[PATCH 1/2] Add Variance filter

[Andy Green]

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Somebody in the thread at some point said:
| This filter is useful to reject clicks that are not reliable. We
|  only care about what happens when we receive DOWN events for the fist
time.
|  If this filter does not reject the first samples then it will change
|  its internal state to "passed" and the remaining samples
|  will be sent to the next filter in the chain.
|
|  First we collect N samples, then then we sort them. We discard the
borders
|  of the vector (with a window) and then compute the variance of the
remaining
|  set. If the computed variance is bigger than a threshold, we reject
the click.
|
|  The parameters of the filter are:
|
|  * size
|  * threshold /* maximum allowed variance */
|  * window
|
|  I'm using values I tuned by trial an error. I noticed we discard a lot
|  of bugus clicks. In fact, it seems harder to get them now.

Thanks Nelson I sent these two on stable-tracking.

- -Andy
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