On Jan 11, 10:25 am, "overgaard" <jor...@antistaten.se> wrote:> >overgaard wrote: > > > ... > > >> I guess I need to fresh up on my maths. TDOA sounds interresting. > >> Well, precision is needed. I do not know what precision/resolution is > >> possible > >> but it would be nice to have an max error for about +- 1mm. > >> Maybe I will start of with the grid solution to test out the hardware > >> and when that is working try to get something with TDOA working for > >> higher precision/resolution. > > >That amounts to a time resolution of 2.91 microseconds at 20C. The speed > >of sound in air depends on temperature. It is 43.0 meters/second at 19C > > 343.6 at 20C, and 344.2 at 21C. > > >Jerry > >-- > >Engineering is the art of making what you want from things you can get. > >����������������������������������������������������������������������� > > Another thing, > > There seem to be at least two approaches to this: > > 1: The first microphone to sense the bullet hitting the target will act as > the trigger, triggering the timer. Then timestamp each remaining > microphones to get the delays. This approach assumes that only the > differences in delay is enough to triangulate. Is this correct? > I mean, the time from actuall bullet hit to the first microphone is > unknown since that microphone is the triggering one. > This way does not require any audiosampling. Just level triggering and a > timer. > > 2: Continously sample ALL microphones and do some really heavy maths, like > TDOA, GCC, fft's, etc. > > Anyway, I've started learning Octave to learn some of the sound analyzing > stuff. > > I do have a long way to go it seems. :) > > I whish there was a function that took the coordinates of the microphones, > the delays and returned the hit coordinates. ;)The way this usually works (at least in the implementations I've worked with) is to record more or less continually, or at least for enough time to catch all of the microphone 'hits.' Then take those recordings, and process them, removing i.e. any extraneous sounds, etc. Do GCC for the recordings from each microphone pair and find the peaks in the cross correlation to get the relative times of arrival (actually, vanilla cross correlation works reasonably well in the absence of statistical information about the sound, variants of GCC that just use the recording data get you narrower peaks (more accuracy), but lowers the effective signal to noise of the main peak to everything else). From those peak times, pick one microphone arbitrarily to be the time 0 reference, and calculate the relative times of arrival to all of the other microphones. Plug these into a TDOA localization algorithm of your choice, depending on whether the algorithm wants relative times of arrival for all the microphones or time differences for microphone pairs. In any case, the time differences are enough to triangulate, but to get the time differences you need to cross correlate, etc. Chris
Triangulation of bullet hitting paper target
Started by ●January 10, 2008
Reply by ●January 11, 20082008-01-11
Reply by ●January 14, 20082008-01-14
>overgaard wrote: > > ... > >> Another thing, >> >> There seem to be at least two approaches to this: >> >> 1: The first microphone to sense the bullet hitting the target will actas>> the trigger, triggering the timer. Then timestamp each remaining >> microphones to get the delays. This approach assumes that only the >> differences in delay is enough to triangulate. Is this correct? >> I mean, the time from actuall bullet hit to the first microphone is >> unknown since that microphone is the triggering one. >> This way does not require any audiosampling. Just level triggering anda>> timer. >> >> 2: Continously sample ALL microphones and do some really heavy maths,like>> TDOA, GCC, fft's, etc. >> >> Anyway, I've started learning Octave to learn some of the soundanalyzing>> stuff. >> >> I do have a long way to go it seems. :) >> >> I whish there was a function that took the coordinates of themicrophones,>> the delays and returned the hit coordinates. ;) > >The first thing you need to do is connect one microphone and figure out >how to get it to trigger something when the bullet impacts. I suspect a >bit of difficulty. When it seems to work, connect two microphones close >together, and see if their trigger signals occur together. I suspect >that a consistent time difference is the best you can hope for, and that>by the time you get it, you will know a great deal more about this than >either of us does now. > >Start your timer from the muzzle blast, That way, all your trigger >circuits can be "identical" > >Keeping the microphones close to the target eliminates false triggering >from reflections but puts them in harms way. If the microphones can be >far enough from the target to that their arcs of constant time subtend >small enough angles across the target, two suffice to delineate a >quasi-rectangular grid. There is an ambiguity with only two microphones >that a third resolves. By properly locating two microphones, the >ambiguity can be located outside the region of interest. (Celestial >navigation works the same way. If observations on two stars allow two >solutions, one within a few miles of where one thinks one is, and the >other halfway around the world, a third sight isn't really necessary.) > >Jerry >-- >Engineering is the art of making what you want from things you can get. >Hi, thanks for your suggestions. :) I will try to set something up at the target range next weekend. Probably I will try with a 192 kHz soundcard and then with a higher speed DAQ / ADC. Will post some testing results here and see where I can go from there.
