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Contant Peak, Constant RMS, How to get a fixed value for peak and RMS
post Dec 12 2012, 21:32
Post #1

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Hi, we are trying to set a default standard for the volume of the audio files we are working on.
We are looking for a tool that, given a bunch of mp3 files, will be able to normalize them at a given db.

For example : 0db for Peak levels and -9db for RMS.

Ideally, we'd love to use sox as we can chain it with other effects we're using without the need to convert multiple time the file from mp3 to wav and back.

We tried sox gain -n , sox gain -ne, sox gain -nb, sox loudness but none of them keep a constant RMS without varying the Peak level as well.
Any help appreciated.
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post Dec 13 2012, 19:15
Post #2

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If you boost the level by 3dB, the average and peak both increase by 3dB (assuming no limiting or clipping.) The ratio* between the peak and average is constant unless you do non-linear processing (clipping, limiting, compression).

1) Peak normalization to 0db using sox
2) normalize-audio -a -9dBFS for a standard RMS of around -9db
You can skip step 1. Peak Normalization is a volume adjustment, and adjusting to
-9dB is another volume adjustment.

3) sox vol 0db 0.01 for a soft limiter to prevent clipping
After adjusting to -9dB RMS, you'll have lots of "signal" above 0dB and several dB of limiting, which will probably cause some files to sound distorted. Soft clipping isn't as noticeable as hard-clipping, but it's still distortion.

Instead, it seems mp3gain does a batch analisys and make some sort of average.
When you change the audio batch, the average change, making the volume inconsistent between batches.
The "Album Gain" is a batch adjustment to keep the same relative levels throughout the album.... Loud songs remain loud, and quiet songs remain quiet the way the album was intended. If you choose "Track Gain", the songs are processed independently.

ReplayGain works great for me!, but nothing is perfect! Two humans might not even agree when two different songs are equally loud, especially if a song has loud-parts and quiet parts. Dynamic compression and limiting can help a lot, but they destroy the dynamic contrast in the music (if over-done).

* Decibels are logarithmic, so the dB is a constant number. If the peaks are twice as loud as the average, that's a 6dB difference and when you increase/decrease the volume (linearly), the ratio remains 2:1 and difference remains 6dB.

This post has been edited by DVDdoug: Dec 13 2012, 19:34
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