[Vorbis] oggenc adds severe distortion

[xiphmont at xiph.org]

On Wed, Aug 13, 2008 at 1:18 PM, Ti Kan <ti at amb.org> wrote:
> Hi all,
>
> I routinesly rips my CDs to WAV and then convert to ogg vorbis format
> for use in my car and portable player.  I don't usually notice anything
> amiss, but on the last track of Mike Oldfield's "Music of the Spheres"
> album ("Musica Universalis", at the very end crescendo), the converted
> .ogg file exhibits terrible distortion (sounds like digital clipping).
> This does not occur with MP3 or WMA formats.  I used a quality factor of 4.
>
> Does oggenc raise the amplitude (to cause digital clippiing)?  Any
> idea why this happens?

No, but it does lose precision.  What is happening is not that Vorbis
is amplifying, it's that the other encoders are attenuating when near
clipping.  Vorbis does not do this; it is capable of representing and
outputiing digital signals of > +0dB, and most players just clip it.

> Also, my previous experience with ogg vorbis format is that it provide
> nice, seamless gapless transitions between tracks.  However, on this
> same Mike Oldfield album, I can hear a click and discontinuity between
> certain tracks.  Since this is a continuous symphonic piece that
> spans across tracks, this is annoying.  What is it with this album
> that taxes the capability of the vorbis encoder?

A DC offset or alot of subsonics that aren't being modelled because
they're inaudible?  (That's a guess).  What do the beginning waveforms
look like?

Monty