slope in

Slope of signal.  Measures the rate of change per second of a
signal.  Formula implemented is:

out[i] = (in[i] - in[i-1]) * sampling_rate

in - input signal to measure.

In the example below a is quadratic noise, b first derivative line
segments, and c second derivative constant segments.

> a <- lfNoise2 KR 2
> let s = 1/2
>     b = slope a * s
>     c = slope b * squared s
> audition (out 0 (mix (sinOsc AR (MCE [a, b, c] * 220 + 220) 0 * (1/3))))