another idea

[Guest kostix]

hey guys

i've been thinking of this little hardware addon. must be easy to build

the thing is, in place of a pot on an analog input, u connect another circuit.

the circuit consists of a bandpass filter, and another gain stage.

first, u input a line level audio,

with the bandpass filter u only get the frequency range u want.

the gain stage is just for normalizing the level to be +5v max peak.

also i assume you would need somekind of a rectifier, to get DC kind of signal out of the AC music signal. this could be easily achieved with a diode (of course, compensating the forward voltage drop of .7v (.3 on a germanium))

with the proper setup u can have a *sidechain* kind of input to "compress" some signal on the presence of another.

or you can simply use it to modulate some controller by another signal..

oh well u get the idea :)

thats bout it

any suggestions are welcome

cheers

[pay_c]

Yepp, in modular synthesizers your idea is called a "EIEF", meaning: External Input Envelope Follower

There are kinda many schematics out there doing that (the most ones without bandpassing!).

Anyhow: The schematic is NOT that easy! The bandpass filtering, the input / output stage, the envelope filtering and so on should fill up at least 1/2 of a 160x100mm PCB in best case I think.

[Guest kostix]

but yet.. its doable :)

it can be an external addon box aswell

but its interesting

i'll try implement it some of these days

[AndrewMartens]

It might be possible to achieve in less space:

- input buffer, single opamp

- bandpass stage, OTA, possibly another opamp

- you might be able to find an IC that measures AC RMS and outputs a DC voltage and use that

But yeah, essentially what you're looking for is an envelope follower schematic.  Tacking a bandpass filter on in front of it just allows you to isolate a certain set of frequencies (which could be handy in some circumstances)...