Hi, I'm using Lame 3.98 and when converting I always see the message:
"using polyphase lowpass filter, transition band 16538-17071"

I didn't find a lot of information on the net about this and what I did find was not in the context of mp3 but related to noise filtering in telephone communication.
So any information about what this is exactly and why it is used by lame by default would be much appreciated! To me it seems as if this message indicates that lame does alter the audio spectrum which I do NOT want. I want to keep my music as similar (quasi-identical) to the original (wave) as possible, wouldn't I ?

Thanks a lot for your help.

EDIT: Hm ... I just figured out the command line parameters that have been used during the conversion:
lame.exe -m j -q 0 -b 320 -cbr %SOURCE_FILE% %DESTINATION_FILE%

Generally speaking, MP3 produces audibly better quality when the very highest frequences are more or less filtered out. In the LAME encoder this filtering happens through a complex system that adjusts several things.

LAME uses the transition band you mentioned when it encodes 128 kbps CBR files (since you used an incorrect setting). For 320 kbps the default is 20094-20627 Hz.

Thanks for your support, Alex B. I understand what you mean. But on the other hand, the best quality is the original audio (wave) material, so wouldn't it be best to leave the frequencies unchanged (i.e. keep the original frequencies?)

Of course the original audio can be "improved" ... background noise reduction, removal of clicks and pops, EQ improvements for old tape audio, etc...
but isn't the goal of lossy audio format conversion to reproduce the original sound with as much fidelity as possible ?

Any lossy encoding will result in a reduction of quality. The issue is how much. In the case of encoding to mp3, trying to encode the highest frequencies, which are inaudible to most humans, causes a reduction of the quality in the audible frequency range, due to the limited amount of bits available for encoding. Applying a low pass filter actually improves the audible quality in most cases.

OK. I didn't think about this aspect of the problem. Thanks a lot for clarifying!