[music-dsp] Noise Cancellation

Charles Henry czhenry at gmail.com
Wed Jun 2 17:38:18 EDT 2010


On Wed, Jun 2, 2010 at 6:03 AM, Victor Lazzarini
<Victor.Lazzarini at nuim.ie> wrote:
> Also, I'm not sure how the filter is supposed to work, since the noise you
> are trying to block is bypassing any of the DSP
> (ie. is made up of  the environment sounds).  I can see how some sort of
> phase cancellation works, so I guess what you
> are trying to say is that the system would generate some signal based on an
> adaptive process that would provide the
> desired effect. Maybe you mean that the filters will be used in an analysis
> stage?

The filters that I mention would be applied during synthesis, rather
than analysis.  If the characteristics don't change, then you could
analyze the system and calibrate it once.

also you gave me a different idea.  A loudspeaker is also a microphone
after all.  Perhaps there could be some kind of simple closed loop
control system that applied voltage to the loudspeaker to counteract
its own movement?

If you applied a small dc voltage to the loudspeaker, it would draw a
constant current and movement of the speaker would show up as
modulation of the current.  Then, apply the closed loop control on the
applied voltage that tries to reduce this current to 0, via a PID
plant or something similar.

>
> This is an area I've always been curious about (but probably too lazy to
> explore). Nice to hear someone that knows about it
> explain the idea.
>
> Victor
>
> On 2 Jun 2010, at 05:24, robert bristow-johnson wrote:
>
>>
>> On Jun 1, 2010, at 9:04 PM, Charles Henry wrote:
>>
>>>>> To be able to cancel noises, you need to use digital filtering.
>>
>> ...
>>>
>>> It's a very technical and difficult problem.  Let me know if you have
>>> any further questions or want to discuss some of the math involved.
>>
>>
>> okay Chuck, i sorta agree with what you're saying, but there are lots of
>> digital filters.  would you be more specific about what such digital filter
>> is doing, mathematically, to cancel noise?  let's say it's one of those
>> noise-cancelling headphones.
>>
>> --
>>
>> r b-j                  rbj at audioimagination.com
>>
>> "Imagination is more important than knowledge."
>>
>>
>>
>>
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>
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