TEF (or TDS)

[Angelo Bello]

>>> 
>>> At 10:27 AM 9/7/97 -0700, Phyllis or Robert Bristow-Johnson wrote:
>>> 
>>> >> A room is actually a whole bundle of linear systems, which are
usually
>>> >> time invariant for audio purposes (exceptions being if you have a
big
>>> >> fan running in front of a sound source, etc)
>>> >
>>> >that, is an interference source and if you can insure that the fan
>>> >frequency is unrelated to the MLS length (i don't think it would
matter
>>> >for TDS which might give it an advantage over MLS) you can treat it as
a
>>> >noise source and if you make your measurement time long enough, the
S/N
>>> >ration of the result gets better and better.
>>> 
>>> No, you can't. Ever put a speaker through a fan? You get Doppler
effects
>>> which don't average out. Try it. It's actually a kind of weird effect.
>>
>>i misunderstood you.  i was thinking more off an additive interference
>>thing rather than souping up the impulse response.  who or where would
>>_anyone_ put a fan directly in front of loudspeakers unless they did it
>>for a musical effect like leslie?
>
>Actually, I heard once of a control room with a big ceiling fan which had
this
>kind of effect. But the only person I know of who actually would
intentionally
>do this sort of thing is Robert Kasper who would use it for effect. It's
kind
>of like the thing he once proposed - running his guitar through a 12"
>speaker as it dropped into a washtub full of oil in front of about 18
>different
>strategically placed mikes. I hear he has his own studio in Pittsburgh and
>God only knows what he's done with it.
>
>Later,
>Andrew Mullhaupt


Hello - i'm a composer working directly with the sound etc., but originally
come from the enginnering world.  I learned about HOS in a DSP course four
years back.  The technique can be used to detect nonlinearities in acoustic
signals, and in one specific area, to correct for reverberation corruption
as well perform echo cancellation, if one wants to detect or correct for
such nonlinearities.  

The extract at the end in this email is taken from the site:
http://www.amsta.leeds.ac.uk/Applied/news.dir/issue2/hos_intro.html
if your interested.

Also...

You may also want to check out the page/site www.atcsd.com for an
interesting new application of sound reproduction without "distortion
contributed by a loudspeaker".  They suggest: "What if a technology could
be developed that would eliminate the undesirable artifacts of loudspeaker
systems. Think about an audio reproduction system that could produce the
broad range of frequencies required for human hearing with no direct
radiating transducer devices, no crossovers, a single point source for the
entire frequency spectrum, no cone or cabinet resonances, no measurable
distortion, 10% electrical to acoustical efficiency, and reduced weight,
size, and cost."  This "appears" to be a very different paradigm approach
to sound reproduction - and I have a funny feeling that a few nonlinearity
aspects in acoustic/sound reproduction are being exploited here.

What do you think?

===========================================================================
===

UK Nonlinear News, Sept. 1995 
Introducing Higher Order Statistics (HOS) for the Detection of
Nonlinearities

S McLaughlin, A Stogioglou and J Fackrell 
Department of Electrical Engineering, University of Edinburgh 


Introduction

Engineering judgement concerning the predictability of a signal is often
based on an examination of the signal spectrum. The conclusion is then
drawn that if a signal has a flat or near-to-flat spectral density that the
quality of the prediction will be poor. While this line of reasoning can
provide useful guidelines in the design of linear predictive systems it is
not true in general not least because it ignores the existence of purely
deterministic mechanisms which generate signals with flat or near-to-flat
spectral densities. 

Speech or music signals are generated mechanically by systems with
nonlinear dynamics. If the prediction and coding quality of such signals is
to be improved then more of the information available in the signal must be
used : the signal higher order statistics (HOS) must be exploited. 

The aim of this article is to introduce HOS to a wide audience (assuming
basic knowledge of signal processing and statistics), and to outline some
of the reasons why they can be useful in practical applications. 

History of Higher Order Statistics (HOS)

Several key papers in HOS were published in the 1960's, but most of these
papers took a statistical and theoretical viewpoint of the subject. It was
not until the 1970's the people started to apply HOS techniques to real
signal processing problems. The last 15 years has seen a revival of
interest in HOS techniques, and there is now a growing number of
researchers around the world working in this field. 

In recent years the field of HOS has continued its expansion, and
applications have been found in fields as diverse as economics, speech,
seismic data processing, plasma physics and optics. Many signal processing
conferences (ICASSP, EUSIPCO) now have sessions specifically for HOS, and
an IEEE Signal Processing Workshop on HOS has been held every two years
since 1989 (the most recent one took place in June 1995 in Spain). 

What are HOS ?

HOS measures are extensions of second-order measures (such as the
autocorrelation function and power spectrum) to higher orders. The
second-order measures work fine if the signal has a Gaussian (Normal)
probability density function, but as mentioned above, many real-life
signals are non-Gaussian.