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Thread Subject:
Wavelet: What is it I'm seeing?

Subject: Wavelet: What is it I'm seeing?

From: Sebastian Gatzka

Date: 9 Mar, 2011 14:06:15

Message: 1 of 5

Hello World.

As I'm currently stepping into the web of Wavelets I'm somehow lost at the momente.
I did a continous wavelete decomposition using the wavemenu.

As input I gave the tool a sinus signal with a frequency of 620 Hz.
The output was this:
http://therealsega.th.funpic.de/glr/wavelet.pdf

So now I have absolutely no idea what I'm seeing.
I don't even recognise the "pseudo frequency".

Any ideas where to start using wavelets?
Any how to's or tutorial? Beside the MATLAB help.

Thank you!
S.

Subject: Wavelet: What is it I'm seeing?

From: Wayne King

Date: 9 Mar, 2011 14:49:32

Message: 2 of 5

"Sebastian Gatzka" <sebastian.gatzka.NOSPAM@stud.tu-darmstadt.de> wrote in message <il81gn$p56$1@fred.mathworks.com>...
> Hello World.
>
> As I'm currently stepping into the web of Wavelets I'm somehow lost at the momente.
> I did a continous wavelete decomposition using the wavemenu.
>
> As input I gave the tool a sinus signal with a frequency of 620 Hz.
> The output was this:
> http://therealsega.th.funpic.de/glr/wavelet.pdf
>
> So now I have absolutely no idea what I'm seeing.
> I don't even recognise the "pseudo frequency".
>
> Any ideas where to start using wavelets?
> Any how to's or tutorial? Beside the MATLAB help.
>
> Thank you!
> S.

Hi Sebastian, one thing you should do is to provide the correct sampling period as an input to the GUI. You have provided the sampling period of 1 for your signal.

If you have a sine wave of 620 Hz, you should know how many samples given your sampling frequency would constitute one period. For example, assume you are sampling at 2 kHz. In that case, 620 Hz would appear at approximately scale 3, since 620 Hz has an approximate period of 3 samples here.

t = linspace(0,1,2e3);
% sine wave only over the 1st 1/2 second
x = cos(2*pi*620*t).*(t<0.5);
scales = 1:.01:10;
coeffs = cwt(x,scales,'sym8');
surf(coeffs); shading('interp');

You can see the sine wave is localized in the first 1/2 of the time interval.

In the MATLAB documentation, there is a section in the Getting Started Guide on interpreting CWT coefficients.

Hope that helps,
Wayne

Subject: Wavelet: What is it I'm seeing?

From: Sebastian Gatzka

Date: 10 Mar, 2011 14:01:06

Message: 3 of 5

"Wayne King" <wmkingty@gmail.com> wrote in message
> Hi Sebastian, one thing you should do is to provide the correct sampling period as an input to the GUI. You have provided the sampling period of 1 for your signal.
>
> If you have a sine wave of 620 Hz, you should know how many samples given your sampling frequency would constitute one period. For example, assume you are sampling at 2 kHz. In that case, 620 Hz would appear at approximately scale 3, since 620 Hz has an approximate period of 3 samples here.
>
> t = linspace(0,1,2e3);
> % sine wave only over the 1st 1/2 second
> x = cos(2*pi*620*t).*(t<0.5);
> scales = 1:.01:10;
> coeffs = cwt(x,scales,'sym8');
> surf(coeffs); shading('interp');
>
> You can see the sine wave is localized in the first 1/2 of the time interval.
>
> In the MATLAB documentation, there is a section in the Getting Started Guide on interpreting CWT coefficients.
>
> Hope that helps,
> Wayne

Hi Wayne.

I have the sine wave with 5001 points in an 0.1 second intervall.
This results from a sample frequence of 500kHz.

The sine wave itself has the previously mentioned frequency of 620Hz .

So, what do I have to do to (in a first step) recognise the 620Hz?
Maybe you can help me find this Getting Started Guide on interpreting CWT coefficients. I have not found it yet in the MATLAB help.

Thanks a lot.
Sebastian

Subject: Wavelet: What is it I'm seeing?

From: Wayne King

Date: 10 Mar, 2011 14:17:05

Message: 4 of 5

"Sebastian Gatzka" <sebastian.gatzka.NOSPAM@stud.tu-darmstadt.de> wrote in message <ilalj2$fqo$1@fred.mathworks.com>...
> "Wayne King" <wmkingty@gmail.com> wrote in message
> > Hi Sebastian, one thing you should do is to provide the correct sampling period as an input to the GUI. You have provided the sampling period of 1 for your signal.
> >
> > If you have a sine wave of 620 Hz, you should know how many samples given your sampling frequency would constitute one period. For example, assume you are sampling at 2 kHz. In that case, 620 Hz would appear at approximately scale 3, since 620 Hz has an approximate period of 3 samples here.
> >
> > t = linspace(0,1,2e3);
> > % sine wave only over the 1st 1/2 second
> > x = cos(2*pi*620*t).*(t<0.5);
> > scales = 1:.01:10;
> > coeffs = cwt(x,scales,'sym8');
> > surf(coeffs); shading('interp');
> >
> > You can see the sine wave is localized in the first 1/2 of the time interval.
> >
> > In the MATLAB documentation, there is a section in the Getting Started Guide on interpreting CWT coefficients.
> >
> > Hope that helps,
> > Wayne
>
> Hi Wayne.
>
> I have the sine wave with 5001 points in an 0.1 second intervall.
> This results from a sample frequence of 500kHz.
>
> The sine wave itself has the previously mentioned frequency of 620Hz .
>
> So, what do I have to do to (in a first step) recognise the 620Hz?
> Maybe you can help me find this Getting Started Guide on interpreting CWT coefficients. I have not found it yet in the MATLAB help.
>
> Thanks a lot.
> Sebastian

Hi Sebastian,
Your sampling rate is just slightly over 50 kHz.
You can find that documentation here:

http://www.mathworks.com/help/toolbox/wavelet/gs/f3-1000759.html

Hope that helps,
Wayne

Subject: Wavelet: What is it I'm seeing?

From: Sebastian Gatzka

Date: 10 Mar, 2011 14:49:05

Message: 5 of 5

"Wayne King" <wmkingty@gmail.com> wrote in message
>
> Hi Sebastian,
> Your sampling rate is just slightly over 50 kHz.
> You can find that documentation here:
>
> http://www.mathworks.com/help/toolbox/wavelet/gs/f3-1000759.html
>
> Hope that helps,
> Wayne

Well, its 500kHz, because I have a 0.01 second intervall, my fault :-)

This documentation looks different than mine in R2010a ...
I will have a close look.

Thanks.

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