>The basic concept of sigma (intregation) delta
>(Change in) has been around since the 1950's but only in recent times has it
>been of use.

I believe the early digital telephone a/d converters were sigma delta types as
it was possible to do the u-law or A-law encoding directly in the a/d converter
by changing the delta size as the digital value changed. This saved doing a high
precision a/d conversion and then doing a conversion to the A-law or u-law
value. I think you will find these converters have actually been in commercial
use longer than you think.