AC3 - figure out dynamic range compression (DRC)

[Lariel]

Hello,

I'd like to figure out what DRC profile modes were used for some ac3 files extracted from a DVD. This is what I got so far: There are programs like mediainfo which can be used to read the meta data of ac3 files. But the DRC settings aren't encoded as meta data. They are converted into "gain words" which come along with the audio data inside the file.

So here's what I tried to do: extract the "raw audio data" without any DRC applied. I used eac3to for that. Now reencode the wav to ac3 applying different DRC profile modes.

Now the only I need is a possibility to compare these new ac3s with the original one. If I find a match, that's the DRC profile I was looking for. I could do an rms normalisation scan in Sound Forge for that. But Sound Forge has no options for decoding ac3. So I need a middle man. A program which enables me to decode all the ac3 files with one of the DRC profiles enabled (which are film standard, film light, speech, none, etc.), for both of the DRC modes (line mode and rf mode). Then I would load the resulting wavs into Sound Forge and compare the loudness rms levels. So far I wasn't able to find a ac3 decoder which let me set the DRC modes (like "off", "line mode", "rf mode"). Anyone?

[virendra]

You can open AC3 file in Nuendo and do the edit. Even normalise option is available. Only problem is the software is not free and damn expensive

[hech54]

the DRC settings aren't encoded as meta data.

Perhaps you could explain why this is so important?

[Selur]

Pumping this old thread,..
Is there an alternative to Nuendo nowadays that can extract the drc infos?

Perhaps you could explain why this is so important?

because when reencode drc is normally lost, at least when using libav (mencoder/ffmpeg/...) as decoder.
A lot of tools which reencode allow to apply custom drc through sox, aften also has some drc options, which are not recommend (siehe: https://forum.doom9.org/showthread.php?t=166872, http://forum.doom9.org/showthread.php?t=154121).
-> so looking for an option to get the intended drc and then ideally apply it through sox (or other software) would be nice.

[Island_Dweller]

Selur, try Ciler's AC3Tool

[Selur]

Thanks, tried it, but sadly it doesn't analyse the stream for the drc infos and uses aften to add drc infos.

[Island_Dweller]

It works on my end. The ac3 analysis tab get the drc info, then you can use that info with your selected encoder.



Or maybe I'm misunderstanding you?

[Selur]

shouldn't drc infos be way more differential and only apply modifications in specific ranges?
The mix levels shown, look more like the level difference to 0dB.

[Island_Dweller]

According to http://www.hydrogenaudio.org/forums/index.php?showtopic=52451&view=findpost&p=469470 , the DRC profile is not specified in the ac3 bitstream. It is only used by the encoder to calculate appropriate DRC values for each block or frame.

I don't think there's any freeware that can do what you're after.

But, as a hack job, if you could decode ac3 to wav with and without drc, you could compare the two wav files and guess which profile was used with the help of this table http://web.archive.org/web/20040716131627/http://www.dolby.com/tech/L.mn.0002.DDPEG1.pdf#page=147

[Selur]

the DRC profile is not specified in the ac3 bitstream. It is only used by the encoder to calculate appropriate DRC values for each block or frame.

Yup, I know.

I don't think there's any freeware that can do what you're after.

okay