filter

[desham]

I tried to filter noisy(white) sine wave.FIlter did not remove noise completely.I want to know how much percentage the filter can remove the noise. Is there any method to calculate the percentage.

I tried different methods but they  doesnt work well.

[Drew_T]

A ideal filter will remove noise completely.  However, ideal filters are not possible.  This webpage gives a good explanation of the ability of filters to remove noise, as well as quantifiable ways of determining a filter's effectiveness.

http://www.ecelab.com/low-pass.htm

Good luck!

Drew T.

[johnsold]

Drew,

I have to disagree with you.  How do you define an ideal filter?

An ideal low pass filter with a cutoff of 1 will pass all noise energy (and all signal energy) with a frequency less than 1 while eliminating all energy (noise and signal) with frequency above 1.  Since white noise is defined as having a flat spectrum over all frequencies, there certainly is noise with frequency less than the cutoff frequency.  Of course 1/f noise has a spectrum which increases in amplitude as the frequency decreases. 

Even a bandpass filter with the center frequency equal to the signal frequency and a very narrow bandwidth will still pass the noise components within that bandwidth.  It may not be very much noise but there will be some.

If you know the spectral characteristics of the noise before the filter and the filter transfer function, you can calculate the noise reduction due to the filter.  But the output noise will never be zero.

Lynn

[Drew_T]

Lynn,

I'm sorry you are correct.  I wan't very clear with my hasty response.  The only noise removed will be that which is above 1 (for the lowpass example you gave).

Drew T.