Monday, September 15, 2008

Wii remote accelerometer bandwidth and sampling

[english version]

Wii remote accelerometer bandwidth and sampling

In the previous post, the Wii remote accelerometer bandwidth was considered as [0Hz 25Hz] because of external capacitors limitations. Indeed, this consideration comes from values of external capacitors. For Cx,Cy and Cz external capacitors, the calculation is given by :
Bandwidth = 1/(2*PI*32kOhms*Capacitor)

In order to check bandwidth limitations, the file fwiine_recording_experiment.m was modified with a new re-sampling time :
t_sample=0.012; instead of t_sample=tps_sample(1,3); %0.02s
The measurement were recorded with a new .wav file as stimulus.
Theorically, the power spectrum of the .wav file is described by the following spectrogram :
[Zoom of area 0-100Hz on theoretical spectrogram of stimulus (.wav file) without considerations about subwoofer and experiment]

The capacitors are going to limit the recorded spectrum. Indeed, the following graph shows the change of spectrum during the recorded measurement :
[AZ(f) Spectrogram of recorded measurement]
[ Left, initial spectrogram / Right, after image processing and area identification(manually)]

The observed spectrum takes in consideration wiimote settings on top the subwoofer and frequency response of subwoofer box itself. Considering frequency responses of subwoofer box and wiimote device can be out of 50Hz (at least) without attenuation, the initial stimulus spectrum is recognized and the noise covers its frequencies up to 33Hz/34Hz.
Then, with this cut-off frequency around of 33Hz, the expected capacitor value is Cz=150nF.
This value is around of an already measured value of 0.2µF. In addition, it also means a min. acquisition step of 0.015s (66 frames per second / 2x33 according to Shannon's theorem) is acceptable and relevant for the "Wiimote Matlab" tool chain.


wiilliam said...

Interested in recording and analysing with the same file ?

Have a look at the .m file :

[Included feature : spectrogram]

Example of recorded spectrogram after manual experiment with the wii controller.

- Wiilliam

wiilliam said...

(Update 09/15/09)
New links :