# Mathematical filter

1. Aug 26, 2008

### Glowfish

Hi
I am working with current meter data (oceanographic) and have to filter my timeseries to remove the tidal component (frequency 12h). I have not worked with filters before and would appreciate any help.
Thanks

2. Aug 26, 2008

### Ben Niehoff

You need to take the Fourier transform of your data; this will give you a representation of your data as a spectrum of frequencies. I suppose there should be a spike at the tidal frequency (twice per day, or 1/12 per hour, or whatever units you use). Then simply erase that spike, and transform your data back into the time domain.

3. Aug 26, 2008

Better yet, design a notch filter with good characteristics, and a center frequency of 12 Hz. Taking an FFT, zeroing out coefficients, and then inverse transforming gives a terrible filter.

Probably this thread should be moved to EE or something like that?

4. Aug 26, 2008

### chroot

Staff Emeritus
If you have access to MATLAB, it makes filtering datasets quite easy. Let us know what tools you have to work with.

- Warren

5. Aug 26, 2008

### Staff: Mentor

Think it's a simple typo -- quadraphonics meant (1/12) Hz.

6. Aug 26, 2008

Indeed; thanks for that.

A simple highpass filter might also be a good choice, if there isn't any interesting information below 1/12 Hz.

7. Aug 26, 2008

### chroot

Staff Emeritus
Actually, this is also a typo. The frequency is 1 / 12 hours, or 2.31481481 x 10-5 Hz.

- Warren

8. Aug 26, 2008

### Staff: Mentor

Ack! I'd better get back to work.....

9. Aug 27, 2008

### Glowfish

Okay, I got a lot of advice but still not sure what to do. Unfortunately I don't have matlab. At the moment i'm working with excel and excelstat which allows me to do FFT and spectral analysis.
How do I design a notch filter or a highpass filter in excel with a frequency of 1/12 hr?
Lisa

10. Aug 27, 2008

### chroot

Staff Emeritus
If you're not familiar with any kind of filter design, we cannot really help you. It's too sophisticated a problem to walk you through on a forum.

If you'd like, you can probably use the free program Octave to perform your actual filtering. (Octave is a free MATLAB clone.) I'd be happy to design your filter coefficients for you.

- Warren

11. Aug 30, 2008

### matematikawan

It will be great if someone could give a basis lesson on filter. I understand that discrete wavelet transform can also be efficiently implemented using filters.

Thanks chroot for the information on Octave. It's new to me. Do you recommend it? What are the advantages of this software? I have some little knowledge about Matlab. Do Octave have good toolbox, in particular for signal or image processing?

12. Aug 30, 2008

### uart

It's a Matlab clone and it's free.

13. Sep 5, 2008

### matematikawan

That's good start. Hopefully there is no copyright violation when using free clone software. Do you recommend any good web site that I can download the Octave.

At the moment, however, I'm trying to understand https://www.physicsforums.com/showthread.php?t=32703" first. Not yet on matlab programming. Hopeful later I will be able to write program on wavelet transform using filters.

Last edited by a moderator: Apr 23, 2017
14. Sep 6, 2008

### CRGreathouse

15. Sep 7, 2008