QUOTE (2Bdecided @ Jul 17 2012, 13:08) <{POST_SNAPBACK}>
I'm concerned that many audio systems don't have a single digital gain control, but have some analogue gain as well (or multiple digital gains). Where this is true, it's wrong to assume that 0dB FS at a given point in the chain represents the loudest sound a system will ever produce. It would be nice to integrate the loudness normalisation into the only volume control (I suggested something like this in 2001), but you've got to anticipate that this won't always be possible.
FWIW (and assuming I understand you!), in pretty much any portable or PC situation (although not necessarily something like HDMI), you will always have both digital and analog gain under you control. So in practice, you can make something like this work for most things outside of home theater.
QUOTE (2Bdecided @ Jul 17 2012, 13:08) <{POST_SNAPBACK}>
I'm thinking about how this would work on my portable mp3 player. I'm listening to a classical track that's basically quiet but has a few loud peaks. These loud peaks mean that the top part of my volume control effectively doesn't work. I visibly change the volume control, and the volume doesn't audibly change - because those peaks (which might not even be in the part of the album I'm listening to at the moment) are preventing it from going higher. Then I skip to a pop track (I got bored with the classical track - I couldn't turn it up loud enough to hear the quiet parts!) and BANG the volume control is working and the level jumps up by 12dB. You could put some logic in to prevent the jump, and/or visibly lock out the top of the volume control range while the classical track was playing - but either way I still can't turn up the quiet parts of my classical track (or the quieter parts of any dynamic album) to listen to them properly.
What we do in rockbox on portable players is to apply replaygain digitally, and then allow analog gain above 0dB (that is, gain that would push a FS signal above the analog levels supported by the amp). In this way a user can apply digital correction to avoid clipping in lossy formats and to perceptually normalize volume, but a user can still pump up analog gain to compensate for digital attenuation. In practice this works somewhat like how you are proposing, you can pump up the volume on quiet tracks if you want, and peaks are (possibly) clipped in the analog domain which is usually mild or at least tolerable.