Try it. Copy a clean MP3. Run one through MP3Gain. Use MC audio analyze on both. The MP3Gain'd file will analyze to a lower level then the other (say -6 vs -11). Play them both. You'll hear no level difference.
Correct... cept the above assumes that mp3gain was run first and ONLY THEN was Replay gain applied via AA.
Yep, and if you know you have analyzed your library, then you'd know to do it afterward.Or, if you are concerned about levels then I'd assume you know enough about this sort of thing to know that the level of the file was changed so any other tool that deals in levels would need to be run again.
What is not clear to me is whether RG uses the volume in the mp3header or gets it from analysing the mp3 ? In other words if a file is mp3gained will RG result in a different value than if it was not mp3gained. It appears from the above that RG gives a different value (than w/o mp3gain) implying RG is affected by mp3gain.
Since there is really no way to determine the original level if the global gain field had been changed I'd assume that MC uses the file as is. My experience with the two show that to be the case anyway.
From the MP3Gain info files:
The mp3 format stores the sound information in small chunks called "frames". Each frame represents a fraction of a second of sound. In each frame there is a "global gain" field. This field is an 8-bit integer (so its value can be a whole number from 0 to 255).
When an mp3 player decodes the sound in the frame, it uses the global gain field to multiply the decoded sound samples by 2(gain / 4).
So if you add 1 to this gain field in all the frames in the mp3, you effectively multiply the amplitude of the whole file by 2(1/4) = 119% = +1.5 dB.
I was referring to the more common case where one has already Replay gained the whole library via AA. If you *THEN* mp3gain these files for the sake of volume normalisation in portable players, the current Replay gain settings they already contain are not acurate and will have to be re-run to be valid when listening in MC.
This is what i mean by tricky to manage, If you did not run AA on the library at all, then i agree with what you said.
Oh, but you know you did something to change the level of your files. You should know to run other items that deal with levels again. And there is no need to have two or more copies of the files. Just a little time to analyze one time. Going forward you know to rip, MP3Gain, then import and analyze.
Simple.