The AES coarse-groove calibration discs are a boxed set of two identical discs, one for routine use, one for master reference. The intent is to characterize the reproduction chain for the mass transfer of coarse-groove records to digital media, much like using a photographic calibration reference in image work. Libraries and archivesaround the world have collections of many thousands of coarse-groove mechanical audio recordings, phonograph or gramophone records, largely 78s or 78 revolutions per minute discs. This is a substantial recorded heritage of mankind's music and spoken word made over a period of 65 years. The 78 rpm disc was largely out of production by 1960. These mechanical recordings won't be available indefinitely since the plastics used in their manufacture are deteriorating slowly but steadily. Preservation programs have been underway by a number of organizations. Decreasing costs of digital storage media now make it possible to consider all mechanical audio recordings for transfer to the digital domain. Thus a widespread need was recognized by the Audio Engineering Society to provide a calibration tool for standard transfer of mechanical coarse-groove audio recordings from the analog to the digital domain.
According to Ted Kendall, maker of the Front Endaudio restoration unit also known as "The Mousetrap", the equalization time constants for post-1955 78s used in the Front End are 3180/450/50 ms. These time constants are identical to those used in the AES Coarse-groove Calibration Discs. Since the 78 rpm record would be obsolete by 1960, this means that there is a very large population of pre-1955 78s requiring different equalization settings depending on the vintage and label of the disc. So, the dilemma is this: should coarse-groove recordings be transferred in mass to digital using an arbitrary phonoequalization curve such as with the AES calibration discs, or should each recording be matched to the curve appropriate to its vintage and label, then transferred to digital media?
Use of RIAA Equalization
Because the RIAA equalization standard has been in use internationally for phonograph records since 1953 and is based on recording practices used for many years by RCA Victor, a dominant record producer, the electronics needed for this purpose are as readily available as record players are. For vintage recordings the Esoteric Sound Re-Equalizer can readily be connected as a standard item to the record playback equipment. The Re-Equalizer is used to modify RIAA. Then, depending on the vintage and label of the 78 rpm record, the appropriate equalizer bass turnover and treble rolloff settings can be easily looked up in a reference guide.
Other Equipment
Another approach to obtaining the rightphonograph record equalization settings for transferring vintage recordings to digital media is to use the Chronologic Equalizer in the Souvenir Vintage Sound Processor – MK-2 made by K-A-B Electronics.
