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Improving ReplayGain, some ideas for Devs etc
2Bdecided
post Nov 18 2003, 17:04
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ReplayGain developer


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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".


Cheers,
David.

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
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SamK
post Jan 8 2004, 17:07
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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.

PS :
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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Posts in this topic
- 2Bdecided   Improving ReplayGain   Nov 18 2003, 17:04
- - Gabriel   I think that a point that should be clarified/upda...   Nov 18 2003, 17:11
- - 2Bdecided   Frank heavily criticised my proposal for storing R...   Nov 18 2003, 17:22
- - Gabriel   You might be interested by: http://sourceforge.net...   Nov 18 2003, 17:32
- - 2Bdecided   Moderators: On this page: http://replaygain.hydro...   Nov 18 2003, 17:35
|- - danbee   QUOTE (2Bdecided @ Nov 18 2003, 04:35 PM)Almo...   Oct 27 2004, 17:54
- - 2Bdecided   QUOTE (Gabriel @ Nov 18 2003, 04:32 PM)You mi...   Nov 18 2003, 17:38
- - Digga   QUOTE (2Bdecided @ Nov 18 2003, 05:35 PM)(It...   Nov 18 2003, 23:26
- - Mike Giacomelli   I would just like a way to write replaygain info i...   Nov 18 2003, 23:49
- - Gabriel   QUOTE Almost everyone is using a reference level o...   Nov 19 2003, 15:28
- - 2Bdecided   That would be fine. Or... Would the following ...   Nov 20 2003, 12:43
- - Gabriel   Seems interesting. It would be nice to hear other ...   Nov 20 2003, 13:04
- - 2Bdecided   Do none of the developers have any comments? Two...   Nov 21 2003, 12:24
- - phwip   QUOTE (2Bdecided @ Nov 21 2003, 12:24 PM)1. I...   Nov 21 2003, 12:31
- - 2Bdecided   QUOTE (phwip @ Nov 21 2003, 11:31 AM)QUOTE (2...   Nov 21 2003, 14:04
- - n68   QUOTE (2Bdecided @ Nov 21 2003, 02:04 PM)QUOT...   Nov 21 2003, 14:43
- - robUx4   Just a question from a user point of view. iTunes ...   Nov 23 2003, 17:03
- - robUx4   QUOTE (2Bdecided @ Nov 18 2003, 05:35 PM)Almo...   Nov 23 2003, 17:05
- - guruboolez   QUOTE (robUx4 @ Nov 23 2003, 05:03 PM)Just a ...   Nov 23 2003, 17:23
- - 2Bdecided   QUOTE (robUx4 @ Nov 23 2003, 04:05 PM)QUOTE (...   Nov 23 2003, 17:30
- - Case   QUOTE (robUx4 @ Nov 23 2003, 06:05 PM)QUOTE (...   Nov 23 2003, 17:55
- - dev0   3. ReplayGain lossy approximation Storing this se...   Nov 23 2003, 18:36
- - /\/ephaestous   QUOTE (2Bdecided @ Nov 21 2003, 06:24 AM)1. I...   Nov 24 2003, 06:02
- - /\/ephaestous   QUOTE (dev0 @ Nov 23 2003, 12:36 PM)3. Replay...   Nov 24 2003, 07:01
- - guruboolez   Try lame abr for exemple (there's a --scale 0....   Nov 24 2003, 11:18
- - 2Bdecided   QUOTE (Case @ Nov 23 2003, 04:55 PM)QUOTE (ro...   Nov 24 2003, 11:50
- - 2Bdecided   QUOTE (dev0 @ Nov 23 2003, 05:36 PM)3. Replay...   Nov 24 2003, 11:51
- - 2Bdecided   QUOTE (guruboolez @ Nov 24 2003, 10:18 AM)Try...   Nov 24 2003, 11:58
- - guruboolez   And what about the idea of a personnal track/album...   Nov 24 2003, 12:26
- - 2Bdecided   See my reply in your other thread, and my suggesti...   Nov 24 2003, 12:54
- - Case   QUOTE (2Bdecided @ Nov 24 2003, 12:50 PM)Sorr...   Nov 24 2003, 18:06
- - 2Bdecided   Another suggestion (this isn't fundamental)......   Nov 26 2003, 11:02
- - andyh   I'm confused as to how the RealLife level is d...   Jan 2 2004, 17:44
- - Lear   QUOTE (2Bdecided @ Nov 18 2003, 05:35 PM)(It...   Jan 2 2004, 19:10
- - Lear   QUOTE (2Bdecided @ Nov 20 2003, 12:43 PM)Fiel...   Jan 2 2004, 22:46
- - SamK   I think it's the right time to switch to absol...   Jan 4 2004, 15:48
- - knik   QUOTE (2Bdecided @ Nov 18 2003, 07:35 PM)Almo...   Jan 5 2004, 12:16
- - Mike Giacomelli   Stupid question: Is 0dB relative also 96 dB in 16...   Jan 6 2004, 06:28
- - SamK   QUOTE (knik @ Jan 5 2004, 12:16 PM)After clos...   Jan 6 2004, 14:52
- - 2Bdecided   QUOTE (Lear @ Jan 2 2004, 06:10 PM)QUOTE (2Bd...   Jan 6 2004, 15:12
- - knik   QUOTE (2Bdecided @ Jan 6 2004, 05:12 PM)I do,...   Jan 6 2004, 18:45
- - knik   QUOTE (SamK @ Jan 6 2004, 04:52 PM)So you...   Jan 6 2004, 20:56
- - 2Bdecided   QUOTE (knik @ Jan 6 2004, 07:56 PM)QUOTE (Sam...   Jan 7 2004, 14:07
- - 2Bdecided   QUOTE (Lear @ Jan 2 2004, 09:46 PM)QUOTE (2Bd...   Jan 7 2004, 14:16
- - Gabriel   Lame is using the fixed point representation from ...   Jan 7 2004, 14:23
- - SamK   QUOTE (2Bdecided @ Jan 7 2004, 02:07 PM)Repla...   Jan 7 2004, 15:57
- - SamK   reading http://home.earthlink.net/~bobkatz24bit/in...   Jan 7 2004, 20:34
- - Lear   QUOTE (2Bdecided @ Jan 7 2004, 02:16 PM)QUOTE...   Jan 7 2004, 20:47
- - knik   Thanks for explanation, 2Bdecided. It really helpe...   Jan 7 2004, 21:32
- - 2Bdecided   QUOTE (SamK @ Jan 7 2004, 02:57 PM)QUOTE (2Bd...   Jan 8 2004, 12:29
- - 2Bdecided   QUOTE (SamK @ Jan 7 2004, 07:34 PM)reading ht...   Jan 8 2004, 13:11
- - 2Bdecided   I remembered something last night. When it stored...   Jan 8 2004, 13:18
- - SamK   QUOTE (2Bdecided @ Jan 8 2004, 01:11 PM)No, b...   Jan 8 2004, 14:15
- - SamK   QUOTE (2Bdecided @ Jan 8 2004, 01:18 PM)I...   Jan 8 2004, 14:27
- - SamK   QUOTE so the relative gain itself will still be br...   Jan 8 2004, 16:29
- - SamK   To sum up my opinion, I am for storing 3 values i...   Jan 8 2004, 17:07
- - 2Bdecided   I think you've got more time to write that I h...   Jan 8 2004, 18:03
- - SamK   QUOTE (2Bdecided @ Jan 8 2004, 06:03 PM)I thi...   Jan 8 2004, 20:31
|- - David Nordin   QUOTE (SamK @ Jan 8 2004, 08:31 PM)QUOTE (2Bd...   Jul 18 2004, 09:10
|- - 2Bdecided   QUOTE (David Nordin @ Jul 18 2004, 08:10 AM)I...   Jul 19 2004, 12:02
- - Iconoclast_a   Well, I think the genre dependent adjustment could...   Jul 13 2004, 10:03
|- - Kuuenbu   There should be an option to turn the equal loudne...   Jul 18 2004, 00:54
|- - 2Bdecided   QUOTE (Kuuenbu @ Jul 17 2004, 11:54 PM)There ...   Jul 19 2004, 12:06
|- - Kuuenbu   QUOTE The peak sample value tells you _exactly_ ho...   Jul 26 2004, 02:18
- - Pio2001   I'd still like to see a plugin that would read...   Jul 18 2004, 08:07
|- - 2Bdecided   QUOTE (Pio2001 @ Jul 18 2004, 07:07 AM)I...   Jul 19 2004, 12:01
|- - Peter   QUOTE (2Bdecided @ Jul 19 2004, 02:01 PM)QUOT...   Jul 19 2004, 21:08
- - Ariakis   For CDs from lossy files in fb2k, you can clean yo...   Jul 18 2004, 08:43
|- - Pio2001   QUOTE (Ariakis @ Jul 18 2004, 08:43 AM)For CD...   Jul 18 2004, 17:32
- - indybrett   If it hasn't been brought up yet, I would like...   Jul 18 2004, 18:09
- - snek_one   only thing i have to get off my chest about replay...   Jul 18 2004, 18:52
|- - Cyaneyes   QUOTE (snek_one @ Jul 18 2004, 01:52 PM)only ...   Jul 18 2004, 19:30
|- - Pio2001   QUOTE (snek_one @ Jul 18 2004, 06:52 PM)only ...   Jul 19 2004, 20:50
- - indybrett   I don't think that is the same as what he is a...   Jul 19 2004, 21:51
- - dev0   This can be done by raising the Pre-Amp.   Jul 19 2004, 22:01
|- - indybrett   QUOTE (dev0 @ Jul 19 2004, 04:01 PM)This can ...   Jul 19 2004, 22:29
|- - Pio2001   QUOTE (indybrett @ Jul 19 2004, 09:51 PM)QUOT...   Jul 19 2004, 22:50
- - Xenno   2B > The peak sample value tells you _exactly_ ...   Jul 19 2004, 23:01
|- - SamK   QUOTE (Xenno @ Jul 19 2004, 11:01 PM)2B > ...   Jul 20 2004, 00:52
|- - SamK   QUOTE (SamK @ Jul 20 2004, 12:52 AM)my opinio...   Jul 20 2004, 00:55
- - 2Bdecided   The RG peak value is the largest absolute value wi...   Jul 20 2004, 13:07
|- - SamK   QUOTE (2Bdecided @ Jul 20 2004, 01:07 PM)The ...   Jul 20 2004, 14:08
||- - 2Bdecided   QUOTE (SamK @ Jul 20 2004, 01:08 PM)Wow, 1.5x...   Jul 20 2004, 14:50
|- - SamK   QUOTE (2Bdecided @ Jul 20 2004, 01:07 PM)isn...   Jul 20 2004, 14:19
- - dev0   CODEreplaygain_track_gain = -10.99 dB replaygain_t...   Jul 20 2004, 14:59
- - 2Bdecided   That's different to the latest issue discussed...   Jul 20 2004, 15:13
- - jcoalson   not a bad idea, e.g. replaygain_reference_level=9...   Oct 27 2004, 18:23


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