🚀 go-pugleaf

m22257@mitre.org (Jeff Correia) writes:

>In article <Bz3vHr.LKK@news.cso.uiuc.edu>, ja51359@uxa.cso.uiuc.edu
>(axelrod) writes:
>> 
>> 
>> Hi.  I know this is possible because I've heard the military does it for
>> helicopter headsets; and I heard that the car companies will be doing it
>> too.  Does someone know of how the basics work behind active noise can-
>> cellation?
>> 
>> What I'm interested in is a system where you have a speaker or two
>> somehow produce a signal that is directly out of phase with the ambient
>> noise.  This seems impossible, since a speaker radiates in all directions,
>> thereby having a different phase at different points in the room.  Is 
>> there some way of getting an average phase or something, in order to
>> decrease the noise in the room/car?
>> 
>> I'd like references to articles on this subject, along with a simple
>> explanation if possible.
>> 
>> Thanks
>> 
>> 

>     The type of cancellation you are referring to is one that I have heard
>most commonly called reference cancelling.  The idea is pretty much as you
>have described it.  You have one primary channel, and 1 or more reference
>channels.  The primary channel contains your desired signal corrupted by
>unwanted noise, the reference channels contain noise which is (hopefully)
>highly correlated with the unwanted noise in your primary channel, but
>contain no trace of your desired signal.  

>     A classic paper on this topic is by Bernie Widrow.  While trying to
>examine the heartbeat of a fetus, the heartbeat of the mother is a strong
>interfering signal.  Using a primary channel of mother+fetus and being able
>to locate a position to get a "clean" but maybe slightly out of phase
>refernce of the mother's heartbeat only, they were able to obtain the
>desired results.  I'm sorry I don't have the exact reference.  




There is a book "Adaptive Signal Processing," Widrow and Stearns, which deals
with this subject.  There are also many articles in the Signal Processing
and Biomedicl Engineering literature.  As Jeff has already noted the main
requirement for using the algorithms in the book is a set of independantly
measured, correlated noise signal, which may be cancelled from the reference
signal. The algorithms  also assume that the signal are linearly combined.  
If Nonlinear signal processing is needed then Neural Network models may be
needed.  Currently, here at JHU/APL, we are using Widrow's LMS algrothms 
for deriving fetal EKG signals from non-invasive measurements from the
mother.  If there is any interest, please post here or e-mail me.


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_______________________________________________________________

    	John A. Cristion