RIAA Equalization

Also referred to as RIAA curve or RIAA de-emphasis curve. These all refer to a standard initially proposed by the RIAA for vinyl phonograph record mastering and playback. Due to some of the physical limitations of phonograph records and the playback systems available back when they were developed, the curve was set up to allow the record pressing plant (and mastering facility) to “pre-emphasize” higher frequencies, which evened out the size of the grooves making high quality records much easier to manufacture. The curve acts as a sort of equalizer, attenuates low frequencies and amplifies high frequencies (relative to a 1 kHz reference point) in order to achieve the maximum dynamic range for a lateral cut vinyl disc. The grooves in a stereo phonograph disc are cut by a chisel shaped cutting stylus driven by two vibrating systems arranged at right angles to each other. The cutting stylus vibrates mechanically from side to side in accordance with the signal impressed on the cutter. The resultant movement of the groove back and forth about its center is known as groove modulation. The amplitude of this modulation cannot exceed a fixed amount or “cutover” occurs. Cutover, or overmodulation, describes the breaking through the wall of one groove into the wall of the previous groove. Since low frequencies cause wide undulations in the groove, they must be attenuated to prevent overmodulation. At the other end of the audio spectrum, high frequencies must be amplified to overcome the granular nature of the disc surface acting as a noise generator, thus improving signal-to-noise ratio.