5.5.2 Selection of Best Copy

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.