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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: acoustic imager
Date: Fri, 18 Apr 2025 17:47:46 -0000 (UTC)
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Cursitor Doom <cd@notformail.com> wrote:
> On Fri, 18 Apr 2025 10:34:25 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> On 2025-04-17 03:45, John R Walliker wrote:
>>> On 17/04/2025 03:12, john larkin wrote:
>>>> On Wed, 16 Apr 2025 22:01:28 -0400, Phil Hobbs
>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>> 
>>>>> On 2025-04-16 10:41, john larkin wrote:
>>>>>> On Wed, 16 Apr 2025 09:01:00 +0100, Cursitor Doom <cd@notformail.com>
>>>>>> wrote:
>>>>>> 
>>>>>>> On Tue, 15 Apr 2025 15:04:15 -0700, john larkin <jl@glen--canyon.com>
>>>>>>> wrote:
>>>>>>> 
>>>>>>>> 
>>>>>>>> https://www.google.com/aclk?sa=L&ai=DChsSEwjTjaDVg9uMAxW3Hq0GHVmKOlYYACICCAEQARoCcHY&co=1&cce=2&sig=AOD64_3aGs74magNuXwdRGFo7oP8zK-LMQ&ctype=5&q=&adurl=
>>>>>>>> 
>>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> For 42,000 dollars? There's a product there you could develop, John.
>>>>>> 
>>>>>> Seems like it needs maybe a dozen electret mikes, one mux'd ADC, an
>>>>>> FPGA, and some code.
>>>>> 
>>>>> In the last few decades, there's been a lot of work done on imaging with
>>>>> sparse arrays.
>>>>> 
>>>>> A full NxN rectangular antenna array has an enormous amount of
>>>>> duplicated information from an imaging point of view. To make a good
>>>>> image, you need spatial frequency information corresponding to all
>>>>> values of dx and dy, with some regular spacing, i.e. in an NxN array,
>>>>> 
>>>>> dx and dy go from -N/2 to +N/2-1 (or equivalently, from 0 to N-1) in
>>>>> integer steps.
>>>>> 
>>>>> In principle you only need one estimate per spacing, but in a dense
>>>>> array, every pair of adjacent pixels gives an estimate of the dx = +-1
>>>>> components, i.e. essentially the same information as every other
>>>>> adjacent pair.� The redundancy is less at wider spacing, of course.
>>>>> 
>>>>> If one is willing to trade off SNR and computational expense, you can
>>>>> get the resolution of a full array with far less than N**2 antennas--I
>>>>> forget what the the number is, but it's a lot more like N log N than
>>>>> N**2.� A pal of mine in grad school, Yoram Bresler, did his thesis on
>>>>> that problem, which is where I first heard of it.
>>>>> 
>>>>> So a sparse array of microphones can in principle do quite a bit better
>>>>> than one might suppose.
>>>> 
>>>> And it looks like the Fluke acoustic imaging is primitive, like those
>>>> hybrid visual+thermal gadgets.
>>>> 
>>>>> 
>>>>> Cheers
>>>>> 
>>>>> Phil Hobbs
>>>> 
>>>> I'd expect that a bunch of wideband antennas and ADCs listening to the
>>>> world would have the same effect, see everything. Radar without the
>>>> transmitter. No doubt that is being done.
>>> 
>>> It is.� Look up "passive bistatic radar"
>>> For example:
>>> https://sspd.eng.ed.ac.uk/sites/sspd.eng.ed.ac.uk/files/attachments/basicpage/20171219/Session%201.0.pdf
>>> 
>>> 
>>> John
>>> 
>> 
>> For a long time, too.
>> IIRC the first successful radar experiment used the reflection from a 
>> BBC transmitter.
>> 
>> Cheers
>> 
>> Phil Hobbs
> 
> Radar and code-breaking really saved Britain's bacon in WW2. Plus a
> bit of assistance from the old colonies. :->
> 

Yeah, Auntie was useful for something back then. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics