As I posted earlier, audio zoom is pretty intriguing.

However, I wasn’t sure how it could be practical. Come on, having fixed 300 microphones in a stadium? Is it practical?

Then, I think we can have a crude but practical “spy gadget” that can audio zoom by using three cell phones – P1, P2 and P3.

All the three can record all the sounds. Say signals S1, S2 and S3.

Now, say the target T of spying is at (approximate) distances R1, R2 and R3 respectively from P1, P2 and P3.

That means if we scale S1, S2 and S3 by distances R1, R2 and R3 and find the common part, that is like eliminating sound emerging from T.

Then take remaining signals and subtract them again from S1, S2 and S3, all remaining is scaled sound from T as heard by P1, P2 and P3. Reconstructing T from these should be easy.

The idea is quite like stereo recording – with noise cancellation. In some sense, it is triangulation inverse to GPS (3 received signals being triangulated to recreate the source).

I don’t know (rather, have forgotten) necessary DSP math for this. I am sure it must be much easier for those in close touch of the math.

I am not sure whether P1, P2 and P3 need to know their relative positions. If it is needed, RFID based sensing can be added. Robotics has enough research to detect geometry.

***

Where is this useful? Spying and journalism!

Imagine a VIP or a celebrity is traveling in a Rajdhani Express or an airplane. Three journalists (or paparazzi) may travel in adjacent compartments with the software loaded. They just need to add approximate relative distance to the target. Bingo!

***

What can be achieved next?

Adding fourth microphone will make it a 3D capture.

When the paparazzi or the target walks around, we should be able to sense the distance traveled and dynamically calculating the signal. This could be achieved by taking Doppler shift in to account.

I am not sure whether it is possible to link deduction of distance with camera focus. If it is, it is one of the most natural interfaces to Audio Zoom possible!

… and suddenly no talk will be “between you and me only”!

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