Sound Engineer, SABC Radio Archives
Marius Oosthuizen is responsible for the sound restoration of these old acetate discs in the Radio Archives.
Since the 1930s, most blank acetate discs have been manufactured with a base, usually aluminum ,glass was used during the war years and cardboard for inexpensive home recordings), that was coated with nitrocellulose lacquer plasticized with castor oil. Because of the lacquer's inherent properties, acetate discs are the least stable type of sound recording.
Acetates are records, usually recorded at 78 RPM, usually 10 inches in size recorders, which were on the market during the 1940's. They have an aluminum metal base, coated with black lacquer, which the recording stylus etches (cuts) the groove into while recording. Most recorders had a constant-pitch feed screw which moved the arm containing the recording-stylus across the record at a constant rate.
Acetate records for recording have blank labels, which are there to mark the title, artist (or "recorder"), date, speed, and whether the disc plays "outside in" or "inside out". "Outside In" means you put the needle on the outside like most records (and the needle works its way to the center while it plays -- the groove moves the needle along). "Inside Out", or "Center-Start", means you must put the needle on the innermost groove, and the groove will push the needle toward the outside while it plays. "Inside Out" records are quite rare.
Transcription discs recorded by radio stations, however, particularly the 16-inch variety, usually have the second side recorded inside out; it is so the equalization changes are less noticeable....equalization (that is, treble and bass) changes, particularly with diminishing treble response, as the needle makes its way toward the center, and was particularly noticeable on these early records (but not noticeable to the human ear on modern stereo LPs).