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D-2

This post sums up all the work we have done in the last three weeks. It is an indirect reply to the other post http://rose.eu.org/2013/?p=1224.

We have tested each of the parameters on the transmission line between the tweeters and our receiver.

Here is a review of the tests we conducted :

Radio transmission :

System : the master sends a synchro messages to the tweeters and orders them to beep at the same time.
Signal observed : signal entering the tweeter
Result : a maximum of 200µs shift is observed
Conclusion : this is a negligeable parameter on the transmission line

Bip detection (here comes the hard part)

This is the main source of uncertainties.

Filtering

The filtered we have design thanks to Alexis Polti is resistant to type I and type II errors (false positive and false negative errors).
First we apply a pass band filter : to remove the noise from other frequencies.

  • band pass [20800Hz; 21200Hz]
  • stop band (40dB attenuation at least out of [20000Hz, 22000Hz]

We square each samples to get the power of our filtered signal and then we apply a low pass filter in order to get the enclosure of the power.

  • band pass [0Hz, 1600Hz]
  • cut-off 1800Hz

Triggering

The trigger is designed as follow :
If a point is above a given threshold it is a possible bip.
We consider it as a valid bip if for the bip_length 90% of the enclosure is above threshold level.
If the point is valid a debouncer to avoid echoes is then activated.

In the following example you can see the signal (light red), the power at 21kHz (dark red) and the enclosure (black).
Regular beeps are send every 1s. Beep lenght is 10ms.
Abscissa is time in seconds, ordinate is the amplitude of the signal

sound

This is a zoom on the second beep of the same recording.

sound1

Conclusion : we observe that the signal we get is not “clean” and as many echoes.
We also observe that the tweeter takes a some time to enter resonance which results in a possible 3ms shift.

This is a major concern. One possible improvement is calculate several position and apply a low pass-filter to get the position is several measures are alike.

Further development

Fo now we record a sample before analysing it.
We shall implement a “continuous” mode that enables doing all the computing part while recording the audio file.

Jean-Baptiste Lescher

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