Hi Arne,
Thanks for finding this. This was a subtle & rare bug in CBR,
triggered by the -k option.
(-k, by the way, is *always* a bad idea. It overrides LAME's
default lowpass filters. It will cause ringing and twinking
at 128kbs)
Note to developers and -X6 fans:
The problem is in frame 2326, side channel. It has a lot
of energy > 15khz. Trying to encode these frequencies
got LAME stuck in the following loop:
1. increase scalefactor for bands with distortion by 1.
2. decrease global_gain so bits < targ_bits
(decrease in global_gain exactly offsets change in scalefactor)
net effect on bands with distortion: NO CHANGE!
Thus over, over_noise, max_noise are all unchanged,
but tot_noise is increasing.
In this case, -X0 would accept the newer quantization as better.
So LAME just kept increasing the scalefactor bands
and decreasing global_gain until it got into a
bizzare configuation. The fact that scalefac_scale
is now enabled, allowed LAME to further aggravate the
situation and create an audible glitch in the mp3 file.
I just committed the following fix:
If over == best->over, and over_noise == best->over_noise,
then use tot_noise to decide on the best quantization.
This makes -X0 (the default) very close to -X6,
although -X0 still gives precedence to 'over'. Changing
that would be too radical at this time :-)
Mark
>
> Hi Mark,
>
> I accidentially discoverd a bug in lame which I really can't understand. I
> have one wav audio track to which, when encoded, a short period of noise is
> added. I'm not able to describe this, listen yourself:
> http://www.home.unix-ag.org/arne/x/cdda.wav.mp3 (1.1MB)
> the original wav-file is at
> http://www.home.unix-ag.org/arne/x/cdda.wav (12MB)
> The noise appears between time offset 1:00 and 1:01 and is clearly audible.
> What I consider strange is that it doesn't seem to appear when
> * encoding with other parameters than -h -k -b 128
> * only a part of this track is used, i.e. I tried this with a rip which
> started at 0:50 in order to save bandwidth -> no noise
>
> I used cvs-lame, one of today, 2 hours ago and one with 2 days of age.
>
> I'm writing directly to you because the part of the track is too long to be
> considered legal. Please don't pass it unnecessarily around. I guess it's ok
> for technical examination.
>
> Of course I still consider lame the best MP3 encoder around, nice work...
>
> cu arne
> --
> Arne Zellentin
> [EMAIL PROTECTED]
>
--
MP3 ENCODER mailing list ( http://geek.rcc.se/mp3encoder/ )
