Re: [Algorithms] Truncate then average? Or average then truncate?

[Michael B. <mi...@fa...>]

You probably need to filter the ADC results. I suggest a gaussian filter 
approximated using a moving average. Search for "Gaussian and Other Low Lag 
Filters" and you'll find tables of coefficients for different order filters. 
You won't need a ring buffer for a 1st order filter, but you will have to 
use the interrupt timer to get evenly spaced samples.

-michael


----- Original Message ----- 
From: "Robin Green" <rob...@gm...>
To: "Game Development Algorithms" <gda...@li...>
Sent: Friday, May 28, 2010 12:24 AM
Subject: [Algorithms] Truncate then average? Or average then truncate?


> Got a bit of a brain block, and I'd welcome someone with a clearer
> picture of what's the correct thing to do.
>
> I have a hardware controller that I'm writing firmware for,
> specifically debouncing button presses. That's gone fairly well by
> using a word for each sample of the keystate. An interrupt timer fires
> and I sample the button states, one bit per key, dumping the current
> state into a ring buffer of N words. At key scanning time I AND all
> the words in the buffer together and if any of the bits in each
> "column" are set I register it as a keypress. So I get instant
> reaction to a keydown and an N-iteration delay before the columns
> clear so I can register a keyup. This deals with noisy bounces on key
> depressions and releases just fine.
>
> I am now attempting to "debounce" an analog input. The issue is that
> the ADC reads the input and returns a 10-bit value, the bottom two
> bits of which are subject to noise, but that's fine as I only need a
> value in the range 0..127. So my current effort is to read the ADC
> value several times, sum them, divide by the number of samples and
> then lose the bottom 3 bits. This works great for most values in the
> range, except for the smallest values - 0, 1, 2, ... As you move the
> controller through the range down to the smallest values, they exhibit
> the noise and become like an unordered list. Other values where a lot
> of bits flip, like the boundary between 63 and 64, also allow the
> noise to propagate up to affect the value. Technically, there should
> be no reason for even the smallest numbers to show the noise - only
> the bottom two bits exhibit noise and we're removing three bits of the
> final value.
>
> What order should I be doing this and why? Why am I amplifying the
> noise to the point where it's a problem?
>
> - Robin Green.
>
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