I have some old speeches dating back almost 100 years that I converted to mp3 using lame. Some of the files are in mono and some in stereo.

I can easily convert a wav file between mono/stereo by first decoding the mp3 into wav, which I would like to avoided.

I would like to know if there is a lossless way to convert a mp3 file from mono to stereo and vice-versa.

Thanks!

MP3 stereo -> MP3 mono conversion without reencoding is not possible.
MP3 mono -> MP3 stereo conversion without reencoding is possible from a technical point of view but I don't know of any tools that can do that. It seems a bit pointless...

AFAIK there's no software capable of doing it without re-encoding, but it's an interesting question:

If there's a mono file it should be possible (easy?) to create a m/s stereo file from it with m = data from mono file and s = 0
Reducing stereo to mono should work similar too:
If the source file is l/r stereo only, just throw away one channel
If the source file is m/s stereo only, throw away "s" information and keep "m" as mono.
With joint stereo files (mix between m/s and l/r there could be problems on transitions between stereo modes resulting in weird artifacts).
Oversimplified?

I always thought mono recordings were best encoded in mono. I transcribed a few recently that were a bit scratchy. Since it was recorded using a stereo cartridge, I noticed that with a stereo encoding the noise was distributed across the soundstage while the program material was in the center. WIth a mono encoding, the noise was concentrated in the center with the program material and was more bothersome. Maybe running through something like a pro-logic circuit then only encoding the center channel is the best solution (plus mixing the subwoofer channel back in if the program is music)

Is no one else wondering "Why?"

EDIT: "Why?" to the first post, not the one that just sneaked in above mine!

Though I've wanted this for when people encode mono tracks at 192kbps SS. Half the file is pure waste! For anything mono in JS, there can be almost nothing to gain.

Cheers,
David.

Thanks for all your responses, I found this (http://www.foobar2000.com/components.html > DSP-Standard DSP array) plug-in for foobar which claims to do the job.

Since I don't use foobar, can anyone who uses foobar let me know if the above plug-in can batch convert mp3's from sereo to mono and vice-versa and if the conversion is lossless?

Thanks again.

You can use fb2k to batch encode mono->stereo and vice versa but it'll be only lossless if you save the results as .wavs or losslessly compressed. Saving the results as mp3 will be transcoding = lossy. As said before, it might be possible theoretically, but it's pointless - so noone has bothered to write a program capable of doing it.