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Subject: output level equalisation
From: ten Velde family (tenvelde_at_xtra.co.nz)
Date: 2003-11-28


I've seen a number of posts here describing settings for a sound that's
close to the sound produced by a CD player.

I'm trying to use the Jukebox through a PA and found that it's rather thin
when all settings are on 0 or off, so decided to create a series of test
tones of various frequencies and put them on the Jukebox as mp3 files, so I
can adjust the settings to give me a reasonable level frequency response
using the level meter on the PA mixer.

I created a series of files of sine waves of one frequency each in wav
format, then bulk converted them using CDex without normalisation.

To check whether these tones were indeed all of the same level, I used the
analising function of MP3Gain. I thought that would be the best way since
I run all my files through this program to ensure a reasonably "evenness"
of levels.

The results of this are rather alarming: at 50Hz the level detected is 72.9
(below that it's even less, but I'm not too worried about those), the
levels then rise to 98.8 at 3840Hz and drop back down again to 62.0 at
15kHz. The mp3s are encoded at 192kbps to ensure a reasonably flat
response at the higher frequencies.

When I load these three files into Goldwave, the levels are:
50Hz maximum 0.2500 RMS 0.1767
3840Hz maximum 0.2510 RMS 0.1767
15kHz maximum 0.2722 RMS 0.1767
which would indicate no significant variation in levels.

The question is: should I use the files as they are and assume that
MP3Gain would distort the real levels, or should I normalise using MP3Gain
to get a true evenness of levels? Also, since I have processed all the
music files through MP3Gain, and if that has "adjusted" the frequency
response of these files, would I be better off to process my test tones the
same way?

Only one can be right, surely - and a level variation of some 40dB over the
audio spectrum is rather too large to ignore.

----------
John ten Velde



Page was last modified "Jan 10 2012" The Rockbox Crew
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