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Topic: distortion / clipping in mp3 and AAC's converted from FLAC (Read 6282 times) previous topic - next topic
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distortion / clipping in mp3 and AAC's converted from FLAC

I converted my entire collection of cds with autoflac last year.  Thanks to all the help in the forums!  I then transcoded the FLAC's to AAC's using winamp.  I also transcoded them into mp3's with lame in foobar (192 vbr) .  I have a small handfull that have very audible distortion in them.  It sounds like they are clipping at crescendos.  The most prominent are on my ambient albums by the artist Hammock.  There are deep bass-filled crescendos full of nasty crackly clippy sounds.  I checked the original FLACs and it isn't there.  The playback system I am using at the moment is not great though, so I couldn't tell for sure if there was zero distortion in the FLAC's or just much less pronounced.  Any Ideas?  It seems like the conversion is increasing the gain just enough to make them clip -maybe?  Sorry if this has been answered before, but I couldn't really find it.

distortion / clipping in mp3 and AAC's converted from FLAC

Reply #1 –
- Ignore me. I am stupid. -
 

distortion / clipping in mp3 and AAC's converted from FLAC

Reply #2 –
Quote from: j7n on
Use a program capable of scanning for Replay Gain statistics (Foobar2000) to determine by how much the MP3s and AACs are clipping.

How does such a program actually determine "by how much"?

EDIT: I wish I had quoted your entire post, j7n.  Besides the part that I'm unsure about, your post was right on the money.  I know that the max peak is stored, I just question how good of an indicator the number is in terms of audibility.

Anyhow, a similar discussion is going on here...
http://www.hydrogenaudio.org/forums/index....showtopic=65321

To summarize the important points...

When converting to mp3 or aac, the encoder won't change the volume of the files unless specifically instructed to do so (Lame has the --scale command, for instance).  However, it is possible for aac and mp3 to go beyond full-scale as it relates back to the lossless source and is a perfectly normal part of the lossy encoding process.  Internal to these lossy files, the data going beyond full-scale isn't actually clipped; it occurs upon decoding.  Whether or not this clipping is audible really depends on the samples being encoded.

In order to keep these lossy files from clipping, it is possible to lower their gain using a program like mp3gain (works with aac files as well).

distortion / clipping in mp3 and AAC's converted from FLAC

Reply #3 –
Quote from: nexus77 on
I then transcoded the FLAC's to AAC's using winamp.  I also transcoded them into mp3's with lame in foobar (192 vbr) .


I am not 100% sure on that, but i believe that if you had the Equalizer or any other DSP activated in Winamp, the conversion will use it.  In foobar you have to manually tell it to use them (DSP).

Being ambient music, it may happen that the replaygain analisys (if applied) could want to increase the volume. This wouldn't be a problem if you had the option selected in foobar that says apply gain and prevent clipping according to peak. (In Preferences, Playback).

You could also decode the file to wav with foobar (use the "convert to" option aswell) and check in a wave editor if it clips on the places you hear these problems. If it is not clipping, it could help to find out what the problem is.

distortion / clipping in mp3 and AAC's converted from FLAC

Reply #4 –
Thanks for the thoughts and ideas.  I will see if I can give it a shot!  I did rip these to Flac with replay gain enabled, but when I transcoded to mp3 and AAC, I didn't intentionally do anything with the gain.

distortion / clipping in mp3 and AAC's converted from FLAC

Reply #5 –
Is it possible that when you play the Flac files the replaygain information is used so that they are played back at low enough volume that they do not clip, but when you converted to lossy format the replaygain information was lost so they are now played back louder and clip?