Need help!

[Paul Schreiber]

This is a good fundamental question, and I'll take a stab at it:

VCAs: can be linear or exponential. Most are linear, because the envelope generators are outputting exponential
waveshapes. The ear has a more exponential response to relative volume (the "Fletcher-Munson" curve). Linear
envelopes are easier to generate in software (just a counter), so an exponential response VCA makes sense. In
reality neither is "correct", its that a exponential EG feeding a linear VCA is easy to do from a hardware standpoint,
and the 'natural-ness" of the result seems OK.

VCF: this is a tricky one, because sometimes you DON'T want the filter to track the VCOs. The Minimoog is a good
example: you can selrect no tracking, 1/3, 2/3, or tracking. The sounds are different on each setting. If the VCF
is not exponentially tracking the VCO, the timbre ("tone") of the sound will change from octave-to-octave, which MAY
be the desired effect. Most orchestrial instruments only have a useful 2-3 octave range anyway, so you only need
to track a narrow range. That's why the exponential generator in a VCF is not as critical as a VCO: if it drifts with
temperature, the timbre change may not be detected. 

Yes, to get a vibrato effect, the analog LFO output is summed with the incoming CV signal. The input stage of the
VCO does this.

Paul Schreiber
Synthesis Technology


----------
From: 	Chris Prine[SMTP:cprine at bit-net.com]
Sent: 	Sunday, February 22, 1998 5:04 PM
To: 	synth-diy at mailhost.bpa.nl
Subject: 	Need help!

Hello, I'm new to this list and diy but that hasn't stopped me from
attempting to build my own synth.  I was hoping you guys could help me out.
I'm confused about linear and exponential.  I've read about how it works on
a VCO (octaves are exponential), but what about a VCF, and VCA? Why would
need a converter with those?
Here's the layout of my synth: 2 VCOs, a VCF (controlled by an LFO, or
envelope generator), a VCA (controlled by ADSR).
I'm going to control the whole thing with Paia's MIDI to CV unit.
How would I use an LFO to get a vibrato effect? Would I mix it with the CV?



      `~.,;'~/   Chris   Prine   * cprine at bit-net.com \~'`,.~`
"Smash the control images, smash | UIN: 666253      
  the control machine..." -WSB   | http://www.bit-net.com/~cprine
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