I’ve always thought it strange that the Voyager gate needs to be set to ON/EXT for external audio to come through without the envelope(s) closing. Why isn’t there an envelope detector for the external audio to allow these external sources that have an envelope of their own (like a drum machine or voice) to trigger the filter envelope (for example)?
…or is it possible, and it’s just one of those options on the Voyager that I haven’t found yet?
This would need some additional hardware. It is not doable via software, as the audio in is not converted to the digital domain in the Voyager.
That’s what I was afraid of.
You could keep an eye out for one of these.
http://www.rlmusic.co.uk/rlm3/korg-ms-03-signal-processor/
Would the MF101 filter or MF107 freqbox get you some of the way there. They both have env out.
Well, there’s this:
All you really need is an amplifier to get the gain to 5V, a rectifier to remove the negative excursions, and a Cap & Resistor low pass to smooth out the original signal into some kind of envelope that would satisfy the kbd gate condition. Basically an envelope follower without a gate output. Add a one-shot and there’s your gate!
But if it was me, and it will be sometime this year when I add a 5U rack 
, I’ll just buy an envelop follower like GregAE posted.
If you know how to build that, why would you buy something that is only going to cost more?
I value my time more. I did hardware for 35 years, I’m retired
Fair enough.