Zoog: Zero Opus Output Gain

Zoog is a Rust library that consists of functionality that can be used to determine the loudness of an Ogg Opus file and also to rewrite that file with new internal gain information as well as loudness-related comment tags. It also has functionality for purely manipulating comment tags.

Zoog currently contains two tools, opusgain and opuscomment. opusgain can be used to:

• set the output gain value located in the Opus binary header inside Opus files so that the file plays at the loudness of the original encoded audio, or of that consistent with the ReplayGain or EBU R 128 standards.

• write the Opus comment tags used by some music players to decide what volume to play an Opus-encoded audio file at.

It is intended to solve the "Opus plays too quietly" problem.

opuscomment can be used to list, replace or modify the comment tags of Ogg Opus files. Its usage is roughly based on that of vorbiscomment though many options have different naming for improved clarity.

Although zoog exposes a library, its API is unstable and this package is released on crates.io primarily to allow access to the opusgain tool. The API is documented however, and the reading the source may prove useful to anyone else wishing to work with Ogg Opus files.

opusgain

opusgain adjusts the Opus binary header for playback at a specific volume and will always generate the R128_TRACK_GAIN tag and the R128_ALBUM_GAIN tag (when in album mode) such that files will play at an appropriate volume in players that support these tags, and at a more appropriate volume in players that don't. Existing R128_ALBUM_GAIN tags will be stripped when not in album mode.

opusgain (unlike its predecessor zoog) decodes Opus audio in order to determine its volume so that it's possible to be certain that all generated gain values are correct without making assumptions about their existing values.

The following options are available (run opusgain --help for usage):

• -p PRESET, --preset=PRESET

  It is recommended to specify this value explicitly, as the default may change.

  • original: Set the output gain in the Opus binary header to 0dB. In players that do not support R128 tags, this will cause the Opus file to play back at the volume of the originally encoded source. You may want this if you prefer volume normalization to only occur via tags.

  • rg: Set the output gain in the Opus binary header to the value that ensures playback will occur at -18 LUFS, which should match the loudness of ReplayGain normalized files. This is probably the best option when you have a player that doesn't know about Opus R128 tags, but:

  • does support ReplayGain for the other file formats you use, and/or

  • the files you play have been adjusted in a player-agnostic way (mp3gain and aacgain can do this) to the ReplayGain reference volume.

  • r128: Set the output gain in the Opus binary header to the value that ensures playback will occur at -23 LUFS, which should match the loudness of files produced by opusenc from FLAC files which contained ReplayGain information.

  • no-change: Do not change the output gain in the Opus binary header.

• -o MODE, --output-gain-mode=MODE

  • auto: Set the output gain in the Opus binary header such that each track is album-normalized in album mode, or track-normalized otherwise. In album mode, this results in all tracks having the same output gain value as well as the same R128_ALBUM_GAIN tag.

  • track: Set the output gain in the Opus binary header such that each track is track-normalized, even if album mode is enabled. In album mode, this results in all tracks being given different output gain values as well as different R128_ALBUM_GAIN tags, but their R128_TRACK_GAIN tags will be identical. Unless you know what you're doing, you probably don't want this option.

• -a, --album: Enables album mode. This causes R128_ALBUM_GAIN tags to also be generated. These tell players that support these tags what gain to apply so that each track in the album maintains its relative loudness. By default the output gain value for each file will be set to identical values in order to apply the calculated album gain, but this behaviour can be overridden using the --output-gain-mode option.

• -n, --dry-run: Displays the same output that opusgain would otherwise produce, but does not make any changes to the supplied files.

• -j N, --num-threads=N: Use N threads for processing. The default is to use the number of cores detected on the system. Larger numbers will be rounded down to this value. To avoid high disk space usage during processing, or a large number of temporary files left around after an error, only one file will be rewritten at a time regardless of the number of threads.

• -c, --clear: Remove all R128 tags from the specified files. The output gain of each file is unchanged, regardless of the specified preset.

If the internal gain and tag values are already correct for the specified files, opusgain will avoid rewriting them.

opusgain supports Unix shell style wildcards under Windows, where wildcards must be handled by the application rather than expanded by the shell.

opuscomment

opuscomment can be used to delete, append, replace and list the comments located in an Ogg Opus file.

The following options are available (run opuscomment --help for usage):

• -l, --list: List all tags in the file in NAME=VALUE format. This will be to standard output unless -O is specified.

• -m, --modify: Tags specified using -t or -I will be appended to the specified file. Tags matching patterns specified using -d will be removed from the existing tags on the file.

• -r, --replace: All existing tags in the file will be removed and will be replaced with those specified using -t or -I.

• -t NAME=VALUE, --tag NAME=VALUE. The specified tag is will be added to the file in modify or replace mode.

• -d NAME[=VALUE], --delete NAME[=VALUE]. Specifies either a tag name, or a name-value mapping to be deleted. All tags that match the pattern will be removed, not just the first. This option is only valid in modify mode.

• -e, --escapes: In all tag input/output either on the command-line or to/from a file escapes will be used for line-feeds (\n), carriage returns (\r), backslashes (\\) and the null character (\0). All other escapes are invalid. This option makes it possible to specify tags which contain newlines which would otherwise fail to be parsed correctly from a comment file.

• -I COMMENT_FILE, --tags-in COMMENT_FILE: In the modify and replace modes, the tags to added will be read from this file in addition to those specified on the command line. Tags are read in NAME=VALUE format, with one tag per line. If - is specified for the file name, tags will be read from standard input.

• -O COMMENT_FILE, --tags-out COMMENT_FILE: In list mode, tags will be written to this file. Tags are written in NAME=VALUE format, with one tag per line. If - is specified for the file name, tags will be written to standard output.

opuscomment only has knowledge of UTF-8. Usage on systems where UTF-8 is not the character encoding scheme in use may encounter issues.

Build Instructions

If you do not have Cargo, install it by following the instructions here.

Clone the Git repository:

$ git clone https://github.com/FrancisRussell/zoog.git

Inside the cloned repository:

cargo build

or

cargo build --release

for a release build.

Built binaries can be found in target/debug or target/release.

Installation via cargo

At the command line, simply run $ cargo install zoog

opusgain and opuscomment should now be available in the path.

Releases

Zoog binaries for Windows, MacOS and Linux can be found on the releases page. Only the Linux binaries have undergone any testing at present.

About Ogg Opus Volume Normalization

Background

Opus-encoded audio files contain an ‘output gain’ value which describes a gain to be applied when decoding the audio. This value appears to exist in order to ensure that loudness changes to Opus files are always applied, rather than being dependent on decoder support for tags such as REPLAYGAIN_TRACK_GAIN and REPLAYGAIN_ALBUM_GAIN which are used in Ogg Vorbis, but not Opus.

The in-header value was intended to correspond to the album gain with RFC 7845 defining the tag R128_TRACK_GAIN for single-track normalization. It seems the original intent of the output gain was to eliminate the need for an album gain tag, however R128_ALBUM_GAIN was later added for album normalization.

The problem

When encoding an Opus stream using opusenc from a FLAC stream which has embedded ReplayGain tags, the resulting Opus stream will have the output-gain field set in the Opus header. The gain value will be chosen using EBU R 128 with a loudness value of -23 LUFS, which is 5 dB quieter than ReplayGain.

The presence of either R128_TRACK_GAIN or R128_ALBUM_GAIN tags will allow players that support these to play tracks at an appropriate volume. However, in audio players that do not support these tags, track will likely sound extremely quiet (unless your entire music collection is normalized to -23 LUFS).

Even more problematically, using opusenc with a FLAC file that does not have embedded ReplayGain tags will produce a file that plays at the original volume of the source audio. This difference in behaviour means that it's not possible for players that do not support R128 tags to assume that different Opus files will play at a similar volume, despite the presence of the internal gain header.

Even if a player does support the R128 tags, this is not enough to correctly play Opus files at the right volume. In the case described above, opusenc will use the internal gain to apply album normalization, meaning that it does not generate a R128_ALBUM_GAIN tag. Without this, it's not possible for a music player to play a track at album volume without again assuming that the internal gain corresponds to an album normalization at -23 LUFS.

Q & A

How is loudness calculated?

Loudness is calculated using ITU-R BS.1770. This is the standard used by EBU R 128 for measuring loudness and the one intended for use when calculating Opus R128 tags.

What happened to the zoog program?

It was deprecated and removed from the repository.

What did zoog do?

zoog modified the internal gain values of Opus files and applied the inverse gain delta to the any R128 tags present in the file. Like opusgain, this enabled targeting Opus-encoded tracks to a particular loudness level on players that did not support R128 tags whilst maintaining the same loudness value for players that used them.

Why was zoog deprecated?

zoog did not decode audio in order to determine loudness. Instead it relied upon existing R128 tags. This was problematic because lack of an R128_ALBUM_GAIN tag does not indicate a track is not album normalized - it might still have been album normalized via the internal gain header (as done by opusenc when encoding from FLAC files containing ReplayGain tags). Such files are problematic for players in general if they wish to play tracks at an album-normalized volume because it's not obvious how to tell if tracks have been album normalized.

zoog had a similar issue. Modifying an album-normalized track's internal gain requires creation of an R128_ALBUM_GAIN tag if there is not one present. If the track is not album-normalized, then adding such a tag is nonsensical.

zoog did not introduce new R128_ALBUM_GAIN tags and It was suggested that a tool like loudgain be used create R128_ALBUM_GAIN tags before applying zoog to album-normalized files. However, failure to do this would likely result in different internal gains being applied to different tracks in an album, losing album-normalization in a way that would likely go unnoticed.

Due to the potential for error, zoog was removed and opusgain was created. Like vorbisgain and similar tools, opusgain decodes the audio to determine loudness and has an option to specify whether the tracks being normalized are part of an album.

When should I use opusgain versus loudgain

If you only play Opus files in players which support R128 tags, then use loudgain.

You should use opusgain if you play Ogg Opus files in players that do not support R128 tags and would like them to play at either their original volume, or at the volumes suggested by ReplayGain or EBU R 128.

Once you have set the internal gains of a set of Opus files to the desired values, then loudgain is likely preferable for any future tag updates related to normalization.

How can I check if opusgain is working correctly?

Applying opusgain to various test files then reviewing the diagnostic output and R128 tags generated by loudgain when applied to the rewritten files is helpful in this regard.

Disclaimer

Please see LICENSE. Unless you have a source you can easily reconstruct your Opus files from, the author recommends making a backup of any files you intend to modify first, and running opusinfo afterwards on any processed files.