MP3: Copy a channel without re-encoding

[kotyczka]

Hello!

Suppose I have a MP3 file which has in the left channel
a good signal and in the right channel a lot of errors.
(Which may happen.)
I would like to discard the right channel and replace it
by the left.
Is there any software allowing this without re-encoding?

Alternative: Generate a mono MP3 file from the stereo MP3 file
which contains the left channel only. Also without Re-encoding.

TIA

[roozhou]

Possible only when the mp3 is encoded in dual-channel mode while most stereo mp3s are encoded in joint-stereo mode, which means you can't.