# Patch-a-Week

## Week 01

The upcoming Chimaera firmware release 0.6.0 will ship with support for velocity signals. This patch-a-week covers the digital filtering technique involved.

### Velocity signal filtering

Although the raw interpolated positional signals are very smooth, the derived velocity signals can be quite noisy. For the velocity signal to be of use as a direct mapping to musical parameters, the velocity signal needs to be smoothed, aka filtered.

Raw x-dimension and corresponding x-velocity of arbitrary sensor array sweeps.

We use a variable cutoff/phase, first-order Infinite Impulse Response (IIR) filter to smooth the velocity signal over time. The equation is rather simple and the filter successfully blocks high-frequency signals. The filters performance can be configured by the variable stiffness $$s$$ which represents the filter sample window (e.g. 1, 2, 4, 8, 16, 32 or 64 samples). The current filter value $$f_n$$ thus depends on previous filter value $$f_{n-1}$$, current and last raw values $$v_n$$ and $$v_{n-1}$$, respectively.

$$\begin{eqnarray} f_n &=& \frac{s}{2} \cdot (v_{n-1} + v_n) + (1-s) \cdot f_{n-1} \end{eqnarray}$$

Raw (red) and filtered (black) x-velocity with stiffness=1 and stiffness=4.

Raw (red) and filtered (black) x-velocity with stiffness=16 and stiffness=64.

### WANTED:Beta Testers

After a high interest at Maker Faire Rome, we are now running a beta-testing campaign to collect more comprehensive feedback of first-hand experiences of our final Chimaera prototype design from interested individuals. Get in contact with us. Now.

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Last update - 16 May 2017

Copyright © 2014-2017, Hanspeter Portner, Open Music Kontrollers, cc-by-sa 4.0. Uses libre javascript.