On 6.4.22 19.51, Ricky wrote:
> On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
>> My guess is that the hum is coming from your patient / target.
>>
>> The capacitance of an average person is about 20 pF to the
>> active phase line and 200 pF to ground. You can assume that
>> the patient has about 10% of the line voltage through a pretty
>> high impedance voltage divider.
>>
>> When we made ECG equipment in the early 1970's, the solution
>> was to make the input of the amplifier differential and as
>> high impedance as possible, including guard bootstrapping the
>> shield braids of the input cables. The impedance of the
>> connection electrodes are hardly ever identical, and this creates
>> voltage dividers with the amplifier input impedances. The
>> imbalance works directly to convert the common-mode hum into
>> differential input.
>
> That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a "reference" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.
>
> I don't know for sure, but I expect the body is a relatively low impedance voltage source. I'm willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.

That is true, but we do not have contacts to the low impedance
source. The electrodes can have impedances of tens of kohm,
with mismatch between the electrodes. We do not hit contact
spikes through the skin.

A notch filter is not a good idea, as much of the interesting
frequency components are in the mains frequency range. The
ECG signal is spiky by its very nature, and narrow band filters
will spoil the information looked for.

--

-TV

On Wednesday, April 6, 2022 at 5:45:30 AM UTC-7, Cursitor Doom wrote:
> On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
> <tauno....@notused.fi.invalid> wrote:
>
> >My guess is that the hum is coming from your patient / target.
> >
> >The capacitance of an average person is about ...

> Yeah, I know what you mean but no cigar for you this time. I was able
> to clean up the hum by using good old fashioned ferrite beads inside
> the box at the point where the power lead comes in. Worked like a
> charm. Sometimes the best solutions are the old ones...

Didn't the original post say that the problem occurred with battery power?
Sounds like you've found PART of the solution, not all of it. Ferrite
beads don't do much at power-line frequency.

On Wednesday, April 6, 2022 at 2:21:53 PM UTC-4, Tauno Voipio wrote:
> On 6.4.22 19.51, Ricky wrote:
> > On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
> >> My guess is that the hum is coming from your patient / target.
> >>
> >> The capacitance of an average person is about 20 pF to the
> >> active phase line and 200 pF to ground. You can assume that
> >> the patient has about 10% of the line voltage through a pretty
> >> high impedance voltage divider.
> >>
> >> When we made ECG equipment in the early 1970's, the solution
> >> was to make the input of the amplifier differential and as
> >> high impedance as possible, including guard bootstrapping the
> >> shield braids of the input cables. The impedance of the
> >> connection electrodes are hardly ever identical, and this creates
> >> voltage dividers with the amplifier input impedances. The
> >> imbalance works directly to convert the common-mode hum into
> >> differential input.
> >
> > That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a "reference" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.
> >
> > I don't know for sure, but I expect the body is a relatively low impedance voltage source. I'm willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.
> That is true, but we do not have contacts to the low impedance
> source. The electrodes can have impedances of tens of kohm,
> with mismatch between the electrodes. We do not hit contact
> spikes through the skin.

The body is still low impedance with respect to the amplifier inputs. They use paste and a large contact area to minimize skin resistance. Given the high input impedance of the amp, the mismatch in impedance is of little consequence.

> A notch filter is not a good idea, as much of the interesting
> frequency components are in the mains frequency range. The
> ECG signal is spiky by its very nature, and narrow band filters
> will spoil the information looked for.

You seem to be confusing a notch (band reject) with a narrow band-pass filter. An adequately narrow band reject filter would have virtually no impact on the signal of interest. It only need be wide enough to accommodate the normal variations in mains frequency.

--

Rick C.

+- Get 1,000 miles of free Supercharging
+- Tesla referral code - https://ts.la/richard11209

On Wednesday, April 6, 2022 at 4:42:48 PM UTC-4, whit3rd wrote:
> On Wednesday, April 6, 2022 at 5:45:30 AM UTC-7, Cursitor Doom wrote:
> > On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
> > <tauno....@notused.fi.invalid> wrote:
> >
> > >My guess is that the hum is coming from your patient / target.
> > >
> > >The capacitance of an average person is about ...
> > Yeah, I know what you mean but no cigar for you this time. I was able
> > to clean up the hum by using good old fashioned ferrite beads inside
> > the box at the point where the power lead comes in. Worked like a
> > charm. Sometimes the best solutions are the old ones...
> Didn't the original post say that the problem occurred with battery power?
> Sounds like you've found PART of the solution, not all of it. Ferrite
> beads don't do much at power-line frequency.

I'd like to see what the output looks like with various inputs, all contacts shorted, resistors between contacts, etc. If the problem really has disappeared, I expect it will return and not by removing the ferrite beads.

--

Rick C.

++ Get 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209

On Wed, 6 Apr 2022 16:06:28 -0700 (PDT), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

>On Wednesday, April 6, 2022 at 2:21:53 PM UTC-4, Tauno Voipio wrote:
>> On 6.4.22 19.51, Ricky wrote:
>> > On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
>> >> My guess is that the hum is coming from your patient / target.
>> >>
>> >> The capacitance of an average person is about 20 pF to the
>> >> active phase line and 200 pF to ground. You can assume that
>> >> the patient has about 10% of the line voltage through a pretty
>> >> high impedance voltage divider.
>> >>
>> >> When we made ECG equipment in the early 1970's, the solution
>> >> was to make the input of the amplifier differential and as
>> >> high impedance as possible, including guard bootstrapping the
>> >> shield braids of the input cables. The impedance of the
>> >> connection electrodes are hardly ever identical, and this creates
>> >> voltage dividers with the amplifier input impedances. The
>> >> imbalance works directly to convert the common-mode hum into
>> >> differential input.
>> >
>> > That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a "reference" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.
>> >
>> > I don't know for sure, but I expect the body is a relatively low impedance voltage source. I'm willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.
>> That is true, but we do not have contacts to the low impedance
>> source. The electrodes can have impedances of tens of kohm,
>> with mismatch between the electrodes. We do not hit contact
>> spikes through the skin.
>
>The body is still low impedance with respect to the amplifier inputs. They use paste and a large contact area to minimize skin resistance. Given the high input impedance of the amp, the mismatch in impedance is of little consequence.
>
>
>> A notch filter is not a good idea, as much of the interesting
>> frequency components are in the mains frequency range. The
>> ECG signal is spiky by its very nature, and narrow band filters
>> will spoil the information looked for.
>
>
>You seem to be confusing a notch (band reject) with a narrow band-pass filter. An adequately narrow band reject filter would have virtually no impact on the signal of interest. It only need be wide enough to accommodate the normal variations in mains frequency.

Leaving aside that the problem has now been resolved, I would have
thought Tauno was right in what he said. The wanted and unwanted
components of the waveform are just too close together to target one
without degrading the other to some meaningful extent.

Ricky wrote:
==========

> An adequately narrow band reject filter would have virtually no impact on the signal of interest.

** Shame you have never looked at the phase and amplitude curve of such a filter.

> It only need be wide enough to accommodate the normal variations in mains frequency.

** About 0.1Hz is all the AC supply varies by.

....... Phil

On 06/04/2022 9:42 pm, whit3rd wrote:
> On Wednesday, April 6, 2022 at 5:45:30 AM UTC-7, Cursitor Doom wrote:
>> On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
>> <tauno....@notused.fi.invalid> wrote:
>>
>>> My guess is that the hum is coming from your patient / target.
>>>
>>> The capacitance of an average person is about ...
>
>> Yeah, I know what you mean but no cigar for you this time. I was able
>> to clean up the hum by using good old fashioned ferrite beads inside
>> the box at the point where the power lead comes in. Worked like a
>> charm. Sometimes the best solutions are the old ones...
>
> Didn't the original post say that the problem occurred with battery power?
> Sounds like you've found PART of the solution, not all of it. Ferrite
> beads don't do much at power-line frequency.

I think that when OP tried battery power he was still examining the
output on a mains connected scope? Interesting would be what is seen
with a battery powered scope and the whole system free of any connection
to mains ground.

No sane amount of ferrite beads will have effect at 50Hz, much more
likely some tens/hundreds of kHz from a mains SMPS in the vicinity is
getting in. The ECG amp will probably be heavily low pass filtered
somewhere along the chain so the HF interference won't be seen at the
output but it's 100Hz modulation rate will be?

piglet

erichp...@hotmail.com wrote:
=======================
>>
> >>> My guess is that the hum is coming from your patient / target.
> >>>
> >>> The capacitance of an average person is about ...

> I think that when OP tried battery power he was still examining the
> output on a mains connected scope? Interesting would be what is seen
> with a battery powered scope and the whole system free of any connection
> to mains ground.

** That is my contention too, posted 2 days ago.
>
The ECG amp will probably be heavily low pass filtered
> somewhere along the chain so the HF interference won't be seen at the
> output but it's 100Hz modulation rate will be?

** Huh ? 100Hz ?

Think is is just simple capacitive coupled injection of the AC supply voltage wave.
The bane of many poorly shielded electric guitars .

....... Phil

On 07/04/2022 09:19, Phil Allison wrote:
> erichp...@hotmail.com wrote:
> =======================
>>>
>>>>> My guess is that the hum is coming from your patient / target.
>>>>>
>>>>> The capacitance of an average person is about ...
>
>> I think that when OP tried battery power he was still examining the
>> output on a mains connected scope? Interesting would be what is seen
>> with a battery powered scope and the whole system free of any connection
>> to mains ground.
>
> ** That is my contention too, posted 2 days ago.
>>
> The ECG amp will probably be heavily low pass filtered
>> somewhere along the chain so the HF interference won't be seen at the
>> output but it's 100Hz modulation rate will be?
>
> ** Huh ? 100Hz ?
>
> Think is is just simple capacitive coupled injection of the AC supply voltage wave.
> The bane of many poorly shielded electric guitars .
>
>
> ...... Phil
>
Yes, but later on the OP said he solved the problem with ferrite beads.
He hasn't said how many beads he used but it seems unlikely he used
enough ferrite to attenuate mains frequencies directly so the theory now
is that he instead attenuated some higher frequency pickup that was
itself power line modulated?

piglet

erichp...@hotmail.com wrote:
========================
>
> >>>
> >>>>> My guess is that the hum is coming from your patient / target.
> >>>>>
> >>>>> The capacitance of an average person is about ...
> >
> >> I think that when OP tried battery power he was still examining the
> >> output on a mains connected scope? Interesting would be what is seen
> >> with a battery powered scope and the whole system free of any connection
> >> to mains ground.
> >
> > ** That is my contention too, posted 2 days ago.
> >>
> > The ECG amp will probably be heavily low pass filtered
> >> somewhere along the chain so the HF interference won't be seen at the
> >> output but it's 100Hz modulation rate will be?
> >
> > ** Huh ? 100Hz ?
> >
> > Think is is just simple capacitive coupled injection of the AC supply voltage wave.
> > The bane of many poorly shielded electric guitars .
> >
>
> >
> Yes, but later on the OP said he solved the problem with ferrite beads.

** Should have tried Hippy beads or Rosary beads.

> He hasn't said how many beads he used ...

** Nor how much weed he smoke daily.

> but it seems unlikely he used
> enough ferrite to attenuate mains frequencies directly so the theory now
> is that he instead attenuated some higher frequency pickup that was
> itself power line modulated?

** I can see what looks like 50Hz in that slow ECG trace.

Try counting the wiggles.

...... Phil

On 07/04/2022 09:44, Phil Allison wrote:
> erichp...@hotmail.com wrote:
> ========================
>>
>>>>>
>>>>>>> My guess is that the hum is coming from your patient / target.
>>>>>>>
>>>>>>> The capacitance of an average person is about ...
>>>
>>>> I think that when OP tried battery power he was still examining the
>>>> output on a mains connected scope? Interesting would be what is seen
>>>> with a battery powered scope and the whole system free of any connection
>>>> to mains ground.
>>>
>>> ** That is my contention too, posted 2 days ago.
>>>>
>>> The ECG amp will probably be heavily low pass filtered
>>>> somewhere along the chain so the HF interference won't be seen at the
>>>> output but it's 100Hz modulation rate will be?
>>>
>>> ** Huh ? 100Hz ?
>>>
>>> Think is is just simple capacitive coupled injection of the AC supply voltage wave.
>>> The bane of many poorly shielded electric guitars .
>>>
>>
>>>
>> Yes, but later on the OP said he solved the problem with ferrite beads.
>
> ** Should have tried Hippy beads or Rosary beads.
>
>> He hasn't said how many beads he used ...
>
> ** Nor how much weed he smoke daily.
>
>> but it seems unlikely he used
>> enough ferrite to attenuate mains frequencies directly so the theory now
>> is that he instead attenuated some higher frequency pickup that was
>> itself power line modulated?
>
> ** I can see what looks like 50Hz in that slow ECG trace.
>
> Try counting the wiggles.
>
>
>
> ..... Phil
>
>
>

With the timebase unhelpfully blurred out I didn't before count the
wiggles to see if 50Hz or 100Hz but tried just now using the heart rate
as a guide and agree with you - it looks like it is 50Hz direct mains
pickup. Thanks Phil.

piglet

erichp...@hotmail.com wrote:
========================
>>
> > ** I can see what looks like 50Hz in that slow ECG trace.
> >
> > Try counting the wiggles.
> >
> With the timebase unhelpfully blurred out I didn't before count the
> wiggles to see if 50Hz or 100Hz but tried just now using the heart rate
> as a guide and agree with you - it looks like it is 50Hz direct mains
> pickup.

** From the OP's first post.

" This *is* mains interference as it works out at exactly
mains frequency. "

....... Phil

Jan Pan Dipshit <pNaonStpealmtje@yahoo.com> wrote in
news:t2kb5a$kd8$1@dont-email.me:

> I just measured 42 per minute when sitting on a chair in front of
> the table looking at the laptop doing nothing.

I'll bet that you sport flawed measurement capability then, because
that rate claim is bullshit.

> But if I really
> relax its lower.

And you are still unable to obtain a correct reading.

Got a BP cuff? What does it say?

*I* say that you are a liar or simply have no clue how to read your
pulse.

> So it seems I am slower than you earthlings,

Right... the utter retard Jan Pan Dipshit thinks he's from
elsewhere.

> I know about some doctor who could control his heart beat.

So could our man Flint, but you lack the skill to even take a
proper reading, so you controlling what you cannot even read is
unlikely.

Phil Allison <pallison49@gmail.com> wrote in
news:e9c27709-1c68-47d3-b8db-45e594bd59ccn@googlegroups.com:

> Ricky wrote:
> ==========
>
>> An adequately narrow band reject filter would have virtually no
>> impact on the signal of interest.
>
> ** Shame you have never looked at the phase and amplitude curve
> of such a filter.
>
>
>> It only need be wide enough to accommodate the normal variations
>> in mains frequency.
>
> ** About 0.1Hz is all the AC supply varies by.
>
Yep... practically wordlwide the electrical grids sport near zero
variance with regard to frequency.

Phil Allison <pallison49@gmail.com> wrote in
news:02662e66-5f7e-4d87-a583-86f0abbcf9a9n@googlegroups.com:

> ** Huh ? 100Hz ?
>
> Think is is just simple capacitive coupled injection of the AC
> supply voltage wave. The bane of many poorly shielded electric
> guitars .

Yes, and it typically sounds off at twice the line frequency.

So, 60Hz gets 120Hz noise and 50Hz gets 100Hz noise.

On 7.4.22 2.52, Phil Allison wrote:
> Ricky wrote:
> ==========
>
>> An adequately narrow band reject filter would have virtually no impact on the signal of interest.
>
> ** Shame you have never looked at the phase and amplitude curve of such a filter.
>
>
>> It only need be wide enough to accommodate the normal variations in mains frequency.
>
> ** About 0.1Hz is all the AC supply varies by.
>
>
> ...... Phil

Right!

A narrow notch rings as badly as a narrow peak when fed with
a spiky signal.

We did try all kinds of filtering already half a century ago,
and the basic mathematics, physics and electronics about the
filter has not changed.

--

-TV

DecadentLinux...@decadence.org wrote:
================================
>
> > ** Huh ? 100Hz ?
> >
> > Think is is just simple capacitive coupled injection of the AC
> > supply voltage wave. The bane of many poorly shielded electric
> > guitars .
>
> Yes, and it typically sounds off at twice the line frequency.
>

** No it does not.

> So, 60Hz gets 120Hz noise and 50Hz gets 100Hz noise.

** The AC supply has a fair amount of 3rd and 5th harmonics that are often more audible that the fundamental.
But SFA 2nd.
Magnetic hum fields radiated from transformers is mostly 3rd to.

...... Phil

On Thursday, April 7, 2022 at 2:58:46 PM UTC-7, palli...@gmail.com wrote:
> DecadentLinux...@decadence.org wrote:
> ================================
> >
> > > ** Huh ? 100Hz ?
> > >
> > > Think is is just simple capacitive coupled injection of the AC
> > > supply voltage wave. The bane of many poorly shielded electric
> > > guitars .
> >
> > Yes, and it typically sounds off at twice the line frequency.
> >
> ** No it does not.
> > So, 60Hz gets 120Hz noise and 50Hz gets 100Hz noise.
> ** The AC supply has a fair amount of 3rd and 5th harmonics that are often more audible that the fundamental.
> But SFA 2nd.
> Magnetic hum fields radiated from transformers is mostly 3rd to.

Second harmonic is the acoustic (magnetostriction) output from transformers,
and the light from fluorescent fixtures (or used to be; ballasts are more
frequency-rich nowadays). Ripple voltage on a fullwave rectifier, also.

If you apply enough gain, you'll see lots of sneaky signal inputs. Microphonic tubes
are /were a major concern, and that's why old HiFi preamps were on a different chassis
than the power stages.

On Wednesday, April 6, 2022 at 7:37:40 PM UTC-4, Cursitor Doom wrote:
> On Wed, 6 Apr 2022 16:06:28 -0700 (PDT), Ricky
> <gnuarm.del...@gmail.com> wrote:
>
> >On Wednesday, April 6, 2022 at 2:21:53 PM UTC-4, Tauno Voipio wrote:
> >> On 6.4.22 19.51, Ricky wrote:
> >> > On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
> >> >> My guess is that the hum is coming from your patient / target.
> >> >>
> >> >> The capacitance of an average person is about 20 pF to the
> >> >> active phase line and 200 pF to ground. You can assume that
> >> >> the patient has about 10% of the line voltage through a pretty
> >> >> high impedance voltage divider.
> >> >>
> >> >> When we made ECG equipment in the early 1970's, the solution
> >> >> was to make the input of the amplifier differential and as
> >> >> high impedance as possible, including guard bootstrapping the
> >> >> shield braids of the input cables. The impedance of the
> >> >> connection electrodes are hardly ever identical, and this creates
> >> >> voltage dividers with the amplifier input impedances. The
> >> >> imbalance works directly to convert the common-mode hum into
> >> >> differential input.
> >> >
> >> > That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a "reference" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.
> >> >
> >> > I don't know for sure, but I expect the body is a relatively low impedance voltage source. I'm willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.
> >> That is true, but we do not have contacts to the low impedance
> >> source. The electrodes can have impedances of tens of kohm,
> >> with mismatch between the electrodes. We do not hit contact
> >> spikes through the skin.
> >
> >The body is still low impedance with respect to the amplifier inputs. They use paste and a large contact area to minimize skin resistance. Given the high input impedance of the amp, the mismatch in impedance is of little consequence.
> >
> >
> >> A notch filter is not a good idea, as much of the interesting
> >> frequency components are in the mains frequency range. The
> >> ECG signal is spiky by its very nature, and narrow band filters
> >> will spoil the information looked for.
> >
> >
> >You seem to be confusing a notch (band reject) with a narrow band-pass filter. An adequately narrow band reject filter would have virtually no impact on the signal of interest. It only need be wide enough to accommodate the normal variations in mains frequency.
> Leaving aside that the problem has now been resolved, I would have
> thought Tauno was right in what he said. The wanted and unwanted
> components of the waveform are just too close together to target one
> without degrading the other to some meaningful extent.

That's simply not true. You can design a filter to satisfy the requirements. It's not like the signal of interest has a critical component at 50/60 Hz. Look at the waveform, or better yet, look at the frequency spectrum. There's nothing important at that frequency. Tauno also spoke of it incorrectly, referring to "narrow band filters". What is called for is narrow band reject which is not at all the same thing and will not disturb the signal overall, unless there is a critical component at the same exact frequency, which is not the case.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

On Thursday, April 7, 2022 at 4:44:28 AM UTC-4, palli...@gmail.com wrote:
> erichp...@hotmail.com wrote:
> ========================
> >
> > >>>
> > >>>>> My guess is that the hum is coming from your patient / target.
> > >>>>>
> > >>>>> The capacitance of an average person is about ...
> > >
> > >> I think that when OP tried battery power he was still examining the
> > >> output on a mains connected scope? Interesting would be what is seen
> > >> with a battery powered scope and the whole system free of any connection
> > >> to mains ground.
> > >
> > > ** That is my contention too, posted 2 days ago.
> > >>
> > > The ECG amp will probably be heavily low pass filtered
> > >> somewhere along the chain so the HF interference won't be seen at the
> > >> output but it's 100Hz modulation rate will be?
> > >
> > > ** Huh ? 100Hz ?
> > >
> > > Think is is just simple capacitive coupled injection of the AC supply voltage wave.
> > > The bane of many poorly shielded electric guitars .
> > >
> >
> > >
> > Yes, but later on the OP said he solved the problem with ferrite beads.
> ** Should have tried Hippy beads or Rosary beads.
>
> > He hasn't said how many beads he used ...
>
> ** Nor how much weed he smoke daily.
> > but it seems unlikely he used
> > enough ferrite to attenuate mains frequencies directly so the theory now
> > is that he instead attenuated some higher frequency pickup that was
> > itself power line modulated?
> ** I can see what looks like 50Hz in that slow ECG trace.
>
> Try counting the wiggles.

You can count as much as you like. Without knowing the time base of the capture, there's no way to turn a count into a frequency.

--

Rick C.

--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209

On Thursday, April 7, 2022 at 1:29:01 PM UTC-4, Tauno Voipio wrote:
> On 7.4.22 2.52, Phil Allison wrote:
> > Ricky wrote:
> > ==========
> >
> >> An adequately narrow band reject filter would have virtually no impact on the signal of interest.
> >
> > ** Shame you have never looked at the phase and amplitude curve of such a filter.
> >
> >
> >> It only need be wide enough to accommodate the normal variations in mains frequency.
> >
> > ** About 0.1Hz is all the AC supply varies by.
> >
> >
> > ...... Phil
> Right!
>
> A narrow notch rings as badly as a narrow peak when fed with
> a spiky signal.

At what frequency does it ring?

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209

Ricky wrote:
===============
>Tauno Voipio wrote:
> Phil Allison wrote:
>
> > >
> > >> An adequately narrow band reject filter would have virtually no impact on the signal of interest.
> > >
> > > ** Shame you have never looked at the phase and amplitude curve of such a filter.
> > >
> > Right!
> >
> > A narrow notch rings as badly as a narrow peak when fed with
> > a spiky signal.
>
> At what frequency does it ring?

** Ringing (as seen on a scope) is at the same frequency as the notch minimum.
But you will not find it in a spectrum analysis.

...... Phil

On Thursday, April 7, 2022 at 9:31:47 PM UTC-4, Ricky wrote:
> On Thursday, April 7, 2022 at 1:29:01 PM UTC-4, Tauno Voipio wrote:
> > On 7.4.22 2.52, Phil Allison wrote:
> > > Ricky wrote:
> > > ==========
> > >
> > >> An adequately narrow band reject filter would have virtually no impact on the signal of interest.
> > >
> > > ** Shame you have never looked at the phase and amplitude curve of such a filter.
> > >
> > >
> > >> It only need be wide enough to accommodate the normal variations in mains frequency.
> > >
> > > ** About 0.1Hz is all the AC supply varies by.
> > >
> > >
> > > ...... Phil
> > Right!
> >
> > A narrow notch rings as badly as a narrow peak when fed with
> > a spiky signal.
> At what frequency does it ring?

The reason I ask, is because if the ringing is high enough frequency, it is out of band and can be filtered out. I'm willing to bet there is an inverse relationship between the half bandwidth of the notch and the ringing frequency. If it is narrow enough (which is what is desired) it will produce ringing that is far out of band and easily filtered. I believe I read the ECG bandwidth is 150 Hz. I expect it would not be too hard to push the ringing frequency up to a range that is easily filtered.

On doing a bit of research on the matter, it seems to be a much discussed topic, but with no definitive solution. However, here is a paper that seems to say the solution is really in the way the electrodes are attached.

https://www.aami.org/docs/default-source/bi-t/bit/2012-bit-nd-ecg.pdf

--

Rick C.

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On Thursday, April 7, 2022 at 10:01:41 PM UTC-4, palli...@gmail.com wrote:
> Ricky wrote:
> ===============
> >Tauno Voipio wrote:
> > Phil Allison wrote:
> >
> > > >
> > > >> An adequately narrow band reject filter would have virtually no impact on the signal of interest.
> > > >
> > > > ** Shame you have never looked at the phase and amplitude curve of such a filter.
> > > >
> > > Right!
> > >
> > > A narrow notch rings as badly as a narrow peak when fed with
> > > a spiky signal.
> >
> > At what frequency does it ring?
> ** Ringing (as seen on a scope) is at the same frequency as the notch minimum.
> But you will not find it in a spectrum analysis.

Let's construct a mind experiment to analyze that idea. If you stimulate a notch filter with an impulse, a signal containing all frequencies in equal amplitude, what will emerge from the notch filter is a ring at the frequency at which the filter is supposed to have a null?

Is that logically consistent with the frequency response of the filter?

--

Rick C.

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Phil Allison <pallison49@gmail.com> wrote in
news:70523b25-2f3e-45b5-994f-23abc24669e1n@googlegroups.com:

> DecadentLinux...@decadence.org wrote:
> ================================
>>
>> > ** Huh ? 100Hz ?
>> >
>> > Think is is just simple capacitive coupled injection of the AC
>> > supply voltage wave. The bane of many poorly shielded electric
>> > guitars .
>>
>> Yes, and it typically sounds off at twice the line frequency.
>>
>
> ** No it does not.
>
>> So, 60Hz gets 120Hz noise and 50Hz gets 100Hz noise.
>
> ** The AC supply has a fair amount of 3rd and 5th harmonics that
> are often more audible that the fundamental.
> But SFA 2nd.
> Magnetic hum fields radiated from transformers is mostly 3rd
> to.
>
> ..... Phil
>

But if it gets rectified, the ripple on a badly designed supply
exhibits noise at f*2.