Reply by Bhaskar Thiagarajan●February 9, 20052005-02-09
"Jim Thomas" <jthomas@bittware.com> wrote in message
news:110k88lpnotvr1b@corp.supernews.com...
> Rune Allnor wrote:
> > Ouch! That leaves me out. I know I've seen my father's slide rule
> > in a box where he keeps his old stuff, but I never saw it used.
> >
> > Rune
> >
>
> Time spent playing with a slide rule is time well spent. This page has
> some PDF's of paper slide rules.
>
> http://chris.gillings.com/collect/slide/index.html
FYI, fred harris is a collector of old slide rules. So if you have a rare
piece he might be interested in chatting with you.
Cheers
Bhaskar
Reply by Jerry Avins●February 9, 20052005-02-09
Jerry Avins wrote:
> Rune Allnor wrote:
>
> ...
>
>
>>Ouch! That leaves me out. I know I've seen my father's slide rule
>>in a box where he keeps his old stuff, but I never saw it used.
>
>
> It is simple and elegant. What's more, there are some startlingly
> elegant procedures you can learn that will make you feel smug. For
> example, calculating sqrt(a^2 + b^2) and phase angle with one push of
> the stick and one alignment of the cursor.
One push after an initial set-up. I could argue that that's two pushes.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jerry Avins●February 9, 20052005-02-09
Rune Allnor wrote:
...
> Ouch! That leaves me out. I know I've seen my father's slide rule
> in a box where he keeps his old stuff, but I never saw it used.
It is simple and elegant. What's more, there are some startlingly
elegant procedures you can learn that will make you feel smug. For
example, calculating sqrt(a^2 + b^2) and phase angle with one push of
the stick and one alignment of the cursor.
I can teach you how to use one and to do that trick even by email if you
like. (You need trig scaled to do trig. Duh!) My large Dietzgen rule is
a "Maniphase Multiplex (Log Log Vector)". The Keuffel and Esser
equivalent is a "Log Log Duplex Vector", the archtypical engineering
rule in my time.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jim Thomas●February 9, 20052005-02-09
Rune Allnor wrote:
> Ouch! That leaves me out. I know I've seen my father's slide rule
> in a box where he keeps his old stuff, but I never saw it used.
>
> Rune
>
Time spent playing with a slide rule is time well spent. This page has
some PDF's of paper slide rules.
http://chris.gillings.com/collect/slide/index.html
You can download them, print them, cut them out, assemble them, and then
use them. There are hundreds of websites with slide rule tutorials.
--
Jim Thomas Principal Applications Engineer Bittware, Inc
jthomas@bittware.com http://www.bittware.com (603) 226-0404 x536
Quidquid latine dictum sit, altum sonatur.
Whatever is said in Latin sounds profound.
Reply by Mike Yarwood●February 9, 20052005-02-09
"dkuhta" <deankuhta@yahoo.com> wrote in message
news:1107918417.946400.260870@g14g2000cwa.googlegroups.com...
>
>> I'm missing something somewhere - did you actually put a load of
> samples into your FFT then just pick one of the outputs to show in your
> example ?
>>
>> Puzzled Mike.
>
> um, yea. Why is that puzzling? Maybe I'm missing something somewhere...
>
O.K. dkuhta - it's clear now, thanks. Mentioning one of the frequency
components at the output of your N point FFT didn't really have any bearing
on your problem but I mistakenly thought it might.
Best of Luck - Mike
Reply by Rune Allnor●February 9, 20052005-02-09
Jerry Avins wrote:
> Rune Allnor wrote:
>
> ...
>
> > One really needs to be made aware of the "e-13" on the far right
> > end, and be inspired to contemplate that term, to actually
understand
> > the numbers. As one needs to be made aware that the real part is
> > correct to 15 significant digits. As you say, it's not a big deal
> > but it could be very confusing the first couple of times one sees
it.
> >
> > I don't remeber if Rick commented on this particular point in his
> > book. If he didn't do so already, I think it would be another one
> > of those detail gems that could be included in a 3rd edition.
>
> Isn't exponential notation taught in high school algebra any more?
The
> only new thing to be learned for reading computer printouts is that
> 1E-13 is shorthand for 1*10^-13.
I don't know about high school, but that ought to get taught at least
in college. Sorry if I'm confusing you Americans, what I think
of as "high school" is the last years of compulsary school where
the students leave at 16. I think of college as an alternative
to university, where students enter at age 19 - 20. In between we
wave something that translates to "extension school", where the
student can choose to either prepare for university-college
or get vocatinal traing to become a carpenter, electrician,...
> Imagine that I pay for 256 liters of fuel, but get only
> 255.99999999999918. I would be embarrassed to complain that the
delivery
> was .082 picoliters short. If the imaginary part is a contaminant,
even
> .188 picoliters of it leaves the fuel quite pure.
Sure. As for myself, I didn't acquire that pragmatic attitude to
numbers until I started to work with numerical stuff. I implemented
some routines and had to contemplate the results. The very first
routine I implemented was the FFT where I, too, had to stop and
think through the very same questions that spawned this thread.
With other routines I had some reference runs with known input
and output. It takes a very short time to become aware of numerical
accuracy when working like that. But it does require a concious
effort to contemplate the numbers.
> All people who use numbers should be required to gain competence with
a
> slide rule.
Ouch! That leaves me out. I know I've seen my father's slide rule
in a box where he keeps his old stuff, but I never saw it used.
Rune
Reply by dkuhta●February 8, 20052005-02-08
> I'm missing something somewhere - did you actually put a load of
samples into your FFT then just pick one of the outputs to show in your
example ?
>
> Puzzled Mike.
um, yea. Why is that puzzling? Maybe I'm missing something somewhere...
Reply by Jerry Avins●February 8, 20052005-02-08
Rune Allnor wrote:
...
> One really needs to be made aware of the "e-13" on the far right
> end, and be inspired to contemplate that term, to actually understand
> the numbers. As one needs to be made aware that the real part is
> correct to 15 significant digits. As you say, it's not a big deal
> but it could be very confusing the first couple of times one sees it.
>
> I don't remeber if Rick commented on this particular point in his
> book. If he didn't do so already, I think it would be another one
> of those detail gems that could be included in a 3rd edition.
Isn't exponential notation taught in high school algebra any more? The
only new thing to be learned for reading computer printouts is that
1E-13 is shorthand for 1*10^-13.
Imagine that I pay for 256 liters of fuel, but get only
255.99999999999918. I would be embarrassed to complain that the delivery
was .082 picoliters short. If the imaginary part is a contaminant, even
.188 picoliters of it leaves the fuel quite pure.
All people who use numbers should be required to gain competence with a
slide rule.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Mike Yarwood●February 8, 20052005-02-08
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> Bhaskar Thiagarajan wrote:
>> "dkuhta" <deankuhta@yahoo.com> wrote in message
>> news:1107794474.626567.31530@l41g2000cwc.googlegroups.com...
>> > Hi, first off I'm fairly new to DSP. That said, here is my
> situation.
>> >
>> > 1.I have an input array of time domain samples.
>> > 2.I feed this array into a FFT, then do an inverse FFT.
>> > 3.From the inverse results, I extract the "real" component.
>> > 4.I plug the iFFT real components into a new array.
>> >
>> > Here is output from my program looking at one data sample:
>> >
>> > time domain sample: -256.0
???????
>> > fft of sample: 622910.4945926662 + 869652.1640951419i
???????
How do you work this out? If I provide a single sample to an FFT I just =
get the same result at the output. =20
>> > inverse fft : -255.99999999999918 + -1.8865049541588732E-13i
>> > extracted real component: -256.0
>> >
>> >
>> > question 1:
>> > Isn't the point of an inverse FFT to get back to the time-domain
>> > sample? I haven't applied any filters, so since I started with
>> > -256.0, don't I want -256.0 after the iFFT?
>>
>> Yes - that's correct.
>> Ideally yes. Your actual result is so close to -256.0 that I'm
> surprised
>> that you are bothered by the result you got.
I'm missing something somewhere - did you actually put a load of samples =
into your FFT then just pick one of the outputs to show in your example =
?
Puzzled Mike.
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<DIV><FONT face=3DArial size=3D2>> Bhaskar Thiagarajan =
wrote:<BR>>>=20
"dkuhta" <</FONT><A href=3D"mailto:deankuhta@yahoo.com"><FONT =
face=3DArial=20
size=3D2>deankuhta@yahoo.com</FONT></A><FONT face=3DArial size=3D2>> =
wrote in=20
message<BR>>> </FONT><A=20
href=3D"news:1107794474.626567.31530@l41g2000cwc.googlegroups.com"><FONT =
face=3DArial=20
size=3D2>news:1107794474.626567.31530@l41g2000cwc.googlegroups.com</FONT>=
</A><FONT=20
face=3DArial size=3D2>...<BR>>> > Hi, first off I'm fairly new =
to DSP. That=20
said, here is my<BR>> situation.<BR>>> ><BR>>> > =
1.I have=20
an input array of time domain samples.<BR>>> > 2.I feed this =
array into=20
a FFT, then do an inverse FFT.<BR>>> > 3.From the inverse =
results, I=20
extract the "real" component.<BR>>> > 4.I plug the iFFT real =
components=20
into a new array.<BR>>> ><BR>>> > Here is output from =
my=20
program looking at one data sample:<BR>>> ><BR>>> > =
time=20
domain sample: -256.0</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial color=3D#800080 size=3D2>???????<BR>>> =
> fft of=20
sample: 622910.4945926662 + 869652.1640951419i<BR>???????</FONT></DIV>
<DIV><FONT face=3DArial size=3D2><FONT color=3D#800080>How do you work =
this out? If=20
I provide a single sample to an FFT I just get the same result at =
the=20
output.</FONT> </FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>>> > inverse fft : =
-255.99999999999918 +=20
-1.8865049541588732E-13i<BR>>> > extracted real component:=20
-256.0<BR>>> ><BR>>> ><BR>>> > question=20
1:<BR>>> > Isn't the point of an inverse FFT to get back to the =
time-domain<BR>>> > sample? I haven't applied any filters, so =
since I=20
started with<BR>>> > -256.0, don't I want -256.0 after the=20
iFFT?<BR>>><BR>>> Yes - that's correct.<BR>>> Ideally =
yes.=20
Your actual result is so close to -256.0 that I'm<BR>> =
surprised<BR>>>=20
that you are bothered by the result you got.<BR></FONT></DIV>
<DIV><FONT face=3DArial size=3D2>I'm missing something somewhere - did =
you actually=20
put a load of samples into your FFT then just pick one of the outputs to =
show in=20
your example ?</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT> </DIV>
<DIV><FONT face=3DArial size=3D2>Puzzled Mike.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2> </DIV></FONT></BODY></HTML>
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Reply by Rune Allnor●February 8, 20052005-02-08
Bhaskar Thiagarajan wrote:
> "dkuhta" <deankuhta@yahoo.com> wrote in message
> news:1107794474.626567.31530@l41g2000cwc.googlegroups.com...
> > Hi, first off I'm fairly new to DSP. That said, here is my
situation.
> >
> > 1.I have an input array of time domain samples.
> > 2.I feed this array into a FFT, then do an inverse FFT.
> > 3.From the inverse results, I extract the "real" component.
> > 4.I plug the iFFT real components into a new array.
> >
> > Here is output from my program looking at one data sample:
> >
> > time domain sample: -256.0
> > fft of sample: 622910.4945926662 + 869652.1640951419i
> > inverse fft : -255.99999999999918 + -1.8865049541588732E-13i
> > extracted real component: -256.0
> >
> >
> > question 1:
> > Isn't the point of an inverse FFT to get back to the time-domain
> > sample? I haven't applied any filters, so since I started with
> > -256.0, don't I want -256.0 after the iFFT?
>
> Yes - that's correct.
> Ideally yes. Your actual result is so close to -256.0 that I'm
surprised
> that you are bothered by the result you got.
Hi Bhaskar
I agree with you, the OP appears to have done everything correctly.
As for the close results, it takes a bit of training to evaluate
the numbers. Seeing an imaginary component where one supplied a
real number does confuse people the first few times they see that.
One really needs to be made aware of the "e-13" on the far right
end, and be inspired to contemplate that term, to actually understand
the numbers. As one needs to be made aware that the real part is
correct to 15 significant digits. As you say, it's not a big deal
but it could be very confusing the first couple of times one sees it.
I don't remeber if Rick commented on this particular point in his
book. If he didn't do so already, I think it would be another one
of those detail gems that could be included in a 3rd edition.
Rune