removing "hum" from an audio recording

[[email protected]]

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.

Any other suggestions?

Michael D.

 

[Ban]

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.

Any other suggestions?

Michael D.

select part of the wavefile where there is only the hum.
Click Effects/Noise Reduction/Noise Reduction
click "Get Profile from Selection" and adjust the slider in the preview
mode to the desired value.
click close and select the whole wave, then call that filter again and
click OK.

 

[Luhan]

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.

Any other suggestions?

If the hum is at 120 Hz, you need to filter everthing below about 200
Hz to get enough attenuation.

Best is usually to use a notch filter at 60 or 120 Hz (or both).
However, some hum signals contain harmonics that exists at much higher
frequencies, these cannot be readily filtered.

Luhan

 

[GregS]

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.

Any other suggestions?

How did you record it? Can you listen to the tape deck with phones plugged
into the tape deck. How are you playing it back. What AC connections
are made during recording and playback?

greg

 

[Joerg]

Hello Micheal,

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.

Any other suggestions?

You really need what is called a "notch filter". In fact you'd need
several because most hum isn't just 60 Hz but also 120Hz, 180Hz and
maybe higher. This is due to dimmers, cheap motors, switch mode supplies
and all that.

I am not familiar with audio software or what filter features they have.
However, you might want the ask the pros in this newsgroup:
alt.audio.pro.live-sound

Stay cool. Heard it's going to be 107F this coming weekend. Whew...

 

[[email protected]]

How did you record it? Can you listen to the tape deck with phones plugged
into the tape deck. How are you playing it back. What AC connections
are made during recording and playback?

greg

How did we record it? Co-worker also brought along two microphones
(one UHF wireless, another wired), along with his video camera. Then
we discovered that for legal reasons we couldn't videotape the
conference.

However, we found an ancient Sony tape recorder (with analog signal
meter!) and power supply in a box. Plugged this right up. We did some
tests - everything sounded ok. Then I had this great (horrible?) idea
- let's move the equipment closer to the laptop, which was driving our
Powerpoint presentation. Then we unexpectedly started right up - no
chance to test. (No headset to monitor the input, unfortunately.)
Then, after the presentation, re-wound, and... hum!

Possibly from the power strip, or the projector, or the laptop... or
maybe from the wired microphone.

md

 

[[email protected]]

Hello Micheal,



You really need what is called a "notch filter". In fact you'd need
several because most hum isn't just 60 Hz but also 120Hz, 180Hz and
maybe higher. This is due to dimmers, cheap motors, switch mode supplies
and all that.

I am not familiar with audio software or what filter features they have.
However, you might want the ask the pros in this newsgroup:
alt.audio.pro.live-sound

Stay cool. Heard it's going to be 107F this coming weekend. Whew...

No kidding... sounds like South Lake Tahoe is the place to be...

Thanks,

Michael

 

[Joerg]

Hello Michael,

However, we found an ancient Sony tape recorder (with analog signal
meter!) and power supply in a box. Plugged this right up. We did some
tests - everything sounded ok. Then I had this great (horrible?) idea
- let's move the equipment closer to the laptop, which was driving our
Powerpoint presentation. Then we unexpectedly started right up - no
chance to test. (No headset to monitor the input, unfortunately.)
Then, after the presentation, re-wound, and... hum!

Possibly from the power strip, or the projector, or the laptop... or
maybe from the wired microphone.

Oh boy. Could be direct coupling from the laptop power brick. Those can
be nasty. I guess you really need software with freely programmable
multiple notch filters. Google search the ham radio community, for FFT
and sound card software. They often have similar problems to tackle,
usually trying to fend off on-the-air noises that can be similar.

 

[[email protected]]

select part of the wavefile where there is only the hum.
Click Effects/Noise Reduction/Noise Reduction
click "Get Profile from Selection" and adjust the slider in the preview
mode to the desired value.
click close and select the whole wave, then call that filter again and
click OK.

Thanks! That noise reduction trick removed my synthetic 60Hz sine wave
tone.

Will try this on the tape when my co-worker comes back in to work -
unknown to me, he took Tues-Thurs off too. (And, he has the tape.)

Michael

 

[martin griffith]

On 20 Jun 2006 13:12:44 -0700, in sci.electronics.design

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.

Any other suggestions?

Michael D.

Try and download a copy of Cooledit 2000, its gat a notch fliter
system that is rather excellent. Low pass is not the right tool for
this sort of thing.

For example my Canon XL1s produces shit sound, nice bit 150Hz hum, I
just knock the fundamental down by 30dB, no problems


martin

 

[Wes Stewart]

On Tue, 20 Jun 2006 20:37:48 GMT, Joerg


[snip]

Stay cool. Heard it's going to be 107F this coming weekend. Whew...

Hello Joerg,

What's the big deal about a balmy 107? It hit that in Tucson
yesterday and summer doesn't start until tomorrow morning [g]. It
cooled off today... only 102.9 at 3:50 PM.

 

[Don Lancaster]

If the hum is at 120 Hz, you need to filter everthing below about 200
Hz to get enough attenuation.

Best is usually to use a notch filter at 60 or 120 Hz (or both).
However, some hum signals contain harmonics that exists at much higher
frequencies, these cannot be readily filtered.

Luhan

A comb filter might work, phase locked to the hum fundamental.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Jim Thompson]

On Tue, 20 Jun 2006 20:37:48 GMT, Joerg


[snip]

Stay cool. Heard it's going to be 107F this coming weekend. Whew...

Hello Joerg,

What's the big deal about a balmy 107? It hit that in Tucson
yesterday and summer doesn't start until tomorrow morning [g]. It
cooled off today... only 102.9 at 3:50 PM.

They're predicting 116°F in Phoenix on Friday. Time for you
Easterners to drop by ;-)

...Jim Thompson

 

[Luhan]

On Tue, 20 Jun 2006 20:37:48 GMT, Joerg


[snip]

Stay cool. Heard it's going to be 107F this coming weekend. Whew...

Hello Joerg,

What's the big deal about a balmy 107? It hit that in Tucson
yesterday and summer doesn't start until tomorrow morning [g]. It
cooled off today... only 102.9 at 3:50 PM.

They're predicting 116°F in Phoenix on Friday. Time for you
Easterners to drop by ;-)

Thats why I moved here from Quartzsite: normal mid-summer temps there
peak at 130. No records are set, because there is no official
temperature taken there.

Luhan "The Desert Rat"

 

[Jim Thompson]

On Tue, 20 Jun 2006 20:37:48 GMT, Joerg


[snip]

Stay cool. Heard it's going to be 107F this coming weekend. Whew...


Hello Joerg,

What's the big deal about a balmy 107? It hit that in Tucson
yesterday and summer doesn't start until tomorrow morning [g]. It
cooled off today... only 102.9 at 3:50 PM.

They're predicting 116°F in Phoenix on Friday. Time for you
Easterners to drop by ;-)

Thats why I moved here from Quartzsite: normal mid-summer temps there
peak at 130. No records are set, because there is no official
temperature taken there.

Luhan "The Desert Rat"

Does anyone live in a house there? All you can see from I10 is
trailers ;-)

...Jim Thompson

 

[Glenn Gundlach]

martin griffith wrote:
<snip

Try and download a copy of Cooledit 2000, its gat a notch fliter
system that is rather excellent. Low pass is not the right tool for
this sort of thing.

For example my Canon XL1s produces shit sound, nice bit 150Hz hum, I
just knock the fundamental down by 30dB, no problems


martin

Good choice but hard to get. After Adobe bought Syntrilium, they
renamed CoolEdit Pro Audition and removed CoolEdit 2000. I have looked
around for it but I haven't found it. Audition sells $349. Its a shame
they got rid of the $69 2000. It was really good.

GG

 

[Luhan]

On Tue, 20 Jun 2006 20:37:48 GMT, Joerg


[snip]

Stay cool. Heard it's going to be 107F this coming weekend. Whew...


Hello Joerg,

What's the big deal about a balmy 107? It hit that in Tucson
yesterday and summer doesn't start until tomorrow morning [g]. It
cooled off today... only 102.9 at 3:50 PM.

They're predicting 116°F in Phoenix on Friday. Time for you
Easterners to drop by ;-)

Thats why I moved here from Quartzsite: normal mid-summer temps there
peak at 130. No records are set, because there is no official
temperature taken there.

Luhan "The Desert Rat"

Does anyone live in a house there? All you can see from I10 is
trailers ;-)

The houses there are all north of I-10; about 1 square mile. I was in
a 32 foot motorhome with no ac (the compressor was already in 'thermal
cutout' from the ambiant temperature). I had a makeshift swamp cooler
that put out 110 degree's if you sat right in front of it.

Even most of the locals leave town for the summer. A few years before
I was there, they had a 'hot one' - hitting 136 reqularly.

Now the worst I have to deal with, is the walk to the mailbox on a hot
day.

Luhan

 

[Robert Baer]

Hello Micheal,



You really need what is called a "notch filter". In fact you'd need
several because most hum isn't just 60 Hz but also 120Hz, 180Hz and
maybe higher. This is due to dimmers, cheap motors, switch mode supplies
and all that.

I am not familiar with audio software or what filter features they have.
However, you might want the ask the pros in this newsgroup:
alt.audio.pro.live-sound

Stay cool. Heard it's going to be 107F this coming weekend. Whew...

The three frequencies you mentioned will kill almost all of it; a low
pass with 3dB point near (say) 5KHz will take care of the spikes which
are individually not too energetic, but add up to most of the balance of
the noise.
This all ASS-u-MEs that the fundamental is 60Hz; modify the three if
50Hz instead.

 

[martin griffith]

martin griffith wrote:
<snip

Good choice but hard to get. After Adobe bought Syntrilium, they
renamed CoolEdit Pro Audition and removed CoolEdit 2000. I have looked
around for it but I haven't found it. Audition sells $349. Its a shame
they got rid of the $69 2000. It was really good.

GG

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

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


martin

 

[Roger]

They're predicting 116°F in Phoenix on Friday. Time for you
Easterners to drop by ;-)

With drop being the key word!