Is there any way to reverse compression and is it worth it?

[pdun459]

I've started noticing compression and intentional clipping on many of my music files. I attached one example from an Alan Parsons CD. Practically the whole CD looks like that. My question is does anyone know if there's a way to reverse these effects and would it make that much difference in sound quality? It sounds okay as it is, but I'm wondering if it would be worth the trouble to try and improve it. Anyone have any experience here?

[kozikowski]

No, not really. That's the result of the Loudness War where everybody knows you need to issue music as loud as possible in order to maximize sales.

The trick is to make it sound half-way reasonable while you're doing that. The process is to reduce the loudness variations and change volume on the fly in a show depending on content. This puts it in the same league as a security encryption scheme. The only way to put it back the way it was is to know the way it was -- the circular reasoning thing.

Koz

[DVDdoug]

There is a Clip Fix effect you can try. I believe it's one of the optional LADSPA plug-ins.

When I tried Clip Fix once or twice, it made the waveform look better but didn't really fix the audible distortion. You're not hearing distortion, but Clip Fix may improve the overall sound.

There are a few other clipping repair tools around. I believe Izotope RX and Adobe Audition both have them. SeeDeClip (~$25 USD & up) is a stand-alone program.

In theory, you can un-do compression with expansion. But, you can't un-do clipping or limiting because it's impossible to know the original height or shape of the waveform. These repair programs have to "guess".

In the real world, expansion doesn't work* because we don't know the original compression parameters (attack, release, threshold, ratio), the individual tracks could have been compressed before mixing, and there can be multiband compression and other unknown variables.




Note that MP3 and vinyl records "distort the distortion" so the clipping is no longer perfectly flat, making it harder to detect with software. (But if it's audible, you'll still hear it.)

People will often claim that the vinyl is less compressed because the waveform looks better and the dynamic range often measures "better" when it's all really from the same master. But in your case, this is an older recording and there is a good chance that the audio has been re-mastered and the original vinyl was more dynamic. But, I'm not saying that scratchy 30-year old vinyl is actually going to sound better than a CD.



* In the analog days, DBX had a noise reduction system that worked by compressing before recording and complementary expansion during playback and phone systems used a similar technique.

[DVDdoug]

P.S.

Expansion can be fun to play with, and you MIGHT get some useful results, but Audacity doesn't seem to come with an expansion effect so you might have to look for a plug-in or try some different software. Maybe someone else can suggest a plug-in...

I also use GoldWave ($60 USD). It has a compressor/expander effect. There is a fully-functional free trial if you want to play around with it. The GoldWave compressor/expander is a little weird, because it sometimes says it's compressing when it's really expanding. But, there are presets such as "Boost Loud Parts" or "Reduce Quiet Parts" which are both examples of dynamic expansion.

If you do expand your peaks, remember to normalize (bring the volume down so your peaks are 0dB or less) to prevent clipping when you save.

[pdun459]

What I've been doing is removing DC offset and applying Clip Fix then normalizing. I'll probably just stick with that as the sound isn't that bad and it sounds like the results might not be worth the extra trouble.

Thanks for the insights and suggestions.

[Robert J. H.]

Chris' Compressor allows expansion if the compression factor is negative.
The parameters for a master limiter could probably be guessed. The attack/release is usually very short. The ratio goes towards infinity (Hard Limiter, not recoverable), at least 1:10 and the threshold is often a integer value such as -3 dB, -6dB or even -10 dB.

[kozikowski]

Chris' Compressor allows expansion if the compression factor is negative.

I did not know that.
Koz

[Gale Andrews]

There is a Clip Fix effect you can try. I believe it's one of the optional LADSPA plug-ins.

To clarify, Clip Fix is a Nyquist plug-in and is shipped with Audacity: http://manual.audacityteam.org/o/man/clip_fix.html .


Gale