[sdiy] cheapest DIY phaser. Was: band reject filter.

[Dave Krooshof]

Phasers are everywhere...
>> I've noticed that using a band reject filter (I use negative gain on
>> a parametric equalizer) and sweeping the frequency sounds a lot like
>> a low resonance phaser.  Is this due to the negative interference of
>> a phaser cutting out different frequencies as it comes in and out of
>> phase?  How exactly does a phaser work, anyway?
>>
>> -Tavys
>A phaser is one or more notch filters, that have there frequency swept up
>and down by an LFO, so what you were doing is essentially making a very
>crude phaser.
>
>Seb Carr

In a phaser, a delayed signal is added to the original.
The delay is tiny, you usually want it less then a wavelength of the input.

While adding a delay over an original, you'll end up having a pattern
of frequencies that are cancelled out, and added in.
This happens, because a certain frequency might be delayed just so much
that it appears to be the opposite of it's original. If this happens at -say-
300Hz, It will also happen at 600, 900, 1200, etc... if the phaser behaves
according to the books.
This is called a combfilter.  _/\/\/\/\/\/\

As the ear perceives patterns rather then actual frequencies, the
'distance' between the cancellations is audible as a tone.
so if there's a 300Hz distance between each cancellation, you'll hear
that tone, and not 600, 900, 1200, or higher.

But if the phasing is not to heavy, it will be perceived as something else.
If someone is speaking to you at some distance from a wall, you'll hear the
voice directly and reflected via the wall, which is thus delayed.
And yes, here is a combfilter as well. But you do not perceive it as such,
nor would you perceive it as a tone. Your brain reworks it into a spatial
3D image of a room with a soundsource in it, and -alas- doesn't lecture
your concience with mathematical thoughts on combfilters.

But as soon as the speaking person starts moving about, you might hear
the combs shifting, you might start hearing not only a moving soundsource
in a space, but also the fenomena itself: combfilters, phasing.
And if it happens real close, you'll notice the comb is a tone.

If you can't talk the speaking person into banging his head to the wall,
you can also fix the distance of the speaker to your ears
and move the reflecting wall at a close distance:
Move your hand to and away from your mouth while you sing.
If you'll do it slow, you'll hear your hand a moving. If you do
it fast, you just made the cheapest DIY phaser.


Dave Krooshof




Ps.1
Bigger hands phase better. You can use cd cases or books as
hand extensions. Do it alone and switch off the webcam, people
might think you're going mad.




Ps.2
Take care: You'll might start noticing that you can hear trees and
streetlights as you pass them by the filtercharacteristics of their
reflections. You can't tell anyone about it as people will think you're
mad. (It all started when he began discussing his synth on the web)








Ps.3
"I'm not mad, I was f.i. never aware of moving combfilters while making love.
Come to think of it, your voice must be phasing while at it. So this is why
http://www.till.com/articles/moog/patents.html#US03800088 sounds so
intimate"








http://www.combfiltersociety.org/psycology-tragedies/freedavenow.html









http://www.xs4all.nl/~krooshof