Multiplexing circuit in CV interfaces.

[msi]

I've been looking at schematics for a couple of synths namely the Juno 106 and the Wildcat modular synth. The Wildcat synth schematic can be found at http://synthdiy.com/files/2008/efm-wildcat.pdf. The Juno service manual can be found at http://www.hinzen.de/midi/juno-106/manual/juno-serv.pdf. On page 6 of the Wildcat schematic and on page 8 of the Juno schematic are the areas I'm speaking of. The wildcat and Juno both use a CD4051 to multiplex the CV signal into multiple control voltages. My understanding is the capacitor attached to the opamp immediately following the CD4051 maintains the voltage. Does the CD4051 have to continuously switch back to each of the seven outputs to maintain the CV voltage or is there something else I'm unaware of going on?

Following that, is there a way to implement this with the Midibox in conjunction with the AOUT_LC. How fast does the switching need to be to have it be seamless?

[Imp]

The CD4051 can't keep voltages on outputs, which aren't active, AFAIK.

I think those outputs behave like they aren't connected to anything.

So you need that cap to maintain the voltage until that output goes active again and the voltage is renewed or changed.

These multiplexers are used on the AINx4, to increase the number of available analog inputs. See here.

It hasn't been used for CV output for performace reasons, i think. Maybe the switching leads to zipper noise.

[jojjelito]

The capacitor output voltage will droop slightly over time so it needs to be refreshed (just like the capacitors in memory cells in a DRAM for instance).

Normally you connect those outputs to FET inputs somewhere else to mitigate that effect.

The zipper noise won't be that bad if you update often, so Nyquist is your friend as always. But here comes the performance issue:

Something (interrupts or a loop) needs to periodically update these capacitors

This is how it was done in most polysynths and the occasional mono as well. See the Prophet VS for an over-the-top example of S/H taken to the extreme.

[xarolium]

On the juno 106, there is AOP with the capacitor how S/H the voltage from the 4051.

Look at here

Edited August 16, 2012 by xarolium