[quote author="StephenGiles"]Thanks xvlk, I thought we might be in line for a new phaser sound here!
Stephen[/quote]
The second order allpass from Orban s optimod is here:
http://sweb.cz/vavak3/semestralka/orfc.tif
Orban uses it to improve crest-factor of the speech.
Posibility to modulate second order allpass is this.
If you know reactancy transform, you can all devices in some circuit
with voltage transfer /hard feed, soft load/ multiply with p without change
of transfer. If you do it inthis allpass with group of R and C in the bottom,
you change capacitors to resistors and resistors to inductors.
Note, that resistors are two with the same value.
If you do substitution (p=1/p), the inductors became capacitors
and frequency became inversion, but allpass is still allpass.
This second order allpass can be modulated via paired optocoupler.
Only capacitors must be accurate and you must combined, but
for one piece you can do it, for serial production you must have it wounded.
This is circuit:
_____ ____
---------|_____|--X-|____|--|
| | |
| ---|- |
| | | | |____x_
--------X-----| |-----------X--| | |
| Rf | | Rf | |- |
| _____ ____ | |
|--|_____|-x-|____|-| __|__
| _____
| |
x-----------------------
|
__|__
_____
|
__|__
It is hard to make semigraphics with proportional font, but think, that
I explained it for You. In Orban schematics it is R24,R25,R26 which
changes to capacitors and C6 and C7 which changes to optoresistors
/matched optocouplers/ To compute values of capacitors you must
work on normalised prototype (this is filter with unity frequencies
and unity resistors/. You can read Huelsmann, if you don t know what
normalisation is.