ReplayGain

[Servatius]

It might be interesting to see what foobar2000 is doing. Its ReplayGain scanner creates tags REPLAYGAIN_ALBUM_GAIN, REPLAYGAIN_ALBUM_PEAK,
REPLAYGAIN_TRACK_GAIN, REPLAYGAIN_TRACK_PEAK.

The foobar2000 playback settings for ReplayGain are:

• Source mode: none / track / album / by playback order
• Processing: none / apply gain / apply gain and prevent clipping according to peak / prevent clipping according to peak
• Preamp with RG info: -20.0 dB ... +20.0 dB (default: 0.0 dB)
• Preamp without RG info: -20.0 dB ... +20.0 dB (default: 0.0 dB)
  

Source mode tells which gain values are applied. "By playback order" allows for a party mode without having to change the settings. When playing normally or in Shuffle (Album) mode, it uses album gain. When playing in Shuffle (Track) mode, it uses track gain (that would be the party mode). I am not sure whether such an automatic selection would be possible with MinimServer as it would require the control point to tell the server its play mode.

Preamp without RG info can be used to select a default loudness for albums/tracks not having ReplayGain info. I don't think this needs to be supported, because a user wanting to use ReplayGain could easily tag all his audio files with RG info (in foobar2000, select the media root directory and use ReplayGain -> Scan selection as albums (by tags)).

However, the Processing settings to apply gain and prevent clipping and the Preamp with RG info setting should be considered to be supported since only a small preamp value gives some headroom for audio files that require to increase the loudness according to the _GAIN values but still have a high _PEAK value that could cause clipping then.

More technical information and links are on the Hydrogenaudio ReplayGain page.

For me as a listener of classical music it is not an issue to adjust the loudness once when I start listening to a long musical work with multiple movements. The variance in album gain values is not so big either. But pop music suffered from extreme compression due to the loudness war in the last two decades so that a collector of pop music may have quieter CDs from the 80s and much louder CDs bought more recently. It must be very annoying to change loudness all the time. For them, honouring ReplayGain in the transcoding process would be an important feature.

As the average loudness for radio station is being regulated in many states now (see this article), the loudness war might stop and we might see again pop music CDs with lower loudness and wider dynamic range in future. However, as this article mentions, sound engineers might still be enforced to produce different masters for loudness-normalised destinations (radio stations) and what the music industry considers peak-normalized destinations (CDs). The more loudness-normalised playback systems exist, the more reason the music industry will see to produce uncompressed material in future. So support for ReplayGain could help pop music listeners to get better audio quality.

Simon, in case you decide to support this, I would have a question about effects of the ReplayGain processing on the sound quality: would it be possible to do the signal processing in high resolution (24 bit), at least when choosing a hires output format? And what about a DSD stream? Could they be processed without transcoding to PCM?

Because there are still radio stations available with a higher loudness level, I would also suggest to consider a possibility to specify a ReplayGain value for an internet stream, e.g. in the #EXTINF description of the playlist entry in an .m3u/.m3u8 file.