Bugbrand DD1 LPG is an envelope follower!

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Bugbrand DD1 LPG is an envelope follower!

Post by otoskope » Thu Nov 10, 2011 4:43 pm

Hi all.
I've been wanting an envelope follower in my bugmod system for a long time. But for now I have to rely on various tricks. I just found out a new one, best so far, I think.

This relies on a DC-coupled LPG being abused, using the vactrol as both a rectifier and a smoother. Since the bug LPG's are advertised as AC-coupled, meaning filtering out incoming DC with a capacitor on the input, I thought I had to bypass this capacitor... but - it turns out the rev.2 DD1 LPG is already DC coupled (thanks Tom for pointing this out)! :banana:

*Feed +DC into the signal in (I use a DC from the joystick module, but any source within the system should do).
*Feed the audio to be followed to CV in.
*Set the Init knob to 0, and use the Mod knob to adjust the output level.
*Set it to "lp" or "both" mode (slightly different response)
*Voila - output gives an envelope follower with quite some slew on the downslope...

Here's an oscilloscope picture - lower signal is fed into the CV in of the LGP-env-follower, upper signal is the the output from the LPG module. This was done in "both" mode, but it seems "LP" mode is slightly better, actually.


Yes, the down-slew is slow, and it takes a few seconds to reach zero. Three work-arounds for that:
* offset it downwards, possibly through a diode, to remove the very low parts
* use an exp VCA to modulate something else
* square the signal with the modulator that is conveniently located in the same module! Do this by simply sending the LPG out to both X and Y modulator inputs and setting both levels to max. This will transform the response, scaling down the low levels. This is the best solution, I think - see this oscilloscope pic, using LP mode, and showing the squared env CV on top:


If you set the LPG to "gate" mode, the response is too fast, so you get AM ripples on top of your output CV. Which might be interesting in another way...

Some implications:
* The DC in to the env-follower-LPG could be varied, allowing gating or shaping of the CV without any extra modules.
* Feeding negative DC instead of positive gives an inverted envelope output, which can be patched into another LPG to create a simple compressor or ducker.

The brain is always greener on the other side.

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