Line noise removal

On 11/28/2010 12:06 PM, acat wrote:
> Hi, I need some help on removing the line noise (50Hz) from my data. I know
> the method conventionally adopted is to fit a sinusoidal function and
> subtract from the original data --- the so called notch filter, I have two
> more questions with this:
>
> 1. In my data, not only 50Hz, but the frequencies nearest to it also have
> abnormal values, this may be caused by the leakage of the power in 50 Hz (I
> used the simple periodogram), but after I subtract the 50 Hz, it's
> neighbors   are still there. So I am thinking whether the 50 Hz is stable
> or not (I took only 4 seconds of data). And do have to remove the
> neighbors?
>
> 2. The notch filter seems not very good I think, is there any way to remove
> this line spectrum while preserving the spectral continuum?

I just realized that no one mentioned the obligatory caveat: DSP isn't 
magic.  Often the best way to remove artifacts in your collected data is 
to get grease under your fingernails and change the way you're 
collecting data.  If there's a way you can get what you need without the 
50Hz hum -- try it, it may be cheaper and better than trying to remove 
the noise after the fact.

Consider also that power line hum is usually rich in harmonics, so 
you'll have 50Hz, 100Hz, 150Hz, etc. -- often up into the kHz.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Nov 28, 3:06&#4294967295;pm, "acat" <newthinker@n_o_s_p_a_m.126.com> wrote:
> Hi, I need some help on removing the line noise (50Hz) from my data. I know
> the method conventionally adopted is to fit a sinusoidal function and
> subtract from the original data --- the so called notch filter, I have two
> more questions with this:
>
> 1. In my data, not only 50Hz, but the frequencies nearest to it also have
> abnormal values, this may be caused by the leakage of the power in 50 Hz (I
> used the simple periodogram), but after I subtract the 50 Hz, it's
> neighbors &#4294967295; are still there. So I am thinking whether the 50 Hz is stable
> or not (I took only 4 seconds of data). And do have to remove the
> neighbors?
>
> 2. The notch filter seems not very good I think, is there any way to remove
> this line spectrum while preserving the spectral continuum?
>
> Thank you for your attention, any suggestions will be welcome!

Have you tried running your hardware on a battery?

Rune Allnor wrote:

> The problem is that there is a theorem which is very central
> to DSP and that I don't remember the name of, off the top of
> my head, that states that one can not set a continuous
> frequency band to zero. That will cause all sorts of havoc,
> e.g. that the system will be impossible to implement.

Paley-Wiener criterion of causality.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Okay, thanks again for all the advices, I will try them now!

Best

>On 11/29/2010 02:07 AM, Rune Allnor wrote:
>> On Nov 29, 9:50 am, "acat"<newthinker@n_o_s_p_a_m.126.com>  wrote:
>>
>>> (3) Is there any reference on this topic?
>>
>> The literature on DSP.
>>
>>> (4) I intended to put all the index of annoying frequencies to 0, is
this
>>> way a little unnatural?
>>
>> No and yes: The *intention* is not unnatural; that's what
>> every newbie and neophyte want to do.
>>
>> The problem is that there is a theorem which is very central
>> to DSP and that I don't remember the name of, off the top of
>> my head, that states that one can not set a continuous
>> frequency band to zero. That will cause all sorts of havoc,
>> e.g. that the system will be impossible to implement.
>
>In this case, practically, what would happen is that you'd bring the 
>average of the 50Hz signal to zero, but in many places that would leave 
>a lot of "50 Hz-ish" signal there to trouble you.
>
>Hence my suggestion to zero out some bins, but to taper the filtering 
>effect.
>
>-- 
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com
>
>Do you need to implement control loops in software?
>"Applied Control Theory for Embedded Systems" was written for you.
>See details at http://www.wescottdesign.com/actfes/actfes.html
>