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Old 7th December 2013, 04:26 PM   #1
Doogie is offline Doogie  Canada
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Default Difference attenuator? Sameness Amplifier?

Hello,

I have built a circuit that amplifies the difference between sounds in two adjoining rooms. Pretty straight forwards circuit using a INA134 Difference Amplifier.

The second part of the project should amplify shared sounds in the two different rooms. Can you attenuate/remove the difference in sounds from two microphones? I guess what I need is a Sameness Amplifier. This has me stumped. Can't find any schematics on the web or more likely I don't know what to ask for.

Any pointers in the right direction would be most appreciated.

Thanks Doogie
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Old 7th December 2013, 04:42 PM   #2
nezbleu is offline nezbleu  Canada
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You have already identified the difference, now just remove that from the summed sound, leaving the common sound.
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Old 8th December 2013, 07:13 PM   #3
Doogie is offline Doogie  Canada
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Thanks Bluenose,

I'm not so great with op amp theory. Do you mean Sum the two sounds together. Put that into a second Diff amp input and then the difference sound (from the first Diff amp) into the other input on the second Diff amp?

Yurs Doogie
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Old 8th December 2013, 08:20 PM   #4
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It seems straight forward to think about it but it is like trying to isolate the common sounds between two stereo channels with summing and differencing - it doesn't work.

The algebra, say the sounds are from room A and B:

A+B=All the sounds from both rooms summed together
A-B=The difference between the sounds of both rooms

So, we are led to believe that:

The sum (all the sounds of both rooms) minus the difference (the sounds unique to the rooms) equals all the sounds common to both rooms. Sounds plausible...

But the reality is:

(A+B)-(A-B)
=A+B-A+B
=A-A+B+B
=2B

You end up the the sounds in room B at twice the amplitude. If you simply reverse your initial differencing you just end up with room A sounds at twice the amplitude. I went down this road trying to make a pseudo surround system where I wanted to have 'common' information in the centre and was initially puzzled when it didn't work as expected. I found the above to end up being the truth. You simply can't unscramble the egg.
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Old 8th December 2013, 08:28 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

You have the sum and the difference of two channels, that is it.

Adding the difference to the sum will restore one of the channels,
substracting the difference will restore the other channel, its how
FM stereo/mono works.

http://www.ti.com/lit/ds/sbos071/sbos071.pdf

Shows a summing amplifier.

rgds, sreten.

crossed posts : (A+B) + (A-B) = 2A, (A+B) - (A-B) = 2B
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Last edited by sreten; 8th December 2013 at 08:32 PM.
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Old 8th December 2013, 09:50 PM   #6
DF96 is offline DF96  England
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Room1 has A+B, Room2 has A+C. Easy to get rid of A - the difference is (A+B)-(A+C)=B-C. Harder to go the other way. You would have to find correlations. Not impossible, given certain assumptions.
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Old 8th December 2013, 11:26 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

Trying to complicate something simple doesn't work.
So you have a sum of 2A+B+C and a difference of B-C.
Add the difference = 2A+2B, subtract = 2A+2C.

Its impossible to extract A, B or C as signals, unless
you define say A as the input to the loudspeakers.

rgds, sreten.
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