Filtering by amplitude, not frequency??

D

Don A. Gilmore

Guest
Hi guys:

I have an audio signal that has a low-level 120 Hz interference in it. In
my application there is no way to eliminate the 120 Hz at its source. The
interference is at a low level in comparison to the audio signal, but is
still an annoyance, especially at high gain. Since my audio signal (music)
can often involve desirable signals around 120 Hz, I can't really notch it
out without harming the music.

Is there an easy way to filter out waves according to their amplitude rather
than their frequency? In other words, if a signal is not above a certain
threshold, can it be filtered out while allowing the louder signal through?

This may be a stupid question with a simple answer, or you may tell me that
it is impossible, but fire away nonetheless. Thanks.

Don
 
Don A. Gilmore wrote:

Hi guys:

I have an audio signal that has a low-level 120 Hz interference in it. In
my application there is no way to eliminate the 120 Hz at its source. The
interference is at a low level in comparison to the audio signal, but is
still an annoyance, especially at high gain. Since my audio signal
(music) can often involve desirable signals around 120 Hz, I can't really
notch it out without harming the music.

Is there an easy way to filter out waves according to their amplitude
rather
than their frequency? In other words, if a signal is not above a certain
threshold, can it be filtered out while allowing the louder signal
through?

This may be a stupid question with a simple answer, or you may tell me
that
it is impossible, but fire away nonetheless. Thanks.

Don
Click to expand...
Sounds like what you need is a multi band compressor/expander, particularly
the expander. I have been usign one to clean up some cassette recordings
and basicaly I have used just the expander settings for this. SO for
example where a track has low level 50/60Hz hum but also bass guitar yo can
set one band of the expander to reduce signals below a certain threshold
and leave anything above it untouched. Works a treat with hiss to.

HTH

Ian

--
UKFS
 
On Sat, 17 Jan 2004 20:59:52 GMT, "Don A. Gilmore"
<eromlignod@kc.rr.com> wrote:

Hi guys:

I have an audio signal that has a low-level 120 Hz interference in it. In
my application there is no way to eliminate the 120 Hz at its source. The
interference is at a low level in comparison to the audio signal, but is
still an annoyance, especially at high gain. Since my audio signal (music)
can often involve desirable signals around 120 Hz, I can't really notch it
out without harming the music.

Is there an easy way to filter out waves according to their amplitude rather
than their frequency? In other words, if a signal is not above a certain
threshold, can it be filtered out while allowing the louder signal through?

This may be a stupid question with a simple answer, or you may tell me that
it is impossible, but fire away nonetheless. Thanks.
Click to expand...
---
If you get the filter sharp enough I don't think it will hurt the music
at all, since the 120Hz content due to the music will be fleeting.

--
John Fields
 
On Sat, 17 Jan 2004 20:59:52 GMT, "Don A. Gilmore"
<eromlignod@kc.rr.com> wrote:
Is there an easy way to filter out waves according to their amplitude rather
than their frequency? In other words, if a signal is not above a certain
threshold, can it be filtered out while allowing the louder signal through?
Click to expand...
Well, you would need a multiband gate or downward expander... But you
will have a hard time finding (and affording) such a unit.

Other suggestion is much better - kill the 120 Hz noise with notch
filter. If the notch is very narrow you won't hear _any_ difference
with music! All you would need is one parametric equalizer and that
should be much easier to come up with.


Regards,
Goran Tomas
 
Why is that you say you can't eliminate the 120 Hz, tellus its source?
tnx
hank wd5jfr
"Don A. Gilmore" <eromlignod@kc.rr.com> wrote in message
news:cnhOb.91357$fq1.10372@twister.rdc-kc.rr.com...
Hi guys:

I have an audio signal that has a low-level 120 Hz interference in it. In
my application there is no way to eliminate the 120 Hz at its source. The
interference is at a low level in comparison to the audio signal, but is
still an annoyance, especially at high gain. Since my audio signal
(music)
can often involve desirable signals around 120 Hz, I can't really notch it
out without harming the music.

Is there an easy way to filter out waves according to their amplitude
rather
than their frequency? In other words, if a signal is not above a certain
threshold, can it be filtered out while allowing the louder signal
through?

This may be a stupid question with a simple answer, or you may tell me
that
it is impossible, but fire away nonetheless. Thanks.

Don
Click to expand...
 
Don A. Gilmore wrote:
Hi guys:

I have an audio signal that has a low-level 120 Hz interference in
it. In my application there is no way to eliminate the 120 Hz at its
source. The interference is at a low level in comparison to the
audio signal, but is still an annoyance, especially at high gain.
Since my audio signal (music) can often involve desirable signals
around 120 Hz, I can't really notch it out without harming the music.

Is there an easy way to filter out waves according to their amplitude
rather than their frequency? In other words, if a signal is not
above a certain threshold, can it be filtered out while allowing the
louder signal through?

This may be a stupid question with a simple answer, or you may tell
me that it is impossible, but fire away nonetheless. Thanks.
Click to expand...
Assuming its not a ground loop problem which might well be fixable by
removing various grounds, you could take a bit of the power supply
ripple and add/subtract it in to the signal with a pot to cancel the
existing signal. You might need to have a bit of phase shift on it to do
it accurately.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"That which is mostly observed, is that which replicates the most"
http://www.anasoft.co.uk/replicators/index.html

"quotes with no meaning, are meaningless" - Kevin Aylward.
 
It's not from the power source. This is a battery-powered device. The 120
Hz interference is literally 120 Hz sound. It is inherrent in this
particular environment and can't be consistently eliminated in all cases.

I may try the notch filter that was suggested. Any ideas on the best way to
get a good, sharp notch without a lot of components? A "twin T" maybe?

Don

"Henry Kolesnik" <wd5jfr@oklahoma.net> wrote in message
news:100jm0eiibhppc7@corp.supernews.com...
Why is that you say you can't eliminate the 120 Hz, tellus its source?
tnx
hank wd5jfr
"Don A. Gilmore" <eromlignod@kc.rr.com> wrote in message
news:cnhOb.91357$fq1.10372@twister.rdc-kc.rr.com...
Hi guys:

I have an audio signal that has a low-level 120 Hz interference in it.
In
my application there is no way to eliminate the 120 Hz at its source.
The
interference is at a low level in comparison to the audio signal, but is
still an annoyance, especially at high gain. Since my audio signal
(music)
can often involve desirable signals around 120 Hz, I can't really notch
it
out without harming the music.

Is there an easy way to filter out waves according to their amplitude
rather
than their frequency? In other words, if a signal is not above a
certain
threshold, can it be filtered out while allowing the louder signal
through?

This may be a stupid question with a simple answer, or you may tell me
that
it is impossible, but fire away nonetheless. Thanks.

Don
Click to expand...
 
On Sun, 18 Jan 2004 16:04:24 GMT, "Don A. Gilmore"
<eromlignod@kc.rr.com> wrote:

It's not from the power source. This is a battery-powered device. The 120
Hz interference is literally 120 Hz sound. It is inherrent in this
particular environment and can't be consistently eliminated in all cases.

I may try the notch filter that was suggested. Any ideas on the best way to
get a good, sharp notch without a lot of components? A "twin T" maybe?
Click to expand...
---
A twin "T", yes.

http://www.national.com/ms/LB/LB-5.pdf

--
John Fields
 
"Don A. Gilmore" <eromlignod@kc.rr.com> wrote in message
news:c8yOb.92485$fq1.35077@twister.rdc-kc.rr.com...
It's not from the power source. This is a battery-powered device. The
120
Hz interference is literally 120 Hz sound. It is inherrent in this
particular environment and can't be consistently eliminated in all cases.

I may try the notch filter that was suggested. Any ideas on the best way
to
get a good, sharp notch without a lot of components? A "twin T" maybe?
Click to expand...
If you could get a clean signal of the pure 120Hz in some way,
you might be able to invert and add it to the channel at the
appropriate amplitude, effectively canceling it.
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top