A standard equalization filter
that is applied to all vinyl recordings
. It's primary purpose is to reduce the amplitude
of low frequencies
on the groove
that would make the stylus wobble
so much that record
, and the walls between grooves be too thin.
Also, it helps the signal to noise ratio by boosting the high frequencies to drown the hiss noises (much like Dolby B/C tape noise reduction).
The equalization curve is somewhat straight on a logarithmic frequency/decibel scale. It is centered at 0dB at 1kHz, the dampening starts below that and goes to -13dB at 100Hz. The boosting goes up from 1kHz to +14dB at 10kHz.
An inverted curve (boost low, cut high) is used on playback. 'phono in' connections on amplifiers and mixers are already setup correctly.
This is good to know if you want to hook-up a turntable to a sound card's audio inputs. The resulting digital audio file has to be corrected before encoding to MP3. If you don't have a phono amplifier, wave editor programs like Cool Edit Pro or Audacity (Effect-Equalization-RIAA) can be used for that purpose.