# Audio Jack Sampling Rate?

Charles Pax charles.pax at gmail.com
Sun Sep 21 01:18:27 CEST 2008

```What is the highest sampling rate I can get from the audio jack? What is the
frequency range?

I'm considering the plausibility of a little hardware project that would
turn a Freerunner or any computer with an adequate sound card into a mimio
[1] device.

The principle is an expansion on the way you calculate the distance (d) of a
lightning strike from you.You see the light instantly, but hear the sound
with a delay (delta t) because sound travels at a much slower speed (v_s) of
approximately 340 m/s. You can use the following equation to find the
distance from you to the lightning strike

d = (delta t)(v_sound)

With this information you can draw a circle of radius d with you at the
center. Observing the same lightning from two known points would allow us to
draw two circles whose intersection would indicate where the the lightning
hit the ground. This gives two points, but knowing the general direction of
the strike would give us a single point. This is basically what I want to do
with the stereo input of the Freerunner. However, instead of observing
lightning and audible sound, I want to observe infrared light and sound at a
higher frequency than humans can hear. The mimio device works in this way. A
dry erase marker is placed into a case what emits IR light and sound when
the marker is pressed down.

I basically want to connect both an appropriate microphone and an IR
detector on each of the two input channels of the Freerunner. There would be
one mic-IR pair at each of points A and B. Points A and B would be seperated
by some distance h along the vertical axis. Below is a simple hardware
schematic diagram.

/--micA
|    |
|   GND
|
I--IR_detA
|     |
|    GND
\------------------(to left channel)

/------------------(to right channel)
|--micB
|    |
|   GND
|
\--IR_detB
|
GND

Software would then monitor the two channels and compare the input. The
software would know that a set of square waves from both the right and left
channel occuring at the same time indicate a flash of IR light. It would
then begin measuring on each channel the time interval to the middle of the
next set of waves.

Sample input
(left channel)
__|-|_|-|_|-|_|-|_|-|___________|-|_|-|_|-|______________

|<--from IR_detA->|           |<--micA->|
|<---delta tA--->|
|<--from IR_detB->|                       |<--micB->|
|<--------delta tB---------->|
(right channel)
__|-|_|-|_|-|_|-|_|-|_______________________|-|_|-|_|-|__

The following two equations would give the position of the dry erase marker
relative to point A. (I may have mixed up my positive and negative
directions somewhere, but you get the picture.)

d_Ax = (v_sound)sqrt(t_A^2 - t_B^2)
d_Ay = h - (v_s^2)(t_B^2 - t_A^2)/(2h)

My first step in testing my idea is to connect ir detectors and mics to a
headphone cable, plug it into my computer, and see if Audacity displays
something similar to the sample input above. Then I'll hack around with that
until I can make a reliable piece of hardware. I'm more of a hardware guy
than a programmer, so writing this software would be very challanging to me.
Is there anyone out there willing to hack some code if I were to supply you
with the appropriate hardware?

Any input from the community?

-Charles Edward Pax

[1] http://www.mimio.com/products/interactive/index.php
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