by ZvezdanD » Thu Apr 23, 2009 5:45 pm
I don't know exactly how MM works, but I suppose if you choose the option "Update tags when editing properties" from the Options dialog and if you choose "Analyze Volume" that MM stores the calculated value to some tag in mp3 file (this is what you say that could be undone). Such value could read only players that support Replay Gain value entered to tags. On another side, if you choose "Level Track Volume", the global gain flag would be adjusted for all mp3 frames (this is what you say that cannot be undone) and such mp3 file could be reproduced equally even on players which don't support Replay Gain standard. However, even in that case there is not re-encoding as you wrote, and actually you could revert back volume level for mp3 file without sacrificing its quality. You only need to know the starting value for the volume level, before its modification.
If you turn off the option "Ignore (do not read or write tags)" in the MP3Gain program and when you choose "Apply Track Gain" (which is same as "Level Track Volume" in MM), after that you could undone this process with its option "Undo Gain changes" if you are not satisfied with the result. MM doesn't have have such option, and this is one of reasons why I prefer MP3Gain. However, as I said, you could always return to the previous state without audio degradation, even in MM, but only if you know the original track volume.
By the way, the global gain flag is just one value for each mp3 frame (IIRC 2048 bytes) which determines audio volume for that frame. If you change that value for some frame of mp3 file, this doesn't mean that you are changing music since the frame with the audio data remains unchanged.
If you don't have MP3Gain installed, I recommend it, just to be able to read its help. Here is some excerpt from it, Glen is much better in explanation than me:
The bad news: MP3Gain can only adjust the volume of your mp3 files in steps of 1.5 dB.
The good news: 1.5 dB is a small enough step for most practical purposes. Most humans can just barely hear a volume change of 1 dB.
The other good news is that this volume adjustment is completely lossless. In other words, if you adjust an mp3 by -6 dB and then change your mind, you can adjust it again by +6 dB and it will be exactly the same as it was before you made the first adjustment.
Here's the technical reason why it's lossless, and also why the smallest change possible is 1.5 dB:
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.
Likewise, if you subtract 1 from the global gain, you multiply the amplitude by 2(-1/4) = 84% = -1.5 dB.
I don't know exactly how MM works, but I suppose if you choose the option "Update tags when editing properties" from the Options dialog and if you choose "Analyze Volume" that MM stores the calculated value to some tag in mp3 file (this is what you say that could be undone). Such value could read only players that support Replay Gain value entered to tags. On another side, if you choose "Level Track Volume", the global gain flag would be adjusted for all mp3 frames (this is what you say that cannot be undone) and such mp3 file could be reproduced equally even on players which don't support Replay Gain standard. However, even in that case there is not re-encoding as you wrote, and actually you could revert back volume level for mp3 file without sacrificing its quality. You only need to know the starting value for the volume level, before its modification.
If you turn off the option "Ignore (do not read or write tags)" in the MP3Gain program and when you choose "Apply Track Gain" (which is same as "Level Track Volume" in MM), after that you could undone this process with its option "Undo Gain changes" if you are not satisfied with the result. MM doesn't have have such option, and this is one of reasons why I prefer MP3Gain. However, as I said, you could always return to the previous state without audio degradation, even in MM, but only if you know the original track volume.
By the way, the global gain flag is just one value for each mp3 frame (IIRC 2048 bytes) which determines audio volume for that frame. If you change that value for some frame of mp3 file, this doesn't mean that you are changing music since the frame with the audio data remains unchanged.
If you don't have MP3Gain installed, I recommend it, just to be able to read its help. Here is some excerpt from it, Glen is much better in explanation than me:
[quote]The bad news: MP3Gain can only adjust the volume of your mp3 files in steps of 1.5 dB.
The good news: 1.5 dB is a small enough step for most practical purposes. Most humans can just barely hear a volume change of 1 dB.
The other good news is that this volume adjustment is completely lossless. In other words, if you adjust an mp3 by -6 dB and then change your mind, you can adjust it again by +6 dB and it will be exactly the same as it was before you made the first adjustment.
Here's the technical reason why it's lossless, and also why the smallest change possible is 1.5 dB:
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.
Likewise, if you subtract 1 from the global gain, you multiply the amplitude by 2(-1/4) = 84% = -1.5 dB.[/quote]