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https://github.com/davidje13/echolocator


https://github.com/davidje13/echolocator

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README 

        
# Echolocator



This project demonstrates using a computer speaker and microphone to

determine distances to objects using echolocation.



Note that the distance found is a crude measure; differences in

computer hardware and speaker placement make accurate distance

detection impossible without prior calibration. The true distance to

an object will be `true_distance = a * measured_distance + b`, where

`a` is an unknown constant (close to 1) controlled by the distance

between the microphone and the speaker, and `b` is another unknown

constant controlled by the time taken between generating a signal and

the speaker producing it, plus the delay between the microphone

receiving a singal and the program processing it. There will also be

small errors due to slight differences in the speed of sound due to,

for example, air pressure.



## Setup



The makefile will attempt to install dependencies in known

environments:



```shell

make environment

```



Alternatively, you can install the dependencies yourself.



### MacOS with Homebrew



```shell

brew install fftw portaudio

```



### MacOS with MacPorts



```shell

port install fftw-3 portaudio

```



### General



This project will build and run on unix-based systems, and possibly

Windows too (all dependencies are cross-platform).



The required libraries are:

* [the Fastest Fourier Transform in the West](http://www.fftw.org/)

* [Portaudio](http://www.portaudio.com/)

* OpenGL and GLUT



## Build and Run



```shell

make

build/main

```



The main application window will open and the speakers will begin

producing rapid clicks. After a few seconds of calibration, the display

will show vertical bars representing echos received. This typically

involves one strong bar (for the sound travelling directly from the

speaker to the microphone), then a cluster of fuzzy bars (various

reflections from the computer itself) then an emptier area. You can try

moving flat objects (card, a hand, etc.) near the computer to see them

appear in this area.



## Theory



Sadly I can't find the original website which inspired this

implementation, but I have found a page which summarises a lot of

interesting research on how bats use echolocation to determine

position, velocity, approximate shape, and even elevation.

https://www.hscott.net/the-dsp-behind-bat-echolocation/



The general approach used here is to send out a "chirp" sound (a short

tone with rapidly changing pitch), then use fourier transforms to

deconvolve the returned audio. By deconvolving using the original

chirp, it is possible to detect reflections with high accuracy while

dismissing background noise. This allows a map of distance to

reflection intensity to be produced (where time-since-chirp is used

as a measure of the distance), and this map is displayed to the user.



## Structure



The vague structure of this project is:

* `chirps.hpp`: contains signal generators (tone, chirp and repeating

  chirp)

* `fourier.hpp`: simple object-based wrapper around FFTW

* `audio.cpp`: the main logic for the echolocation process

* `renderer.cpp`: a simple wrapper around OpenGL / GLUT for

  displaying bitmap data

* `main.cpp`: main entrypoint for the program and orchastration