Nobody's interested ?

http://w3.mit.edu/cheever/www/cheever_thesis.pdf

"Lionel" wrote in message


Nobody's interested ?

http://w3.mit.edu/cheever/www/cheever_thesis.pdf

It's based on Stereophile reviews, sighted listening evaluations and the
idea that there's some unique desirable something about tubed amps. That
would be three strikes against. The author is kind enough to discredit
himself early in the paper.

If you want to read something on the same topic that is actually credible,
try these papers:

Audio Engineering Society Convention Paper 5890 Auditory Perception of
Nonlinear Distortion - Theory Earl R. Geddes 1 and Lidia W. Lee

Audio Engineering Society Convention Paper 5891 Auditory Perception of
Nonlinear Distortion Lidia W. Lee 1 and Earl R. Geddes This paper presents
practical results related to the application of the ideas proposed in paper
5890.

Basically, they propose weighting nonlinear transfer function coefficients
based on the cosine of the amplitude times order squared.

The claim is made that the audibility of nonlinear distortion is inversely
related to amplitude (nonlinearities at low levels are more audible) and
also related to order squared (nonlinearities that generate spurious
responses that are more broadly dispersed from the original signal are far
more audible). They back their new metric up with listening test results
showing that this criteria does a better job of fitting a variety of
synthetic distortion sources to perceptions of degraded sound quality.

In short, the Geddes/Lee papers show that the way most people formally
characterize audio gear nonlinear distortion today, which focuses on high
outputs and unweighted harmonics, is about as close to irrelevant as one
could imagine. The papers report experimental studies of current metrics
that support the idea that they are irrelevant or at least uncorrelated with
human perceptions of sound quality.

My own simplistic research into the subject and the scientific literature
was highly supportive of Geddes/Lee general thinking before they got the far
more polished and complete results that are published in the articles. My
point here is that it seems reasonable to view the Geddes/Lee results as
being orthodox and reasonable, as far as they go. To their credit they seem
to have found a far better metric, but maybe not the best metric. I'll take
better as long as it is the best we have!

Today, most audible nonlinear distortion comes from loudspeakers and other
electromechanical/electroacoustic transducers. Nonlinear analysis of
speakers is in its infancy, but two methods for characterizing speaker
nonlinear distortion are currently being used pretty widely. One is due to
Klippel, and one is due to Clark and Geddes. AFAIK the Klippel methodology
only produces results for the second and third order, which would appear to
be a significant failing. The Clark & Geddes methodology (AKA Dumax)
produces results for higher orders as well which would appear to be a
significant strength.