[sdiy] Phase correctors

[cheater cheater]

Magnus,
that's all exactly why I want to start experimenting with phase correction.
Would you be able to direct me to some papers or simple circuit
schematics? Unfortunately I'm very electronics-agnostic but anything
you throw at me I'll understand in (0,+oo) years.

Cheers
Damian

On 7/18/08, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> cheater cheater wrote:
>
> > Hey guys,
> > does anyone know of any variable phase correctors?
> >
> > By phase correctors I mean the things usually put in EQs that correct
> > the filters' phase response. Basically, the phase correctors have a
> > flat frequency response, and the phase shift rises (or falls) as
> > frequency rises (or falls).
> > Unfortunately, I wasn't lucky enough to find one schematic ...
> >
> > Those things look interesting, though:
> > http://www.springerlink.com/content/g67w778950831075/
> > http://www.springerlink.com/content/r5537667429tu61k/
> >
> > http://sound.westhost.com/pcmm.htm Look at fig 2.5 - the
> phase delay
> > drops (and becomes 'negative') with frequency rising, at a certain
> > rate. The idea is to be able to control this rate.
> >
> > I briefly thought about just taking such a simple all-pass filter and
> > simply varying the 'wet' level, so the mix between the output of the
> > APF and the original (dry) signal, but then figured that it would just
> > create something to the effect of an EQ instead.
> >
>
>  By doing that you have created a null, just as with a phaser... you don't
> want to do THAT.
>
>
> > What do you guys think? Is this possible? Even by simply using
> > different element values (and switching between them), so not being
> > fully variable, but still allowing some degree of freedom.
> >
>
>  You can do it. You can create one-pole and two-pole fully parametric phase
> correctors and they are not even hard to control. They are just as easy as a
> fully parametric EQ, but with the twist that the level control is at a fixed
> value. For one-pole you have only a frequency control, for the two-pole you
> have frequency and Q/width control. It is essentially a special case of an
> EQ. You do want a bypass-switch for each section.
>
>
> > What are your favourite designs of all-pass filters? Maybe some of
> > them would be more pliable for modifying the phase rolloff.
> >
>
>  For one-pole, a very simple one-opamp design is possible, which is the
> standard of the shelf 1 pole AP found in many design examples.
>
>  For a two-pole, a state-variable design might be the best choice. Need to
> think about it.
>
>  The trouble with phase correctors is that they can be a bit hard to verify
> their tuning. If you don't have the right tools you might just fuck things
> up more than you repair them.
>
>  Well used full-parametric EQs can be used to "move" troublesome poles or
> zeros out of range. This requires very good tools and experience, but can
> help cleaning up responses. Using the propper tools could also with help of
> an imaginative mind and lots of experience help solving STRANGE resonances
> and cancelations. Start to think in terms of where poles and zeros are in
> frequency and damping and ways to move them or damp them out makes you look
> very differently on acoustics and transient responses. I encourage you to
> learn more about it.
>
>  Cheers,
>  Magnus
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