Re: Openeeg-list Digest, Vol 133, Issue 1

Hi Dami

I have no time to check the design and look at calculations, but I would
recommend that you stay with the original filter design for the following
reasons.

1. No filter is a brick wall.  A 3rd order filter is really not very sharp,
so if you choose a 49.5 Hz cut-off, your 50Hz mains noise will only see a
bit more than 3db reduction, which will not help you.

2. If you reduce the cutoff point as you intend, your filter pass-band will
be negatively effected.  I.e. your higher frequency EEG signal will start
to be attenuated and your processed data will be inaccurate.  No EEG
amplifier has a perfectly flat transfer function, simply because filters
are far from ideal.  High order filters produce all sorts of phase delay
and other problems and are thus also not a solution (not to mention the
cost).  We all need to accept we live in an in-perfect world.  You should
not act in a way to further compromise the accuracy for reasons that are
not that important.

3. From all my readings about neurofeedback, I have learned that everyone
has 50 or 60Hz interference and that most people use it to judge their
electrode conductivity.  If they have high 50/60 Hz, they know that their
electrode is not well seated, or the electrolyte has started to dry out, or
whatever.  If the mains is fairly suppressed, they know the electrode
contact is sound.  There will always be some mains hum, and they simply
learn to ignore it.

4. I think the original design team spend many 100s of hours analysing the
filter response and carefully considered where best to position the
-3db filter cut-off point.

I KNOW you want to understand the design.  And that is good.  You should
find discussions and explanations in old openeeg forum messages.  This msg
will not help you with the filter analysis, but I just want to make you
aware that the openEEG design does not need to be changed for EU mains.  In
fact, the design team was mainly from Europe.   Joerge, one of the original
team members, later made certain modifications regarding safety, leakage,
etc. to pass/qualify EU medical grade standards for a commercial spinoff
for a student start-up (based in Berlin, if I remember correctly).

Regards
Stefan

On Thu, Feb 1, 2018 at 2:02 PM, <[email protected]>
wrote:

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> Today's Topics:
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>    1. Low.pass filter (Dami?n Filo)
>    2. Re: Low.pass filter (salsero)
>    3. Re: Low.pass filter (Dami?n Filo)
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 31 Jan 2018 16:58:39 +0100
> From: Dami?n Filo <[email protected]>
> To: "Discussion of EEG hardware, software, training protocols, NFB
>         theory - for developers and users."
>         <[email protected]>
> Subject: [Openeeg-list] Low.pass filter
> Message-ID:
>         <CALPZnbFYtf2_D0XqJArAUOJ-tGUENbKi0258N05DkWj2Q_0cyA@
> mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hello everybody.
> As I am finishing up my project of customizing EEG-SMT I encountered some
> things I can't seem to understand about the OpenEEG. One of those things is
> the 3rd order besselworth filter which I cannot successfully
> reverse-engineer in several Bessel filter calculators. Every time when I
> enter the provided values of cutoff frequency, order... I get really
> different values of the parts used. It is really important to me,  I'd like
> to set it to 49Hz cutoff because of european electric network of 50Hz.
> That's why I get too much 50Hz ripple. I will share the schematics, board
> and electrode designs, but at first I need to understand the filter. Is
> there someone, who will be able to explain the value selection and formulas
> used for this filter? I'd be really thankful. I spent hours maybe days
> figuring it out, but I can't seem to understand it. I used multiple
> calculators, but this one looks the best:
> http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html
>
> Thank you in advance.
> --
> Dami?n Filo
> -------------- next part --------------
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>
> Message: 2
> Date: Thu, 01 Feb 2018 11:38:58 +0100
> From: salsero <[email protected]>
> To: "Discussion of EEG hardware, software, training protocols, NFB
>         theory - for developers and users."
>         <[email protected]>
> Subject: Re: [Openeeg-list] Low.pass filter
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
>
> Hi Damian,
>
> Did you try a twin-T notch filter? An RC low pass filter is not really
> that sharp. May these articles will help you:
>
> http://www.g3ynh.info/circuits/Bob_Batey/LB-5.pdf
> http://fourier.eng.hmc.edu/e84/lectures/ActiveFilters/node4.html
>
> Hope this helps a bit,
>
> Leo.
>
>
>
>
>
> Dami?n Filo schreef op 2018-01-31 16:58:
> > Hello everybody.
> >
> > As I am finishing up my project of customizing EEG-SMT I encountered
> > some things I can't seem to understand about the OpenEEG. One of those
> > things is the 3rd order besselworth filter which I cannot successfully
> > reverse-engineer in several Bessel filter calculators. Every time when
> > I enter the provided values of cutoff frequency, order... I get really
> > different values of the parts used. It is really important to me,  I'd
> > like to set it to 49Hz cutoff because of european electric network of
> > 50Hz. That's why I get too much 50Hz ripple. I will share the
> > schematics, board and electrode designs, but at first I need to
> > understand the filter. Is there someone, who will be able to explain
> > the value selection and formulas used for this filter? I'd be really
> > thankful. I spent hours maybe days figuring it out, but I can't seem
> > to understand it. I used multiple calculators, but this one looks the
> > best: http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html [1]
> >
> > Thank you in advance.
> >
> > --
> >
> > Dami?n Filo
> >
> > Links:
> > ------
> > [1] http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html
> >
> > ------------------------------------------------------------
> ------------------
> > Check out the vibrant tech community on one of the world's most
> > engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> >
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> > subscription options, e.g unsubscribe.
>
>
>
>
>
>
> ------------------------------
>
> Message: 3
> Date: Thu, 1 Feb 2018 12:54:03 +0100
> From: Dami?n Filo <[email protected]>
> To: "Discussion of EEG hardware, software, training protocols, NFB
>         theory - for developers and users."
>         <[email protected]>
> Subject: Re: [Openeeg-list] Low.pass filter
> Message-ID:
>         <CALPZnbF9AUmUqFrGx6+c8+sXufd2yvTp=XyLDpKDLmDOWPfRpw@
> mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Thank you for your reply, but as I am using the original filter/amplifier
> design it would be really time consuming to modify it to another type of
> filter. I just need to get to understand what have been used
>  in the original design. The original design is here:
> https://imgur.com/a/MzSs2
>
> If you would be able to point me to the right direction I'd be really
> grateful.
>
> 2018-02-01 11:38 GMT+01:00 salsero <[email protected]>:
>
> >
> > Hi Damian,
> >
> > Did you try a twin-T notch filter? An RC low pass filter is not really
> > that sharp. May these articles will help you:
> >
> > http://www.g3ynh.info/circuits/Bob_Batey/LB-5.pdf
> > http://fourier.eng.hmc.edu/e84/lectures/ActiveFilters/node4.html
> >
> > Hope this helps a bit,
> >
> > Leo.
> >
> >
> >
> >
> >
> > Dami?n Filo schreef op 2018-01-31 16:58:
> >
> >> Hello everybody.
> >>
> >> As I am finishing up my project of customizing EEG-SMT I encountered
> >> some things I can't seem to understand about the OpenEEG. One of those
> >> things is the 3rd order besselworth filter which I cannot successfully
> >> reverse-engineer in several Bessel filter calculators. Every time when
> >> I enter the provided values of cutoff frequency, order... I get really
> >> different values of the parts used. It is really important to me,  I'd
> >> like to set it to 49Hz cutoff because of european electric network of
> >> 50Hz. That's why I get too much 50Hz ripple. I will share the
> >> schematics, board and electrode designs, but at first I need to
> >> understand the filter. Is there someone, who will be able to explain
> >> the value selection and formulas used for this filter? I'd be really
> >> thankful. I spent hours maybe days figuring it out, but I can't seem
> >> to understand it. I used multiple calculators, but this one looks the
> >> best: http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html [1]
> >>
> >> Thank you in advance.
> >>
> >> --
> >>
> >> Dami?n Filo
> >>
> >> Links:
> >> ------
> >> [1] http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html
> >>
> >> ------------------------------------------------------------
> >> ------------------
> >> Check out the vibrant tech community on one of the world's most
> >> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> >>
> >> _______________________________________________
> >> Openeeg-list mailing list
> >> [email protected]
> >> https://lists.sourceforge.net/lists/listinfo/openeeg-list
> >> Go to the above address to change your
> >> subscription options, e.g unsubscribe.
> >>
> >
> >
> >
> >
> > ------------------------------------------------------------
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>
>
>
> --
> Dami?n Filo
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