Under some listening conditions, it's useful to have every track sound equally loud. However, sometimes we want to leave the interntional loudness differences between tracks in place, whilst still correcting for unmusical and annoying changes in loudness between discs.
The Replay Gain proposal suggests that two different gain adjustments should be stored in the file header, as follows.
This will make all the tracks sound equally loud (as they do on the radio, hence the name!). If the ReplayGain is calculated on a track-by-track basis (i.e. an individual ReplayGain calculation is carried out for each track), this will be the result. This is something that ReplayGain does very well. Take a listen.
The problem with the "Radio" setting is that tracks which should be quiet will be brought up to the level of all the rest. For casual listening, or in a noisy background, this can be a good thing. For serious listening, it would be a nuisance. You don't want a solo flute track blasting at the same loudness as Iron Maiden!
To solve this problem, the "Audiophile" setting represents the ideal listening gain for each track. ReplayGain can have a good guess at this too, by reading the entire CD, and calculating a single gain adjustment for the whole disc. This works because quiet tracks then stay quiter than the rest, since the gain won't be changed for each track. It still solves the basic problem (annoying, unwanted level differences between discs) because quiet or loud discs are still adjusted overall - so the pop CD that's 20 dB louder than the classical CD will be brought into line.
Where ReplayGain will fail is if you have an entire CD of quiet music. It will bring it up to an average level. This is why the "Audiophile" Replay Gain adjustment must be user adjustable. The ReplayGain whole disc value represents a good guess, and should be stored in the file. Later, the user can tweak it if required.
If the file has originated from the artist (e.g. download from mp3.com), then the "Audiophile" setting can be specified by the artist. Naturally, the user is free to change the value if they desire.