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From: Andreas Leitgeb on 28 Oct 2009 08:17
Andreas Leitgeb <avl(a)gamma.logic.tuwien.ac.at> wrote:
> harryos <oswald.harry(a)gmail.com> wrote:
>> On Oct 28, 2:13 pm, Arved Sandstrom <dces...(a)hotmail.com> wrote:
>> > the argument is in radians, not degrees. So cos (20pi) = 1.
>> if I use m1=Math.cos(Math.PI*2*10*t*180/Math.PI)
>> will that do the conversion?
> If with 20pi you really thought of an angle in degrees,
> (that would be slightly larger than the angles of an
> equilateral triangle), then Math.PI*2*10*t*180/Math.PI
> or just: 3600*t does it.

Damn, slipped into the trap, myself.
if 20pi was an angle in degrees, then the argument to
cos had rather be Math.PI*2*10*t*Math.PI/180, that
is (pi²/9)·t

sorry for confusion.

From: jimgardener on 28 Oct 2009 09:13
On Oct 28, 5:17 pm, Andreas Leitgeb
> if 20pi was an angle in degrees, then the argument to
> cos had rather be Math.PI*2*10*t*Math.PI/180, that
> is (pi²/9)·t
>

the equation looks like an expression for a stationary signal with
frequencies 10,25,50 and 100.(.I am not very familiar with signal
processing stuff.but I remember seeing this as an example for
stationary signal.)If you plot the x(t) against different values of t
(time) then you will only get a straight line parallel to t axis if
you use
m1=Math.cos(Math.PI*2*10*t);
m2=Math.cos(Math.PI*2*25*t);
m3=Math.cos(Math.PI*2*50*t);
m4=Math.cos(Math.PI*2*100*t);
y=m1+m2+m3+m4;

May be you need some conversion to get a good plot..Can someone
familiar with this domain comment?

jim
From: Andreas Leitgeb on 28 Oct 2009 09:59
jimgardener <jimgardener(a)gmail.com> wrote:
> the equation looks like an expression for a stationary signal with
> frequencies 10,25,50 and 100.(.I am not very familiar with signal
> processing stuff.but I remember seeing this as an example for
> stationary signal.)If you plot the x(t) against different values of t
> (time) then you will only get a straight line parallel to t axis if
> you use
> m1=Math.cos(Math.PI*2*10*t);
> m2=Math.cos(Math.PI*2*25*t);
> m3=Math.cos(Math.PI*2*50*t);
> m4=Math.cos(Math.PI*2*100*t);
> y=m1+m2+m3+m4;
> May be you need some conversion to get a good plot..Can someone
> familiar with this domain comment?

Of course it depends on the sample rate of t: is it watched only in
discrete units?

"Hey, I marked the top position of a wheel and then I rolled it two
complete rotations forward, and guess what? The mark was on top,
not only afterwards but also at halftime!"

From: Patricia Shanahan on 28 Oct 2009 10:49
jimgardener wrote:
> On Oct 28, 5:17 pm, Andreas Leitgeb
>> if 20pi was an angle in degrees, then the argument to
>> cos had rather be Math.PI*2*10*t*Math.PI/180, that
>> is (pi�/9)�t
>>
>
> the equation looks like an expression for a stationary signal with
> frequencies 10,25,50 and 100.(.I am not very familiar with signal
> processing stuff.but I remember seeing this as an example for
> stationary signal.)If you plot the x(t) against different values of t
> (time) then you will only get a straight line parallel to t axis if
> you use
> m1=Math.cos(Math.PI*2*10*t);
> m2=Math.cos(Math.PI*2*25*t);
> m3=Math.cos(Math.PI*2*50*t);
> m4=Math.cos(Math.PI*2*100*t);
> y=m1+m2+m3+m4;
>
> May be you need some conversion to get a good plot..Can someone
> familiar with this domain comment?
>
> jim

The multiplication by Math.PI*2 in

m1=Math.cos(Math.PI*2*10*t);

may itself have been a messed up conversion from degrees to radians.
Sampling angles of the form 10*t degrees would be quite reasonable, and
yield different cosines.

I would do the conversion use Math.toRadians. However, from first
principles a complete rotation is 360 degrees and 2*pi radians, so x
degrees is 2*pi*x/360 radians. Forgetting the division by 360 would make
a big difference in how the angles behave.

Patricia
From: Roedy Green on 4 Nov 2009 10:20
On Wed, 28 Oct 2009 02:07:08 -0700 (PDT), harryos
<oswald.harry(a)gmail.com> wrote, quoted or indirectly quoted someone
who said :

>hi
>I was trying to calculate the value of
>x(t)=cos(2*pi*10*t)+cos(2*pi*25*t)+cos(2*pi*50*t)+cos(2*pi*100*t) for
>some values.I wrote this code

for the usual gotchas, (e.g. confusing radians/degrees) see
http://mindprod.com/jgloss/trigonometry.html
--
Roedy Green Canadian Mind Products
http://mindprod.com

An example (complete and annotated) is worth 1000 lines of BNF.
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