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Fundamentals of AV Preservation - Chapter 3

Section 1: the digitization signal path

While the exact technologies and workflows vary from project to project, it is helpful to understand the general setup and process that a vendor will use with your AV materials in the reformatting process in order to determine your internal priorities and specifications. The diagram below shows a very generic signal path with associated AV inputs and outputs. This diagram assumes that the original item has already been inspected and any necessary physical repair and stabilization has already been performed. This diagram also only speaks to AV and not to metadata.

A diagram of the digitization Signal Path
Credit: AVP

This diagram is extremely simplified in order to show the high-level, salient components of a digitization signal path. An original physical item is played back on a machine. This machine must be calibrated and aligned to the original item in order to achieve a faithful reproduction of the original recording. The signal processor inserted in this diagram could be any number of different devices depending on what the original item format is. For audio discs this may be a preamp and equalizer. For video formats this may be a time base corrector. For audio open reel this may be noise reduction decoders. Principally, signal processors in a preservation reformatting signal path are not about enhancing the signal. They are devices that are necessary in order to reproduce the original recording with integrity. If they were not present in the signal path, a faithful reproduction could not be achieved. In some cases, their absence may mean that no reproduction could even be achieved.

The output of the signal processor(s) is fed into the analog to digital converter (ADC), responsible for transforming the signal from its analog source to a digital approximation. The quality of the ADC is critically important in the ability to create a digital copy with high precision and integrity. There are video ADCs and audio ADCs. The digital stream of bits created by the ADC is routed to an encoder which takes this raw digital stream of data and converts, maps, and packages it into a file format or wrapper. When this is written to the hard drive as a file, the Preservation Master file is produced as a result. In order to create derivative files such as Mezzanine and Access Copy files, a transcoder is needed to decode the Preservation Master file, thereby creating a data stream that can then be used to convert, map, and package the AV into the appropriate specifications for the Mezzanine and Access Copy file formats.