I am wondering about the theoretical side of editing an AC3 file. I have ripped a 6-channel AC3 file from a DVD, and I want to make some very slight edits to it, like silencing/trimming, nothing fancy. When I then export the new AC3 file, is this a recompression of the original? Will I lose quality in any way? I have tried this, and it seems to be fine quality-wise, but it also seems to lose some volume after exporting also. Does anyone know what causes this and how to work around it?
Yes I’m exporting as a 6-channel AC3. My goal is to keep it as close as I can to the original in terms of bitrate and sample rate and everything.
Is there a better way to edit so as to preserve as much quality as possible? Like say, if I convert the AC3 to something like an AIFF, make the edits, then export that as an AC3? I assume that there is no way to make simple cuts without recompressing, but is there one way that’s better than the other? Would this also maintain the same volume level as the original and, if not, how can I fix this?
It’s the And Everything that kills you. Did the original have DialNorm applied?
That’s the Dolby Surround feature that automatically increases and decreases volume as appropriate. This is the “explosion” filter. Explosions would never fit in a digital bitstream, compressed or not, so Dolby AC3 has an “extra track” that keeps track of the theatrical loudness.
It makes editing or even creating Dolby AC3 very entertaining.
Audacity always works with uncompressed PCM audio data so there will always be recompression losses.
There’s a program called “free audiodub” that claims to be a able to edit AC2 files without decompressing them. I’ve not used that program and I have no further information about it other than it runs on Windows. I don’t know if it supports multichannel AC3.
Koz, I have no idea if this DialNorm is in the original file or not, how do I find out?
Steve, I gave Free Audiodub a spin and it looks pretty sweet. I found another one like it called Delaycut. These programs will both cut out a piece of AC3 files and shorten the file length without recompression, which is nice. I was hoping for the ability to also make other edits like silencing certain stretches of the audio without shortening the length, but I think to do that, recompression is necessary. Is that accurate?
There only way to tell if DialNorm is applied is to listen – or have a decoder that tells you. The bad news is that DialNorm is always there. The trick for simple video editing is to adjust it so it doesn’t work most of the time – it stays out of the way. Commercial movies and shows tend to use it.
Dolby Tools tend to be a part of the Dolby Production Suite or a high end video editor. That’s how Dolby makes their money.
Advanced compression techniques work by taking the sound apart in tiny chunks and managing them so they take up less room and, depending on the amount of compression, throws away parts that the compressor thinks won’t be missed. Everybody thinks the sound gets muffled and that it. It’s not nearly so simple.
If you go completely nuts with high compression, the compressor starts throwing away parts you can hear. If you recompress a show, the compressor starts throwing parts away – twice.
You can’t directly edit a compressed file because you change the sound when you do. The tiny chunks needed for compression change and you need to do the whole process again – recompress – or it will sound pretty awful. There are certain simple edits you can do such as cuts and some volume control jobs that can slide by because the basic sound doesn’t change all that much, but the instant you apply an effect, filter, or heavy production tool, you’re dead.
So if I convert the AC3 file into, say, an AIFF file, would the DialNorm effect also be baked into that file? If so, then I could convert, make the edits I want, then export the AIFF as a new AC3 and it should all work, right?
What workflow would you recommend for editing an AC3 file and maintaining as much fidelity as possible to the original?
Hi, I know this thread is old, but I would like to clarify for other people who would come here.
Before editing your AC3 tracks, remember to remove the dialnorm, because it may be applied permanently on the track by your soft audio. You can remove the dialnorm with EAC3to or EAC3to Gui (command on EAC3to GUI: _% . Ac3 the dialnorm will be automatically removed), or with MKVtoolnix too.
AC3 works in 32ms fragment, we can edit fragments of 32ms without reencoding the track with some software.
Delaycut allows it, EAC3to too.
Maybe Free Audio Dub works like that too but I have never tried.