ReplayGain: How to find which gain VLC applies

[hendriks73]

Hey there,

I'm a software dev and am having a little trouble figuring out what ReplayGain tags are read by VLC.

Where in the VLC UI can I confirm that the tag is read correctly? I.e. where does it show me something like e.g. -8dB for a specific file. (I have turned track replay gain on in the audio preferences and playing files.)

Is the tag format for AAC documented somewhere?
I'm guessing it REPLAYGAIN_TRACK_GAIN and REPLAYGAIN_TRACK_PEAK in the com.apple.iTunes atom, but cannot find any valid docs.
Also, do the tag names have to be uppercase or is lowercase tolerated as well?

Thanks so much!

-hendrik

[Lotesdelere]

VLC applies the specifications:
https://en.wikipedia.org/wiki/ReplayGain
http://wiki.hydrogenaud.io/index.php?title=ReplayGain

[hendriks73]

Unfortunately, the specification does not define the exact tag format for AAC.

Or does it and I'm just missing it?

[hendriks73]

That aside: Does VLC show anywhere in its UI, what ReplayGain has been applied to the currently playing song?

[Lotesdelere]

More to read:
http://wiki.hydrogenaud.io/index.php?ti ... cification
http://wiki.hydrogenaud.io/index.php?ti ... cification

Long story short:
Raw AAC can not be tagged so the AAC stream must be embedded into an MP4 container.
Then the tags standard format being used is ID3v2 in ID32 box. VLC can read and apply those ReplayGain tags.

That aside: Does VLC show anywhere in its UI, what ReplayGain has been applied to the currently playing song?

Not that I know, maybe in the messages (Ctrl+M or Tools -> Messages).

[hendriks73]

Thank you—that definitely answers the second part of my question!

[hendriks73]

Not that I know, maybe in the messages (Ctrl+M or Tools -> Messages).

Couldn't find it there.

[hendriks73]

This http://wiki.hydrogenaud.io/index.php?title=ReplayGain may be out of date, but it looks to me like no scanner actually uses the ID32 atom to write ReplayGain info. Probably because the non-standard ID32 atom is not widely used to begin with. I'd love to get my hands on a ReplayGain scanner that uses it.

Are you aware of any (I'm not)?

I played around with FooBar2000. Its seems to write the ReplayGain tags into the iTunes metadata tag under "----":"replaygain_track_gain". This makes sense, as it is super-easy to do, especially, if you already have a parser for those iTunes tags. beaTunes does the same thing.

Perhaps this would be a nice feature for VLC: Look for ReplayGain in several places that seem to be used in the wild. Especially, if there is no scanner that adheres to the spec and writes to ID32 (i.e., if there is no ReplayGain scanner that writes to the ID32 atom, it makes no sense for VLC to *only* look there).

This order would make sense to me:

- ID32 Atom (because it's in the spec)
- APEv2 tags, as that's what many scanners seem to write to according to http://wiki.hydrogenaud.io/index.php?title=ReplayGain
- iTunes MP4 tags, as at least FooBar2000 and beaTunes read/write there

Also: It would be nice, if VLC displayed the currently used gain somewhere. I did some more digging, but couldn't find it anywhere. Transparency, i.e. the possibility for the user to understand what's going on, is typically a nice to have.

[Jean-Baptiste Kempf]

Samples needed.