5.2.8 Calibration Discs

5.2.8.1 Calibrating an audio system involves applying a defined input and measuring the corresponding output over a range of frequencies. A pre- amplifier/equaliser may be calibrated by supplying the input with a constant signal of variable frequency while loaded with the correct impedance, and the measurement consists in plotting (or data-logging) the output against frequency. Automatic apparatus exists for this. In use the input comes from a pickup cartridge, a transducer that converts a mechanical input to electrical output, and for this we need a mechanical calibrating signal.When mechanical recordings were commercially available test discs were produced for this purpose. The Audio Engineering Society (AES), via its Standardisation Committee, runs an ongoing and active project of developing and publishing a series of simple test discs, both for coarse groove work and for microgroove. The AES 78 rpm Calibration Disc Set: ”Calibration Disc Set for 78 rpm Coarse-Groove Reproducers. AES Cat. No. AES -S001-064” is available from the AES website. http://www.aes.org/standards/data/x064-content.cfm

5.2.8.2 If the calibration by means of a test disc has been performed with sufficient resolution, the plotted curve may be regarded as a plot of the transfer function of the pickup or the pickup-preamplifier¡equalizer combination. Apart from the fact that visual inspection of the curve will tell the operator of gross deficiencies, it may actually form the basis of a digital filter that may filter the digitised signal from the mechanical record, so that it becomes independent of the actual pickup (and preamplifier and equaliser) used. All it takes is to be certain that no adjustment has been changed between using the test disc and the mechanical record to be transferred (and ideally that the record materials for those two inputs behave the same way). (For further discussion see Brock-Nannestad 2000).