Filtering by amplitude, not frequency??

[Don A. Gilmore]

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

 

[Ian Bell]

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

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

 

[John Fields]

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.

 

[Goran Tomaš]

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?

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

 

[Henry Kolesnik]

Why is that you say you can't eliminate the 120 Hz, tellus its source?
tnx
hank wd5jfr

 

[Kevin Aylward]

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.

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
[email protected]
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.

 

[Don A. Gilmore]

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

 

[John Fields]

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?

 

[Greg Neill]

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?

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.