[Lame-dev] [ lame-Feature Requests-737998 ] Highpass problems?

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Feature Requests item #737998, was opened at 2003-05-15 01:54
Message generated for change (Settings changed) made by bouvigne
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Category: None
Group: None
>Status: Closed
Priority: 5
Submitted By: Phil Glau (filmotheklown)
Assigned to: Nobody/Anonymous (nobody)
Summary: Highpass problems?

Initial Comment:
when using --highpass, I get kind of crummy results. 

First of all, most people are probably going to want to high pass from 
0 to 150 hertz (not kilohertz). This is the normal range of 'rumble' 
that can be distracting on field recordings (i.e. lectures, interviews, 
etc.) the documentation say to enter a Khz number not a hz number 
(i.e. 150 hertz = .150 Khz ???)

when I 'lame -b 40 -m m --resample 22.05 -S --highpass .150', I get 

Warning: highpass filter disabled.  highpass frequency too small
LAME version 3.92  (http://www.mp3dev.org/)
Using polyphase lowpass  filter, transition band:  9336 Hz -  9602 Hz
...

I also tried 

'lame -b 40 -m m --resample 22.05 -S --highpass 300'

which seems to work somewhat (converts to hertz automatically ??)

Using polyphase highpass filter, transition band:     0 Hz -   267 Hz
Using polyphase lowpass  filter, transition band:  9336 Hz -  9602 Hz

But this makes it sound tinny as the frequency is too high. 

using

'lame -b 40 -m m --resample 22.05 -S --highpass 150'

also give the:

Warning: highpass filter disabled.  highpass frequency too small

Since there's no documentation on highpass ;) I'm not sure what's 
going on here.

It seems to me the only use of a high pass would be if it could work 
on any frequencys from 0 to 1000 hertz (0 to 1Khz). 

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Comment By: Takehiro TOMINAGA (takehiro)
Date: 2003-05-30 15:30

Message:
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This is LAME's spec. LAME uses subband filter for 
lowpass/highpass. This subband filter is defined in the MP3 
spec. Because it has only 32 bands(whose width is about 
340Hz when the sampling rate is 22kHz), we cannot use "fine-
grained" frequency setting.

We need another algorithm to implement such fine highpass 
filtering and it is the almost the last item of TODO list. Till 
then, use another "FINE" filtering software before encoding.


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