bungalowbill

audio-tracking

Quote: bungalowbill wrote on Thu, 10 January 2008 06:47

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Quote: bungalowbill wrote on Thu, 10 January 2008 06:47
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> hi, 
> i


i thought about using contact-microphones (attached directly to the floor) and use the step "noises" (excuse my english).

Contact mics seem like a good idea. You'll still have to work out onset detection, but it might work.

Let's assume for a moment, you've got perfect onset detection, the next thing you need is the time difference between the same event in different mics. Let's imagine just two mics for now. Let's put them 10 meters apart. The time difference will range from zero ms (when something is exactly between the mics) to about 29.4 ms (with sound moving at 340 m/s) when something is right on one mic. By the way, that's the speed of sound in air; I don't know the speed of sound through floor. You could calibrate your system experimentally, though.

Whatever the actual values, the time difference tells you how much closer the sound is to one mic. If the time difference is 50% of the maximum time difference, than the object is 50% of the way from the middle point to the mic where the sound arrived first. (In the example, 2.5 meters from one mic and 7.5 meters from the other mic, assuming the person stays on a line between them.) By adding a third mic, you could position a sound inside a triangle by comparing pairs of mics as above.

Am I making any sense? Accuracy will be important in this project. Below is a quick patch to compare the time difference between two clicks.

Let's assume for a moment, you've got perfect onset detection, the next thing you need is the time difference between the same event in different mics.  Let's imagine just two mics for now.  Let's put them 10 meters apart.  The time difference will range from zero ms (when something is exactly between the mics) to about 29.4 ms (with sound moving at 340 m/s) when something is right on one mic.  By the way, that's the speed of sound in air; I don't know the speed of sound through floor. You could calibrate your system experimentally, though.  

Whatever the actual values, the time difference tells you how much closer the sound is to one mic. If the time difference is 50% of the maximum time difference, than the object is 50% of the way from the middle point to the mic where the sound arrived first. (In the example, 2.5 meters from one mic and 7.5 meters from the other mic, assuming the person stays on a line between them.)  By adding a third mic, you could position a sound inside a triangle by comparing pairs of mics as above.

Am I making any sense?  Accuracy will be important in this project.  Below is a quick patch to compare the time difference between two clicks.

> i thought about using contact-microphones (attached directly to the

> floor) and use the step "noises" (excuse my english).

The sound travels probably faster through the floor than through the 

air, you need to measure the times separately.

It will be tricky, but worth to try, I'd take more than three mics, the 

difference in the arrival time will be a few milliseconds. You might 

need to take a higher sampling rate to get it more precise...

And what if there are several people walking???

bungalowbill schrieb:
> i thought about using contact-microphones (attached directly to the
> floor) and use the step "noises" (excuse my english).

The sound travels probably faster through the floor than through the 
air, you need to measure the times separately.
It will be tricky, but worth to try, I'd take more than three mics, the 
difference in the arrival time will be a few milliseconds. You might 
need to take a higher sampling rate to get it more precise...

-- 
Stefan Tiedje------------x-------
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----------()--------www.ccmix.com

mzed: thanks for posting the patch, i copy & pasted it into a text editor & tried to open the file with max, but that doesn

I'm working on the same type of position tracking.

However I'm trying to use the microphone output amplitudes to do the math.

I was wondering if you got your patch stable and working at all?

I'm working on the same type of position tracking.
However I'm trying to use the microphone output amplitudes to do the math.

the basic concept seems a simple thing to me, you just measure the 

time difference between the "attacks", the resrt is a bit of math.

something like tj´s analyzer~ (or some custom code like that) 

on the data layer you can now easily check for the delay between

convert those milliseconds to centimeters

use simple geometric calculations to find position of sound

source in room.(of curse you need to know where the mics are)

the basic concept seems a simple thing to me, you just measure the 
time difference between the "attacks", the resrt is a bit of math.

1. 
something like tj´s analyzer~ (or some custom code like that) 
could be used to find attacks. 

2.
the attacks will be converted to bangs.

3.
on the data layer you can now easily check for the delay between
the mics.

4.
convert those milliseconds to centimeters

5.
use simple geometric calculations to find position of sound
source in room.(of curse you need to know where the mics are)

you can easily make good onset detection doing the following (sorry don't have max here):

- mic put your mic signal in an average~ 44100

- send commands "441" and "rms" to the average~

- subtract atodb output from bucket output

now you have the rise and fall time of the sound, it indeed travels much faster through a floor than through air (+- 1000m/s vs 343 m/s) but is also much louder through the floor to te contac mic, so I think it should work.

you could use > some threshold in dB/s with a select 1 to generate trigger events

if you get retriggers, just change the "441" to some higher number and/or make the threshold higher

- mic put your mic signal in an average~ 44100
- send commands "441" and "rms" to the average~
- connect out to snapshot~ 1
- connect to atodb
- connect to a bucket 
- subtract atodb output from bucket output

audio tracking