I have increased volume levels many times in Audacity, but all were mono or stereo.
How do you do it with Dolby AC3 5.1 channels? Just mark them all to go up the same or is there another proper way to do it?
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I always decompress AC3 5.1 to W64 with eac3to, and import that in Audacity.
You have to set "Use Advanced Mixing Options" at the export settings to be able to output multichannel.
After editing you can convert the W64-file back to AC3 or maybe better a lossless format. Keep in mind that editing an already lossy source and encoding it back with a lossy encoder, will yield a certain loss of quality.
Maybe you first want to look if your source file is encoded with dialog normalisation. When so, it would be a lossless step to remove this. Eac3to can do this for you.
...and YES, they should all go up by the same amount.
Some years ago I downloaded a movie from the Internet Archive. The audio was very quiet when
played back on my TV, much too quiet.
I opened the file in Audacity and the levels looked OK to me.
Turns out there is something called "dialogue normalization" and the TV was honoring it. It is metadata
that sets how the audio is attenuated during playback.
I remember at the time I was unable find out if/how this metadata could be changed, so I had
to re-encode the file before I could watch my movie.
DIALNORM is an important and standard function with AC3 files. In a sense, it should be setting it to that which is standardized for all broadcast output, and if that meant your setup was too quiet, what it officially should really mean is that you need to up your overall output and fix those files that are "too loud".
But I know most won't want to work that way - too many files that are peak normalized, and that's how they've set their system up to accommodate.
Dialnorm is metadata, so you should be able to re-adjust the flag (with the proper tool) to fix those files without re-encoding. But I don't know which tool does that properly.
yet looked normal when the audio was opened in Audacity; average dB in the typical range, about -25dB average.
That was when I started to research the phenomenon and found out about the dialogue normalization.
Never did find a program to change the metadata, hence the re-encode.
I could have just exported the files as-is, to aac for example. Levels on the TV would have been fixed then