Reading waveforms and unclipping music files

Special Interest Groups Audio Processing
1

First, thnks to this forum. This is my first visit and a have a few doubts. My name is Renato and I’m a 56 y.o. brazilian guy with some bits and bytes experience. As I’m new with sound digital files maybe these doubts could be primarily questions. Sorry for that. Well, lets go to the questions: Looking into the MP3, FLC or WAV files with Audacity I think I discovered the reason for distortions and noises even with calm music: it seems to be “clipped”. I discovered too that I can use some resource of Audacity to fake an unclip: I amplify negatively to -12db, equalize reforcing bass and treble (to try to recontruct what was clipped) with a follow curve: +6db from 0 to 400Hz, a descendent curve from here to 0db (no change) to 1000Hz, keep flat till 2000Hz and then an ascendent curve to +6db till 7000Hz. Then keep +6db from 7000Hz to the end. After this i amplify the whole file with to get -0,5db as the louder. It helps a little 'cause the cliped parts is commonly very hard basses or trebles. But I know this is very fail and fake way to do that, and I understad that there is flies mortally clipped… Now I dicover an Audacity resource called “Clip Fix”, and now - finally - the questions:

1 - How does this Clip Fix works? I applied this effect as the manual says (firt decreasing the volume with -10db) and the waveform doesn’t seems to change at all…

2 - How can I understand the graphic showed by Audacity for a music file? I see that stereo files have two “sets” of graphic: the left and the right ones. But what are the dark (external) and the light (internal) blue areas in visualization mode = waveform? Why, taking a single channel as example, it’s waveform have assymetrical form? Tracing a horizontal line in the midle of the waveform, why the superior segment is not equel the inferior one, if this waveform refers just one chanel?

3 (and more important question) :wink: - If I’m boring you with my assume and very not-too-good english (excuse for that, please) or with dummy questions, where I could find answers for them for begginers, especially the way to read and understand wavforms?

Thanks!

P.S.: If this is not the right “room” in forum to post doubsts as the mine, please excuse again and indicate me the right place.

2

Versions as required:
Audacity 2.0.5 - Windows 7

Excuse but I didn’t find a way to edit the previous post. Thanks again.

Moderator note: I moved it for you.

3

Thank you, waxcylinder.
And, excuse but… nobody? The main question is how can I understand the waveform graphic? Where can I find instruction about, just to begining studies about?
Thanks.

4

ClipFix looks for flattened peaks - this sort of thing:
firsttrack006.png
and if found, can “restore” them like this:
firsttrack007.png
Note that it only works if the peaks have been flattened (as may occur if the audio is digitally clipped). Analog clipping tends to be less clearly defined, but the slider control in ClipFix can allow for a small amount of irregularity in the “flattened” (clipped) part. If the clipping is not reasonably flat, then ClipFix will not be able to detect the clipping.

Note also, that this effect is not “magic” :wink:
It can only guess at what the audio might have been like if it were not clipped. In cases of very mild clipping, it can guess quite well, but in cases where the clipping is severe there is no chance of recovery - the results of “repair” (when the clipping is really bad) may sound worse than un-repaired.

5

Looking to the graphics you posted I can understand a lot of things. I can zoom my test-file and find something like the pics you posted. Normaly in a horrible-file sound, severe clipped as you say, we can’t see the peak of the waveform, it goes beyond the margins. And when I decrease the volume, now I can see a lot of curves clipped, practicaly the top and the bottom of the waveform as an horizontal line. In that case, as you said, I better re-rip the music.

Thanks, Steve. And as I commented this afternoon with a friend about my new “adventure” with digital sound, he recommend this site, only for beginers… I read it not deeply, just to see if it could help a beginer and, well… It may help someone: http://www.animemusicvideos.org/guides/avtech/index.html

Thanks again!