[sdiy] Saw DCO

[Fredrik Carlqvist]

Thanks for the suggestions. I think I will try the reference Vbe method
first though, as it requires virtually no change in the hardware. 

Richard, are you suggesting leading the Vbe of another OTA into the
trimmer's position on the integrating OTA? Could that work? Or perhaps even
the OTA's own Vbe into the '-' input!?! At least, the temperature dependency
could be reduced. 

The basic idea was to implement a saw output with minimum additional
hardware. All I added was a cap, an OTA, two resistors and a trimmer. I
already have even amplitude over the octaves, only not perfectly
temp-stable.

I think a control loop will be too slow even though I agree it would solve
many problems (and freeing some precious code memory). When the pitch
changes drastically, it will take at least a couple time constants to
adjust, which would be 100ms with the lower freq limit 20Hz. The intricate
system for speeding it up seems a little complicated to me. But I maybe
missed the point. 

John, the hard synch comes in by multiplying the saw slope by a scale factor
K larger than 1. This makes the saw hit the roof and reset. It is still
reset every DCO cycle. The resulting frequency is a combination, just like
in normal hard sync, where a faster oscillator is reset by a slower one. The
fundamental is at the DCO frequency f, and there is an additional
"fundamental" at f*K. 

This is the greatest list, several good and interesting suggestions on a
post like this. Take care!

Fredrik C