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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: acoustic imager
Date: Fri, 18 Apr 2025 19:10:07 -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. :->
> 

Yes but until the cavity magnetron came to fruition British radar was
technically grossly inferior to German radar. Think of the wurzburg system
with uhf, parabolic dishes, high prf rates, coax cable etc. 

In the early stages the main factor was operational in that the brits
thoroughly integrated radar into air defense whilst the Germans only used
it on an individual basis. The chain home radar was so totally
different/primitive/worse than the Nazis radars that initially they mistook
it for something else.

-- 
piglet