Improving ReplayGain, some ideas for Devs etc
Improving ReplayGain, some ideas for Devs etc
Nov 18 2003, 17:04
Joined: 5-November 01
From: Yorkshire, UK
Member No.: 409
Every now and again I wish I had the time to update the ReplayGain website and add some new ideas, and maybe even clarify some old ones. I don't, so this thread will have to do.
Firstly, the format used to store ReplayGain info in files is not documented correctly on the ReplayGain website, and it would be good to "publish" what has emerged as the standard for each format.
Secondly, what is stored is not documented correctly on the ReplayGain website, and I'd like to re-examine what is stored...
One change has already happened, and I think it's a good change:
Forget Radio and Audiophile - Track and Album are much better names.
(that's an open admission of me being wrong, for anyone who discussed this with me previously!)
So, we store:
ReplayGain Track adjustment
ReplayGain Album adjustment
(ReplayGain) Track peak
(ReplayGain) Album peak
(this last one wasn't in the original proposal, but it has been widely used - I've put it in bold to remind me to include it in the update)
That makes sense, and most software supports this. I'd like to formalise some extensions, some of which were there from the start, and others that have cropped up more recently:
1. (ReplayGain) undo adjustment
- this is written when the gain of the file is changed (e.g. by mp3gain, or by decoding with ReplayGain enabled), and is the gain change required to put the file back to where it started.
e.g. If I apply -8dB gain change using mp3gain, then
(ReplayGain) undo adjustment = +8dB
e.g. If I use --scale 0.5 when encoding (for whatever reason?!), then
(ReplayGain) undo adjustment = +6dB
If the gain of an already ReplayGained file is changed, the original four values (Track and Album adjustment and peak) should be updated so that they are correct for the new audio data. (see an example in this thread: http://www.hydrogenaudio.org/forums/index....topic=15412&hl= )
I can't see any argument against defining this field. It would be zero (or absent) if the audio file hasn't been altered. It's useful in all formats because you can always apply wavgain before encoding, and it would be nice to know that this has been done.
2. ReplayGain calculation method
OK - I've had this argument before, but this really is important. ReplayGain can be improved, but you'll never know whether files are tagged using the old or new ReplayGain calculation unless the calculation method (actually a number which corresponds to the method) is stored. This doesn't increase the complexity of players, as they won't care - it just makes it very easy to pick out files that were tagged with the old version, and update them.
3. ReplayGain lossy approximation
This is just a single bit: 0 or 1.
0= this ReplayGain info has been calculated from the data in this file
1=this file has been lossily encoded/transcoded since this ReplayGain info was calculated.
What's the point of this? If you have a file with ReplayGain info, you can transcode it and copy the RG info across. It'll be close enough to give you excellent loudness equalisation, and you won't have to re-calculate it. Yet they'll be a label there to tell all you anal retentives that it's not quite right, and should be recalculated if you want to be 100% sure (especially important for peak amplitude).
You could (should?) have one “ReplayGain lossy approximation” bit for each of the four values, which gives you the chance (for example) of re-calculating the peak values (quick, and important - so let's do it), but leaving the ReplayGain values (slow, and unimportant - so let's not do it).
4. ReplayGain user adjustment
Instead of suggesting that users should change the calculated values if they wish, give them a field to enter their own value if they really have to. Players should give the option to read the user value in preference to any others (i.e. let it act as an over-ride), and taggers should give the option of removing the user values from all (downloaded) files.
5. ReplayGain RealLife adjustment
The gain required to give the actual SPL of the original event (in a calibrated system), or a human judged sensible replay level (see the explanation behind the original "Audiophile" level and the work of Bob Katz if you think this is an impossible idea). I've found a few DVD-A discs that have this information (it's in the MLP stream), so it would be nice to have somewhere to store it. It's unlikely to get used much, but it would be a useful thing to have. It would be the last link in some of the best recordings out there.
I'd like to come to a consensus of which ones of these (if any/all) should be included, and then get some specs as to how they are/should be stored in each file format (especially APE2.0 tags) finalised and published on-line.
Comments? Suggestions? Offers of help?
btw I've received a couple of suggestions for improving the ReplayGain calculation. One is trivial, and seems like a great idea. I'll post it for testing when the problem of version numbering is solved. If anyone else has slightly or totally re-worked the ReplayGain algorithm/concept, now would be a good time to step forward! We could do listening tests to find the best candidate for "calculation version 2".
Newbie warning: this thread is not for asking questions about ReplayGain that are already answered on www.replaygain.org or in previous threads on HA. (I'm always happy to answer "silly" questions via email – half of them aren't silly at all.)
However, if you do already have some understanding of ReplayGain then this thread is the perfect place for clarifying anything to do with the above proposals which is not clear.
This post has been edited by 2Bdecided: Nov 18 2003, 17:42
Jan 8 2004, 17:07
Joined: 4-January 04
Member No.: 10938
To sum up my opinion,
I am for storing 3 values instead of 2 :
1. assumed_system_gain == it's a gain compared to SMTPE RP200 calibrated, 83dB SPL, system.
(think of SMTPE RP200 as a home stereo on which the volume has been permanently set to a precise level, chosen by a comity so that movies and classical music play loud and clear, but not too loud either)
this value depends on the genre of the music (that's a genre-dependant reference)
For instance :
-> +0dB for wide range dynamics genres (classical, movies, ..)
-> -6dB for pop songs (and that's simply because pop songs are already digitally loud,
so you typically lower your amplifier settings compared to if you were playing classical)
2. replaylevel, in db SPL == the perceived overall loudness if played on a system with gain=assumed_gain. It can range from eg. 80 db SPL to 100 or more.
3. album replaylevel == loudness of the whole album ..
The assumed_system_gain (let's use a more precise name than 'reference gain' if we want to avoid
confusion) might seem uselessly redondant to store in the file.
But it has to be stored if the replay level computation really takes the expected
real world loudness into account and we want to keep the possibility of playing files of different
genres (and different assumed_system_gain) at the same perceived loudness - see my post
with the 'purist replaygain fanatic' example.
[ and if after all the assumed_system_gain is not to impact the psychoacoustics algorithm,
then there's no real need to use SPL numbers at all, the replaylevel is just a
digital domain computation giving a measure of headroom, and would be better expressed in dB FS.
True it's computed with a calibration designed with one correspondance to real world SPL assumed,
but if it's only assumed and fixed, no point in making it appear in the result. ]
Each of those values could also be stored in other forms, but I think this scheme make them all easy to grasp, and really mean what they are.
You can for instance know the intrinseque loudness as
loudness on RP200 system = (replay_level - assumed_system_gain)
storing the real-world SPL obtained on assumed system (83 for classical-tuned system, 77 for pop-tuned) seems a bit less intuitive, as it is good to be able to get total real world SPL (rp-200 calibrated) of a file by simply adding 2 values.
And I also find relative replaygain less intuitive than sound level, because it's confusing to look at a number which varies inversely to the level of the song..
Technically, that makes a complete change of the replaygain values, but I dont see how it would be better to keep the same tag names if changing the values's meaning or encoding.
In fact, the next-generation tag readers will be as happy both ways,
while using a new set of tags make it possible to support previous-generation tag readers smoothly.
users would have the possibility to add new tags, with possibly more info, and generate compatiblity tags if they want. re-generate them later if they want other replaygain settings, etc..
I find this kind of spec update scheme the most comfortable.
the assumed_system_gain value might be even easier to grasp if we stored -assumed_system_gain, and called it
genre_shift, as this value is somewhat a measure of the difference between the typical enjoyed loudness of songs of the given genre, and the typical enjoyed loudness of a wide range dynamics reference material (I think I read they used Star Wars movie sound actually)
'genre_shift' would be a bit easier to grasp, but also a bit further from its precise definition.
This post has been edited by SamK: Jan 8 2004, 17:27
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