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Round tripping audio to repair and sweeten theory question

Larry Watts

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My workflow as a video producer to finish audio is to round trip audio from premiere or FCPX to audition or logic or protools and from there send it to Isotope RX4. I'm fairly new to RX4 so I'm wondering:

If I open an audio/video mpeg4 file directly into RX4 can I save it back INTO the mp4 AV file intact, or does it become only an audio file?

Same question goes for Audition or protools?

Right now I have a 1.5 hour video that needs audio leveling and some spectral repair. It seems faster to just do it all in RX4. 

I just don't understand what an audio editor does to the video portion of the file. 

It seems like I don't have all the same tools when using RX4 as a plugin versus as the standalone program.

So is there a shortcut workflow to fix audio in a video that does not require using a video editing program to keep audio and video together?


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You're right, RX4 is a lot more capable as a standalone than its plug-in implementation. When you're using it with a pro audio app (ProTools or Nuendo), there's a function where you can select a clip as you would with a plug-in, pass it to the standalone, and then return it to your track in perfect sync. That's the way I usually use it for clips; I reserve the plug versions for overall track tweaking.

If your mp4 AV file has compressed audio -- which is the standard -- you'll lose a lot of quality by editing / processing / mixing in that mode. Far better to treat the audio as linear PCM (AIFF or .wav) through all the production steps, then just convert the final version for release. 

Most audio editors keep the video file as a sync reference, edit/process/whatever the audio, and then render the result as an audio file that can be married back to the video (a few will actually plant the audio back on the AV file, but it isn't really necessary; you can do the same thing in your video editor or even QuickTime Player Pro). The advantage of using an audio program is freedom from frame boundaries -- which is a very big deal when cutting dialog or music -- as well as much more flexible processing and mixing options.

The standard workflow IS the shortcut:  

1) edit the video in FCP or Avid or Premiere or whatever, adding a 2-pop/flashframe or universal leader to audio and video

2) pass the edited audio in sync via OMF or AAF, which any competent audio program can open,  

3) edit / clean / premix dialog, then do whatever is needed for sfx and music

4) mix and render in the audio program

5) put the rendered file back with the finished video, using the pop for sync.

If you're working with decent software and all in the same computer, it'll all hold sync -- even over the length of a feature film -- unless you deliberately (or accidentally) do something that'll change the length of the finished audio.


tutorials, books, etc at my website jayrose.com.

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Thanks Jay! (and the great comment,Steven)

I'm jumping in at the tail of a project end helping a non-profit get their video in decent shape for replication, so the mp4 is what I got handed to me. 

The project has already been authored for DVD menus etc and I need to go back and level the audio, take out some pops, clean up one area that peaked and bring it back into Encore to burn the master for replication. (Which is why I was looking for a short cut.)

My normal personal workflow is to shoot on my canon C100 recording on a Ninja 2 in Prores at 200mb/s and record sound on my sound devices 644 as wav files double sound so I get clean sound and video. Sync using FCP sync or clapboard. I'll start using the 2 pop/flashframe technique.

My dilemma here is how to fix things and not introduce any new problems. Because of the compressed audio and with your comment in mind, would you suggest I work on the audio within the mp4 format and not save out a wave file which will recompress again?

What I don't know is if I open a premiere mp4 video and send to audition/RX4 will it uncompress and recompress in the process and therefore diminish the quality?

Thanks Jay, and I went to your website and am going to order your book!

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I did a test and this is what I found.

I opened an mp4 video in RX4. I fixed a really bad audio glitch using the replace feature in RX4 and saved it.

When I opened it in Premiere the audio was fixed and the video was still synced in a single file.

Question: Did RX4 recompress the file when it did that? How much degradation occurred if it did?





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Rick is right. Almost all compression formats work with small windows, sort of like video frames, rather than with one audio sample after another. This way they can look forward and backward to find places where masking or redundancy or variable delta lets them save data.

But it also means you can't really edit them until you convert the windows back to continuous audio. So programs that edit or level things while appearing to pass a compressed format are actually de-compressing under the hood, doing their work, and then re-compressing*. If they're well designed and you're using gentle compression, it's mostly benign. But no different from decompressing on your own, doing the work, and then recompressing using something like SoundGrinder.

Plus, if you do the decompression and recompression yourself, you can archive the intermediate linear version where you did the fixing. Then if more fixes or changes are required, you can go back to the archive and won't have to suffer multiple codec cycles... which can hurt quality.



* A few decades ago, when the original hardware-based DAWs were first getting a foothold, MusicamUSA showed a prototype of an mp3 DAW that allegedly "saved quality by doing all the work in native mpeg". It was klunky, limited you to cutting on frame boundaries or had to decompress/recompress without telling you, and never caught on.

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