[GreenKeys] tone shifting.

[Jeffrey D Angus]

This was discussed as far back as the beginning of amateur RTTY.
One way to accomplish it, and a VERY simple technique was to NOT switch
between two constantly running audio frequencies, but to just add the
correct capacitance across the LC network to drop the frequency.
But a true implementation of tone shifting without sudden dis-continuities
came about with logic and a shift register.
Using a ladder network and a shift register to "stair-step" a sine wave, 
then
changing the clock frequency to shift the final output frequency.

-- 
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com