"ABTSound"

Analogue audio recording began around 1877 when Thomas Edison first invented the wax cylinder phonograph recorder followed, a little later, by the disc gramophone playing accoustically recorded flat shellac discs. The discs revolved at speeds around 60 - 130 rpm and were recorded and replayed using a steel needle attached to a diaphragm and amplified through an accoustic horn. Cylinders and discs competed with each other until the 1910s when the disc finally won the commercial battle. During this period Edison's original 'hill and dale'recording technique, in which the audio modulation was recorded by varying the depth of the groove, gave way to the process of "lateral" recording, where the groove was of constant depth but modulated by lateral movement of the needle in the cutting lathe. The invention of electrical transducers and electronic amplifiers enabled the first electrically recorded discs to be marketed in 1925 and although, for many years, the various recording companies continued to operate their own in-house standards, the recording speed was eventually standardised at 78rpm.

Much attention was directed towards improving the sound quality of the recordings. For a disc cutting lathe revolving at constant velocity, the deviation of the groove increases as the frequency decreases. It was therefore found necessary to limit the amplitude of the low frequency deviations in order to avoid interaction with adjacent grooves whilst attempting to maximise the duration of the recording. It was also found helpful to increase the amplitude at the higher frequencies in order to combat the "surface noise" of the disc. These adjustments were achieved by modifying the frequency response of the amplifier driving the cutting head, by boosting the higher and attenuating the lower frequencies. Then the replay amplifier also needed to have its response modified to the reverse of the recording characteristic, in order to reproduce the orginal sound as closely as possible

Initially, and indeed for most of the commercial life of 78rpm records, up to the 1950s the details of these response curves continued to be set by the individual companies and even became a source of competition between them to produce the best sounding recordings. Decca's full frequency range recordings "ffrr" were an example of this. However, after the introduction, towards the end of the 1940s, of the much smoother and quieter vinyl to replace shellac as the base material for the discs and the launch of 33⅓rpm long-playing records (LPs) and 45rpm 7inch singles, the recording companies, in the early 1950s, eventually managed to reach agreement on an industry wide recording characteristic which is still universally known by the initials of the trade group which published it. RIAA - The Recording Industry Association of America. The RIAA reproduction curve became and has remained the standard response curve used in record player preamplifiers. It is found, in practical form, lurking behind the "phono" input on most of the hifi audio amplifiers produced in the latter half of the 20th century and indeed it even survives in the 21st century "USB record players" sold for the digital transfer of vinyl records.

It should be noted that this curve was quite a lot more extreme in both its high and low frequency deviations, than many of the earlier response curves used for 78 rpm records, which accounts for the rather dull and "bass heavy" sound associated with 78rpm records played on "modern" record players.

For the very best results when digitising old 78rpm recordings, it is therefore advisable not to use an RIAA pre-amp but to use one with a variable response which can be set as closely as possible to match the original company's recording characteristic. Details are available, on the internet, of the curves used by the major companies in the US, Britain and Europe. Rod Elliott of 'Elliott Sound Products', in Australia, has an excellent website for DIY audio enthusiasts and his "project 91" page gives a particularly helpful list of response curves, as well as details for building a variable response pre-amp.