Filter ICs and crossfade circuits

[Andrew Schrock]

On Tue, 25 May 1999, Michael Potas wrote:
> I have noticed that most state variable filter circuits use the CA3080
> and not the LM13600.  Do the CA3080s sound better?  The LM13600s work
> out much cheaper, having dual OTAs and buffers in the same package,
> compared to just a single OTA for about the same price. They also taking
> up a lot less PCB space to implement.
> 
> I want to build a CV controlled crossfade circuit.  The idea is that I
> can smoothly fade between two input waveforms to create a single output
> waveform.  Hence using say Trig and square wave inputs, you can morph
> between one and the other using CV.  My plan is to use two  VCAs fed an
> opposite polarity CV signal.  Has anyone done this, and does it work OK?

Yeah.. I actually want to do a QUAD VC-crossfade circuit, with the audio 
inputs amplitude controlled by default with a joystick. When C/V is
inserted, it bypasses the voltage output by the joystick. This would be in
my mind beautiful for mixing, say, HP/NP/LP or saw/tri/pulse/sine outputs.
I forget who I was discussing mixing waveforms with a few months ago, but
I was trying to argue that it's better to have just output jacks and no
mixing ability.. was I ever wrong... 

For a dual crossfade, just run the second VCA through an inverting opamp
at unity gain. Should work dandy as long as you trim the C/V on both of
them correctly. You could always use the dual-opamp + 3080 method, but
it's a little noisy IMO. Definitely usable tho! 

Andrew

-| Andrew Schrock | aschrock at cs.brandeis.edu |-