Calibration Disc Set for 78 rpm Coarse-Groove Reproducers - A Rationale
In proposing the manufacture of the AES-S001-064 calibration disc set, the following rationale was presented:
A necessary working tool
These calibration discs are intended to be used to verify and characterise the performance of less-than-perfect mechanical/analog transfer chains and as a reference for further treatment of the audio in the digital domain. These discs are not intended primarily to confirm the performance of 'perfect' reproduction chains, although this will also be valuable.
These calibration discs will enable transfers to be made to a known characteristic, so that the resulting digital signal may be corrected to any desired characteristic based on the associated reference signals. The calibration records open the door to consistent and accurate mass digitization with non-specialist operators and a wide range of equipment.
With common, agreed-upon reference standards, it also becomes possible for individuals and institutions to exchange digitized audio-preservation files successfully. Calibration also provides a firm foundation for audio-related metadata to accompany these recordings.
Vanishing opportunity
While most music recordings after the mid-1960s are supported by studio masters on magnetic tape, everything prior to that time is available only in the form of mechanical recordings, most of them coarse-groove records. Libraries and archives around the world have collections of tens of thousands of such audio recordings.
These mechanical recordings will not be available indefinitely. The plastics used in their manufacture are deteriorating slowly but steadily.
The decreasing cost of digital storage means that the time is close when all such mechanical media must be considered for transfer, possibly as a deliberate "last time" transfer into the digital domain. During this transition period, transfers will need to be done rapidly, economically and predictably.
Transfer work can be time consuming and expensive and it is unlikely that records will be considered for re-transfer simply to obtain marginal improvements. Without dependable calibration, the quality of this single digital signal will be ambiguous, and uncorrectable objectively - for differences in recording characteristic, for example. The opportunity to preserve the information in those old grooves will have been lost.
Vanishing expertise
Up to the present day, most transfers of gramophone records were made without calibration. Typically, each disc would be replayed a number of times in order to satisfy local subjective criteria using some unspecified monitoring chain. This obviously exposes the mechanical recording to the risk of excessive wear. The need is for objectively-characterised transfers, with the minimum of replay passes.
There is an informative parallel with photographing artwork. It is conventional in this work to photograph calibration references such as a color chart and grey-scale to characterise the camera and the lighting conditions used in the session. Using information from these reference images, it becomes possible to recreate the original artwork more faithfully.
The frequency characteristic of archived records can vary significantly. Throughout the early decades, there was no single recording characteristic or speed for these coarse-groove discs. The issue extends further than just the few octaves captured by the first recordings, the high frequency content of later 78 rpm discs often exceeds that of the early 33 rpm microgroove records so the transfer chain will need to be calibrated over an extended frequency range.
Increasingly, large-volume transfers will be handled by technicians more accustomed to working with digital audio and computer technology, or by volunteers. These people cannot be expected to master all the knowledge and skills available in the heyday of the mechanical disc. However, using these reference discs to characterise the reproduction chain substantially reduces the need to do so and can reduce the wear on the record to a single replay pass.
Private collectors
In addition to the formally-organised collections and archives, there are a large number of private collectors throughout the world. Private collectors have a role to play in that they may sometimes hold the only known surviving copy of a particularly significant record. The more caring collectors, being aware of the fragility and value of the recorded legacy in their possession, will use more modern reproducers. With a suitable calibration reference, they will be able to contribute to conservation efforts on an equal basis.
Vanishing manufacturing resource
At present (2007), the commercial expertise exists to produce these discs without an investment in development of lost technologies. This may not be available indefinitely. For example, if the fashion for vinyl records in dance clubs declines, the manufacturing capacity for records is unlikely to survive for long.
Prospective users
Users of this calibration disc will include collections and archives, both public and private, and the transfer operations that support them.
The International Association of Sound and Audiovisual Archives (IASA) coordinates the activities of many sound archives internationally. IASA supports this AES calibration disc project. As Kurt Degeller, IASA President notes:
"Our member institutions, as well as private members, hold vast amounts of these records. [...] To ensure correct and authentic reproduction of legacy material, calibration discs are one of the necessities. The IASA, through its Technical Committee, urges the use of calibrating material in any transfer project, and the lack of proper calibrating discs for coarse-grooved media is still an obstacle. "
