VC envelopes, MIDI2CV

Colin Fraser colinf at
Tue Mar 18 14:24:29 CET 1997

I have used a VC slew limiter composed of a LM13700 as the basis of an
envelope generator with no problem. 
I have been told by others that there is a shift in DC offset of an OTA
when the control current changes that can cause glitches in the output
of EG's built this way, but I have never noticed any problem with mine.
The rest of my VCEG comprises a 4053 analogue switch for switching the
target levels and slew rates to the slew limiter, a dual comparator for
detecting gate signals and the end of attack phase, and a D-type
flip-flop to select between attack and sustain phases. I could knock up
a quick schematic on Protel if you're interested.
I would think a few vc slew limiters under processor control would make
a fine multiple EG, and would certainly be preferrable to software
generated envelopes. You would need to give the processor some way of
detecting the output voltage of the slew limiter, either with an ADC or


>From: 	Mikko Helin[SMTP:MHELIN at]
>Sent: 	Tuesday, March 18, 1997 1:04 PM
>To: 	synth-diy at
>Cc: 	MHELIN at
>Subject: 	VC envelopes, MIDI2CV
>   Hello list,
>   Do you know if it's difficult to use a VC slew rate controller as an
>   generator when connected to two DAC's which are then controlled by
>computer or
>   microcontroller? I have an idea to use a multichannel DAC (32 channels) to
>   control four channels (mono or poly) of analog stuff and generate eight
>   envelopes with VC slew rate limiters. For one EG two DAC channels are
>   needed, one for rate and the other for actual data values. I guess
>   or even optocouplers (4N37) could be used to control EG times. I've also
>   a little MC51 compatible MC with 128 bytes RAM and 2 kB FLASH in a 20 pin
>   (Atmel's AT89C2051 -> that seems easy to be used in this
>kind of 
>   project. I think 8-bit DAC is okay, if control range of slew limiter is
>   enough, but only 128 bytes of RAM may be a problem.
>   Mikko

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