removing "hum" from an audio recording

[Glenn Gundlach]

martin griffith wrote:

just found this
http://www.mpex.net/en/software/download/cooledit.html

I dont know if it works, since I already have cooledit


martin

The link doesn't work but the eBay links show CoolEdit for pretty
cheap. I'm like you, probably worse. I have cooledit 96, 2000, Pro 2.1,
Audition 1.0 and 1.5, all legal. I like it a LOT.

GG

 

[mc]

I tuned in late, but for audio processing free of charge, try GoldWave.

 

[Michael A. Terrell]

martin griffith wrote:


The link doesn't work but the eBay links show CoolEdit for pretty
cheap. I'm like you, probably worse. I have cooledit 96, 2000, Pro 2.1,
Audition 1.0 and 1.5, all legal. I like it a LOT.

GG

I have the version referenced in that web page if you want it. Its
8.24 MB. E-mail me if you want it and I'll stick it up on one of my
websites for you.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Mac]

I'm sure you've come across this before.

We had to tape-record a meeting (with real cassette tapes!) yesterday.
I was planning on playing the tape as input to the sound card, and burn
a CD of the meeting for all attendees.

Unfortunately, we have this HUM in the background. Sounds like it's
somewhere between 60 Hz and 120 Hz.

How do I remove this?

As a test, I tried the freeware program Audacity, asked it to produce a
pure 60 Hz tone, then
tried the "low pass filter" feature, cutting off everything below 100
Hz. This just seems to reduce the amplitude of the sine wave.

Then, when I looked up "low pass filter" on Wikipedia, I realized I
might have gotten it backwards (cut off higher frequencies instead of
lower frequencies), so I then ran a "high pass filter", asking Audacity
to cut off everything below 100 Hz. No improvement.

Try moving the cutoff frequency higher. At some point it will make the
voices sound funny, but you can definitely go higher than 100 Hz.

Any other suggestions?

Michael D.

--Mac

 

[Wim Lewis]

We had to tape-record a meeting (with real cassette tapes!) yesterday.
I was planning on playing the tape as input to the sound card, and burn
a CD of the meeting for all attendees.

Unfortunately, we have this HUM in the background. Sounds like it's
somewhere between 60 Hz and 120 Hz.
[....] I then ran a "high pass filter", asking Audacity
to cut off everything below 100 Hz. No improvement.

Try moving the cutoff frequency higher. At some point it will make the
voices sound funny, but you can definitely go higher than 100 Hz.

The pass band for telephone audio is 300 Hz to about 3 kHz. As I understand
it, this wasn't (originally) because of any difficulty in having a wider
pass band, but because that's where most of the information in speech
is, and removing the other frequencies helps intelligibility by removing
distracting background noises.

So if the purpose of recording the meeting was just to hear what people
said (no music or sound effects ) then you might try filtering out
everything below 300 Hz and above 3-4 kHz anyway, and see if that makes
the recording more intelligible, even aside from the hum problem.

 

[Al]

Try moving the cutoff frequency higher. At some point it will make the
voices sound funny, but you can definitely go higher than 100 Hz.


--Mac

If you have access to an adjustable notch filter you might be able to
get rid of the hum if it is at one frequency.

How do you know that it is not your tape reader that is producing the
hum? Have you tryed a second machine?

al

 

[[email protected]]

If you have access to an adjustable notch filter you might be able to
get rid of the hum if it is at one frequency.

How do you know that it is not your tape reader that is producing the
hum? Have you tryed a second machine?

al

Prior to moving the machine, everything sounded crystal clear.

Then I moved it, then we started unexpectedly.

Then on playback, hum...

Oh well.

I tried the "Noise Removal" in Audacity; hum was gone (yay!), but now
the voices sound a bit like aliens.

Management told me not to "waste any more time on cleaning it up". Go
figure.

Thanks all who replied...

Michael

 

[mc]

I tried the "Noise Removal" in Audacity; hum was gone (yay!), but now

the voices sound a bit like aliens.

I'll bet you sampled the whole recording and subtracted that out, leaving
behind, at any moment, only the difference between the instantaneous
spectrum and the average of the whole thing.

Instead, sample a portion that contains only noise (not voice) and use that
as your noise sample. I don't know how to do that in Audacity, but it
should be fairly straightforward. For GoldWave instructions see:

http://www.covingtoninnovations.com/audio/digitizing/#noise

You should get beautifully clear results.

 

[[email protected]]

I'll bet you sampled the whole recording and subtracted that out, leaving
behind, at any moment, only the difference between the instantaneous
spectrum and the average of the whole thing.

Instead, sample a portion that contains only noise (not voice) and use that
as your noise sample. I don't know how to do that in Audacity, but it
should be fairly straightforward. For GoldWave instructions see:

http://www.covingtoninnovations.com/audio/digitizing/#noise

You should get beautifully clear results.

Er... I took a small sample of nothing but noise, told Audacity to
recognize this as noise, then highlighted the whole 30-minute clip and
said, Go Ahead and Remove the Noise.

The problem is, once someone starts talking, the amplitude of the hum
decreases. So, I'm guessing the algorithm is trying to subtract hum
from audio patches which have no hum - resulting in an alien-like
effect.

It's ok, it's quite possible we won't even use the recording, since we
have the questions in writing... (and have to provide the answers in
writing as well...)

Michael

 

[mc]

Er... I took a small sample of nothing but noise, told Audacity to

recognize this as noise, then highlighted the whole 30-minute clip and
said, Go Ahead and Remove the Noise.
Ah.

The problem is, once someone starts talking, the amplitude of the hum
decreases. So, I'm guessing the algorithm is trying to subtract hum
from audio patches which have no hum - resulting in an alien-like
effect.

Could well be. You might try telling it to subtract only 50% of the noise,
or something like that.

 

[Ban]

Er... I took a small sample of nothing but noise, told Audacity to
recognize this as noise, then highlighted the whole 30-minute clip and
said, Go Ahead and Remove the Noise.

The problem is, once someone starts talking, the amplitude of the hum
decreases. So, I'm guessing the algorithm is trying to subtract hum
from audio patches which have no hum - resulting in an alien-like
effect.

It's ok, it's quite possible we won't even use the recording, since we
have the questions in writing... (and have to provide the answers in
writing as well...)

Look at the slider, don't go over the yellow range in the middle. You didn't
follow exactly my instructions there. You can preview the result and put
much less effect into the final. You can also modify the fft settings.
uncheck "live update" and set the fft size to 8192. Click on help and read
about the different parameters and how they affect the outcome. Experiment a
bit before you complain here.

 

[Rich Grise]

Could well be. You might try telling it to subtract only 50% of the noise,
or something like that.

It sounds like it has AVC on the record part. Almost like the recorder
needs a "squelch" control. ;-)

But a 300Hz low-pass will get rid of a lot of crap - people will sound
a little nasal, but not alien. Oh, and putting this filter on the input
of the recorder will keep the crap off the tape in the first place.

Good luck!
Rich

 

[mc]

The pass band for telephone audio is 300 Hz to about 3 kHz. As I
understand
it, this wasn't (originally) because of any difficulty in having a wider
pass band, but because that's where most of the information in speech
is, and removing the other frequencies helps intelligibility by removing
distracting background noises.

Right. My experience, however, having done some speech research, is that it
helps if you go up to about 5 kHz instead of just 3 kHz. A strict cutoff at
3 kHz will sound just a bit muffled.

So if the purpose of recording the meeting was just to hear what people
said (no music or sound effects ) then you might try filtering out
everything below 300 Hz and above 3-4 kHz anyway, and see if that makes
the recording more intelligible, even aside from the hum problem.

It should.

 

[Rich Grise]

Er... I took a small sample of nothing but noise, told Audacity to
recognize this as noise, then highlighted the whole 30-minute clip and
said, Go Ahead and Remove the Noise.

The problem is, once someone starts talking, the amplitude of the hum
decreases. So, I'm guessing the algorithm is trying to subtract hum
from audio patches which have no hum - resulting in an alien-like
effect.

It's ok, it's quite possible we won't even use the recording, since we
have the questions in writing... (and have to provide the answers in
writing as well...)

Well, good grief! You could find a studio, and read them again! ;-)

Cheers!
Rich

 

[Jake]

Read the help file

The depth of noise reduction is to great leaving electronic artifacts.
Reduce the amount of noise reduction until it is as best as possible and
without the artifacts. Test on a small section first to save time.

You could also try the notch filter. Look at the spectrum of noise and try
and determine if there is a single frequency that relates to your hum.

Peter

 

[[email protected]]

Read the help file

The depth of noise reduction is to great leaving electronic artifacts.
Reduce the amount of noise reduction until it is as best as possible and
without the artifacts. Test on a small section first to save time.

You could also try the notch filter. Look at the spectrum of noise and try
and determine if there is a single frequency that relates to your hum.

Peter

I did the high pass filter from 300 hz, and the hum is mostly gone now,
thanks.

However, I still have these strange "peaks" of noise superimposed on my
signal. There would be a peak maybe 1 cm tall, then about 2 inches
later, a peak of opposite sign will show up. Seems immune to the
low-pass filter. It's not a regular sine wave... anyone familiar with
what these are?

We're not really supposed to be installing software on our work
computers, no notch filters. I used Audacity because it bypasses the
Windows installer.

 

[joseph2k]

I did the high pass filter from 300 hz, and the hum is mostly gone now,
thanks.

However, I still have these strange "peaks" of noise superimposed on my
signal. There would be a peak maybe 1 cm tall, then about 2 inches
later, a peak of opposite sign will show up. Seems immune to the
low-pass filter. It's not a regular sine wave... anyone familiar with
what these are?

We're not really supposed to be installing software on our work
computers, no notch filters. I used Audacity because it bypasses the
Windows installer.

Audacity is a good tool, gets better as you learn it more. First
observation the original signal has already been compressed (audio /
instrument sense first), this interferes with the relationship between the
noise and the desired signal. Noise reduction / recognition / removal
works best on un-compressed signals. Try various Dolby as well as DBx
uncompression models. If done correctly the noise properties should match.