GeezerVison

Half-speed mastering is a technique occasionally used when cutting the acetate lacquers from which phonograph records are produced. The cutting machine platter is run at half of the usual speed (16 2⁄3 rpm for 33 1⁄3 rpm records) while the signal to be recorded is fed to the cutting head at half of its regular playback speed.^[1]

The reasons for using this technique vary, but it is generally used for improving the high-frequency response of the finished record.^[2] By halving the speed during cutting, very high frequencies that are difficult to cut become much easier to cut since they are now mid-range frequencies.^[3]

Mobile Fidelity Sound Lab used half-speed mastering for its Original Master Recording LPs.