Lossless mp3 conversion mono -> stereo?

[DRP]

Is there any tool that can losslessly convert a mono mp3 into stereo? Basically just duplicate the one channel and double the bitrate to accomodate it?

I know it sounds pointless, but reason for wanting to do so is because I think this is why AutoGK is rejecting the audio channel as being unsupported.

[El Heggunte]

Is there any tool that can losslessly convert a mono mp3 into stereo? Basically just duplicate the one channel and double the bitrate to accomodate it?

Quite unlikely, AFAIK.

I know it sounds pointless, but reason for wanting to do so is because I think this is why AutoGK is rejecting the audio channel as being unsupported.

What about using AutoGK for encoding only the video part, and then let AVI-Mux GUI (try to) mux the mono MP3 stream

[jman98]

I can't promise this can be done losslessly, but Audacity should be able to do it. Honestly, you probably won't be able to tell the difference anyway even if it's not lossless. You can always use another tool to mux your mono MP3 as El Heggunte suggested. Virtual Dub can do this.

[DRP]

Good suggestion, thanks. I shall try this.

[hello_hello]

AutoGK can be fussy about the MP3 header but it should be fine with mono MP3. When I say fussy..... chances are there's actually something wrong with the MP3 header. I've demuxed "problem" MP3 audio and opened it using foobar2000 before, and while it's supposed to have (for example) a 25 minute duration, foobar2000 might display it's duration as being over two hours.... until I've fixed it.
Foobar2000 has a function via the right click menu for rebuilding MP3 streams and fixing MP3 headers, but anyway.....

When you open the AVI using AutoGK (I assume you're opening an AVI?) if AutoGK displays the audio as CBR MP3 or VBR MP3 you should be fine. If it displays it as (something like) MPEG1/3 then chances are there's a problem with it.

Try demuxing the existing audio using VirtualDubMod. Open the demuxed MP3 file using MP3DirectCut (or if you have foobar2000 you can use it to repair the MP3). With the MP3 loaded into MP3DirectCut, use CTRL+A to select the entire audio, then use "File/Save Selection" to resave the audio as a new file. The process is lossless. With any luck the new version will have an MP3 header AutoGK likes. Replace the original audio with the new version, then try opening the AVI using AutoGK again. If my assumptions regarding what's wrong are correct, AutoGK will hopefully then see it as CBR MP3 or VBR MP3 and you won't get an unsupported audio error.

[AlanHK]

I was about to recommend mp3DirectCut too.

The simplest way is open it and then "File/Save complete audio". It will overwrite the original MP3, and clean up any format or header problems, without reencoding the audio.

[DRP]

If it displays it as (something like) MPEG1/3 then chances are there's a problem with it.

That is precisely the problem I'm seeing. Thanks for the answer.

[El Heggunte]

......
I've demuxed "problem" MP3 audio and opened it using foobar2000 before, and while it's supposed to have (for example) a 25 minute duration, foobar2000 might display it's duration as being over two hours.... until I've fixed it.

Thanks for the useful info

Just out of curiosity, do you know what encoders were stupid enough to generate such pathological files?

[hello_hello]

I think it's due to the VBR header becoming corrupted, and/or it's seen as constant bitrate, or something along those lines. It possibly happens every now and then when muxing. I'm not sure. I really don't understand fully how it all works. I suspect, but don't really know for sure, there's more than one way for a program to identify an MP3's type and duration etc.

As an experiment I just opened a VBR MP3 (CD track) with MP3DirectCut and elected not to save the VBR/Info header when resaving it. It's original duration was 4:49. Foobar2000 showed the new version as having a 2:46 duration (CBR) but played it correctly, except the seek bar reached it's end at 2:46. I opened a random AVI and added the new MP3 to it. VirtualDubMod still had a whine about it being VBR and asked if it could re-write the header, but when I said "no" it loaded it while displaying the correct duration. Foobar2000 went back to displaying the correct duration after I used it's function to repair fix the VBR header.

No matter what I did though, AutoGK didn't seem to mind. It'd display the MP3 audio as being either CBR or VBR (depending whether the header was correct) but I couldn't get it to see it as MPEG1/3 no matter what I did. So while I thought a bit of experimenting might reveal why AutoGK has problems with some MP3 audio, it wasn't to be. I only know how to fix it.....

Given the above, I suspect the VBR/Duration info must get written to a "header" but also saved somewhere else in the stream, as VirtualDubMod still managed to identify it correctly as VBR MP3 even with the VBR header removed. I'd take a guess and say wherever else the info is stored, it can become corrupted for some reason, but I'm still none the wiser as to how it works.