Editing MPEG1 audio in place

November 13, 2007 11:23 PM

How do I edit audio in a MPEG1 without going through a video re-encode?

I assembled a 6-min film for a friend in Premiere 7 and outputted to MPEG1. Now, he needs to mute 3 seconds of audio but I don't have the original raw DV footage as it took up much needed space. Can I work on the MPEG1 without any video degradation? What tools can I use? Just in case, what about the same situation for MPEG-2 and MPEG-4 Part 2?

posted by Gyan to technology (7 comments total)

Sound Forge (now made by Sony) lets you open MPEG 1 and 2 files (with a preview window of video) and lets you edit just the audio portion. I assume since it doesn't have any video editing functions it doesn't re-render.

caveat: I've only ever used older versions of this software, and I've never done this specific task.
posted by tremspeed at 11:29 PM on November 13, 2007

If you're using Windows, you should be able to use virtualdub to select any portion of the mpeg1 and export it as an uncompressed file.
posted by dhammond at 11:33 PM on November 13, 2007

I'm about 99% sure that pulling in the MPEG to Premiere will show you an audio track and a video track - just edit the audio as you would and when exporting, select "None" (or as close to 100% as possible) for compression.

The other option would be to use Quicktime Pro. I don't have a codec for MPEG-1, so the end part about exporting might be a little tricky, but again - no compression and turning off "allow transcoding" ought to do it.

If you have Quicktime Pro, you can do it this way :

1. Open the movie with QT
2. Under "Window", select "Show Movie Properties"
3. Select the audio track, click "extract" at the top of the window
4. Export the audio to an .aif or similar file to work with.
5. Open the finished audio file (make sure the length is still the same as the video)
6. "Select All" (Edit -> Select All) in the audio file
7. Edit -> Copy
8. Open the original MPEG.
9. "Show Movie Properties" again, this time hit "Delete" on the audio track.
10. "Select All" in the original MPEG
11. Under the Edit menu, select "Add to Selection & Scale"
12. Export it using the MPEG-1 encoder you used in Premiere, or pick something like MPEG-4 with little/no compression.

Again, as long as the audio track's length didn't change at all, it'll sync up just fine. I've done this many times myself, though I don't usually work with MPEG-1 files.
posted by revmitcz at 11:38 PM on November 13, 2007

revmitcz, Premiere uses VfW so any MPEG1/2 output requires going through the 'Adobe Media Encoder', which forces a recompress.

I'd be cool with even a tool that lets me edit the actual binary data i.e. the audio frames, within the MPEG-1.
posted by Gyan at 11:55 PM on November 13, 2007

ahh.. sonofabitchpremiere. Well, I tried.

I guess it really depends on where the final output is going - but I'd imagine a 100% quality recompression wouldn't be very noticeable.

Anyway - good luck :)
posted by revmitcz at 12:08 AM on November 14, 2007

1. get TMPGEnc. use the mpeg tools therein to de-interleave your mpeg file into m2v (video) and mp2 (audio) files.

2. use mp3directcut (which also, apparently, supports mp2) to edit the audio as you wish (but be sure to keep it the same length, of course).

3. use TMPGEnc to re-interleave the resulting files.

haven't actually tried this, but it should work.
posted by neckro23 at 12:31 AM on November 14, 2007

neckro23, I ended up using TMPGEnc but mp3directcut (as well as Audacity) can only read MP2s, not save them.

Ultimately, this is what I ended up doing.

1)Did a 'Simple Demultiplex' of the .mpg in TMPGEnc. So I got a .m1v with only the MPEG-1 video and a .mp2 with only MPEG-1 Layer II audio at the source rate of 224 kbps.

2)I silenced the offending portion of the audio in Audacity.

3) I exported the new audio as a WAV since Audacity doesn't do MP2.

4)I converted the WAV to MP2-192 (since 224 isn't an option) using dBPowerAmp.

5)In TMPGEnc, I did a 'Simple Multiplex' of the video stream and the new MP2 to get a new .mpg
posted by Gyan at 1:09 AM on November 14, 2007