[sdiy] jitter analysis

[Magnus Danielson]

From: "Czech Martin" <Martin.Czech at Micronas.com>
Subject: RE: [sdiy] jitter analysis
Date: Fri, 9 Jul 2004 15:51:24 +0200
Message-ID: <D9D56E8FA1A73542BE9A5EC7E35D37FF01C4EFBF at EXCHANGE2.Micronas.com>

> > The ear, being a bandpassesque analyserbank, can naturally 
> > sniff the sidebands,
> > but some of it will be masked by normal masking procedure. 
> > Analysis done by
> > among others the late Julian Dunn has provided insight into 
> > where a jitter
> > tolerance curve would be given the knowledge we have about 
> > masking effects and
> > hearing the sidebands. (I am actually missing the paper, so 
> > if someone digs it
> > out, let me know). This tolerance curve goes below the ns 
> > level, just to give
> > you an indication... I don't recall it in detail right now.
> 
> 
> and the same thing can the DFT do to a sampled version of the signal,
> since the side bands will all be there.
> I think the reall con against the spetral measurement is
> its slowness. I have to average somehow and then I loose track of the
> short time changes. Heisenberg biting again.
> 
> But what about Parsevals theorem? Can't that help?

First of all, a DFT is _NOT_ equalent to a filterbank. A filterbank with
filters have a certain transientresponce of their own, but the detector
mechanism weighs in and this is not necessarilly RMS as DFT provides with
propper scaling. A continous time filterbank has detectors providing continous
time monitoring and in the ear also the continous time correlation of the
detectorsignal. This correlation excists and for instance the masking effect is
one of the results of this correlation.

Going straight to the DFT is not supported by what we know about the hearing to
start with. Again, doing models is easy, verify their validity to real life is
much much harder. The quick and dirty engineering assumption of tossing a DFT
on a problem is not allways even a wise thing to do, you can end up analyzing
all the wrong artifacts.

Also, when trying to do more fundamental research one has to be carefull not to
waste the wrong type of data, but the hole purpose is to establish what makes
it tick and only when you do that you know which shortcuts you can do. Much
work actually become useless due to lack of methology or lack of analysing the
methology. Looks darn nice on the paper, but when you really think about it at
least I end up thinking how much efforts where wasted for the poor addition to
knowledge. That accounts for so many articles a year that I get really scared!

Cheers,
Magnus