[Lame-dev] CBR bug (rumble/distortion) in Lame 3.98.x

[Fabian S. <f-s...@gm...>]

Dear LAME developer team,

for years I've been using LAME, as a stand-alone tool and inside different applications that come with it bundled or as the "under-the-hood" encoder.
I know that VBR encoding is much more powerful, efficient and generally provides better quality vs. file size. But for several reasons I often still want to encode CBR files at 192 kbit/s, which generally gives a very good quality and high compatibility to older devices, such as car radios, DVD players and so on.

For a very long time, I was using version 3.96 as it was the version that came with different pro audio apps such as Steinberg WaveLab 5 etc. But I wanted to switch to a new version, 3.98.2, recently. This is also the version that comes with Winamp (and is - unfortunately, as you will see below - also used by last.fm to encode their samples).

The thing is that I noticed a strange behavior in version 3.98.2 (and I think also in 3.97) when encoding CBR files: A strange, rumbling background noise is added to the low frequencies. This has definitively not been there in versions before 3.97 and - for a listener with focus on quality, like me - very disturbing. It is audible especially in classical, vocal / choral, and orchestral pieces.

Maybe you can consider this and have a look at the psychoacoustical routines for CBR mode. Note that I for myself will just stick to 3.95 which does not suffer from this bug, but all the many people out there who use LAME in CBR mode to mass-produce files without quality-checking them afterwards (as, for example, last.fm, and also all the people ripping their CD library into MP3 using Winamp!) are definitively producing broken files at the moment.
I personally feel a little bit unhappy about this.

To me, it sounds a little bit like LAME tries to "mask" or "retouche" over the possible artifacts and has lowpass filter driving mad... That could cause the rumbling (like in some early ogg vorbis encoders).

To give you an example of what it sounds like, please take some moments to listen to the following files (especially the beginning of the sample):
LAME 3.95, 128 kbit/s CBR: http://stashbox.org/672372/sample395.mp3
LAME 3.98.2, 128 kbit/s CBR: http://stashbox.org/672373/sample398.mp3

I know that you have really good psycho-acoustics devs out there that will hear the difference and the rumbling, bubbling lower frequencies....

Just listen at the very first, powerful brass chord. You will notice that in the 3.98 version, there is an indifferent rumbling and bubbling in the background, while in the 3.95 version it is all clear and without rumbling. Of course, at this bitrate, you will encounter some little artifacts in the high frequencies on those "sharp" trumpet sounds, but 3.95 definitively does NOT show that indifferent backround bubbling.

Thank you very much for your time and your consideration!

With the best regards from Mainz, Germany,

Fabian Schneider


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