CD emphasis
CD emphasis When the compact disc was first developed, the designers implemented a rather surprising element in its specification. This was a pre- emphasis, de-emphasis cycle. In brief, this permits the treble in the source signal to be boosted before the CD is mastered (pre-emphasis), recording this fact by a special bit in the package around each segment of audio data, and cut again by the CD player (de-emphasis). When cut, it also had the advantage of reducing any noise due to the recording medium. In practice, there is no such noise, so initially it can be hard to see the purpose of this. Then when you consider that for most music the amplitude of the signal reduces as the frequency increases at around 6dB per octave, you can see that the amplitude of the higher frequency components would be very low and, consequently, subject to increased quantization noise. Boosting the higher frequencies significantly reduces this noise accordingly. The frequency response of the signal is pre-emphasised by boosting the signal from 50μs (microseconds -- which is what engineers used to specify frequency in some contexts) or 3,183 hertz and levels out at 15μs, or 10,610 hertz, with a maximum boost of 10.45dB. CDs that actually use this are now very rare.
Source: http://www.astintrew.co.uk/support/The%20Astin%20Trew%20Dictionary%20of%20Hi-Fi%20Terms%202008.pdf
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