See the attached PDF file for the schematic.

You can effectively cancel much (but not all) of the unwanted
background noise by using two identical microphones and a
simple instrumentation (differential) amplifier.

Each of the microphones needs to be identical so they have the
same frequency response, etc.

The two microphones are separated by a distance that can be
anywhere from five or six inches to several feet. You only speak
into one microphone. This microphone picks up the background
noise at 100% level, and your voice at 100% level.

The microphone that is further away, and into which you do NOT
talk directly will pick up 100% of the background noise and
only a small percentage of your voice. Let's say 10%, just for
the sake of discussion.

The outputs of the two microphones are applied to the inputs
of an instrumentation amplifier, which will take the difference.
(100% noise + 100% voice) - (100% noise + 10% voice) = 90% voice.

There will be a small amount of "whoosh" type noise that is
caused by phase-shifted sound, but it is usually barely
audible.

The further away the source of noise is, the greater the
separation you can have between microphones.






-----Original Message-----
From: Duilio Foschi <foschid@SHUTTLE.ETRURIA.NET>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Thursday, March 09, 2000 9:49 AM
Subject: recording w/o noise


>I am preparing a tutorial for my new application using Lotus' ScreenCam.
>
>I have done all the video recording and now have to add the voice track.
>
>This is easily done (at least in theory) using the mic attached to the
>sound card of my PC.
>
>The problem is:
>
>all sort of noises enter my room from down the street and in all the tests
>I made the noise is well audible in the recorded track.
>
>Still I have all my things here and would hate to migrate to a different
>place.
>
>Also I guess that if I don't rent a professional recording room - what
>would be expensive and troublesome - I will find noise - more or less -
>everywhere.
>
>Is there a hw solution (a special mic ?) to filter out the noise when
>recording and/or a sw solution (Goldwave ?) to filter out the noise from
>the WAV file after the recording ?
>
>Thank you
>
>Duilio Foschi
>

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