5.5.3 Cleaning, Carrier Restoration
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
