Deterioration of magnetic tapes can be minimised by appropriate storage conditions. Standards for long-term digital magnetic tape storage are generally more stringent than for analogue tapes, due to their greater fragility and susceptibility to data loss through relatively minor damage or contamination. Higher than recommended temperature or humidity will promote chemical deterioration. Cycling of temperature and humidity will result in expansion and contraction of the tape and may damage the tape base. Dust or other contaminants can find its way onto the tape surface resulting in data loss and possibly physical damage during replay.

5.5.3.6 After cleaning and/or repairing measures or prior to the reproduction it may be advisable to first measure the magnetic digital tape’s error rates. The organisation of the data and the type of error correction used varies according to the tape format. For DAT for example, the error correction process uses two Reed-Solomon codes arranged in a cross code system, C2 horizontally and C1 vertically. Also, each block of data has a value assigned, known as a parity byte. Counting the Block parity errors are known as CRC errors, or sometimes as the block error rate. The sub code of the DAT (Digital Audio Tape) is also subject to errors. Error measurement should include, as a minimum:

5.5.3.6.1 C2 and C1 errors.

5.5.3.6.2 CRC or Block error rate.

5.5.3.6.3 Burst Errors (derived from C1).

5.5.3.6.4 SUBC1 correction.

5.5.3.7 If any of the error measurement reveals a sample hold, interpolated or mute level error the tape should be cleaned and the tape path checked. If after cleaning and repair one or more of the error rates exceed these thresholds refer to 5.6.3 “Selection of Best Copy.” (above).

5.5.3.8 There are very few error measuring devices available for DAT or other magnetic carriers. Any transfer, however, should include a measurement of the errors produced at the error correction chip of the replay machine and this information must be recorded in the metadata of the resultant audio file.

2. Vacuum ovens reduce the air pressure in the oven chamber and so better control moisture content

5.5.4.1 Replay equipment must comply with all specific parameters of a given format. Digital tape formats are mostly proprietary in nature, with only one or two manufacturers of suitable equipment. Latest generation equipment is preferred, but for older or obsolete digital formats there may be no choice but to purchase second- hand equipment.

5.5.4.2 The high recording density of R-DAT(Rotary Head Digital Audio Tape) has ensured that applications other than audio-recording-only were developed. The DDS (Digital Data Storage) format, based on DAT technology, was developed by Hewlett-Packard and Sony in 1989 and was dedicated to the storage of computer data. Steady increases in data integrity of the basic system resulted in developments which allow for signal extraction from audio DAT tapes.Various types of software are available which allow the extraction of the audio as separate files based on ID’s on the tape. Dedicated data extraction software can also generate metadata files for each program, including clock, start and end ID positions, durations, file size, audio properties, etc. Additionally the DDS format allows double speed capturing of audio material.

5.5.4.3 Nevertheless, the important questions such as format incompatibilities (e.g. the different long play modes, high resolution recordings, time code extraction etc.), proper data integrity checking, pre-emphasis handling and especially all matters concerning mechanical and tracking problems are still not yet solved by such systems and therefore need individual treatment.

5.5.5.1 The R-DAT (commonly referred to as DAT) is the only common system to use a cassette format specifically developed for digital audio recordings. DAT tapes have been widely used in field and studio recording, broadcasting and archiving. New DAT equipment is now virtually unavailable. Second hand professional DAT machines are a solution, but present maintenance problems as parts supplies become exhausted.

5.5.5.2 Some last generation recorders operate outside the specification, allowing high resolution recording at 96 kHz and 24 bits (at double speed), others provided Timecode (SMPTE) recording, or Super Bit Mapping, a psycho-acoustic principle and critical band analysis to maximize the sound quality of 16-bit digital audio. 20-bit recordings are quantized to 16 bits using an adaptive error-feedback filter. This filter shapes the quantization error into an optimal spectrum as determined by the short-term masking and equi-loudness characteristics of the input signal. Through this technique, the perceptual quality of 20-bit sound is available on a 16-bit DAT recording. Full quality can only be reached with signals containing frequencies lower than 5-10 kHz. Super bit mapping does not require special decoding on playback.

Table 1 Section 5.5 Specifications for various record/playback modes of DAT for both blank and pre-recorded tapes:

5.5.5.3 Phillips DCC (Digital Compact Cassette) system was (unsuccessfully) introduced as a consumer product and offered limited compatibility with analogue compact cassettes through the ability to replay analogue cassettes on DCC equipment. DCC is now considered obsolete.

Table 2 Section 5.5 Digital Audio Cassettes

5.5.6.1 SONY and Mitsubishi have both produced open reel digital systems for the recording studio market, and NAGRA produced a four-track field recording system, the NAGRA-D.

5.5.6.2 Sony/Studer’s DASH (Digital Audio Stationary Head) system has numerous variants, based on common formats for the digital tracks on tape. DASH I provides 8 digital tracks on ¼” tape and 24 digital tracks on ½" tape. DASH-II provides 16 digital tracks on ¼” tape and 48 tracks on ½" tape. Twin DASH formats are commonly used for ¼” stereo digital recordings and utilise twice the normal number of data tracks for each audio channel to increase the systems error correction capability so that tape splicing can be used for editing. Low speed formats double recording time by sharing data for each audio channel across multiple data tracks, halving the number of audio tracks available.

5.5.6.3 Nagra still support NAGRA-D Sony DASH and Mitsubishi Pro-Digi format machines are no longer manufactured. These formats are/were intended for high-end professional use and as a result were extremely expensive to support.

Table 3 section 5.5 Open Reel Formats

5.5.7.1 There are two variants within this category: systems using videotape in a standard VCR to record digital audio encoded on a standard video signal, and systems using videotape as the storage medium for proprietary digital audio signal formats.

5.5.7.2 Sony has produced a range of formats using VCR systems as a high bandwidth storage device. More recently Alesis introduced the ADAT system, which used S-VHS videocassettes as high capacity storage media for their proprietary format of digital audio, and Tascam released the DTRS system using Hi8 videocassettes as the storage medium.

5.5.7.3 Formats using video recorders were based on interface devices that incorporated A-D and D-A converters, audio controls and metering, and the hardware required to encode the digital bit stream as a video waveform. Sony’s professional system specified NTSC standard (525/60) Black-and-White U-Matic VCR, and these were manufactured specifically for digital audio use. The semi-professional PCM-F1, 501 and 701 series worked best with Sony Betamax recorders, but were generally compatible with Beta and VHS. Machines in this series supported PAL, NTSC and SECAM standards.

5.5.7.4 Reproduction of VCR based recordings requires availability of a VCR of the correct standard, plus the appropriate proprietary interface. There is normally backwards compatibility within related systems, so purchase of later generation equipment should facilitate replay of the widest range of source material. As some of the video based PCM adaptors had only one A/D converter for both stereo channels, there is a time delay between the two channels.When the tapes are replayed and the audio data is extracted the signal processor delay should be corrected in the digital domain. Transfers should be made only with equipment which allows the output of a digital signal.

5.5.7.5 Early digital recorders sometimes encoded in what are now uncommon sampling rates, such as 44.056kHz (see table 4 Section 5.5). It is recommended that the resultant files be stored at the encoding levels at which they were created. Care should be taken to ensure that automatic systems do not misrecognise the actual sampling rate (eg a 44.056kHz audio stream may be recognised as 44.1kHz, which alters the pitch and speed of the original audio). Second files can be created for users in common sampling rates using appropriate sampling rate conversion software. Nonetheless, the original file should be retained.

5.5.7.6 In addition, third-party equipment for systems based on domestic VCRs can provide useful extended functionality, including better metering and error monitoring facilities and professional inputs and outputs.

5.5.7.7 VCR based systems are obsolete, and equipment will need to be sourced second-hand.

Table 4 section 5.5 Digital Audio on Videotape – Common Systems

5.5.8.1 Precise identification of the format and detailed characteristics of the source material is essential to ensure optimum reproduction, and is complicated by the variety of formats with outwardly similar physical characteristics but different recording standards. Machines should be cleaned and regularly aligned for best signal reproduction. Any operator-controlled parameters such as de-emphasis must be set to match the original recording. For VCR based formats the video tracking may need to be adjusted for best signal, and any dropout compensation on the video signal must be switched off.

5.5.9.1 Misalignment of recording equipment leads to recording imperfections, which can take manifold form.While many of them are not or hardly correctable, some of them can objectively be detected and compensated for. It is imperative to take compensation measures in the replay process of the original documents incurred, as no such correction will be possible once the signal has been transferred to another carrier.

5.5.9.2 Adjustment of magnetic digital replay equipment to match misaligned recordings requires high levels of engineering expertise and equipment. The relationship between the rotating heads and the tape path can be adjusted on most professional equipment, and for DAT recordings especially, this can lead to significant improvement in error correction or concealment, even making apparently unplayable tape audible. However, such adjustments require specialised equipment and only trained personnel should undertake them. Equipment should be returned to correct setting by trained service technicians after completing the transfer.

5.5.10.1 It is preferable in most cases to minimise the storage related signal artefacts before undertaking digital transfer. Digital tapes should be re-spooled periodically if possible, and in any case always re-spooled before replay. Re-spooling reduces mechanical tension, which can damage the tape base or decrease performance during replay. Open reel digital tapes that have been left unevenly wound for some time may exhibit deformations, particular of tape edges, which may cause reproduction errors. Such tape should be rewound slowly to reduce the aberrations in the wind and rested for some months, which may aid in reducing replay errors. Though cassette systems can be similarly affected, the ability to influence the pack through reduced wind speed is not as great with such equipment.

5.5.10.2 Magnetic fields do not decay measurably in a period of time that is likely to affect their playability. The proximity of adjacent tracks or layers will not cause self erasure on analogue tapes, and in the unlikely event that it may cause issues with older digital tapes this is rarely critical as any resulting errors are within the limits of the system. Some loss of signal may be measurable in the oldest video based tapes when used to record digital audio. In these circumstances the lower coercivity of the magnetic particles and the apparent short wavelength on the tape caused by recording digital information using a rotating head combine to create the conditions where this may occur, at least in theory. This may make it difficult for replay equipment to track the information on the tape. All but the very earliest video tape formulations have a much higher coercivity, combined with systems which have better error correction technology, which made this problem largely irrelevant. In any event, attention to the cleanliness of the heads of the replay machine and tape will maximise the possibility of replay, as will careful alignment of the tape path.

5.5.10.3 Seriously damaged tapes may be recoverable using techniques that could be characterised as “forensic” due to their dependence on high-level skills from a range of scientific and engineering disciplines (see Ross and Gow 1999). Management of digital tape collections should aim to ensure copying occurs before un-correctable errors become a problem, as options for restoration of failed digital tapes are very limited.

5.5.11.1 The time needed for copying contents of audio material varies greatly, and is highly dependent on the nature and status of the original carrier.

5.5.11.2 Preparation time will vary depending on condition of the source copy. Set-up time depends on details of facilities and formats in use. Signal transfer is generally slightly more than actual running time for each recoded segment, and time taken for management of metadata and materials management will depend on details of the archiving system in use. Most audio specific tape based digital recording formats do not allow upload of the data at greater than real time, with the exception of those mentioned above. However, capture systems that accurately measure error levels and warn operators when set levels are exceeded may allow for multiple systems to be run simultaneously.