7. Optimal signal retrieval from original carriers

Optimal retrieval of an analogue signal, where replay distortions are kept to an absolute minimum, can only be achieved by modern, well maintained replay equipment, ideally of the latest generation. When replaying historical formats, replay parameters (such as speed, playback equalisation, track format, type of time base stabilisation, etc.) must be chosen objectively, and be based upon knowledge of the given historical format.

Certain adjustments to replay equipment may be necessary, in order to align with original recording characteristics and to optimize the retrieval of the recorded signal. For example, azimuth error is common in analogue magnetic tape recordings, and can only be corrected during the replay of the original carrier at the time of digitisation. Similarly, storage-related print-through must be minimised at the point of signal extraction. Other minute inaccuracies in the tape path adjustment of original recordings can also cause a considerable and avoidable rise in errors.

For video, certain types of dropouts are best compensated for at transfer time. Where motion picture film is being copied, some scratches can best be eliminated or suppressed by the use of liquid bath in the film printer when the transfer takes place. In a digital scanning transfer, the use of specialised diffuse light sources can have the same effect.

In order to minimise possible damage to the original carriers, replay equipment must be regularly maintained to professional standards. To aid in this and to diagnose emerging problems, calibration media suitable for the replay equipment must be used whenever obtainable.

For digital carrier-based formats, different players or readers may retrieve data from the same carrier in varying ways, not all of which will successfully present the bitstream for transfer. In order to evaluate and detect such problems, error monitoring during realtime replay, or error reporting after high-speed ripping, is imperative. The presence of uncorrectable errors copied to resultant files for preservation must be documented.

Digital carrier-based formats may contain various types of sub-code information, that is, secondary information written in parallel with the primary information bitstream. Incompatibilities between recording and replay devices can result in this information being retrieved incorrectly or not at all. Understanding the properties of a given format or collection, including any sub-code information, and defining the minimum required combination of primary and secondary information prior to its digitisation, is of utmost importance (see section 2).

It is not always an easy task to assess the correct replay parameters for a given analogue audiovisual document if objective information on the recording format parameters is missing. As in other fields of historical research, the use of cautiously chosen approximations is permissible when necessary. As a matter of principle, however, all such decisions must be documented, and irreversible steps should be avoided. All unnecessary subjective treatments must only be applied to access copies.


Inadequate signal retrieval from original documents is very often the result of a lack of professional knowledge, or the use of inappropriate equipment. It is hard to overstate the importance of operator skill and experience, as well as the availability of specialised equipment, when reformatting challenging materials. Optical sound tracks for film-based motion pictures, for example, can be very challenging to transfer, and the role of highly specialised equipment can be crucial.

In some circumstances it may be appropriate to take a multi-layered approach to choosing replay parameters. This might involve digitisation and creation of master preservation files without playback equalisation, and applying equalisation either in the creation of access files, or as a software process at the time of access.

For example where one-light transfers from film are deemed appropriate, the RGB output should be adjusted to get the maximum colour information from each channel, to correct for colour fading without introducing any clipping. Best practices for the transfer of motion picture film for preservation are still in development, with some cutting edge work advancing under the auspices of the Academy of Motion Picture Arts and Sciences (AMPAS) and the Society of Motion Picture and Television Engineers (SMPTE). These new developments will help standardise approaches that will have a special impact on the capture of colour and the representation of tonal variation in the original film. However, systems that implement these new developments are not yet widely available and the approach is not yet employed in memory institutions.

The systematic retrieval of sub-code information from digital carrier-based formats, as a method of safeguarding useful secondary information, is still a widely neglected subject. This is largely due to incompatibilities between the sub-code formats of different players and interfaces. As yet, few if any standards have been widely adopted for the further retention of this information in file-based formats. Compatibility problems can also often be encountered in the replay of recordable or rewritable optical disks.

The principles described in this section apply unambiguously where primary information exists in the form of documentary records, whether documenting artistic performance or other forms of actuality. Where the primary information exists as part of an art object however, for example where sculpture or installation art has an audiovisual component, there may be an additional ethical requirement to preserve original reproduction distortions, and therefore diverge from these principles, in order to honour the intentions of the artist. Determining the intentions of the original creator(s) may be necessary in choosing how such art objects may best be represented in a file-based environment.