OPUS gain is applied incorrectly/undocumented

[hknutsen]

Context:

• Most files (e.g. FLAC and MP3 files) use the ReplayGain standard for volume normalization. ReplayGain adjusts to -18 dB LUFS by default.
• Opus files use the EBU R 128 standard for volume normalization. EBU R 128 adjusts to -23 dB LUFS by default.
• In short: Opus files are 5 dB quieter than files that use the ReplayGain standard.

Also:

• ReplayGain stores album gain in the REPLAYGAIN_ALBUM_GAIN tag, and track gain in the REPLAYGAIN_TRACK_GAIN tag.
• Opus stores album gain in the header (referred to as "header gain" here), and additional track gain in the R128_TRACK_GAIN tag.
• In short: Opus files have built-in album gain.

I've been trying to figure out how Auxio applies header gain to Opus files by digging through issues, PRs and commits in this repository, however I'm still confused.

Would it be possible to document how Auxio applies gain to Opus files? For example in the wiki?

What could be documented:

• Are Opus files automatically adjusted to target -18 dB by default (i.e. +5 dB added to Opus files), to ensure consistency with files that use the ReplayGain standard instead?
• Is header gain always applied? I often see header gain referred to as "base gain" in this repo, which implies that it's always applied. See also this issue comment.

[OxygenCobalt]

re Opus files automatically adjusted to target -18 dB by default (i.e. +5 dB added to Opus files), to ensure consistency with files that use the ReplayGain standard instead?

Auxio does, but I don't know if the system media codec handler does.

Is header gain always applied? I often see header gain referred to as "base gain" in this repo, which implies that it's always applied. See also #521 (comment).

That is my belief. The system media codec handler is documented to already process and handle the header gain. Auxio handles the additional R128 tags.

Does this help @hknutsen?

[hknutsen]

Thank you for the quick response!

As a result of your very helpful comment, I did a couple of quick tests where I compared the volume in Auxio with the volume in Spotify.

Test 1

• Opus files target -23 dB LUFS by default.
• Spotify targets -14 dB LUFS by default (referred to as the "normal" level here)
• Assuming that Auxio applies +5 dB by default to Opus files, I added +4 dB to the pre-amp to target the same loudness as Spotify (23 - 14 - 5 = 4).

With this configuration, Auxio plays back the exact same song at a noticeably lower volume than Spotify (that song being Fossegrim by Kvelertak, streamed on Spotify here and bought on Qobuz here. The UPC and ISRCs match between Spotify and Qobuz, so these are the same release).

Test 2

• Assuming that Auxio does not apply +5 dB by default to Opus files, I added +9 dB to the pre-amp to target the same loudness as Spotify (23 - 14 = 9).

With this configuration, Auxio seems to play back at the same volume as Spotify (or at least close enough that I'm unable to make out a difference).

Findings

I'm fully aware that this is not a very formal testing method, but these are my findings for now:

• The system media codec does seem to apply header gain by default.
• Auxio does not seem automatically add +5 dB to Opus files.

[hknutsen]

I'm fully aware that this is not a very formal testing method, but these are my findings for now:

• The system media codec does seem to apply header gain by default.
• Auxio does not seem automatically add +5 dB to Opus files.

Just to clarify:

This issue was not created to argue whether Auxio should apply +5 dB gain to Opus files or not. I created it simply to clarify how gain is currently applied to Opus files, and that it should be documented to prevent future confusion, since gain for Opus files can be a confusing topic (hence my own confusion) 🙂

[OxygenCobalt]

Will need some time to consider these results, as far as I know I'm doing everything right, but there may be a gotcha I'm not seeing @hknutsen.

[hknutsen]

FWIW @OxygenCobalt, I think you're doing everything right as well 🙂 See this comment.

[OxygenCobalt]

Okay, I updated Supported Audio Formats to document the OPUS base gain behavior @hknutsen. Thanks for the heads-up.

[hknutsen]

Documentation looks perfect, thanks @OxygenCobalt !