Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.design Subject: Re: acoustic imager Date: Fri, 18 Apr 2025 22:18:33 +0100 Organization: A noiseless patient Spider Lines: 109 Message-ID: References: <7rltvjhmbdi7213aqa1d6qrv9sepn567k6@4ax.com> <05c71fa8-af08-c88c-2e02-30c99364249d@electrooptical.net> <9lo00kl6bgchmo0h10bn83p2gcvl48thb7@4ax.com> <9bca663e-278d-1716-7ad4-0d4fb8ffd409@electrooptical.net> <9f150kll8rosqtuojmh80at1ed8c5t1dgh@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Injection-Date: Fri, 18 Apr 2025 23:18:35 +0200 (CEST) Injection-Info: dont-email.me; posting-host="4a2ae449cf51163deba60360be568a0c"; logging-data="4064089"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19i4fcyjFZVg3Fy4/ySci7KoXhDuRHOq4c=" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:9aCHTTjIHsNdIaB61jmbMRB8Skg= Bytes: 5552 On Fri, 18 Apr 2025 19:10:07 -0000 (UTC), piglet wrote: >Cursitor Doom wrote: >> On Fri, 18 Apr 2025 10:34:25 -0400, Phil Hobbs >> 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 >>>>> wrote: >>>>> >>>>>> On 2025-04-16 10:41, john larkin wrote: >>>>>>> On Wed, 16 Apr 2025 09:01:00 +0100, Cursitor Doom >>>>>>> wrote: >>>>>>> >>>>>>>> On Tue, 15 Apr 2025 15:04:15 -0700, john larkin >>>>>>>> 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. Scene: Churchill's War Room under Whitehall:- "Bandits incoming! Bandits incoming! Dispatch 810 Squadron to intercept immediately!" "Tally-ho chaps! Let's get those Spitfires up! Where do the radar chaps say gerry's position is? North West Europe?? Ginger, Corky - get the bloody binoculars out again!"