| The idea of preemphasis is based on three things:
The first is the ears relative sensitivity to low vs high frequencies
(you must have seen the so called Fletcher Munsen curves at some time).
At low levels its quite easy to perceive a low level of hiss whereas
rather large amounts of hum or rumble may be present and not be
noticible.
The second is the fact that quantization noise of a properly dithered
digital recording is nearly white noise, taking into account the
inverse sin(x)/x filtering in the player.
The third is the crucial fact that statistically the extreme high
frequencies are not as intense (on average) as the midrange and low end.
So the idea is to boost the highs a bit during recording (since on the
average we can get away with it because there is more headroom there
to use), and then restore the balance during playback, thereby cutting
the perceived dither noise.
The CD makers had the good sense to make the preemphasis optional in
the recording, since there may be some cases where the tradeoff of
reduced high end noise against reduction of high end headroom would
be bad.
The preemphasis of CD is much milder than the 75 uS curve used on phono
and FM broadcasting, its more like 15 uS or so, and is not nearly as
drastic.
- Jim
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