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

[Ross Levis]

Hi Beni

To alter the frame volume would be the most useful if possible.  It 
would provide for fading which I've found useful on occasions with mp3 
files.  It would also allow selective volume changes over a portion of 
the file.  There are 2 or 3 graphical MP3 editors around which can do 
the above very nicely.  The only hassle is the mp3 frame is limited to 
1.5db increments due to an 8 bit field.  Hopefully a vorbis frame volume 
can be altered using at least 16-bits.  Would Monty like to comment on 
whether this is possible or not.

Regards,
Ross Levis.

Beni Cherniavksy wrote:

>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?
>
>  
>

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