RIAA equalization
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RIAA equalization is a specification for the correct playback of vinyl records, established by the Recording Industry Association of America. The purpose of the equalization is to permit greater playback times, improve sound quality, and to limit the physical extremes that would otherwise arise from recording analogue records without such equalization.
The RIAA equalization curve has operated as a de facto global industry standard for the recording and playback of vinyl records since 1954. Prior to that time - mainly between 1940 and 1954 - each record company applied its own equalization; there were over 100 combinations of turnover and rolloff frequencies in use, the main ones being AES, LP, NAB and FFRR.
Before 1940, most records were cut flat, with a low-pass turnover of 6 dB/octave below 300 Hz to 800 Hz. This included broadcast recordings (transcriptions) and motion picture recordings before sound-on-film. If you play a pre-WWII 78rpm record through a modern preamp, you will effectively be playing it with a scratch filter whose cutoff begins at 2200 Hz, giving lack of high frequencies and muffled voices.
RIAA equalization is a form of preemphasis on recording, and deemphasis on playback. A record is cut with the low frequencies reduced and the high frequencies boosted, and on playback the opposite occurs. The result is a flat frequency response, but with noise such as hiss and clicks arising from the surface of the medium itself much attenuated. The other main benefit of the system is that low frequencies, which would otherwise cause the cutter to make large excursions when cutting a groove, are much reduced, so grooves are smaller and more can be fitted in a given surface area, yielding longer playback times. This also has the benefit of eliminating physical stresses on the playback stylus which might otherwise be hard to cope with, or cause unpleasant distortion.
A drawback of the system is that rumble from the playback turntable's drive mechanism is greatly amplified, which means that players have to be carefully designed to avoid this.
RIAA equalization is not a simple low-pass filter. It carefully defines transition points in three places - 75 µs, 318 µs and 3180 µs, which correspond to 2122 Hz, 500 Hz and 50 Hz. Implementing this characteristic is not especially difficult, but more involved than a simple linear amplifier. The phono input of most hi-fi amplifiers have this characteristic built-in, though it is omitted in many modern designs due to the gradual obsolescence of vinyl records. A solution in this case is to buy a special preamplifier which will adapt a magnetic cartridge to a standard line-level input, and implement the RIAA equalization curve separately. Some modern turntables feature built-in preamplification to the RIAA standard. Special preamps are also available for the various equalization curves used on pre-1954 records.
