[vorbis] just an idea about quality evaluation

[Lorenzo Prince]

The problem with using technical methods to judge the quality of a
perceptual codec is that the codec is designed to be tuned to the human ear.
For example, an mp3 or Vorbis file encoded at a bitrate (mp3) of 128 or
quality setting (Vorbis) of 4, which are roughly the same file size, when an
application is applied to them that removes center material from the stereo
image, will have some swishing in the left-over vocals/removed material that
remains due to an echo or similar occurance.  However, the human ear, when
the center material is not removed doesn't even hear this effect.  However,
a difference between the two file formats does become aparent when this is
done.  Incidentally, when using a vocal remover application on both files,
the swishing that occurs in a Vorbis file has no affect on the rest of the
music.  However, when you listen to the mp3 file, the swishing that occurs
greatly distorts the surrounding music.  However, these are all just
technical differences.  You have to let the ear hear what it hears to
actually judge the perceptual quality of the encoded file.  It just so
happens that some difference in the quality of the two above mentioned files
is actually apparent to most listeners.  At this bitrate/quality setting,
the Vorbis file actually does perceptually sound much better than the mp3.
but this was meant to be an example of how much the application of technical
equipment and operations on decoded files can greatly distort the actual
perceptual quality of the encoded file when heard by the human ear.
However, when comparing two file formats, this method may be used to judge
the technical aspects of the encoded files.  It can give you a good idea of
just how much of the actual music is being taken out by the compression
scheme and the psychoacoustic models of the encoders.  Most likely, the
differences you described are caused by the channel coupling of the encoder.
The encoder compares the left and right channels and couples them according
to the perception of the listener in an attempt to achieve greater quality
in a smaller file.  But when technical applications such as phase inverters
and vocal removers are used, the mathematical differences are exploited and
the sound you hear becomes both perceptually and mathematically much
different from the original.
Regards,
Lorenzo

----- Original Message -----
From: "Stoffke" <stoffke at directbox.com>
To: <vorbis at xiph.org>
Sent: Friday, January 24, 2003 11:16 AM
Subject: [vorbis] just an idea about quality evaluation

<p>Hello

I had an idea about judging the quality of ogg vorbis
(or any other lossy codec)

I took a wave-file and encoded it to ogg.
Then decoded ogg to wav and inverted it's phase. When mixing the original
wav with the phase-inverted decoded ogg-file, any identic parts of
compressed and uncompressed audio should be eliminated.
Of course there's always a "rest" of sound because the encoder is not
lossless.
Could this "rest" be an indicator for the quality of the encoder ?

I know that evaluating the quality of a perceptual encoder with any
technical equipment is nearly impossible and it would be better to let the
ear do this.
But I'd like to know what you think about this idea .

Greetings
Stoffke

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