[sdiy] audio to CV

[media at mail1.nai.net]

At 4:39 AM -0800 01/02/02, Derek Holzer wrote:
>
>DIYers,
>
>perhaps this is a 'newbie' question...

Don't worry, I ask questions so obtuse, the EE's on the list must think I'm
using crayons :)

>i'm working on a project to use audio sources derived from radio signals as
>control voltage. (i.e. patch from radio receiver audio out to synth CV in)
>i've investigated a sample-hold module (thanks, rene schmitz!), which i am
>building right now. what other devices are good for this...

You might want to look at the ESP section of an MS-20.  Sending the audio
output of a television into it was a bit of a fad awhile back.  Both pitch
followers and envelope followers turn audio into CV.

>a comparitor?

A comparator will produce a pulse waveform.  It will swing all the way one
way if the input is above threshold, and swing back all the other way if
the input is below threshold.  It's one way of getting a gate or trigger
from the output of an envelope follower.

>if i build a simple AC/DC diode recitifier, should i follow it with a
>>smoothing cap? other suggestions?

It sounds like you are trying to build an envelope follower.  Imho, the
best way to do this is to use an audio specific "VCA" chip (which are
expensive and very hard to buy), or an optical element, such as a vactrol
(which imho are used to drive the best envelope filters, such as the Mutron
III).  You might want to look at schematics for envelope followers, and
devices that contain them, such as compressors, envelope filters, octave
pedals, etc.  Yes, following a rectifier with a lag circuit is one way to
do this.  The compromise is that if it is used to control a VCA or VCF you
will end up with "ring modulation" or "FM" if the output is too fast, and
it won't track closely enough if the output is too slow.