Sat Dec 25 03:43:12 EST 2010

1/2 range Sigma Delta

Maybe it's best to simply always do this: if switching between 0 and
1, never use the range outside of [1/4, 3/4].  This avoids large
period cycles as it places the max at 4 time units.

To combine this with a low-level PWM we could use the low bit of the
PWM to "center" the SD stage such that it operates in 1/2 range.

I.e. for an output around 3 it could switch between 2 and 4, while for
an output around 4 it could switch between 3 and 5.