>
> Jerry Avins <jya@ieee.org> wrote in message news:<3F61CC5E.BF7E91DD@ieee.org>...
> > Vanamali wrote:
> > >
> > > allnor@tele.ntnu.no (Rune Allnor) wrote in message
> > >
> > > > That's a succint formulation of my immediate reaction... I always thought
> > > > that "[causal] IIR filter" and "linear phase" were contradicions in terms?
> > >
> > > I browsed through comp.dsp after a long gap and came across this topic
> > > and the above quoted post. In case others have not already pointed
> > > this out, as the Clements paper shows, you can have causal IIR filter
> > > with precise linear phase. The catch is that the system transfer
> > > function is not rational. If one is restricted to the class of
> > > rational transfer functions, then causal, stable IIR system with
> > > precise linear phase is not possible.
> >
> > What is an irrational transfer function? Surely, it can't be h(x) =
> > x�sqrt(2)! Why would I care if a transfer function were rational or not?
>
> A transfer function that is rational (i.e. is a polynomial in frequency
> domain) separates easily into a feed-forward part and a feed-back loop.
> Check out the link between difference/differential equations and
> representations in the discrete/continuous Fourier domains.
>
> Rune
I know what is meant. I wasn't aware of the terminology.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Rune Allnor●September 13, 20032003-09-13
allnor@tele.ntnu.no (Rune Allnor) wrote in message news:<f56893ae.0309120934.46b22238@posting.google.com>...
> A transfer function that is rational (i.e. is a polynomial in frequency
> domain)
A rational function is, I believe, one that is expressed as a *fraction*
of polynomials in frequency domain. The denominator in that fraction is
related to feedback, the numerator is related to feed-forward.
Rune
Reply by Vanamali●September 12, 20032003-09-12
allnor@tele.ntnu.no (Rune Allnor) wrote in message news:<f56893ae.0309120934.46b22238@posting.google.com>...
> Jerry Avins <jya@ieee.org> wrote in message news:<3F61CC5E.BF7E91DD@ieee.org>.>..
>
> >
> > What is an irrational transfer function? Surely, it can't be h(x) =
> > x�sqrt(2)! Why would I care if a transfer function were rational or not?
>
> A transfer function that is rational (i.e. is a polynomial in frequency
> domain) separates easily into a feed-forward part and a feed-back loop.
> Check out the link between difference/differential equations and
> representations in the discrete/continuous Fourier domains.
Practically implementable discrete-time systems have rational transfer
functions, i.e., H(z) is a polynomial in z (or equivalently in 1/z).
This means that they can be implented in the time-domain by using
simple delays.
Any zeros of rational transfer functions are isolated. If the TF is
zero over a band of frequencies, it cannot be rational, the ideal
brickwall LPF being a case in point (which is a linear phase
non-causal IIR filter).
Reply by Rune Allnor●September 12, 20032003-09-12
Jerry Avins <jya@ieee.org> wrote in message news:<3F61CC5E.BF7E91DD@ieee.org>...
> Vanamali wrote:
> >
> > allnor@tele.ntnu.no (Rune Allnor) wrote in message
> >
> > > That's a succint formulation of my immediate reaction... I always thought
> > > that "[causal] IIR filter" and "linear phase" were contradicions in terms?
> >
> > I browsed through comp.dsp after a long gap and came across this topic
> > and the above quoted post. In case others have not already pointed
> > this out, as the Clements paper shows, you can have causal IIR filter
> > with precise linear phase. The catch is that the system transfer
> > function is not rational. If one is restricted to the class of
> > rational transfer functions, then causal, stable IIR system with
> > precise linear phase is not possible.
>
> What is an irrational transfer function? Surely, it can't be h(x) =
> x�sqrt(2)! Why would I care if a transfer function were rational or not?
A transfer function that is rational (i.e. is a polynomial in frequency
domain) separates easily into a feed-forward part and a feed-back loop.
Check out the link between difference/differential equations and
representations in the discrete/continuous Fourier domains.
Rune
Reply by Jerry Avins●September 12, 20032003-09-12
Vanamali wrote:
>
> allnor@tele.ntnu.no (Rune Allnor) wrote in message
>
> > That's a succint formulation of my immediate reaction... I always thought
> > that "[causal] IIR filter" and "linear phase" were contradicions in terms?
>
> I browsed through comp.dsp after a long gap and came across this topic
> and the above quoted post. In case others have not already pointed
> this out, as the Clements paper shows, you can have causal IIR filter
> with precise linear phase. The catch is that the system transfer
> function is not rational. If one is restricted to the class of
> rational transfer functions, then causal, stable IIR system with
> precise linear phase is not possible.
What is an irrational transfer function? Surely, it can't be h(x) =
x�sqrt(2)! Why would I care if a transfer function were rational or not?
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Vanamali●September 12, 20032003-09-12
allnor@tele.ntnu.no (Rune Allnor) wrote in message
> That's a succint formulation of my immediate reaction... I always thought
> that "[causal] IIR filter" and "linear phase" were contradicions in terms?
I browsed through comp.dsp after a long gap and came across this topic
and the above quoted post. In case others have not already pointed
this out, as the Clements paper shows, you can have causal IIR filter
with precise linear phase. The catch is that the system transfer
function is not rational. If one is restricted to the class of
rational transfer functions, then causal, stable IIR system with
precise linear phase is not possible.
Reply by Keith Larson●September 8, 20032003-09-08
> What scope sizes would be needed? Is this possible at all with smaller
> scopes than the 36" you mentioned? It seems as if it would be very difficult
> to gain the light needed to get sufficient contrast, even in optimal viewing
> conditions.
Suprisingly this does not require that large of a telescope. The trick
is to wait for the moons to be at max distance and use an occulting bar
to block the light from the planet. The limitation is not the dimness
of the moons, but the glare of the planet. As an example a few other
club members caught Deimos and Phobos with a 6" refractor. It is kind
of like trying to spot stars in the great Orion nebula Trapezium. They
are there and would be easy to see if it were not for the other stars
blazing away right next door!
> I'm not sure if I follow you here. You use a CCD to capture stills and/or
> video? And use some sort of correlation to enhance the images? I know some
> ultrasound people who used some sort of across-frames correlation measure
> to enhance medical images. Maybe I should try to get hold of their PhD
> theses...
For Mars, Saturn Jupiter and the moon I capture the output of an NTSC
color security camera to video tape. I then digitize this into an AVI
file. Finally, I stack the images. Though I am still plunking away at
a true corelator, it does not seem to be that much better than simply
co-registering the centroids. I then use a 2-d filter to sharpen the
results.
> Why not... most of the hardware is already there. Apparently, the modern
> astronomy scopes are fitted with motorized mounts, GPS navigators and what
> not, so that the user apparently only keys in what object he wants to view
> and waits for the scope to find it. What would be needed is an extra servo
> to adjust focus, some image processing capability to estimate the image
> quality and some feedback to control those servos.
Read Jerry's post as well. He is right about not being able to use the
main drive for stabilization. Too small of a correction, too much mass,
too much gear reduction. BTW, there are two things to contend with...
Mount/Motor vibration
---------------------
These are true mechanical vibratrions that can be corrected with
mechanical damping or optical path correction. Interestingly quite a
bit of the mechanical problems can be dealt with mechanically, and if
you can improve here, there is a lot less you need to do using brute
force. I for example can see the stepper motors ticking away in my
mount and am considering using rubber O-ring's to isolate them.
Atmospheric turbulance
----------------------
Here nothing is vibrating, so you only option is to change/correct the
optical path. You *could* attempt this with the main drive but as
mentioned too many things are against you. A better solution is to
mount a mirror onto a tip/tilt voice coil and move that.
> Sounds like a nice DSP hobby project. <sigh> I have to move somewhere else.
> Having an astronomy scope here makes only infitesimally more sense than
> bringing a fishing rod to the middle of Sahara: 60 clear nights per year,
> 40 of those during the season of the midnight sun and 15 of the remaining
> contaminated either by the bright moon or the aurora.
I would have to agree. Houston is a might bit warmer but ao is the
humidity. I liken it to the arm-pit of Texas.
> Rune
+------------------------------------------+
|Keith Larson |
|Member Group Technical Staff |
|Texas Instruments Incorporated |
| |
| 281-274-3288 |
| k-larson2@ti.com |
|------------------------------------------+
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Reply by Jerry Avins●September 6, 20032003-09-06
Jerry Avins wrote:
>
...
> The is too much inertia even with a short-tube catadioptric system for
^
re
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jerry Avins●September 5, 20032003-09-05
Rune Allnor wrote:
>
> ... most of the hardware is already there. Apparently, the modern
> astronomy scopes are fitted with motorized mounts, ...
The is too much inertia even with a short-tube catadioptric system for
that. Vibration is inevitable; no mount is infinitely rigid, and even
slight vibration magnified a few hundred times is ruinous. Images are
stabilized not by moving the scope on its mount, but by tilting a small
plane-parallel plate in the image path. (For a 6- to 8-mm plate with
even a short telescope, the spherical aberration introduced is
imperceptible.) Mounting the tilt motors rigidly on the telescope
introduces too much reaction torque for light ones like mine. A better
way is to fasten the motor to a flywheel and allow it to rotate freely,
with only a weak spring to establish an equilibrium DC position. There
is then nothing but bearing friction to excite vibration. I should
patent that, but hereby I put it in the public domain.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jerry Avins●September 5, 20032003-09-05
Keith Larson wrote:
>
> Hi again
>
...
> Now put something hot, like the hand I just mentioned, on a very cold
> night, in front and watch the light bend and twist. The bright waves
> are where the light has been bunched together, while the darker areas
> are where the light has diverged. Its kind of like the eerie effects you
> might see in a Frankenstein movie, but anyhow, thats what the problem
> boils down to.
To throw more light on that -- pun noted -- that's the basis of
Schlieren photography. Sure, collimators and knife edges are used to
increase the contrast to the point where sound waves become visible, but
that's just an (analog) image-processing detail.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������