FM of Tri-Square VCO

[Christopher List]

Howdy DIYers - 

It was mentioned a while back that the Tri-Square VCO (of 
Electronotes/Elctronic Music Circuits) couldn't do "through zero FM". I have a 
vague notion of what this limitation implies - just from looking at the FM'ed 
waveforms on a scope, but I'm 100% sure. 

1. Could someone describe this limitation (or rather, this capablility)?
2. Does anyone know if it would be possible to modify the circuit so that it 
could do through-zero FM? 
3. How? 
- Perhaps Bernie has a suggestion?

- Thanks,
CList