5.3.1 Introduction
5.3.1.1 Long Play (LP) microgroove1 records first made their appearance around 1948, pressed in flexible vinyl2 and hailed as ‘unbreakable’ in comparison to the preceding commercial records pressed from a rigid (and easily broken) shellac base.
5.3.1.2 By the time the vinyl disc was developed there was a greater industry agreement on standards. Grooves were cut at 300-400 to the inch as opposed to the 100 or so grooves per inch that was characteristic of the shellac pressings, and with a standard sized and shaped stylus on a cutting lathe that revolved at a speed of 33 1/3 rpm. 7” vinyl records, both singles and ‘Extended Play’ (EP), were made to be replayed at 45 rpm and in some cases 33 1/3 rpm. Larger diameter discs were on rare occasions produced for replay at 16 2/3 rpm for speech, where up to 60 mins could be recorded on one side. Equalisation characteristics still varied between companies, (see Table 2 Section 5.3 Equalisation Chart for Pre-1955 LP Records) however, many preamps catered for these variations. Eventually agreement was reached and the RIAA (Record Industry Association of America) curve became standardised throughout the industry.
5.3.1.3 Stereo records were commercially available from around 1958, and initially many records were produced in both mono and stereo versions. The groove walls are at right angles to each other and inclined by 45º to the vertical. The inner wall of the groove contains the left channel information, and the outer groove the right channel information recorded perpendicular to the respective groove wall. This has remained the standard, although at the time of its introduction a small number of stereo discs were made with a combination of lateral and vertical technology, an approach that was soon discontinued. Stereo pick-ups may be used to play mono records, but playing a stereo record with a mono pick-up will cause severe groove damage.
1 As some late generation coarse groove recording were pressed in vinyl the use of the term “microgroove” is preferred to using “vinyl” as a collective description.
2 “Vinyl” is a colloquial term for the material of the discs which basically consists of a polyvinyl chloride / polyvinyl acetate co-polymer (PVC/PVA)
