The Enhanced RIAA Curve
The official RIAA standard defines three time-constants with pre-emphasis rising indefinitely above 75 µs, but in practice this is not possible. When the RIAA equalization standard was written the inherent bandwidth limitations of the recording equipment and cutting lathe imposed their own ultimate upper limit on the pre-emphasis characteristic, so no official upper limit was included in the RIAA definition.
Modern systems have far wider potential bandwidth. An essential feature of all cutting lathes—including the Neumann cutting lathes—is a forcibly imposed high frequency roll-off above the audio band (>20 kHz). This implies two or more additional time constants to those defined by the RIAA curve. This is not standardized anywhere, but set by the maker of the cutting lathe and associated electronics.
The so-called "Enhanced RIAA" curve or "eRIAA" curve attempts to provide complementary correction for these unofficial time constants upon playback.
Read more about this topic: RIAA Equalization
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