How can I remove Background Noise Removal after FFT?

Amount of time required to get a basic fft noise removal algorithm running; 1 week
Time required to eliminate " musical noise" artifacts; about 2 years. 

Everyone who succeeds keeps their musical-noise-reduction recipe to themselves. 

Bob

On 7/17/13 12:09 PM, glen herrmannsfeldt wrote:
>
> Also, in audio (and likely others) there are two-sided
> (is that the right term) and single-ended (I believe that
> is right) noise reduction systems.
>

i dunno what the terms are, but i know what you mean.  the two-sided is 
like using pre-emphasis and de-emphasis.  or, in another sense, like 
companding; do something to the signal to make it stand up better to the 
anticipated added noise (or noise generated from other mechanisms, like 
quantization), then undo that something to get your original signal 
back, hopefully with less error than it would have if you didn't do this 
something and unsomething.

> The more popular dolby and dbx systems are two-sided,
> encoding the system before noise (vinyl disk or magnetic
> tape) recording, and decoding it on playback.

the vinyl wasn't dolby or dbx, i think it was called RIAA and it was 
static pre- and de-emphasis.  (dolby was dynamic pre- and de-emphasis.)

>
> Single-ended (on playback only) is more difficult, but
> sometimes must be done. Since audio, and especial spoken
> word, has little in the high frequencies,

except for them nasty fricatives.  but otherwize you're quite right.  at 
least when Fs is 44100 Hz or similar.  when Fs is 8K, then what is meant 
by "high frequencies" gets to be a little different.

but, even for music, the far greater portion of energy is in the bottom 
5 or 6 octaves of normal human hearing.  so the top 2 or 3 or 4 octaves 
have very little energy.  i have used this fact to advantage in 
designing the sinc-like impulse response for sample rate conversion alg. 
  you can inexpensively put notches right on top of all multiples of Fs 
(except for the 0th multiple of Fs) and kill most of the energy in the 
images.  the technique (notches at m*Fs except for m=0) is pretty 
obvious but i can't say it (just infer around it).

> but noise is
> often uniform in frequency space, a low pass filter
> is sometimes a fine noise reduction system.

yup.


-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

robert bristow-johnson <rbj@audioimagination.com> wrote:

(snip, I wrote)
>> Also, in audio (and likely others) there are two-sided
>> (is that the right term) and single-ended (I believe that
>> is right) noise reduction systems.
 
> i dunno what the terms are, but i know what you mean.  the two-sided is 
> like using pre-emphasis and de-emphasis.  or, in another sense, like 
> companding; do something to the signal to make it stand up better to the 
> anticipated added noise (or noise generated from other mechanisms, like 
> quantization), then undo that something to get your original signal 
> back, hopefully with less error than it would have if you didn't do this 
> something and unsomething.

There are references to single ended, but I don't see any reference
naming the other one. Sometimes they aren't so obvious. Note that
the opposite of single-ended SCSI is differential, as that is the
way it works electrically. 
 
>> The more popular dolby and dbx systems are two-sided,
>> encoding the system before noise (vinyl disk or magnetic
>> tape) recording, and decoding it on playback.
 
> the vinyl wasn't dolby or dbx, i think it was called RIAA 
> and it was static pre- and de-emphasis.  
> (dolby was dynamic pre- and de-emphasis.)

There were dbx coded vinyl records, though I never saw one.
I have a dbx tape deck that has the ability to decode them.
I beleive they were somewhat expensive, so didn't sell well enough
to get popular. Also, as with dbx tapes, they sounds bad played
without the decoder. 

The big advantage of dolby B is that it doesn't sound too bad
without the decoder, and might even sound a little better,
it you turn the treble down a little bit. I did used to have
a car tape player with a dolby B decoder, but they weren't
all that popular. A little compression helps in a car, to 
keep above road noise.
 
>> Single-ended (on playback only) is more difficult, but
>> sometimes must be done. Since audio, and especial spoken
>> word, has little in the high frequencies,
 
> except for them nasty fricatives.  but otherwize you're quite right.  at 
> least when Fs is 44100 Hz or similar.  when Fs is 8K, then what is meant 
> by "high frequencies" gets to be a little different.

Seems that AM radio is limited to 10.2kHz in the US. 
I sometimes listen to baseball games on a noisy AM radio, adjusting
the tone control until it sounds best.  I suppose I could work on
the radio, instead. It is part of a Nutone intercom system, and
I do have the service manual for it. (Installed by the previous
owner, who also supplied the service manual.)
 
> but, even for music, the far greater portion of energy is in the bottom 
> 5 or 6 octaves of normal human hearing.  so the top 2 or 3 or 4 octaves 
> have very little energy.  i have used this fact to advantage in 
> designing the sinc-like impulse response for sample rate conversion alg. 
>  you can inexpensively put notches right on top of all multiples of Fs 
> (except for the 0th multiple of Fs) and kill most of the energy in the 
> images.  the technique (notches at m*Fs except for m=0) is pretty 
> obvious but i can't say it (just infer around it).

Besides, other than babies most of us don't hear anywhere near 20kHz.

There are stories of kids with ring tones on cell phones above the
teachers hearing, but below the kids' hearing limit.

-- glen

On Wed, 17 Jul 2013 01:45:30 -0700 (PDT),
muhammadusman.khalil@gmail.com wrote:


>I studied until now that there's no generic way for removal of noise from c=
>ontaminated signal.
>

That is right.  As Tim Wescott said in another posting, successful
noise removal methods depend on specific differences between the noise
and the signal.  The more you know about the signal and/or the noise
in a particular instance the more you can separate them.  But as long
as they are both "generic", there is no way.

The FFT methods rely on the fact that your signal has one general
distribution and your noise has a different distribution.  If you
favor the frequency bands where your signal/noise ratio is the
highest, the overall signal/noise ratio will be higher.

But don't forget other methods of noise removal that are not based on
conversion to the frequency domain.  One example is when the noise is
highly-structured, such as ignition noise in an automobile electrical
system.  In this case the noise pulses are of very short duration and
have high energy.  If you look at this noise in the frequency domain,
it may spread throughout your desireable signal frequency range and be
difficult to separate from the signal.  But if you look at it in the
time domain, a simple clipper that limits time domain amplitude will
be very effective at reducing the noise.

Are you still looking for generic methods?  Or does your project allow
the detailed consideration of the characteristics of the noise and the
signal?

Robert Scott
Hopkins, MN

On Wed, 17 Jul 2013 13:05:57 -0700 (PDT), dbd
<dbd%ieee.org@gtempaccount.com> wrote:

>On Tuesday, July 16, 2013 7:09:30 PM UTC-7, muhammadus...@gmail.com wrote:
>> Hi mates,
>> I need to know somehow, the parts of overall structure setup for removing background noise and determining background noise sources.
>>
>> I am working on a problem to solve background noise emitted from test objects.
>> 
>> Here's written all on the document, I have 
>> https://www.dropbox.com/sh/38hbzpufqiajm7n/D_vTPEP9mb
>> ...
>
>The type of structure you show is common to many signal processing applications. A general example is the class of instruments called dynamic spectrum analyzers. Combining that term in a Google with: application notes points to a variety of resources supplied by many venders if instrumentation.
>
>
>### General Background ###
>
>http://www.measurement.net.au/g/5788/application-notes.html
>
>The Fundamentals of
>Signal Analysis
>Application Note 243
>http://literature.agilent.com/litweb/pdf/5952-8898E.pdf
>
>### Example Instruments ###
>
>http://www.me.ua.edu/me360/docs/HP-DSA.pdf
>Effective Machinery
>Measurements using
>Dynamic Signal Analyzers
>Application Note 243-1
>
>The SR785 Data Sheet
>SR785c.pdf
>available from site:
>http://www.thinksrs.com/products/SR785.htm
>User Manual
>available through web site:
>http://www.thinksrs.com/mult/SR785m.htm
>
>### Consider looking at the library at:
>http://www.bksv.com/library/
>for examples of applications, algorithms and instruments
>for example:
>http://www.bksv.com/doc/TechnicalReview1996-2a.pdf
>
>Browse and learn. Come back with better questions.
>
>Dale B. Dalrymple

Hi Dale,
   Wow!  Those are great PDF files.  Thanks.

[-Rick-]

Robert Scott <no-one@notreal.invalid> wrote:

(snip on noise removal)
> The FFT methods rely on the fact that your signal has one general
> distribution and your noise has a different distribution.  If you
> favor the frequency bands where your signal/noise ratio is the
> highest, the overall signal/noise ratio will be higher.
 
> But don't forget other methods of noise removal that are not based on
> conversion to the frequency domain.  One example is when the noise is
> highly-structured, such as ignition noise in an automobile electrical
> system.  In this case the noise pulses are of very short duration and
> have high energy.  If you look at this noise in the frequency domain,
> it may spread throughout your desireable signal frequency range and be
> difficult to separate from the signal.  But if you look at it in the
> time domain, a simple clipper that limits time domain amplitude will
> be very effective at reducing the noise.

I could never afford one, (maybe now the price is down) but there
used to be boxes that would remove the pop noise from vinyl disk
recordings. When the stylus goes over a piece of dust, or scratch,
you get a sharp peak that is easy to see in the time domain.

Somewhat similar to CD error concealment (when ECC fails), you
smooth over the pop. Might work for ignition noise, too, but I
haven't heard of anyone doing it.

-- glen