Strong pass audio filter, with adjustable setting of minimum pass level.

Bad input  gives bad output ,  but there are exeptions when the output result depends on human hearing.
Our ears are not sensitive at all to volume change.  3dB is the absolute minimum audio volume difference we can hear.
+3dB  equals to Signal voltage x 2  , using a linear scale.

By using our strong pass audio filter to eliminate background noise, all signals smaller than the minimun pass level are reduced by a  factor, typical  20dB .

This will also result in some  small audio volume errors,  but
just that is not a problem. 
Our ears are not designed to hear it.

Idea & solution by Joar Falstad.

Filter function:  The weakest parts of the total signal, smaller than the preset minimum pass level,  are redused by a factor. 
Up to 100x or -20db reduction is used in our filter .
   You can clearly hear that noise reduction.
The result is natural sound  and the background noise is reduced to a minimum.

No audio signal delay.

Compared to the original  signal , tone balance, max volume  and pitch are not changed in any way,

only the disturbing background noise has almost vanished.

Prototype hardware. 

Internal & demo use only.

Later prototype version

Line  in / out  signal  in consumer equipment has a standard max value :  0.316  Vrms 
Tape / Aux signals often equals to line signals.
The strong pass udio filter  audio signals max value : 4Vrms

All versions have  galvanic insulated transformer output.

A similar stereo ( possible commersial ) version  is in the making.  

Prototype video's :

ON/OFF function switch
and  adjustments to show the function  " set minimum pass level "  .

  Video  1 , Early version with experimental circuitry

  Video 2      whispering  . noise removal

  Video 3      normal voice .   noise  reduction

  Video 4      realistic audio background noise reduction 

Measured  audio volume reduction outside pass level : up to  100x or  - 20db .
Adjustable volume reduction to come.

Suitable in / future suggestions :
chipcorders ,  hiss and background noise  reduction / removal / cleaner /  limiter .
Hearing aid systems , background / ambient noise reduction .
PA  message systems,
strong background  noise reduction .

If the noise is bad the audio transformation  is not  perfect, 
but what is important , your ears will hear it as an improvement .

More video's to come , to show  how to set the minimum pass level controller
Different audio noise levels.

Live   audio background noise reduction or removal , hiss cleaner , minimum signal filter .

One channel prototype item.
 Line in - out  balanced  , 600 ohm impedance.
Aux in   unbalanced,  high impedance.

Easy  adjustment :  
A LED.indicator for visual control of the strong pass filter.
And for best noise cleaning  result, an  adjustable controller to set minimum pass level .

( registered on 20-03-2001 )

Contact :      Joar Falstad

phone:          +47  99718408



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