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Handling high frequencies, Please help pushing my tiny MP3 know-how
post May 16 2003, 04:03
Post #1

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Hi! - I had thought... But first tell me please:

1.) --ns-sfb21 # : Using a higher the value # will DEcrease all the high frequencies of the scale factor band 21
(above 16 kHz) in my MP3, right ?

2.) The "polyphase lowpass filter transition band : 19383-19916 Hz" will use a 'dynamical' lowpass filter border
(here between 19,4 and 19.9 kHz) depending on any other encoding restriction (eg. space / filesize), ok ?

Now, would create a Lame setting with ...
case A) "-- lowpass 19.3 --lowpass-width 0.5 --ns-sfb 2.5" (=> polyph. LP filter trans. band: 18.7-19.9kHz)
... equvalent results like...
case B) "-- lowpass 19.6 --lowpass-width 0.3 --ns-sfb 3" (=> polyph. LP filter trans. band: 19.4-19.9kHz)
... ???

Well, You see surley, case B) is very similar to the consequences of "--alt-preset-extreme (-Z)" setting.

But this should be no general critism on the ape setting. I wonder simply - guessed 1) and 2) are true -
why reducing ALL high frequencies by using a quite high lowpass filter [=>B)] insteed of a 'more untouched'
scalefactor band 21 in combination with a more dynamic polyph. LP filter trans. band [=>A)] ...?
...specially respecting the fact than people can here a sound at 17 kHz rather than one at 19.5 kHz.

I hope anyone could check this out - I would be very interessed on his opinion.

Bye, 384kbps
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post May 16 2003, 21:08
Post #2

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From: Germany
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I can only comment on the "polyphase lowpass filter". It is not dynamic. It means that frequencies below 19383 Hz will remain untouched while there shouldn't be any frequencies left above 19916 Hz. Think of it as a fade out. It is difficult to design "brick wall filters" that have a very narrow transition band since often they will add audible filter ringing. Allowing a soft fade out reduces the ringing.
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