6. Selection of best copy and carrier restoration

If several copies of a sound document are available, the best must be selected for the further preservation of its content. Additionally, cautious and appropriate cleaning and restoration procedures must be performed to optimise signal retrieval.

Comment:
With mass replicated carriers - specifically mechanical and optical carriers - the replay quality of different copies may vary considerably because of the way that they have previously been handled and stored. It may, therefore, be advantageous to extend the search for the best copies to other collections on a national,or even international scale.

Archival holdings in a narrower sense, ie non-replicated recordings, may often be available in two or more versions. For example, an original carrier (tape, cassette, instantaneous disk) and an archival master which is a copy of the original. While archival masters, or younger carriers in general, may often be in a better state of physical preservation, they may be of inferior audio quality due to poorer earlier transfer technology and the inescapable losses in the analogue copying process. Consequently, the retrievable quality of the various available copies must be checked against each other. It is a common experience that signal retrieval from well preserved early original tapes yields better results than that from their analogue archival master tape copies.

When choosing cleaning and restoration procedures, utmost care must be taken to balance improvement of signal retrieval against possible further deterioration, even the entire loss of the carrier. Therefore, in the transfer of any historical and/or endangered carrier, the use of the originals must be kept to the minimum at all times. Heavily deteriorated carriers may even be lost completely in the attempt to replay them. In such critical cases it is necessary to safeguard the sonic content by making a straight copy of the first replay which may, in the case of successful restoration, be changed for a later transfer. In addition, the straight copy should be used for the experimental calibration of parameters.