[vorbis] RE: Hlp in finding a native Ogg trim, fade & nomalise tool

[Beni Cherniavksy]

On 2002-10-02, Ross Levis wrote:

> Hi John
>
> A trimer/vcut type application is possible but unfortunately not a
> fade/normaliser.  Not without decoding/encoding.  I asked about this a
> couple of years ago.  Unlike MP3, the Ogg Vorbis format doesn't store a
> global gain setting per frame.
>
I think this:

http://www.xiph.org/archives/vorbis-dev/200105/0139.html

comes from the thread which you refer to but it's the part which was
crossposted to vorbis-dev where some more detailed answers were given.
Look for the following messages by Monty and Segher...  To sum up, there
actually is a per-frame volume in Vorbis (but twiddling it might be a bit
complicated and no tools were ever written for this) and the volume of the
whole stream can be changed by very easy codebook manipualtion (it might
have become less easy with the introduction of channel couplig).  The
later (amplifying the whole stream) is now implemented in ReplayGain by
most self-respecting vorbis players so there is no need for the codebook
manipulation anyway...

> I have often downloaded MP3 files from mp3.com and other sources for
> broadcasting, and with the type of music I need, it is often not
> normalised before encoding so I find it very useful to be able to
> normalise the volume of the MP3 file without losing quality.It is a
> shame that in the future when Ogg Vorbis is the codec of choice, I'll
> have to re-encode.
>
>From following messages I understand that you don't just want to normalize
(== scale the whole sound) but to compress it (== variable scaling
depending on local volume average).  The first is already done by
ReplayGain well.  The second is best achieved by player tricks (like
ReplayGain with extra amp and a limiter or generic compression plugins).
I don't quite see the reason for compressing the vorbis file itself (even
if it was lossless) since compression generally lowers audio fidelity.
Store the vorbis file as is and compress before playing through media
requiring it (like FM).

However (read above thread), there is a theretical possibility to
losslessly manipulate volume locally in Vorbis.  The only problem is that
the envelope is defined with packet granularity (somewhat smoothed by the
window function).  This will make the envelope sort of "wavy" which might
distort the frequency domain (any modulation does but this adds
higher-frequency componenets to the envelope than usually).  I'm don't
know whether this can introduce audible distortions with typical
fade-in/out transition lengths; I suppose it would be OK.  I don't fill
very good about doing it all throughout the song (for compression) - but
you'll need somebody more knowledgeable than me to clarify this....

> Hopefully some more work can be done to reduce the artifacts from
> re-encoding.
>
Another trick once suggested for fade in/out at ends of the pieces (or
even cross-fading) is to reencode only the ends and copy the middle as-is.
This should leave the degradation almost unnoticable.  Obviously it
doesn't solve compression but why do you want it?


-- 
Beni Cherniavsky <cben at tx.technion.ac.il>

Awake! Awake! Fear, Fire, Foes! Awake! Fire, Foes! Awake! [Tolkien, LOTR]
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