Rockbox Development > Feature Ideas

FS#10199 - Dynamic Range Compression

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Chronon:
So, we already have ReplayGain for normalizing perceived loudness between tracks (or albums).  But as has been pointed out, there are other situations where compressing the dynamic range within a file can be desirable -- for example in listening to tracks with large dynamic range in noisy environments. 

ykevinw:
For anyone wishing to implement this here is some info on an algorithm that works quite well:
http://vlevel.sourceforge.net/about/

There is a foobar 2000 plugin made from it:
http://stephan.kochen.nl/proj/foo_dsp_vlevel/

Hydrogenaudio thread:
http://www.hydrogenaudio.org/forums/index.php?showtopic=22057


I have used this as a dsp in foobar2k when making mp3s from lossless to play in my car or to play during dinner. It is more of a sophisticated micro volume adjuster rather than a true dynamic compressor. It doesn't seem to mung the sound at all and is very effective for classical music in high noise environments.

-kev

gevaerts:

--- Quote from: ykevinw on September 11, 2008, 12:20:21 PM ---For anyone wishing to implement this here is some info on an algorithm that works quite well:
http://vlevel.sourceforge.net/about/

--- End quote ---

"It's different from other dynamic compressors because it looks ahead. "

Depending on how far it needs to look ahead this could be a problem

edit: forget that. Only a few seconds seem to be enough

ykevinw:
It does look ahead, by default 2 sec. The algorithm works well for classical music which tends to ramp between loud and soft passages. It really does work just like an automatic intelligent "hand on the volume".

One odd effect is when a sudden loud sample comes up - you can notice the level ramp down just before. You can adjust this time window. This is not a big deal as most pieces tend to get quiet before a big trumpet blare to heighten the effect anyway.

Another effect is with the silence before and after a piece. The filter will ramp up to the noise level (tape hiss) if you let it. You can adjust the max gain to take care of this nicely. A gui would have to be made for these settings.

Check it out in foobar2k with a few pieces, I've been very happy with it and it would be great to be able to do this within Rockbox.

mborus:

--- Quote ---Anyone willing to take a look at implementing this? It's been out there for a couple years now and sounds like a killer idea!!! Especially for those of us who use Rockbox in the car/ on the highway.
--- End quote ---

On the Archos models there's the "Auto Volume" feature. If you set the AV Decay Time to 20ms, you get a very poor, but working compressor (which is great if you want to listen to speach in noisy environments like airplanes)

It sounds pretty awful with music, but is acceptable if it's the only way to get the volume high enough.

Regarding the look-ahead compressor: I prefer a one that lowers the volume at the last moment possible, because that's how normal sounding compressors work. For classical music, this might work fine. For dance and pop music, it sounds very strange to me.

BTW, for Windows Winamp-Users the best free compressor I know is Stereotool. There's also a DOS command line version you can use to batch compress your music before putting it onto your Rockbox player...

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