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Rob Hutchinson wrote:

> What is the preferred method for removing 60 hz hum from a signal without
> wiping out signal info around 60 hz?  A 60 hz notch filter would not be
> useful because it would attenuate the signal as well.  I'm interested in
> doing this for sampled data, so all filtering would be done in the digital
> domain.
> Thank you in advance,
> Rob
> 
> 

Your signal processing algorithm won't be able to tell the hum from the 
real data -- you're stuck with either a really narrow 60Hz notch (which 
you can do digitally if you're really careful about numeric precision) 
or trying to remove the hum before you acquire.  Neither of these is a 
good prospect, but there you are.

Perhaps you should move to Germany, or Brazil?  Then you'll only have to 
worry about 50Hz, or 25 :).

-- 

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

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Rob Hutchinson wrote:

> What is the preferred method for removing 60 hz hum from a signal without
> wiping out signal info around 60 hz?  A 60 hz notch filter would not be
> useful because it would attenuate the signal as well.  I'm interested in
> doing this for sampled data, so all filtering would be done in the digital
> domain.
> Thank you in advance,
> Rob
> 
> 
You could try using some active noise reduction. Use a 60 Hz 'generator'
of which you can alter phase & amplitude and a little freq and a small
bandpass filter around 60 Hz. Now adjust the parameters so that the
output of the filter minimizes. (but I guess its earier said than done)
For me 150 Hz is always annoying me, they manage to deliver a clipped mains.

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A notch can be made almost arbitrarily narrow if care is taken in the
implementation.  The best bet of course is to remove the hum in the analog realm
before the signal is recorded, etc..  But if that isn't possible, then I would
say a very narrow notch filter is a decent method.  If the hum is uniform in
level, another possibility is some sort of adaptive noise removal algorithm.
Don't have much personal experience with this myself, but maybe others do?

"Rob Hutchinson" <r...@kdsi.net> wrote in message
news:1...@corp.supernews.com...
> What is the preferred method for removing 60 hz hum from a signal without
> wiping out signal info around 60 hz?  A 60 hz notch filter would not be
> useful because it would attenuate the signal as well.  I'm interested in
> doing this for sampled data, so all filtering would be done in the digital
> domain.
> Thank you in advance,
> Rob
>
>

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What is the preferred method for removing 60 hz hum from a signal without
wiping out signal info around 60 hz?  A 60 hz notch filter would not be
useful because it would attenuate the signal as well.  I'm interested in
doing this for sampled data, so all filtering would be done in the digital
domain.
Thank you in advance,
Rob

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Rob Hutchinson wrote:

> I was thinking that an adaptive scheme would be commonly used.  Anybody know
> about this approach?  Is this the common method for this problem??
> 
> 
> Jon Harris <g...@hotmail.com> wrote in message
> news:2...@uni-berlin.de...
> 
>>A notch can be made almost arbitrarily narrow if care is taken in the
>>implementation.  The best bet of course is to remove the hum in the analog
> 
> realm
> 
>>before the signal is recorded, etc..  But if that isn't possible, then I
> 
> would
> 
>>say a very narrow notch filter is a decent method.  If the hum is uniform
> 
> in
> 
>>level, another possibility is some sort of adaptive noise removal
> 
> algorithm.
> 
>>Don't have much personal experience with this myself, but maybe others do?
>>
>>"Rob Hutchinson" <r...@kdsi.net> wrote in message
>>news:1...@corp.supernews.com...
>>
>>>What is the preferred method for removing 60 hz hum from a signal
> 
> without
> 
>>>wiping out signal info around 60 hz?  A 60 hz notch filter would not be
>>>useful because it would attenuate the signal as well.  I'm interested in
>>>doing this for sampled data, so all filtering would be done in the
> 
> digital
> 
>>>domain.
>>>Thank you in advance,
>>>Rob
>>>
>>>
>>
>>
> 
> 
If the frequency is nice and constant the best "adaptation" that you can 
get would be a nice narrow notch.

-- 

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

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"Tim Wescott" <t...@wescottnospamdesign.com> wrote in message
news:1...@corp.supernews.com...
> Rob Hutchinson wrote:
>
> > Jon Harris <g...@hotmail.com> wrote in message
> > news:2...@uni-berlin.de...
> >
> >>A notch can be made almost arbitrarily narrow if care is taken in the
> >>implementation.  The best bet of course is to remove the hum in the analog
> > realm
> >
> >>before the signal is recorded, etc..  But if that isn't possible, then I
> > would
> >>say a very narrow notch filter is a decent method.  If the hum is uniform
> > in
> >>level, another possibility is some sort of adaptive noise removal
> > algorithm.
> >>Don't have much personal experience with this myself, but maybe others do?
> >>
> >>"Rob Hutchinson" <r...@kdsi.net> wrote in message
> >>news:1...@corp.supernews.com...
> >>
> >>>What is the preferred method for removing 60 hz hum from a signal
> > without
> >
> >>>wiping out signal info around 60 hz?  A 60 hz notch filter would not be
> >>>useful because it would attenuate the signal as well.  I'm interested in
> >>>doing this for sampled data, so all filtering would be done in the
> > digital domain.
> >
> If the frequency is nice and constant the best "adaptation" that you can
> get would be a nice narrow notch.

Assuming a perfectly constant hum, how about adding a 60Hz sine of the exact
same magnitude/opposite phase?

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I was thinking that an adaptive scheme would be commonly used.  Anybody know
about this approach?  Is this the common method for this problem??


Jon Harris <g...@hotmail.com> wrote in message
news:2...@uni-berlin.de...
> A notch can be made almost arbitrarily narrow if care is taken in the
> implementation.  The best bet of course is to remove the hum in the analog
realm
> before the signal is recorded, etc..  But if that isn't possible, then I
would
> say a very narrow notch filter is a decent method.  If the hum is uniform
in
> level, another possibility is some sort of adaptive noise removal
algorithm.
> Don't have much personal experience with this myself, but maybe others do?
>
> "Rob Hutchinson" <r...@kdsi.net> wrote in message
> news:1...@corp.supernews.com...
> > What is the preferred method for removing 60 hz hum from a signal
without
> > wiping out signal info around 60 hz?  A 60 hz notch filter would not be
> > useful because it would attenuate the signal as well.  I'm interested in
> > doing this for sampled data, so all filtering would be done in the
digital
> > domain.
> > Thank you in advance,
> > Rob
> >
> >
>
>

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Jon Harris wrote:

> "Tim Wescott" <t...@wescottnospamdesign.com> wrote in message
> news:1...@corp.supernews.com...
> 
>>Rob Hutchinson wrote:
>>
>>
>>>Jon Harris <g...@hotmail.com> wrote in message
>>>news:2...@uni-berlin.de...
>>>
>>>
>>>>A notch can be made almost arbitrarily narrow if care is taken in the
>>>>implementation.  The best bet of course is to remove the hum in the analog
>>>
>>>realm
>>>
>>>
>>>>before the signal is recorded, etc..  But if that isn't possible, then I
>>>
>>>would
>>>
>>>>say a very narrow notch filter is a decent method.  If the hum is uniform
>>>
>>>in
>>>
>>>>level, another possibility is some sort of adaptive noise removal
>>>
>>>algorithm.
>>>
>>>>Don't have much personal experience with this myself, but maybe others do?
>>>>
>>>>"Rob Hutchinson" <r...@kdsi.net> wrote in message
>>>>news:1...@corp.supernews.com...
>>>>
>>>>
>>>>>What is the preferred method for removing 60 hz hum from a signal
>>>
>>>without
>>>
>>>
>>>>>wiping out signal info around 60 hz?  A 60 hz notch filter would not be
>>>>>useful because it would attenuate the signal as well.  I'm interested in
>>>>>doing this for sampled data, so all filtering would be done in the
>>>
>>>digital domain.
>>>
>>
>>If the frequency is nice and constant the best "adaptation" that you can
>>get would be a nice narrow notch.
> 
> 
> Assuming a perfectly constant hum, how about adding a 60Hz sine of the exact
> same magnitude/opposite phase?
> 
> 
Good idea.  If you use a unity-gain highly resonant bandpass filter to 
acquire your 60Hz sine wave and subtract it from your signal you'll have 
-- a notch filter!  (I've been through this before).

I suppose that you could attempt to do this with a PLL of some sort, but 
I expect that after you make it so it can identify the hum's magnitude 
and phase, and correctly identify when you are operating off of battery 
power in the middle of the Sahara desert and therefore don't have any 
hum, and take all the other odd little corner cases into account, you'd 
be better off just making a _really good_ notch filter.

But I've become a luddite in my old age.

-- 

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

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"Jon Harris" <g...@hotmail.com> wrote in message
news:2...@uni-berlin.de...
> "Tim Wescott" <t...@wescottnospamdesign.com> wrote in message
> news:1...@corp.supernews.com...
> > Rob Hutchinson wrote:
> >
> > > Jon Harris <g...@hotmail.com> wrote in message
> > > news:2...@uni-berlin.de...
> > >
> > >>A notch can be made almost arbitrarily narrow if care is taken in the
> > >>implementation.  The best bet of course is to remove the hum in the
analog
> > > realm
> > >
> > >>before the signal is recorded, etc..  But if that isn't possible, then
I
> > > would
> > >>say a very narrow notch filter is a decent method.  If the hum is
uniform
> > > in
> > >>level, another possibility is some sort of adaptive noise removal
> > > algorithm.
> > >>Don't have much personal experience with this myself, but maybe others
do?
> > >>
> > >>"Rob Hutchinson" <r...@kdsi.net> wrote in message
> > >>news:1...@corp.supernews.com...
> > >>
> > >>>What is the preferred method for removing 60 hz hum from a signal
> > > without
> > >
> > >>>wiping out signal info around 60 hz?  A 60 hz notch filter would not
be
> > >>>useful because it would attenuate the signal as well.  I'm interested
in
> > >>>doing this for sampled data, so all filtering would be done in the
> > > digital domain.
> > >
> > If the frequency is nice and constant the best "adaptation" that you can
> > get would be a nice narrow notch.
>
> Assuming a perfectly constant hum, how about adding a 60Hz sine of the
exact
> same magnitude/opposite phase?
>

It depends on how the hum is picket up but I bed the amplitude is not
constant. Most likely it is not a perfect sine wave either. Depending on
your setup sometimes it helps to match the impedance and remove the signal
you are interested so you can look at the hum only.

Evert Rozendaal
www.adacs.com

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Tim Wescott wrote:

   ...

> But I've become a luddite in my old age.

You're learning. The only excuse for filtering is that the signal is
already acquired and you can't go back and do it over. An adaptive
filter may be useful if the interfering frequency drifts slowly, but
power lines rarely do.

Boston really is like a small town. One of my customers had an old tube
FM tuner with weak AFC. It worked well enough on strong stations, but
WXHR broadcast at modest power from Woburn, and needed frequent
retuning. One day, my crotchety customer called the station manager,
John Thornton, to complain that his station drifted too much. He
replied, "Madam, I know. You should know that nowadays, you can get a
tuner that drifts with the station." John had the chutzpah to request a
commission.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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