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Technology

Principles of copying and restoration

The Swiss National Sound Archives creates, for archiving purposes, backup and working copies of any audio carriers that are in a precarious state or that may be requested by the users. If the time and means available allow it, a backup copy will be made of every original audio carrier. This copy will be stored in a separate place

 

Copying

78 rpm discs (shellac, acetates, etc.)

The disc, if its state of preservation allows it, is first of all washed. The state of preservation of the materials, the type of disc and the number of copies to be treated determine the choice of the type of washing process. Two Keith Monks devices are available for the brush washing of single discs and an ultrasonic device for the simultaneous treatment of several discs together. During this first treatment, the original cover of the disc, which is often in a bad shape, is replaced with an acid free paper envelope lined with polyethylene and an acid free cardboard cover.

All the materials that come into contact with the discs, including the liquids used for washing, have been studied and selected with care by the Swiss National Sound Archives and subsequently tested by EMPA (Federal Laboratory for Materials Testing and Research) to certify their suitability for long-term archiving.

Once the washing phase is concluded, the disc moves on to the audio studio, where it is examined under a microscope to determine the dimensions and shape of the groove and to verify the physical state of its surface. These measurements and checks allow us to choose the most suitable stylus for playing the disc. The indications on the label (brand, catalog and matrix numbers) allow us to set the equalization and rotation speed correctly. Lastly, in difficult cases, to obtain even better results, the geometry of the reading arm is optimised.

The turntable signal is converted from analog to digital and "leveled" to ensure the best dynamics and signal/interference relationship. The digital signal is recorded on high resolution WAV files. Those files will contain only what is extracted from a single document (disc or set of discs), and they will be considered as the first generation "archive copy". Any subsequent copying or processing will be derived from this copy.

All the working phases, as well as the equipment used and the adjustments made are documented to allow the exact copying process to be reconstructed even in the future. The time necessary to copy a 78 rpm disc can be quantified as 4 times the total duration of the disc. On average, therefore, considering a duration of 8 min for both sides, it takes 32 minutes of working time.

 

Microgroove discs (LPs, singles, maxi-singles, etc.)

The process is similar to that described for 78 rpm discs, but simpler. Microgroove discs can be washed without exceptions and in the re-packing phase only the inner envelope is replaced, as the outer cover often provides valuable information concerning the audio carrier. The stylus, equalization and rotation speed are standard. The time necessary for copying is generally equal to double the total duration of the disc.

 

Analog tapes (cassettes of various types, reels)

Every tape is first examined and tested physically. If it is very dirty and/or fragile, it is cleaned and treated with a lubricating liquid. It is then unwound and rewound to reduce the print through effect and to restore the winding tension. In a later step, the track configuration, equalization and reproduction speed are determined. In order to be able to handle every type of tape several reproduction devices are required with different mechanical and electronic characteristics. Once the most suitable device has been chosen and the necessary settings made, the tape is read and the signal converted from analog to digital. If necessary, the tapes are wound onto new reels made of a material that does not become electrostatically charged and that complies with archiving requirements. The rest of the process as well as the working times are the same as those described for microgroove discs, that is to say double the recorded time. In difficult cases, the times may be considerably extended.

 

Optical discs (CDs, DVDs, etc.)

The disc is examined visually and, if necessary, cleaned or a dedicated machine. The disc is copied directly in digital format onto a series of WAV files. This direct transfer allows the working times to be contained and, at the same time, assures the consistency of the recorded information. To avoid quantization errors due to the inclusion of digital converters in the copying chain, all documents that are originally created in digital form are copied maintaining their original format.

 

Other digital formats (Mini-Disc, cassettes DAT, etc.)

These audio carriers do not require, as a rule, any physical maintenance, so they can be read directly. In this case also, the transfer is done entirely in digital format.

 

Notes

When it began its activity, the Swiss National Sound Archives chose DAT cassettes, which had just been introduced on the professional market, for the creation of backup copies. The DAT cassettes are regularly tested by our technicians and even now continue to hold their own in the area of backup copies. In 2007, however, DAT cassettes, as happened with other formats before them, disappeared from the market. It has been therefore necessary to develop new solutions.

In 2005 we introduced a mass storage system, which replaced the use of DAT cassettes. This kind of solution offers many advantages, and we will list just a few of the most obvious among them: with mass storage it is possible for instance to save any original format in the form of a computer file; several users can work at the same time on the same document, even online; it is easier to plan and guarantee the preservation in time of the information, as computer tools enormously facilitate the necessary operation of periodic migration of the system content, imposed by technological evolution.

 

Restoration

The basic procedure for restoring an audio carrier is practically identical for every type of carrier. In the first phase - listening - the more obvious necessary actions are identified; the process then moves on to high definition copying on a computer system and spectroscopic analysis. Some (semi-)automatic modules exist that are used for reducing or eliminating common problems: they are the so-called declicker and decrackler modules, etc. Other operations are performed manually, such as, for instance, the elimination of transients through cuts, or the non linear compression/expansion of the sound and its filtering through equalizers of various types. The manual operations are carried out at the discretion of the technician and the end user and may require working times lasting up to 60 times the duration of the track (to correct 1 minute of modulation can take up to 1 hour, for instance).

The Swiss National Sound Archives remains consistent with its mandate even it does jobs of a commercial nature. The fundamental principle on which it bases its work is: the extraction of the best possible sound from the original carrier through the use of the most suitable reproduction device, and subsequent processing aimed at reconstructing a sound that is the most faithful possible to the original performance or production. Any other type of intervention is possible, but does not come within the scope of the tasks of the Swiss National Sound Archives.

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