Swap AC3 channels without reencoding?

[r0lZ]

I have a commercial DVD (Fly Me To The Moon, French version) with AC3 5.1 channels, and they have the left and right channels swapped. I would like to fix the problem without re-encoding the AC3 stream. Is it possible? All AC3 muxers/demuxers I have tried so far output to WAV, and then re-encode to AC3, and therefore introduce some quality loss. I suppose it is possible to output the AC3 raw channels, and then remux them in the right order.

Someone knows the right tool to use? Thanks in advance.

[Skelsgard]

I have a commercial DVD (Fly Me To The Moon, French version) with AC3 5.1 channels, and they have the left and right channels swapped.

Unlikely. If one original DVD has bad audio, all of the DVDs burned in the batch with it will have the same problem too.
It is more likely that your speakers are swaped or that your decoder has a matrix with swaped channels (let's say you're using AC3filter or ffdshow with custom matrixing for decoding).
And to answer your question, no. It's not possible to strip a 5.1 AC3 into its mono channels and reorganize them back into a 5.1 file.

What is telling you that the channels are swaped? Maybe some moving objects with the sound in an inverted panning, like say, a car passing by from left to right while the sound phases from right to left?

[r0lZ]

Yes, the voices of a character running to the right passes from the left speaker to the right one. The problem occurred with my DVD player connected to my hi-fi amp, so the decoder's matrix cannot be the culprit, as I know that the player works fine.

But you are right. I've recently cleaned some connectors, and I have reconnected two of them in the wrong order! :o

I haven't suspected that, because the TV (connected to the amp with another cable) has not that problem, and only the DVD player was wrong. I should have tried to feed the audio via the TV before posting!

Anyway, it's fixed now, and I have learned that the AC3 muxers and demuxers are actually encoders and decoders.

Thanks for your quick reply!

[Skelsgard]

Yes, they are, and you're welcome.

[Ghitulescu]

http://forum.doom9.org/showthread.php?t=144969

[r0lZ]

Thanks, Ghitulescu. But I've read rapidly that thread before posting here, and it deals with conversion. What I was searching is a tool to modify the internal order of the channels in an AC3 file to remap the channels without having to re-encode, not a tool to convert the AC3 file to another format and remap the channels on the fly. That's easy if you demux/decode to WAV first, rename the WAV files and then remux/re-encode them.

Anyway, I don't need to do it any more.