21 Apr
2010
21 Apr
'10
4:51 a.m.
Hi,
On Wednesday 21 April 2010 03:34:38 Ken Restivo wrote:
Kind of a general mastering question, but obligatory
Linux screenshots of
JAPA are included, I promise.
I've noticed with some professional cd's/oggs/mp3s I have, the high end is
rolled off at around 20Khz.
The question is: why do they roll off like that, and is there some reason I
should do it in this day and age?
Ogg and mp3 roll off before the nyquist-frequency because of their algorithm.
So I wouldn't take that as a reference.
CD's actually don't cut off sharp at the nyquist-frequency of half the
sampling-rate.
The highest frequency possible to reproduce -with correct amplitude- is half
the sampling-rate _only_ if the phase is aligned to the sampling-clock so that
minima/maxima of the sinus are correctly sampled. If its out of phase, the
amplitude is not reproduced correctly.
It is easy to understand that this correlation between phase and correct
amplitude also affects frequencies below half the sampling-rate. Might be as
low as quarter of the sampling-rate, which in case of the CD is 11kHz. Below
that you will have more then four samples to reproduce the sinus wave.
That is in fact another reason to do the recording, mixing and mastering in
more then 44kHz...
Your own music is probably done in 48kHz sampling-rate which moves this effect
upward (into the region japa is not showing you). Down-sampling to 44kHz will
introduce the roll-off in the range you see.
Have fun,
Arnold